6.3.1 Heavy-liquid bubble chamber for WIMP detection

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Low background Screening and Prototyping Facility
at the Soudan Underground Mine.
4.1
L. Baudis D. Bauer , J. Beaty , G. Bengtson4, J. Collar5, P. Cushman3,
4
J. Davis , D. Demuth6, L. Duong3, R. Gaitskell 7, D. Hestetune4, A. Lu8, M. Marshak3,
J. Meier3, C. Michael4, W. Miller3, H.Nelson9, E. Peterson3, J. Reeves10, A. Reisetter3,
R. Schnee11, T. Shutt12, A. Sonnesheim5
1
2
3
1
University of Florida, 2 Fermilab, 3 University of Minnesota-Twin Cities, 4 Short Elliot
Hendrickson Engineers, 5 University of Chicago, 6 University of Minnesota-Crookston, 7
Brown University, 8 U.C. Berkeley, 9 U.C. Santa Barbara, 10Reeves & Sons, LLC., 11
Case Western Reserve University, 13 Princeton University
Table of Contents
1. Project Summary
2. NUSEL
3. Overview of Low Background Counting
3.1
3.2
3.3
3.4
Introduction
Gamma counting
Alpha and Beta Counting
Prototype NUSEL screening
4. The Soudan Underground Laboratory
5. The Low Background Counting Facility
5.1 Overview
5.2 Active Veto Shield
5.4 Veto Shield DAQ
5.4 Water Tank
5.5 Purification System
5.6 Shielded Multi-purpose Room
5.7 Radon Reduction
5.8 Copper Electroforming
5.9 High Purity Germanium Detectors
5.10 Beta Cage
5.11 Cloud Chamber
6. Scientific Program
6.1 Gamma Screening
6.2 Beta and Alpha Screening
6.3 Staging new Prototype Experiments
6.3.1 Heavy-liquid bubble chamber for WIMP detection
6.3.2 Liquid Xenon Dark Matter Prototype Experiment: XENON
7. Management
8. Outside Users
9. Timeline
1. Project Summary
By capitalizing on the active veto shield and experimental hall of the Soudan2 detector,
screening instruments of unparalleled sensitivity to low level radiation can be established
at the Soudan Underground Mine at a modest additional investment. This would serve
several purposes:
1) Provide alpha, beta and gamma screening of materials on a rapid timescale for current
users in the underground science community (e.g. Majorana, CDMS, XENON,
ZEPLIN...) as well as the larger community (geomicrobiology, semiconductor
industry, homeland security, public health and environmental monitoring)
2) Test and participate in developing a new counting instrument that advances the current
technology for alpha and beta screening (either a cloud chamber or a wire chamber)
that is immediately useful for the CDMS II detectors and their upgrades.
3) Provide experimental facilities with deep characteristics (via active muon and
passive neutron shielding) for prototype tests of the next generation dark matter,
double beta decay and proton decay experiments using novel technologies such as
liquid noble gas and liquid bubble chamber technology.
4) Advance the techniques of low background screening and shielding for the next
generation NUSEL facility.
The removal of the Soudan2 Proton Decay detector will immediately free up considerable
real estate in a detector hall of dimensions 40 ft x 30 ft x 235 ft, located at a depth of
2090 mwe. An active muon shield encloses 100 ft of the hall with muon rejection
efficiency of 99.5 % on the ceiling and walls, and 95% from the floor where structural
supports interfere, producing a volume that is at effectively twice the Soudan depth with
respect to the residual muon rate. The shield is in working order, but requires an upgrade
to the readout system.
Additional passive shielding is needed for neutrons and gammas from both cavern
radioactivity and muon interactions in the cavern rock. We will build a unique large
purified water shield into which detectors are lowered. The background will be limited
only by water purity, and thus will provide a lower background environment than
standard Pb and plastic shielding. This will provide as much as 4 meters of shielding in
the center of the tank, reducing the high energy neutron flux by two orders of magnitude.
It will eliminate the need for expensive lead (including ancient lead) shielding around the
instruments. A clean room will be assembled on top of the tank with an adjoining
mezzanine for assembly and staging. The beta and new gamma screener will be installed
in the tank with access from the tank clean room. There will also be room for cubic
meter size experiments to be lowered into the tank.
A clean room will also be built adjacent to the tank on the main floor. Since the water
tank already provides excellent shielding from the south wall, a set of shielded bays will
be located at its base, inside the clean room coverage. This room will serve as an
experimental area for prototypes which require deep underground environments, reducing
or eliminating their need for expensive shielding. Since we will tailor the bays to the
immediate needs of the incoming experiments, we expect that much of the additional
shielding will be provided by the experiments themselves.
Existing 7.5-ton crane coverage will be used to construct the tank, the shielding bays, and
mezzanine. In addition to the prefab clean rooms, sample preparation, storage facilities,
and radon reduction must be put in place, in order to keep particulate and airborne
sources below the newly-established background levels. An existing surface building
provides remote computer access to the underground experiments and additional space
for offices, storage and preparation for users.
The CDMS II experiment requires immediate use of a beta screening instrument for the
solid state dark matter detectors, in order to measure backgrounds in existing detectors
and improve the fabrication process for new detectors. Two devices for beta screening are
under consideration: a drift chamber referred to as a beta cage, and a cloud chamber. The
new beta cage design is a neon drift-chamber with multi-wire proportional readout.
Samples are placed directly in the gas. A set of trigger grids directly above the sample
surface tags events that originate at the sample. A drift region beyond this is read out by
a set of wire readout grids at the top of the chamber. Crossed grids will be used for x-y
position determination, and timing can be used to determine the spatial profile of the
track in the z dimension. A cloud chamber technology is also being developed because it
is a continuously sensitive volume and provides detailed tracking information. The
principal obstacles in the cloud chamber involve long term stability of the device.
A gamma screening facility using two HP Ge detectors is already operating at Soudan. It
is being used to screen materials used for the Majorana and CDMS II experiments. There
is significant backlog on this program as well as additional demand from other users for
gamma spectra counting facilities, and so we propose increasing the facility by 2
additional detectors. In addition, users will be able to install their own screening
instruments (e.g. alpha counting or NAA) as desired in the multi-purpose clean room, and
utilize the shielding and infrastructure provided.
Ultrapure copper is required for shielding the lowest background counting stations, for
building water enclosures, for the Xenon cryostat and for CDMS III shielding needs. Yet
the radiopurity of copper is limited by radionuclides produced by cosmic-ray induced
interactions above ground. We will therefore house a station for electroforming copper
underground in order to produce our own supply of ultrapure copper.
2. NUSEL
Underground science is a multidisciplinary field of scientific and technological studies. It
comprises both the study of the underground environment itself which covers geology,
rock mechanics, geophysics, geochemistry, geohydrology and geomicrobiology, as well
as utilizing the unique properties of the underground environment to probe for extremely
rare events: experiments which search for dark matter, nucleon decay, or neutrino-less
double beta decays, and experiments which measure neutrino properties.
Although there are opportunities for significant progress in basic and applied
underground science, the US now trails the world in the number of facilities where such
science can take place, with only Soudan and WIPP (Waste Isolation Pilot Plant) in
operation. Since the beginning of 2001, at least five panels and committees have
recommended the establishment of a National Underground Science and Engineering
Laboratory (NUSEL) to address this lack, but the timescale for site selection and actual
funding continues to slip. Thus it is important to realize that progress in underground
science can only be made by utilizing our existing resources in the most efficient manner.
We are therefore proposing to begin work on an identifiable NUSEL component
immediately by housing it an existing US underground lab and building a prototype that
will advance the state of the facility technology while serving a variety of near-term
physics goals, such as improving the sensitivity reach of the ongoing Cryogenic Dark
Matter Search, deep site characterization and first physics run of the XENON prototype,
and developing bubble chamber technology for dark matter searches.
Every NUSEL proposal includes a state-of-the-art low background counting facility1,2. It
is impossible to achieve the full benefit of the large overburden which eliminates cosmic
ray-induced backgrounds, without eliminating natural, manmade, and cosmogenic
activities which may also contribute to the detector background and interfere with the
physics measurement. A screening facility to identify and provide quality control in low
activity materials for detector fabrication is essential to the success of the next generation
of dark matter searcher, solar neutrino and double beta-decay experiments. An on-site,
underground electroforming facility, which is being developed in collaboration with two
successful SBIR’s (Reeves & Sons, Inc), will both improve the screening instrumentation
itself by providing cost-effective ultra-pure components, as well as producing clean
shielding for experimental prototypes.
Such a facility can also serve a multitude of needs outside the nuclear and astro-particle
physics community, such as monitoring trace radioactive elements in the environment,
serving nuclear nonproliferation needs, and aiding in the manufacture of commercial
ultrapure materials. It is expected that once such a facility is in operation, the costs can
be recouped by fees from a broad consortium of users. The last section of this proposal
contains a first step toward identifying actual users. Their interest is indicated by their
letters of support.
3. Overview of Low Background Counting
3.1 Introduction
In the US, Lawrence Berkeley National Laboratory operates an underground counting lab
inside the Oroville dam. This facility is now more than 15 years old, and it is not clear
how to modernize it in order to reduce the background further. Pacific Northwest
National Laboratory has operated Ge detectors in various underground locations and
some university groups and national laboratories perform low background counting either
above ground or at very shallow sites 3. What is lacking is a coordinated program giving
access to the broader scientific community. This immediate need would be addressed by
the conversion of the old Soudan2 cavern into a multi-user facility, along with developing
a web-accessible database with inter-site scheduling functionality. The addition of onsite neutron and gamma shielding, combined with the existing active muon shield,
upgrades the moderately deep Soudan site into a facility with deep counting
characteristics, unique in the United States. In order to do deep-site counting now,
groups must go overseas. Often this is not a viable option for the security-related
applications. Although NUSEL will eventually address this issue, the need for additional
sites is immediate. A staged approach to the final NUSEL background facility will open
up badly needed screening options on a short time scale, as well as give the community
experience in setting up such facilities with a view towards improving their design in a
final NUSEL proposal.
The new generation of experiments will require material testing which is a factor 100 or
even 10000 more sensitive than what can be done with existing Ge detectors. The
construction of what may be termed a ``next generation'' counting facility, utilizing large
plastic scintillators shielded by a water tank or a large liquid scintillation detector
modeled on the Borexino collaboration's Counting Test Facility at Gran Sasso, could
serve these needs. The Soudan facility would provide a first step toward understanding
how to build the final facility by employing some of the same techniques as an R&D
project, while actually serving the counting needs of several immediate users. We
therefore propose to dedicate one of the ports in the water tank to a liquid scintillator
lined acrylic box read out by low-activity MRS APD’s. It will monitor the neutron
background inside the tank as well as samples placed within. The box can move the
length of the tank in order to measure the gradient of the neutron flux.
3.2 Gamma counting
Low background gamma ray spectroscopy using Ge semiconductor detectors is a welldeveloped and mature technology which has served as the prime tool for material
selection. Sensitivities down to a few hundred ppt of U and Th are routinely achieved
using commercially available detectors 4. The outstanding energy resolution gives these
detectors high diagnostic power. This makes them an excellent choice for counting
applications where radioisotope identification is important. All current generation solar
neutrino, dark matter and double beta decay experiments have been relying heavily on
this detection technique. 5,6 High sample through-put (equivalent to the availability of
multiple counting stations) and good diagnostic power are needed to fulfill this task.
Currently we are operating two high purity Ge detectors in a conventional lead shield
with nitrogen purge behind the MINOS detector. As is the experience of all who put
together these systems, the need far outstrips the capacity. Both CDMS and Majorana
screening needs are now seriously backlogged and screening of photodetectors and
chamber materials for the XENON prototype are also waiting for slots. We propose to
enhance the existing capacity with the addition of two more detectors as well as
eventually improving the background capability of the existing set of two Ge detectors by
moving them under the muon shield. We expect that the new Ge detectors will be able to
probe the few ppt domain and at least one will reside within the water shield.
3.3 Alpha and Beta Counting
Compared to gamma screening, techniques for low background counting of betas are not
as well developed. Thus, experiments that require screening for alpha, beta or x-ray
emission that is not accompanied by gamma emission are not well served. Low energy x
rays are a background for many types of low-background experiments, especially solar
neutrino projects. Assaying materials for low-energy x-ray emission in order to eliminate
such activities or to quantify the effect is required. Another example is identification of
210
Pb contamination. 210Pb and its progeny do not have a penetrating radioactivity
signature. Since Pb is often used in circuitry and alpha activity within the Pb can create
difficulties for the circuit, there is a need to search for samples of Pb that are low in 210Pb.
210
Po will eventually result from any 210Pb contamination, and its alpha decay can cause
single-site upsets in any circuit that contains the host Pb.
The dominant internal background in CDMS comes from low energy betas from surface
contaminants such as potassium or carbon. Inefficient charge collection produces an
ionization signature that resembles that of the potential WIMP signal. Screening of the
detectors would provide information to improve fabrication of the next generation of
cryogenic silicon and germanium detectors, as well as to understand the background of
the existing towers. Thus, beta screening is a high priority item for CDMS, and efforts
have already produced two possible technologies which we would like to pursue and
which would eventually be located at Soudan to serve CDMS and other users. The wire
chamber beta cage design has been separately proposed by Tom Shutt (CWRU) and
Steve Elliot (Los Alamos). We will seek other funding for its development in
collaboration with the above institutions as well as Caltech. The shielding is provided by
the water tank and would not have to be separately procured. It is important to note that
the cost of conventional lead/poly shielding appropriate for the beta cage turned out to be
comparable to the total water tank cost, yet the water tank will serve more than just one
screener or experiment. A second, large-volume multi-wire device for atmospheric
samples will be designed using the experience gained from the smaller instruments. This
screening device will most likely remain in one of the shielded bays. The cloud chamber
technology is being pursued by Harry Nelson (UCSB) and Priscilla Cushman
(Minnesota), possibly as an SBIR with Jeff Radtke of Supersaturated Environments, Inc.
3.4 Prototype NUSEL Screening
The next generation low background experiments (solar neutrino, dark matter and double
beta decay) will need to reach background levels that are far below what can be screened
for even in the best Ge counters. To achieve these levels a variety of screening
techniques will be employed, including specialized use of "chemical" methods such as
mass spectrometry and NAA to check for certain radioactive isotopes. These techniques,
while powerful, do not provide an ultimate check on the total activity from all isotopes in
the material, including short-lived isotopes which are essentially impossible to detect
chemically.
An advanced direct counting screening technique with orders of magnitude improvement
sensitivity would therefore be extremely important for essentially all next-generation low
background experiments. For the case of experiments based largely or purely on
advances on material purity, such screening would play an essential enabling role. We
propose to begin development work on one of the ideas presented at the Conference on
Underground Science. We propose that one of the detectors which would be available to
lower into the tank would be an acrylic sample box lined with liquid scintillator. Light
guides, which also are the structural support for the sample box, direct light to external
PMTs. For a sample volume is 0.5 sq meters, the estimated sensitivity, for a plastic, is
better than 1 ppt U and Th. This scheme has the advantages of low cost, 1-2 order of
magnitude sensitivity improvement over current Ge counters, and large sample volume.
4. The Soudan Underground Laboratory
The University of Minnesota has operated the Soudan Laboratory in northeastern
Minnesota since 1980. At the mine, science is a symbiotic activity with a State Park,
which hosts about 40,000 visitors a year for both its historic and science tours.
Minnesota State Parks has maintained access to the mine for historic tours since 1962 and
added the science tours in 2001. The underground complex of the Soudan Laboratory
currently consists of two fully-equipped laboratories at a depth of 710 m. One laboratory
(15m x 15m x 70m ) houses the Cryogenic Dark Matter Search (CDMS II) and the
decommissioned Soudan 2 Proton Decay Detector. The adjacent laboratory (15m x 16 m
x 90m ) contains the 5,500 tonne MINOS Far Detector for the Neutrinos at the Main
Injector (NUMI) Oscillation Search and a small gamma screening facility for Majorana
and CDMS. In addition to the experimental offices and counting rooms, there is a 2-story
office complex at one end of the CDMS II hall which houses the lunch room, conference
facilities and the computer networking infrastructure. A second-floor visitors’ gallery
lines the edge of the MINOS hall, with MINOS and lab management offices off the
gallery. There is a machine shop, welding facilities, detector assembly and storage areas.
Both halls have 7.5 ton crane coverage
Figure 1. The Cavern Layout of the Soudan Underground Science Laboratory.
There is an existing 640 m2 Soudan Surface Building, which has been serving as a
receiving facility and warehouse for MINOS. On the mezzanine level it now also houses
the remote counting room and computing farm for the CDMS II experiment, with a direct
fiber link to the underground laboratory. This under-utilized space can easily be
converted into sample storage and preparation areas or to provide remote access to
underground experiments for new users of the counting facility.
5. Low Background Counting Facility
5.1 Overview
The southernmost 35 m of the Soudan2 cavern are encased in a 1700 m2 working muon
veto shield. The kiloton Soudan2 proton decay experiment is a modular steel/drift tube
array which presently occupies the central space. Catwalks to the east and west, and a
northern mezzanine provide access to the muon shield electronics. The removal of this
detector would take several months at the cost of up to $100k. Due to the soaring steel
prices, this cost will be recovered or exceeded by selling it for scrap. This would leave a
4300 m3 room at 2080 mwe with full muon veto coverage. A modest investment in
infrastructure would provide a new venue for a host of experiments and screening
instruments which service existing and future experiments as well as ongoing research
programs in other disciplines.
A cross section of the existing cavern is shown in figure 2. Steel I-beams support the
proton decay experiment a meter above the muon shield with a crawl space below for
repair and replacement. Since the floor muon shield is less efficient due to penetrations
by the structure, prone to damage by any new construction, and hard to repair, the
decision was taken to remove tubes from the floor to free up another 5’ of vertical space
for the tank and prototyping space. These tubes are in good condition and could be placed
near the roof as the second coincidence plane. All proposed clean rooms and structures
will fit within the existing cavern structure with 4 ft clearance on all sides to maintain
access to the muon shield and to satisfy safety requirements. The height is constrained by
access to the shield and air handling systems. The 7.5 ton crane adds another meter to the
required clearance. Removing the last set of overhead air ducts will not affect air
ciculation. This, plus restricting crane access to certain parts of the cavern, can give us a
maximum of 30 ft of vertical extension.
Figure 2. Cross section of the Soudan2 Cavern with intact proton decay calorimeter
Figure 3. The proposed plan and elevation of the Low Background Facility
A 28 ft diameter, 19 ft deep stainless steel water tank will be constructed underground by
a commercial water storage tank manufacturer. This tank will be located at the southern
end of the cavern. It will provide the best shielding for low background screening
applications as well as a proving ground for the next generation facility at NUSEL. A
purification system will recirculate water originally brought down from the surface in
specially-designed containers. The water collected at the 23rd and 27th level has
percolated through rock. Although it appears clean and sediment could easily be
removed, it is most likely contaminated with trace radioactive elements from the
surrounding rock. Tests at SNOLab will be conducted, but it is likely that we will need
to transport surface water. The containers will travel down by hoist and be emptied to a
holding tank during the fill procedure. A clean room plus anteroom will be constructed
over the tank. Three slots with movable covers and a portable 1.5 ton internal lowering
crane will be installed inside the clean room. In order to maximize the height of tank and
accompanying clean room, outside crane coverage will not be extended over the top of
the clean room.
Next to the tank is an experimental hall which consists of a clean room surrounded by a
passive neutron shield. The shield material will be determined by the members of this
collaboration after proper evaluation and sample counting. Since the clean room is
installed first, the sample evaluation might take place in the room itself using one of the
screening instruments described below. Candidates are plastic extrusions with
recirculated purified water, sealed water-filled plastic tanks, and commercial plastic
composite lumber. – electroformed copper only if we get a company involved who also
wants to install electroforming underground - . This multipurpose room will be used to
test prototype dark matter experiments/technology. It will also house additional HPGe
detectors for gamma screening. Testing of the prototype beta cage will take place here,
as well as its final installation as a user-oriented beta and alpha screener.
5.2 Active Veto Shield
Most of the veto shield is made of extruded aluminum proportional tube modules7 filled
with 95-5 Ar-CO2 at 1.12 bar. The module cross section is a honeycomb pattern of
hexagonal cells arranged in two 4-cell interlocking layers. The overall thickness of the
module is 9.5 cm with thin (2.2 mm) tube walls. The modules range in length from 6.6 to
7.9 m. Each tube has one 60 um sense wire running down the middle, operated at 2.4 kV
with a maximum drift length of 2.5 cm. Each set of 4 cells is ganged together in readout
to form an element 0.18 m wide and approximately 7 m long.
The wall modules are hung from a steel framework supported by bolts inserted into the
rock walls and lie within 0.5 m of the rock. The 9.5 x 31 m2 east and west walls are each
constructed from four panels of modules mounted horizontally. The gaps between the
panels, required for electrical and gas connections, are covered by curtains of modules
mounted vertically. These curtains can be slid to the side for access to the horizontal
module panels. The 9.5 x 13.4 m2 south wall consists of two panels of horizontal
modules, with the center gap covered by movable vertical curtains. The north wall is
similar, except that its west panel has been broken into two subpanels with one set back
to allow general access to the veto-lined room. It has also been modified over the years
for ease of access into the mezzanine area and will have to be supplemented.
The ceiling modules rest on a steel framework built to support the central proton decay
calorimeter, lying 2 m below the roof at the central highest point, but adjacent to the roof
at the east and west edges. The array consists of two 7.0 x 31 m2 sections which overlap,
yet allow the ventilation ductwork to pass through. The floor array is mounted on a
platform welded to the support structure, lying only 0.1 m above the cavern floor. The
shield floor consists of two 6.6 x 31 m2 sections, oriented with wires running east-west in
the east half and north-south in the west half. The center gap is covered by a 0.7 m wide
strip oriented north-south. There are 63 gaps in the shield floor corresponding to the 0.2 x
0.2 m2 posts which hold up the steel floor framework.
The tubes mentioned above were built by Tufts University and form the principle veto
shield system. Additional veto modules were obtained from Harvard (the HPW tubes)
and Oxford (the TASSO tubes). The HPW tubes are 2” x 6” in cross section and run
from 6 feet to 21 feet. They have a single channel consisting of two parallel sense wires
and a pair of field shaping wires. They were arranged perpendicular to the Tuft’s tubes on
the ceiling and floor with approximately 60% coverage. The TASSO tubes are 12” x
1.5” in cross section. Each module consists of 8 cells with a sense wire read out for each
channel. There are three steel pallets of these tubes resting on top of the proton decay
calorimeter. Each pallet is a double layer with twenty-four 4-meter modules above
fourteen 8-meter modules. Each pallet covers ~32 square meters. These pallets would
have to be removed before disassembling the calorimeter. It is unlikely that we would
bother to restore the TASSO tubes to working order.
The veto shield is operated in threshold mode. The current pulse from each set of 4 sense
wires is amplified and converted to a voltage pulse with gain of 190 mV/uA (determined
by the resistor values). A comparator returns a +5 volt pulse whenever the input pulse
rises above 100 mV. A multivibrator stretches the pulse to 1.1 us, followed by a
differential line driver to a 100 twisted pair. The components for the two channels are
mounted on a 6.0 x 14 cm2 printed circuit board mounted inside the exterior well of the
module endcap. A metal coverplate shields the card from electromagnetic interference.
The two interleaved 4-channel layers can be used in coincidence mode, if so desired.
Differential signals from the digital output cards are transmitted to single-width 64channel CAMAC modules via twisted pair. The input signals are sampled at 1 MHz.
The results are twofold multiplexed in time and then written to a random access memory.
The address is incremented at 2 MHz with the address cycling continuously through 256
locations. Thus each input/output port of the memory serves as a pair of 128-element
delay lines for the hits recorded within the previous 128 us interval. Upon receiving an
external trigger, the modules scan through the delay lines for hits recorded within the
previous 128 us interval. The information for each hit (channel and time) is coded and
stored in another memory. Following completion of the data scan in 1 ms, the readout
module sets a flag to indicate data-ready.
The rate in a 7 m long element is about 270 Hz near the ceiling and 320 Hz near the floor.
The rate of coincidences between two four-cell elements in a module is roughly 3 Hz.
Due to local radioactivity, this coincidence rate is far in excess of the 3 x 10-3 Hz rate
expected from the cosmic ray muon flux in the mine. The efficiency is measured to be
99.5% on walls and ceiling and 80-90% on the floor due to the gaps caused by the floor
supports.
Since the shield was turned off in July 2001, nothing has been disassembled. In order to
confirm its health, a pressure test was performed this March (2004) on one of the oldest
Veto panels (Panel 5: the southernmost panel on the east wall). It is constructed of fifty
7-m long Tufts tubes covering some 70 square meters. The panel was pumped up to 5
psig with an old bottle of C-10 mixture, sealed off and left over the weekend. The
absolute pressure gauge showed a drop of 128 mbar in the course of 95.5 hours or
1.3mbar/hr. The original standard for acceptability was .3mbar/hr. From experience we
know that 1.3mbar/hr for an entire panel corresponds to a single small leak that should
not be hard to find or fix. There is no evidence of catastrophic failures of the epoxy
sealing on the chamber ends (16/tube) or joint failures in the gas manifold piping. The
panel was then brought up to bias voltage and signals observed.
Since that time, all the panels have been pressure tested and a database maintained under
the direction of University of Minnesota (Crookston) as part of student work-study
grants. Although the electronics is generally in working order, the removal of the proton
decay calorimeter may disturb the thousands of feet of 32-twisted pair signal cables
running from the individual tubes to the CAMAC crates. While easy to repair once the
break is found, we have budgeted considerable time to thoroughly checking all channels.
We anticipate minor repairs. There is an existing test/repair stand for the preamps and
there are boxes of spare components. There are also a number of pre-assembled spare
preamp cards and spare CAMAC cards.
5.3 Veto Shield DAQ
Although the data acquisition VAX cluster is still working, we plan to replace the VAX
and CAMAC with a modern DAQ to improve throughput and speed, as well as to avoid
parts obsolescence. We will maintain the VAX cluster only for the initial shield checkout, since it will speed up this process to have an existing software interface. The final
DAQ must be developed as an integrated system with flexible extension to multiple users
and their screening instruments. Thus we are designing a system which assigns a
geographical location and a time-stamp to every muon event. Any experiment or
screener can then use this data via ethernet to provide their own veto or test trigger. The
finalization of the DAQ design and implementation is covered by supplementary funds
already acquired in 2004.
Each readout module will be able to handle incoming data from 96 tube modules.
Signals from each channel first pass through a Coincident Logic Network (CLN), which
is a simple on-board network of AND and OR gates which select coincidences between
the back and front 4-wire planes of the same and adjacent tube modules, thus
determining muon candidates and rejecting singles noise. The 3 microsecond variation in
proportional tube drift times require one-shots on each input. The output of the CLN will
be latched until the Serialization Module (see diagram above) can read it and reset the
latch. We are in the process of modifying this design to use off-the-shelf components and
FPGA’s, but the original schematic is useful in demonstrating the overall concept of the
new DAQ. In either case, a 1 MHz oscillator will provide the 1 microsecond timestamp
precision. It will be reset every second using the TTL signal provided by the MINOS
GPS receiver. A chain of 4-bit counters (4029’s) create a 32-bit clock which provides the
timestamp. Epoch information for each pulse (second, minute, day, year) will be
provided from the MINOS timing PC which is decoding the same receiver. All computer
clocks will thus be synchronized using NTP protocol.
A 50 MHz Ubicom SX processor will read and store the data and the corresponding
timestamp from the multiplexer arrays. A coincidence occurring in the veto shield will
interrupt the processor and tell it to begin reading. The 74150 multiplexers have a 36 ns
propagation time, so that 16 channels can be read out in just over ½ microsec. All of the
multiplexers will be cycled simultaneously to allow a parallel data read, starting from the
least significant bit, resulting in 16 cycles for 128 bits of data (32 bits from the clock and
96 bits from the CLN). Since the SX processor has 8 banks of 16-byte RAM, a complete
read from the clock and the CLN fits exactly in one RAM bank, allowing the banks to act
as an 8-event FIFO. While the processor is not collecting data from a triggered event, it
will dump the contents of its event buffer to the host computer system via an RS-485
serial network. Since each event will require 30.5 microsec to transmit, and any single
CLN will be generating hit events at a rate less than 1 Hz, the processor’s idle time is
sufficient.
Should more than 8 events occur in less than 30.5 microseconds, the FIFO will flood and
the incoming event will be dropped. A flood situation will only occur for a “hot” tube.
In this case, the processor will poll for the tube and ignore further events from that tube
until such time as the tube is fixed. The decision time is less than one second.
Once sent to the host computer, the data will be made available through an API that will
be designed to allow the projects under the shield to access the data from the shield. It
will also have the ability to determine the detected particle’s path (panel region
coincidences) in order to check whether that particle passed through any of the
experiments under the shield or not.
5.4 Water tank
Two commercial storage tank companies have bid on this item. One company has visited
the mine, but both have explored the detailed plans required to transport materials and
weld the tank underground. Both a carbon steel with liner or epoxy paint and stainless
steel options have been evaluated. The stainless steel option is about 1/3 higher in price,
but is the safest bet for maintaining the highest standards of purity in the water. The tank
can be assembled in place using commercial techniques relatively cheaply, since most
commercial tanks are built on location. The size of the unassembled steel sheets needs to
be slightly narrower and shorter than usual due to the cage constraints, but this is not a
large perturbation. The height of the tank plus clean room is constrained by the size of the
cavern itself. With only 1 upward-going muon a week, we have decided to remove the
shield from under the tank and construct it directly on the floor. The shield can be
reinstalled above the clean room to provide additional coincidence information if desired.
The expected neutron flux inside the water tank was estimated using a GEANT4.5 Monte
Carlo. Neutrons with energies of 20 MeV, 100 MeV and 1 GeV were tracked from the
cavern walls through the water tank geometry. The number of muons making it to a
central 1 m detector volume is shown below. 87 % of the 20 Me neutrons are removed
when water, rather than air, fills the tank. Another simulation was run to estimate the
expected neutron spectrum due to cosmogenic muons at the Soudan depth, propagating
the estimated muon spectrum starting at a depth well above the cavern through 30m of
rock surrounding the cavern, using the QGSC physics list and the additional
G4MuNuclearInteraction process. We have not finished this study, but the Boulby Mine
cosmogenic neutron spectrum is displayed below for a rough idea of how many low and
high energy neutrons will impinge on the tank. Once we have finished generating our
own muon and neutron spectrum, we will run the entire spectrum in order to determine
the expected neutron flux at the immersed detector space. Neutrons due to natural
radioactivity in the rock surrounding the cavern do not have to be simulated, as they will
thermalize in the outer meter of tank volume.
At present, we plan to purchase a pre-fab class 10,000 clean room from Clean Air
Products or Gateway Air Filter, Inc. Installation on site is included in the bids. This is
the price included in the budget. Another option is to extend the steel supports around
the tank and basically build a taller tank, the top piece of which will be the clean room.
We will explore both alternatives to find the most cost efficient plan.
5.5 Purification System
The goal of the Soudan purification is to provide low radioactivity water. A necessary
condition is that the water must be very pure. Absolutely pure water has a resistivity of
18 megohm-cm. If a solute is present at the 1ppm level, the resistivity plummets to 0.5
megohm-cm; i.e., resistivity is a good indicator of dissolved material. Since the final
purity will be determined by the nature of the solutes and the mass and type of shielding
immersed in the water, it is best to plan for a high quality purification system with the
flexibility for upgrades built in. The tank itself will be made of a material able to
withstand very pure water, either stainless steel or a lining of Teflon (C and F chain) or
Kynar (C and F and H chain). If the tank is an unlined stainless steel, the welds must be
covered with Teflon tape.
A high purity water system is installed for the DIRC at the BaBar detector at SLAC. This
system has about 5 tons of water and is capable of recirculating 6 times a day. The
purpose of this system is to maintain light transmission at better than 1 meter at 280nm.
The input water is 18 megohm-cm and the returning water is typically 12-14 megohmcm. Another system is installed at Milagro in New Mexico. This system holds
approximately 20 ktons of water, capable of recirculating once a month. The goal is
again light transmission but only down to 320nm. At Soudan, a system capable of
recirculating once or twice a week is adequate.
The purification plant would have a series of components. A rough screen filter would
remove large particulates. This would be followed by some charcoal filters leading to a
reverse osmosis unit. Next would come a series of ion-exchange units followed by a degasser. An ultra-violet lamp will be required to kill bacteria, followed by a filter for the
dead bacteria. Since this is a recirculating system, the return water would come in after
the reverse osmosis unit, which is not needed after the initial pass. Any new makeup
water would go through the whole loop. Since units can fail, and even in normal
operation, filters need to be cleaned or changed and resin beds need replacement, there
should be two parallel lines for everything in the system except the reverse osmosis unit
and the de-gasser. Most of these units are standard in the industry with the exception of
the de-gasser which would be a Teflon microtube system in which the air in the water
diffuses out into a vacuum surrounding the microtubes. The main purpose of the degasser is to eliminate oxygen which promotes bacterial growth and to cut down on radon
in the water. The footprint of such a purification system would be roughly 6 ft by 12 ft
for the steel box tubing base plus additional space for the return reservoir and pumbing.
5.6 Shielded Multi-purpose Room
The water tank provides an excellent passive shield for neutrons and gammas from the
south wall of the cavern. A set of shielded bays can be built against the side of the water
tank to provide a low background staging area for prototype dark matter and double beta
decay experiments. Experiments that will eventually opt for the ultra-low background of
the water shield can refine their material selection (using the on-site screening facilities)
while taking data in a deep site environment and preparing for the next step. Other
experiments may find that their needs are already met by additional shielding in the bays.
High density plastic extrusion material or HPDE (like the plastic lumber for decks) can
be used for the outer shielding since it is available in quantity, cheap ($1,400/cubic
meter) and easily rearranged and stacked for various geometries. A good shield would be
enough plastic to thermalize the neutrons, then a thin cadmium layer to capture the
thermals, then lead for the 2614 keV gamma (thorium chain), ending with ultra-pure
copper from the electroforming facility to get rid of the Pb210. This is a fairly
conventional low background shield. The question is how much we will provide through
the facility, recouped by lease fees, versus how much we expect experimenters to bring
themselves. One option is to surround the outside of the bay area with a 0.5 meter
thickness of the inexpensive plastic and then let the experimenters provide their own
shielding, which might include purchasing electroformed copper produced locally.
The entire shielded bay complex and adjoining assembly area will be contained in a class
10,000 clean room with HEPA filter modules, sealed light fixtures, anti-static vinyl floor
and a class 100 laminar flow workstation. Additional radon–free storage space for
samples and shielding materials will be available. An anteroom provides a place to suit
up and to clean items to be brought into the clean room.
5.7 Radon Reduction
One challenge underground is the high ambient Rn levels. If not reduced, these will lead
to the Rn daughters 210Pb, 210Bi, and 210Po being deposited on any samples or detector
surfaces that are exposed to air. Local reduction techniques, such as glove boxes, can be
effective, but obviously have limited applicability. Instead we plan to provide two clean
room areas with Rn-reduced air.
Figure 5. The Borexino radon-scrubbing unit at Princeton
While readily-available liquid nitrogen boil-off is low in Rn, there is no simple means to
have a large volume of low-Rn breathable air. Air synthesized from liquid nitrogen and
oxygen, is conceptually simple, but very expensive, and the liquid oxygen would present
serious safety issues underground. Storage of air long enough for Rn to decay away
(with 5.5 day half life) is also impractical. We will instead use a copy of the system
developed by the Princeton Borexino group8. This is a room temperature process that
uses two large (250 Kg) charcoal columns that strongly retard the flow of Rn relative to
air. A cyclic process in employed in which one column cleans the air, while a small
fraction of the cleaned air is used to regenerate the other column using a “vacuum swing”
technique.
This system supplied ~ 100 m3/hr of air with the Rn reduced from an ambient level of 30
Bq/m3 to ~ 0.3 Bq/m3. Since this value is likely to have been limited by the measurement
technique, it is believed that overall Rn reduction factor is more like 104, Thus a Rn
value this low should be achieved even starting with the much higher Rn-content air in
Soudan. At Princeton, this air provided the make-up flow which creates the necessary
overpressure in a 770 m3 clean room. The final Rn level of 1.5 Bq/m3 is believed to be
the result of Rn emanation in the room, and thus depends on the air turnover rate (room
volume / make-up flow). Thus, with a similar size Rn scrubber, we will expect at least
comparable performance for the proposed rooms with 300 m3 volume each. We estimate
a cost of $120 K for this system when commercially fabricated, of which ~ $50K is for
equipment (dominated by a gas throughput pumping system).
Typical clean rooms of size we are considering have make-up air flows much larger than
100 m3/hr. To achieve a low make-up flow, the seams in walls must be better sealed
than is typical, and the air handling system should be situated in an interior plenum space.
Based on the Princeton experience, we expect these to be done for a modest additional
cost.
5.8 Copper Electroforming
It is also important to further reduce the level of radioactive contamination in at least one
of the major materials (copper) used in low background experiments. As experiments go
to larger mass in order to increase sensitivity to the signal of interest, the larger mass of
support materials and internal electronics will increase the background as well, unless
purer materials are found. Radionuclides are produced in copper from cosmic ray induced
interactions when copper is above ground 9. Therefore, electroforming will be performed
underground in order to reduce this source of radioactivity. Under research funded by an
SBIR, Reeves & Sons, LLC. expects to produce copper containing only one
microBecquerel (or less) per kilogram. This would already be a great advantage. A
second objective is to enhance the electroplating technique to produce material thicker
than the approximately 1 to 3 millimeters currently attainable before the copper must be
removed from the bath and machined.
Copper sulfate will be purified by repeated dissolution/precipitation steps and ultra pure
sulfuric acid can be purchased from suppliers. Then, beginning with a relatively pure
form of copper (OFHC) as anodes and using the purified materials for the plating bath,
new anodes will be electroformed, and these will contain significantly less radioactive
contamination. These anodes will be used in the bath producing the final copper product.
It would take about 30 days to purify the copper sulfate and prepare anodes. With 4 baths
running, approximately 500 pounds/week could be electroformed.
The proposed design for the electroforming facility includes a modular system that will
have each electroforming bath individually enclosed and with a cover gas exhausted
through a filter designed to trap any corrosive fumes that might otherwise escape. The
low-background counting facility specified in the SBIR for testing the material is itself a
proprietary design that should rival the MEGA detector. It will be used primarily for the
copper testing, but after the procedure is perfected, it will be available for use as a general
screening device. The enclosure for each bath would be made from plastic such as PVC
or acrylic or a combination of metal and plastic if required. It is important to be able to
see inside each enclosure from at least one location since manipulations inside each
enclosure would be accomplished through glove ports. The footprint for a single typical
bath is about two feet square. A larger bath with a footprint of three feet square may also
be needed. A room height of about eight feet is desirable.
The maximum power required will be typical 60 cycle ~117 volts AC. A total of eight 20
amp circuits would be needed to provide lights, air conditioning, vacuum system, plating
power, and power for a small combination lathe/milling machine. For large tasks such as
providing large quantities of ultra-pure copper for gamma-ray shielding, continuous
power at near 20 amps per circuit would be required for up to four circuits for several
days at a time. However, for smaller tasks, the power requirements would be less since a
typical DC power supply providing both voltage and current control requires less AC
current when operated at low DC current.
Very little water is required and could be brought to (and removed from) the lab in plastic
carboys. However, if water is available, it would be used as input to a small water
treatment unit that would produce the 18 megohm water needed. Alternatively, some
amount of the purified water used in the large tank could be diverted to this use.
Fresh air requirements are minimal since the plating itself will happen within closed
modules and the vapor pressure of the copper sulfate plating bath is low. However, air
suitable for breathing and sufficient to sink the heat from the power supplies and a small
air conditioner will be necessary. If a water supply sufficient to handle the heat load is
available, then only sufficient breathing air for two people would normally be required.
The primary waste is plastic and paper. Plastic gloves are used extensively as are plastic
bags and paper products. Otherwise, there is very little waste material. Machining
produces copper turnings and shavings but it is believed that a large part of this material
can be used as anodes in the plating baths, especially since it will be the cleanest copper
available if not contaminated during machining. Each plating bath is composed of ~ 8%
high purity sulfuric acid and ~ 150 grams per liter high purity copper sulfate (somewhat
similar to a lead-acid battery but lower acid concentration and copper rather than lead)
and high purity water. Very small quantities of cobalt and barium (a few milligrams per
liter) are the only other chemicals being used in the bath at this time. A typical plating
bath would be about 10 to 100 liters and be contained in a plastic vessel, generally a right
circular cylinder. A second plastic vessel provides secondary containment and the floor
section of the main room will be assembled in such a manner to provide backup
containment. The main room will be maintained at a slightly positive pressure and the
exhaust can be directed in the most suitable direction and pass through appropriate filters
to remove any trace amounts of sulfates, if any. A monitor with alarm capabilities will
continually monitor the air and sound the alarm if a corrosive or dangerous level is
reached.
5.9 High Purity Germanium Detectors
Currently there is gamma screening facility named SOLO, operating in the Soudan Lab
behind the MINOS detector. A 10 ton, 14” thick Pb castle with a 2” thick liner of 50
mBq/kg German lead houses two HP Ge detectors borrowed from a previous double beta
decay experiment. The “TWIN 2124” is 1.05 kg and “Diode M” is 0.5 kg, each with its
own (8”)3 sample chamber. A double layer of 100 um mylar and boil-off from the liquid
nitrogen dewar provides a radon shield. The spectra shown below provide a brief history
of the shielding improvements, starting with the cavern background at the top, followed
by the first run without radon elimination and with a few “radioactive” lead bricks from
FNAL, followed by the radon purged,final shield configuration over 40 kg-days.
Figure 6. Background spectra from the SOLO-TWIN HP Ge detector at Soudan.
To extend the existing SOLO facility, Laura Baudis will purchase a large volume p-type
HP coaxial Ge detector using University of Florida startup funds. The detector is being
constructed by Canberra Semiconductors (USA) with selected low background materials
provided by the company. However, the background of this detector will most likely be
dominated by the cryostat made of high-conductivity, oxygen-free copper. To increase its
sensitivity after an initial period of operation below ground, the cryostat and the internal
fittings would have to be replaced with new components made of electroformed copper
produced in the underground electroforming facility.
The new cryostat will be constructed at Florida in close cooperation with Canberra.
To establish a competitive Ge spectroscopy facility, we propose to construct a second
100% efficency HPGe detector, in parallel to constructing the new cryostat system of the
first one. We also propose to build new sample chambers for these detectors. The sample
chambers will have inner dimensions of 25 cm x 25 cm x 35 cm, allowing one to place
more than 10 l of sample material around the cryostat housing. The chambers would be
formed by 5 cm thick plates of electropolished copper surrounded by several layers of
low radioactivity lead (a total of 20 cm). The innermost Pb layer with a 210-Pb
contamination of 5Bq/kg. The chambers would be closed on top with large Cu plates
carrying the Pb. The entire shield would be surrounded by an airtight steel box containing
also an operating box and an airlock for accessing the samples. The entire system would
be flushed permanently with gaseous nitrogen. The samples would be first stored above
the closed sample chamber in nitrogen, allowing the trapped radon and the plate-out
radon progenies to decay before the chamber is opened and the sample measured.
The operating box would contain 2 windows and a glove system. A crane would be
mounted on top of this box to handle heavy loads and lower them into the chamber.
5.10 Beta Cage
There are two types of technology which we plan to develop in order to screen for alpha
and beta particles. The first technology is a large area gaseous detector for measuring
surface contamination of low-energy beta emitters. The principles which guide its design
are as follows:
1. Use the minimum amount of gas needed to stop electrons of interest in order to
minimize background from ambient penetrating gammas.
2. The detector should have the minimum possible surface area which itself can be a
source of betas.
3. There must be sufficient spatial information about events to distinguish those coming
from the sample surface.
drift grids
trigger grids
sample
bottom guard grids
Figure 7. Side view of the Beta Cage. Each grid set consists of an anode plane where
charge multiplication occurs, plus two field shaping grids. One field grid plane and the
anode plane will be read out for the trigger grids to determine x-y position.
These goals are met by using a gas drift-chamber with multi-wire proportional readout.
A sketch of the detector is shown in Figure 7. Samples are placed directly in the gas. A
set of trigger grids directly above the sample surface tags events that originate at the
sample. A drift region beyond this is read out by a set of wire readout grids at the top of
the chamber. The depth of the drift region sets the energy of betas whose full energy is
contained in the detector. Crossed grids will be used for x-y position determination, and
timing can be used to determine the z-profile of the track. The region below the sample is
also an active veto region. The gas of choice (neon gas at STP), and detector thickness
are determined by electron stopping based on Monte Carlo studies. The trigger region
will be as small as is practical, roughly 0.5 cm, and the drift region will be 15 cm, which
should contain ~ 90% of 150 keV betas. The region below the sample is also an active
veto region. Initially we plan to segment the x-y readout with only a few channels in
order to set a guard region at the edges of the detector. The wire pitch on the trigger grid
will be roughly 2.5 mm, so much more fine-grained readout could be used if it proves
beneficial.
The layout of the grids is based on distinguishing events from the sample surface from all
others. Until the level of surface contamination of beta-emitters is systematically
reduced, we can expect betas coming from the surfaces of the detector itself. However
those from the outer walls of the chamber are readily distinguished by use of the bottom
veto, the drift region, and the x-y position. Good events are located above the sample in
x-y, deposit energy in the trigger region, and an have a small enough (or zero) energy in
the drift region that they could not have penetrated the drift region from above. (A veto
region above the drift region could also readily be employed). By design, the only
surface of the detector near the sample is that of the wires, which are a few % of the
sample surface.
Each grid structure consists of a center anode grid, surrounded by two sense grids. The
anode and one sense grid will be read out, to provide x-y information. The grid structures
will have a grid spacing and wire pitch of 5 mm, though for the trigger grid structure this
distance may be reduced to 2.5 mm. Anode wires will be 20 µm Ø.
The dominant background is expected to come from penetrating gamma rays that interact
(primarily Compton scattering) in the trigger region, or in the sample or drift region such
that a recoil electron or X-ray interacts in the trigger regions. These photons come from
contamination in the shield, and are present in all low-background environments.
Having as small a gas mass in the trigger region as possible minimizes these
backgrounds, except for the case of electrons emerging from the sample (ejectrons)
which are indistinguishable from surface beta decays. Further reduction of these
backgrounds will be achieved by use of a passive water shield and an active muon veto
provided by the refurbished Soudan2 veto shield in the new Low Background Counting
Facility at Soudan.
The gas choice, and detector thickness are determined by electron stopping. Based on
Monte Carlo studies described below, we intend to use Ne gas at STP. The trigger region
will be as small as is practical, roughly 0.5 cm, and the drift region will be 15 cm, which
should contain ~ 90% of 150 keV betas. Higher energy betas can be studied in the same
chamber by use of Xe gas.
The demand for a low radioactive background environment requires that the basic
construction materials of the chamber be copper or plastic. Gas handling is simplified,
and non-atmospheric pressures can be utilized if a vacuum tight, and, possibly pressurecapable copper box is constructed. This will be expensive and technically difficult,
especially for large detector sizes. If a plastic chamber at STP is used, initial purging of
the box will be needed. With Ne, this is probably practical. With Xe, a more
complicated, sealed gas system will be needed.
Monte Carlo studies for the beta cage option
Two main issues have addressed with Monte Carlo study using Geant4. The first is the
size of the electron tracks. We find that tracks < 100 keV have completely curled up on
themselves. For Ne, 150 keV events are contained within 10 cm. The second issue is
the background expected inside a shielded environment. We simulated the event rate in
the counter under an ambient background of gamma rays typically of shielded
environments, and scaled to give 1 count/kg/keV/day in Ge at low energy. This is a
typical, if somewhat high background in such setups. The results are shown in figure 8.
Figure 8. Simulated event rate for a beta cage at Soudan. The surface area was
700 cm2, so 2x10-2 cnts/keV/day corresponds to 3x10-5 cnts/cm2/keV/day.
The curve labeled “all events” represents the final background.
Other sources of internal background are expected to be small compared to the photonrelated backgrounds. Several are summarized below:
Source
Rate
(cnts/keV/cm2/day)
3x10-5
4x10-6
5x10-8
Photon, nominal 1 dru (in Ge) environment
Kr rate in full volume. Assume 20 ppb.
5% methane quench gas (assume 10-18 C14/C12).
Rate in full volume.
Wires: Internal beta rate < 100 keV. U, Th
~ 10-8
contamination is typical for dirty metals, e.g. SS.
Wires: Rate from gammas in full volume.
~ 10-13
30 resistors: Rate from gammas in full volume.
~ 10-6
Assume ~ ppm U, Th, typical of ceramics
The Kr and methane backgrounds will become important if the photon background is
reduced to 0.1 dru equivalent in Ge. The Kr value is near the limit of commercial
measurement techniques, but should be readily reduced by use of a charcoal trap operated
at 77 K. The 14C/12C ratio assumed for methane is typical of petrochemical values.
Reduced concentrations or non-organic quench gasses such as SF6 can be explored. The
rate from bulk contamination of the wires based on Ge-counter based screening of metals,
does not appear important. We may learn, however, that wire surfaces are contaminated
with beta emitters that we must remove. The radioactivity of resistors should dominate
the rest of the electronics, and could become an important background. If lower activity
components cannot not be found, it is possible to place the resistors outside the shield.
Sensitivity of the beta cage.
The sensitivity of the detector depends on the sample area and final background. In
figure 9 we show the 3  (99.8 % CL) sensitivity for 30 days counting time. Gaussian
statistics are assumed. For large enough exposures, there is non-zero background due to
photons, especially at low energy. It is standard in low-background counting to subtract
the background after having measured the blank background with no sample present. We
will do this here, but more care must be taken as photon interactions in the sample
leading to escaping electrons (and X-rays) are not measured in a blank run. Instead,
gamma calibration sources or samples that have already been found to have low surface
contamination could be used.
Figure 9. Sensitivity of the beta cage as a function of linear dimension of the sample
area, and for several photon backgrounds. Our target background is at least 1 dru, and
ultimately 0.1 dru. The heavy lines indicates < 1 background count in each energy bin,
so that no background subtraction is applied.
In order to reach the sensitivity goal of 0.1 cnts/m2/keV/day with a rather long 30 days of
counting, a sample area of at least roughly 30 cm on a side is indicated. At this scale,
background subtraction (in 30 keV bins) is just becoming necessary for a gamma
background in the range of 0.1 and 1 dru equivalent in Ge.. A smaller detector will
require very long counting times. A larger detector will allow faster counting, and would
be desirable. As our understanding of background subtraction improves, and especially if
the photon background is low, a detector of 1 m2 in area can lead to 0.01 cnts/m2/keV/day
sensitivity. A sample area 31 cm in diameter can hold 12 x 8 cm Ø samples, for 550 cm2
= (23.4 cm)2. Significantly larger areas will require a large number of samples.
5.11 Cloud Chamber
The second technology is a triggerable cloud chamber. This device will be developed by
UCSB using other funding. It will be installed in one of the shielded bays for evaluation
of its efficiency and operation at low background.
The concept of the chamber is shown in Figure 10. The chamber would be of the
diffusion type (15), and thus be continuously sensitive. Digital images from two cameras
with stereoscopic views would be continuously read out, and a software trigger based on
the number of hits above threshold would trigger the retention of interesting candidates.
Since the rate of events is extremely low, this strategy should be effective. Subsequent
offline analysis would select the beta candidates.
A prototype device has already been built; an image of a roughly 100 keV electron from
this device is shown in Figure 11. Electrons at this energy multiple scatter quite a bit,
and the sense of their motion can be determined by dE/dx. Backgrounds would be
rejected principally through exploitation of the exquisite information on the track
provided by the images. Electrons from beta decay in the sample would have a track that
emanated from the sample, and that multiple-scattered like a low energy beta. The
energy of the beta would be reconstructed from the total length of the track.
The chamber and the alcohol would be maintained at the highest degree of cleanliness,
for the good operation of the device, and so as not to contaminate the sample. The inert
gas would be nitrogen. Alcohols are used in the final cleaning of the CDMS detectors.
Cloud chambers built for low backround applications have achieved good sensitivities in
the past, for example, to measure the naturally occurring tritium concentration in water.
We anticipate that the dominant background will come from conversion of ambient
gammas near the surface of the sample. The rate of this background depends primarily
on the ambient flux of gammas; the canonical flux in a good lead shield gives one
Compton electron recoil per keV per kilogram in standard materials. The portion of those
that can escape the surface indicates about one false beta every 10 days for one of the
CDMS ZIP detectors, or equivalently, about 110-5 events per cm2 per keV per day. This
sensitivity is of interest to future incarnations of CDMS, although another order of
magnitude would be optimum. The main obstacle to greater sensitivities is likely to be
gamma flux from radioactivity in the shield and components.
Figure 10. Concept of the cloud chamber for beta screening. The material to be screened
(in this case, a ZIP detector, one of the CDMS dark matter detectors) is placed above the
sensitive region. The cooling (or heating) coils would maintain stable and necessary
temperature conditions for operation of the device. Alcohol from the tray evaporates, and
diffuses throughout the volume. A cooler temperature and the temperature gradient in the
lower portion of the chamber allows the vapor to become supersaturated, and becomes
the area where tracks can be imaged by the droplets they induce. For scale, the ZIP
detector is approximately 8 cm in diameter.
The second prototye chamber would be rather small, a cube of 30 cm dimension. The
main purpose would be to develop the temperature control and digital imaging readout,
and understand stability issues, and the efficiency of the device near the surface of the
material to be screened. The target for operation of this chamber would be September,
2004. We would then move the device to Soudan, and operate it in such space as is
available until a bay in the new lab was ready. Based on the performance of that
prototype, we would plan the third chamber, which we envision to be capable of handling
a significantly larger area.
Figure 11. Image of a recoil electron from a Compton scatter in the UCSB prototype
cloud chamber. The energy of the electron is of order 100 keV. The bright horizontal
line is a reflection from one edge of the chamber.
6. Scientific Program
This proposal is designed to meet near term needs of the astroparticle, nuclear, and high
energy physics communities whose underground experiments are either now running or
in the R&D stage. As such, it has definite physics goals which will be met by its timely
construction, as well as creating an economy of scale that should actually reduce the
overall costs of the experimental program which needs the facility. It is an attempt to
bring together a set of known users under one umbrella, such that shielding requirements
(both active and passive), screening needs, data acquisition, off-site infrastructure and
personnel can be shared. The scientific program outlined below is structured to bring
experiments from our community online quickly in the 8 year hiatus before NUSEL is
available, and maintain a general capacity that will be available to the larger community
on a fee basis.
6.1 Gamma Screening
Immediate screening needs are Majorana and CDMS, as well as XENON. Majorana
contributed to the development of the SOLO HPGe facility and has been using it to
screen – SPECIFICS PLEASE. CDMS also needs to screen samples of the plastic and
copper from its shield to determine whether their higher gamma flux is due to radon
plate-out or not. The XENON collaboration requires immediate screening for phototube
selection and design, as well as other photodetectors such as APD’s or HPD’s which may
be preferable due to their lower intrinsic radioactivity. They also need to count the
various commercial components such as HV resistors and feedthroughs, and finally
characterize their shielding. At the sensitivity required, such tests can take
approximately 30 days. We are backlogged on the current facility and need to extend it
by at least a factor of 2. Eventually, the older HPGe detectors will be moved under the
muon shield and the lead reconfigured to accommodate users from outside the
nuclear/astro-particle physics community on a per-fee basis when the ports are not in use.
At least one of the most sensitive HP Ge detectors will reside inside the water tank.
6.2 Beta and Alpha Screening
The CDMS experiment running at the Soudan Mine is currently among the world leaders
in the direct search for Weakly Interacting Massive Particles (WIMPs), which may
comprise the dark matter in the Universe. CDMS II has already produced the best limit
worldwide from only a short run at Soudan with one out of the five towers. Over the next
couple years, the number of towers will be increased to 5 and eventually to 7 under the
CDMS III experiment. However, there is significant competition from advances being
made elsewhere in the community, including the Edelweiss and Zeplin programs, which
are now operating experiments with sensitivities similar to our best CDMS-I results.
To maintain a leading role, we must begin to explore ways to improve the sensitivity of
our dark matter search. This must occur immediately if the results are to influence the
processing of towers 4 and 5, which will be fabricated and tested soon. The work
proposed here is aimed at reducing the dominant internal background in CDMS: lowenergy betas from detectors or nearby surfaces. These low-energy betas come from
surface contaminants in the tens of keV energy range. Since their charge is inefficiently
collected compared to that of beta’s which interact in the bulk, their ionization yield
(ionization/recoil energy) is lower. On a plot of yield vs recoil energy (see figure above),
they produce a continuum of events (small green dots) which leak out of the band where
we expect electron recoil events (yield = 0.8 – 1.2) and into the band where WIMP-like
nuclear recoils are to be found (yield = 0.2-0.4) , thus compromising our beta rejection.
Figure 12. Data from the CDMS run at the Stanford shallow site.
Significant progress toward a no-background dark matter experiment can be made by
reducing the level of beta-emitting contaminants on the detectors and nearby surfaces. In
CDMS-II, time limitations in the fabrication and testing cycle have precluded fast
feedback on the presence and sources of these contaminants. Diagnosis of the
contamination has relied on operating the detectors themselves at the CDMS-I setup in
the Stanford Underground Facility. In order to develop handling and processing methods
that minimize the introduction of beta-emitting contaminants, or to test cleaning methods,
we need to use faster-turnaround techniques for screening and probing for contaminants.
Screening methods have been used by a variety of low-background experiments.
However, nearly all such screening depends on detecting bulk contamination using
counters sensitive to gamma rays, but not to beta particles. Available surface-analysis
techniques using in materials science are useful for detecting some possible beta
contaminants. However, no low-background screening technology exists with good
sensitivity for surface contamination of low-energy beta emitters. Since such direct
counting is the only way to measure the final background of interest, such screening is
critical. Thus we propose to build a novel dedicated detector for this activity, based on
multi-wire proportional counting.
Screening would be used not only on fully instrumented detectors but also on test wafers
subjected to only some of the processing steps or fabricated under different conditions. If
the results indicate contamination introduced during detector processing, we would
evaluate how to refine handling and processing methods for the next batch of test wafers
and detectors. If the results indicate contamination from handling, methods to clean
identified contaminants would be attempted, or handling most likely to introduce the
contaminant would be altered or removed. Possible cleaning steps range from chemical
methods to light sputtering with argon beams, with possible post-cleaning introduction of
clean buffer layers to reduce the reintroduction of beta-emitting contaminants.
In order to determine how to remove a contaminant, it must first be identified. The
screening must therefore be sensitive enough to detect contaminants at the level that
would cause the raw background goal of 8 x 10-3 events/ (keV kg day) in the 15-45 keV
region. Beta contamination on the surfaces of the detectors themselves is the most
critical to limit. Since the detectors have a mass of 250 g and a surface area of about 100
cm2, the beta screening needs to be sensitive to 2 x 10-5 counts/ (keV cm2 day). Most
surface analysis techniques cannot reach this sensitivity goal and are limited to
radioactive isotopes such as 40K and 14C, which can be tagged by their more common
isotopes. A direct measurement of the ejected beta’s themselves using beta screening is
superior, since it is not limited in this way.
6.3 Staging new Prototype Experiments
The following experiments have either committed or are seriously considering occupying
the multi-purpose room as soon as possible. For most of them, the radon scrubber can be
a later addition. They will take advantage of the local screening facilities for their
material selection prior to installation and for shield characterization during
commissioning. This list is by no means exhaustive and simply represents those who
have already contacted us.
6.3.1 Heavy-liquid bubble chamber for WIMP detection
Recent progress in the application of superheated liquids to WIMP detection at the
University of Chicago indicates that it may be possible to keep bulk volumes in a
radiation-sensitive metastable state for long enough to perform rare event searches. For
certain pressure and temperature operating conditions the vaporization of the liquid can
only be produced by particles having a large stopping power (e.g., nuclear recoils,
Figure 13), making the detector insensitive to most minimum ionizing backgrounds. The
devices are operated at near room temperature and the industrial refrigerants used are
non-flammable, non-toxic and inexpensive, with a chemical composition (e.g., CF3I) that
is optimal to maximize sensitivity to neutralino interactions through both the spindependent and –independent channels. For these reasons the technique seems to be
ideally fitted for the goal of building tonne or even multi-tonne WIMP detectors, able to
prove most of the supersymmetric phase space where the neutralino may abide.
First tests using a ~20 g active mass prototype bubble chamber show an event rate
compatible with the fast neutron background in a shallow (6 m.w.e) underground
laboratory. Calibrations using monochromatic neutron sources have shown good
agreement with theoretical predictions for the response of the detector and a sensitivity to
recoils down to ~7 keV (tests with a Sb-124/Be photonuclear source able to demonstrate
sensitivity to ~1 keV recoils are underway). The construction of a second prototype
chamber, able to house an active 2 kg target mass of CF3I is well advanced. Installation in
the Soudan laboratory is expected during summer of 2004.
Fig. 13. Left: Calibrations using Am/Be and monochromatic Y-88/Be neutron sources show good
agreement with theoretical predictions of recoil energy threshold, while demonstrating
insensitivity to a strong (3 mCi) gamma source. Right: multiple scattering events can produce
more than one simultaneous bubble in the chamber, providing a strong rejection mechanism
against neutron-induced events like that in the image.
The proposed facility offers an ideal enclosure for this type of detector. Superheated
liquids sensitive to low-energy recoils are also sensitive to alpha-daughter recoils from
radon contamination, and therefore any intervention requiring manipulation of the
interior of the chamber should be performed in a low Rn ambiance to avoid plating and
possible contamination. Similarly, a dust-free atmosphere is needed to ensure that dust
motes are not present in the interior volume, if it ever needs to be opened while
underground: untreated motes can act as inhomogeneous nucleation centers, able to
destabilize chamber operation by producing frequent spurious events. Finally, while the
detectors are insensitive to direct ionization from muons, these particles have a small but
finite cross section for direct nuclear displacement production. A residual background
from these can be rejected using the existing muon veto enclosure.
While it is risky to extrapolate the present success with small ~20 g prototypes to the
tonne or multi-tonne detector mass regime, especially before testing of the 2 kg prototype
deep underground, it is worth mentioning that the increase in target mass per module
from this second prototype to an envisioned ten-module farm containing a total of 1 tonne
CF3I is actually smaller than the present transition. If the second prototype is successfully
operated in Soudan, we would feel confident in proposing such a multi-modular
approach, which the proposed facility would be able to house in an optimal environment.
Fig. 14. Left: External recompression chamber (roughly 1m tall) of the 2 kg CF3I bubble chamber
presently under construction. Right: inner quartz vessel and recompression bellows for the same.
The proposed facility can house this intermediate-size prototype and an envisioned 1-tonne
modular detector in an ideal environment.
6.3.2 Liquid Xenon dark matter detector prototype XENON
The elastic scattering of a WIMP within liquid xenon results in a low energy Xe recoil.
The moving recoil produces both ionization electrons and fast UV scintillation photons at
178 nm, from the de-excitation to the ground state of excited diatomic Xe molecules
(Xe2). Under a high electric field, a nuclear recoil will yield a very small charge signal
and a much larger light signal, compared to an electron recoil of the same energy. The
distinct charge/light ratio is the basis for nuclear recoil discrimination in a LXe detector.
To detect the small charge signals involved, the process of electroluminescence is
typically used. The free ionization electrons are extracted from the liquid to the gas phase
where in the strong electric field they induce proportional scintillation light. The number
of photons generated by one drifting electron is sufficiently large to be detected by
PMTs, although other photodetectors, such as APD’s and HPD’s are being explored.
The XENON collaboration is approved to produce a prototype liquid Xenon Time
Projection Chamber (TPC) for dark matter detection. The prototype module containing
the active xenon target is formed by a sandwich of Teflon spacers (UV diffuse reflector)
and copper rings for electric field shaping. The structure is closed at the bottom by a
copper plate, which is coated with CsI to convert Xe scintillation photons into free
photons and a electrons, while at the top there are 7 PMT’s to observe the UV
scintillation light and a wire structure for the proportional scintillation process in the gas
phase. The structure is designed for a maximum field of 5 kV/cm and a 10 cm LXe drift
gap. The entire module is enclosed in a copper vessel as shown in figure 15.
Figure 15. The 10 kg XENON dark matter TPC. Left: Xe gas system,
Center: Schematic of prototype, Right: Photo of the prototype
The plan is to finish a 10 kg prototype chamber by summer of 2005, at which point it will
need an underground location for what may eventually be a physics run, since the reach
should be equivalent to CDMS II. In any case, it will be important at that point to reduce
the fast neutron background to?? . An active veto shield as is available at Soudan, in
combination with conventional passive shielding, would reduce the neutron background
to ?? This may even be sufficient for the next step with 100 kg chambers. It certainly
would be, if the modules were to be immersed in the water shield, but at present we have
not budgeted for water-tight containers and need to first operate out 10 kg chamber in the
Soudan environment and understand our backgrounds. Xenon requires 100 KW of power
and a dust free environment and ?? of space.
7. Management
There is a demand for underground sites with infrastructure and user amenities. We have
identified short term users and the physics goals which will be accomplished over the
next several years while the facility is being commissioned and operated. However, the
final goal of this facility is to be a multi-user facility serving the needs of a broad variety
of communities for many decades. A true user’s facility will need to be operated in a
manner consistent with guidelines already established by the existing experiments in
coordination with Fermilab, The University of Minnesota, and the Department of Natural
Resources, as well as establish new procedures for outside users.
The Soudan Underground Laboratory is the name of the organization hosting the testing
facility. It is a part of the School of Physics and Astronomy of the University of
Minnesota. The University leases the space occupied by the laboratory from the
Minnesota Department of Natural Resources (DNR), and any activities must conform to
the terms of the lease agreement. The University has formed, with Fermilab, a Soudan
Laboratory Management Group, which meets periodically to review laboratory operation,
finances and future plans. The group is advisory to the Laboratory Director, a Minnesota
physics department faculty member.
The proposed organization of the testing facility meshes with the current laboratory
structure. In addition to the Laboratory Director, a testing facility administrator (also a
Minnesota faculty member) will be appointed by the Management Group. The overall
operation of the facility will be the responsibility of this administrator, with consultation
by the Director. Day-to-day activities will be coordinated by the on-site Laboratory
Supervisor, and all activities will be monitored by the laboratory safety officer.
The lease with the DNR specifies that any new initiatives must be approved in advance
by that agency. A short description of the activity and a list of chemicals, reagents and
other materials must be furnished. All specialized equipment must arrive at the mine’s
headframe on pallets suitable for transport in the cages. Documents describing these
requirements are available for prospective users.
The facility budget provides for one full-time staff member to assist users in setup and
operation of their tests. The facility budget will include an operations budget which is
intended to cover expenses for lights, power, administration and communication services.
If more staff assistance is required, it will be charged at prevailing rates (approximately
$40/hour). We also envision users sending samples for screening which will be entirely
managed by on-site persons for a per sample fee. Mine cage trips for equipment delivery
and casual access will incur a $30/trip charge. Common shop supplies and tools will be
available, but unusual items or continuous usage will be charged at cost. All of these
charges should be paid via a purchase order to the University of Minnesota.
Application to use the testing facility should be made, in writing, to the administrator
with a copy to the Laboratory Supervisor. This should include the description of the test
and material list noted above, for transmittal to the DNR for approval. Once the DNR
has granted approval, and a PO issued, the schedule can be arranged with the Laboratory
Supervisor.
8. Outside Users
Soudan has already had a request from Jeffery Wilkinson, an engineer with Medtronics,
to place a self-contained experiment underground for long-term experiments in
semiconductor memory and logic testing. A low background environment is needed to
establish a baseline value for these tests, to compare with data taken at sea-level and
moderate altitudes. The results of these experiments are used to guide the development of
high reliability electronic components for implanted medical devices. The only space
required is for a small suitcase sized device that only needs 200W of AC power and an
ethernet communications link. In the future, alpha counting of the components may be
also relevant.
Professor Claudiu Lungu at the University of Minnesota is interested in using a new
Soudan facility for research in the field of Environmental Health Sciences. If the facility
were available, he and colleagues would seek additional funding for a series of
experiments related to environmental and occupational health. Projects would be
1. Cancer epidemiology studies of uranium workers in DOE facilities. The dose-effect
relationship at low radiation exposure levels is still a mater of debate among scientists.
Current epidemiological studies related to radiation exposure rely on sparse data with
significant uncertainty. A critical problem in evaluating the dose is due to the
uncertainties in measuring low activities. Uranium underground miners and workers in
the uranium industry were exposed for long periods of time (years) to radioactive
uranium ore or more refined uranium containing products. By measuring lung tissue
samples in a low background facility containing a sensitive Ge detector the activity
content of the lungs could be determine and a better estimation of the cumulative dose
could be expressed.
2. Estimate of exposure to fuel burning products. Exposure to particular matter resulting
for coal, kerosene, and diesel fuel represents an important health concern in particular in
countries that rely daily on these fuels for heating and meal preparation. However, the
exposure assessment needed to evaluate the health effects of these practices is based on
intensive sampling and intrusive methods such as lung spirometry. Because all fuelburning products contain elemental carbon a method for evaluating the lung particulate
burden based on the measurement of radioactive C14 in expectorated samples could be
developed. This method requires a sensitive counting system and a very low background
to produce reliable results.
Dating with 14C and shorter-lived isotopes such as 210Pb are important aspects of
disciplines as diverse as geology, archeology, and oceanography. The best 14C dating
uses accelerator mass spectroscopy (AMS) to achieve 0.3% precision on 14C: 12C ratios of
10-14 for samples at the microgram level. Such small samples would take years at a
counting facility. The Soudan facility becomes competitive for bulk or gaseous samples.
For a 1 m3 volume of gas, we could push the 14C: 12C ratio down to 10-16 with 3%
precision in 100 days of counting in the proposed MWPC beta screener, which
corresponds to 10,000 years earlier than is available by any other method.
However, there is another way in which radiocarbon dating becomes a viable option for
Soudan and that is with respect to standard samples where cost is a factor. A sample for
AMS must be first prepared (processed into a graphite target pellet) and then sent to the
accelerator. The entire process costs roughly $600/sample and most researchers want
complete cores which average 10 samples. This is almost 4 times more expensive than
than counting techniques. Thus, for bulk samples that are not too old, this facility will be
cheap and convenient. The Large Lakes Observatory in Duluth, for example, sends its
14
C samples to the Limnological Research Center’s core lab at the University of
Minnesota, where they are processed into pellets and sent to AMS. Core Lab processes
many samples from all over the country and often redirects customers to conventional
methods when their samples are large. If the Soudan facility were to come online, they
are interested in enhancing their services to also include sample preparation for Soudan.
They will explore chemical digestion and gasification through an MRI grant if the
demand is sufficient.
The St. Croix River Watershed Research Station (SCRWRS), affiliated with the Science
Musuem of Minnesota, presently maintains a counting facility for lake sediment and
geological samples at Marine-on-St-Croix. Their specific research interest is in
environmental history. They do not perform any radiocarbon dating and would send that
out to Core Lab. Lake sediment carbon dating is plagued by the “reservoir effect”,
whereby effects such as ground water influx with old carbon from calcium carbonate
deposits produce water that is no longer in equilibrium with the carbon ratio in the
atmosphere. Thus lake sediment dating relies on finding objects of terrestrial origin, such
as twigs or seeds, in the core, and cannot be approached using bulk sediment samples. A
typical sample might be 50 mg for a date of 10,000 years. The Soudan beta screener
could do this in about 20 days with 2% precision, which is often sufficient and much
cheaper than going the AMS route. Of course, samples of a gram or more can be finished
in less than a day for the same precision.
The SCRWRS does perform on site alpha, beta, and gamma screening of other shortlived isotopes. A popular isotope is 210Pb from organisms, such as coral, that leach
materials which contain daughters from the U/Th chain. Researchers using these
methods are Larry Edward (University of Minnesota- TC) and C. Gallup (MinnesotaDuluth). 137Cs and 241Am from nuclear testing are popular for recent dating, since they
provide a 1963 benchmark. Samples for these three examples of gamma counting are
usually a couple grams. The SCRWRS facility is sensitive to 210Pb at ~0.5 pCi/gm.
Without their limitations coming from cosmics and radon, Soudan can do better. Even
before upgrade, the existing SOLO facility, is able to count 0.01 pCi in a one gram
sample to 3% precision in 100 days. 241Am is difficult to do by SCRWRS, since fallout
produces 100 times less of it than 137Cs. However, 241Am is much more accurate, since is
not as mobile as the cesium. Soudan can already do this 100 times better. According to
SCRWRS’s director, Dr. Daniel Engstrom, they gross approximately $70k - $100k per
year from outside users of the facility and charge ~$2500/core for gamma counting and
~$1800/core for alpha-dating of 210Pb. He estimates that he could redirect between $10k
and $20k of work per annum to the Soudan facility for sensitive counting that is
impossible in his lab.
There is overlap of interest with the CRONUS-Earth Project
(http://www.dal.ca/~cronus/cronus_2062.html), which will address the various
uncertainties affecting the production and accumulation of in-situ cosmogenic isotopes in
rock, including 3He, 10Be, 14C, 26Al, and 36Cl. The build-up of cosmogenic isotopes
provides a measure of near-surface residence time. In the specific case of steady erosion
or deposition, cosmogenic isotopes can be used to estimate the rate of surface denudation
or accumulation. However, because these processes integrate cosmogenic isotope
production over depth, a thorough understanding of the scaling of production rates at
depth is required. Knowledge of the neutron flux and energy spectrum at depth is the first
step towards better calibrating production rates. This falls under the auspices of the
working group “Refinement of Present-Day Neutron Flux and Production Rates,” where
one of the specific goals is direct measurement of neutron flux rates and linking of
neutron fluxes with production rates (http://www.dal.ca/~cronus/cronus_2065.html). The
CRONUS-Earth Project would benefit from collaborations with the background
calculations and measurements performed at Soudan mine, since the background neutron
flux and intensity is their signal. Ideally this would lead to further collaboration in
screening samples and studying backgrounds at Soudan at various levels of the mine.
Lesley Perg (UMinn) is a member of the “Neutron Flux and Production Rate” working
group of CRONUS-Earth and would serve as liaison between the two groups.
Examples from other labs also inform us of the possibilities that exist. The European
Underground Laboratory HADES (Mol, Belgium) has done assays of human lung-tissue
samples collected in German uranium mining areas with the goal of correlating the
activity content with cancer mortality. The same lab assessed the exposure of the public
to fast neutrons around the accident site at the Japanese Tokai-mura nuclear fuel
processing plant by assaying neutron activation products in metal spoons which had been
collected locally around the accident site. These measurements have been performed
using low background Ge detectors. Typical counting sensitivities required for these
applications are of the order mBq/kg or ppb for U/Th. The PIsCES (Precision Isotope
Counting Experimental Setup) of the US Navy Research Laboratory continues R&D
aimed at the ultra sensitive detection of nuclear fission products. Applications range
from verification of compliance with the Comprehensive Test Ban Treaty to nuclear nonproliferation studies and tracking the location and use of nuclear materials for the
International Atomic Energy Agency. The proponents of this project envision the use of
ultra sensitive Ge detector counting, gas proportional counting, and neutron activation
analysis 10.
9. Timeline
Guidelines: Major construction/hoist use in the Mine shall take place during the winter
months for minimum conflict with the tourist season. Commissioning and scientific
input at the mine shall take place during the summer months for maximum faculty and
student involvement. Offsite development, planning and testing can happen during the
academic year at the institutions.
Readying the muon veto is a high priority task. The beta and gamma screeners should be
brought on line as soon as possible using the existing lead shields, but under the muon
veto. The gamma screeners will be in place first, followed by the beta screeners. The
water tank will provide the next stage of background reduction in a year when the
purification system is put in. However the tank itself and the mezzanine need to be
constructed first. By the end of the 2nd year, the screening facilities will already be a
world class facility for users, but even further background reduction may be obtained by
improving water purification techniques as part of the NUSEL (National Underground
Science and Engineering Laboratory) low background working group.
This schedule assumes no limitation due to the funding profile. A complete project file
with schedule, cost, and basis of estimate is available upon request. The project file
reflects current best schedule based on the accessibility of funds.