Replies to MICE Upstream PID Review, February 2007
They are provided throughout the document below the reviewers text,
in italic fonts. A final chapter is devoted to magnetic shielding design.
Executive Summary
Both the Time-of-Flight (TOF) and Cerenkov (CKOV) groups have made excellent progress in the
design of their systems over the past 6 months. Test beam studies of both systems (at Frascati and
Fermilab) have shown that the basic designs are sound and should meet the technical needs of the
MICE experiment. Most technical performance aspects of both systems have now been studied in
some detail. The simulation efforts done to date indicate that the systems should meet the particle
identification (PID) goals of MICE, however, how the time measurement will be determined
relative to the phase of the RF has not been specified and the timing performance needed for the
longitudinal emittance measurement in MICE has not been studied.
Plans will be made in collaboration with the analysis and RF groups to tackle this specific issue
that is beyond the sole competence and manpower of the upstream PID groups. A scoping
workshop should soon be arranged (AB,JC,VP) .
Both groups lack student and/or postdoc resources to provide the necessary support for the
simulation/analysis efforts needed for MICE.
More simulation/analysis effort is definitely needed. The small PID group is forced to raise all
possible help from the collaboration. Chris Rogers and Rikard Sandstrom, that have so far done
most of what has been done, are leaving. Some of the help already obtained is specifically
mentioned below. There is indeed here opportunities for young physicists from all MICE
institutions of very rewarding work at the forefront of MICE, early comparisons with the first
MICE data, etc.
TOF0 and the CKOV will be located behind the shielding wall in the MICE hall and will therefore
have limited access during ISIS operation. Although these detectors are based on an inherently
reliable technology (photomultiplier tubes) the long-term operation reliability of these systems has
not yet been analyzed.
For CKOV I ………………………. Lucien, any preliminary answer possible?
For TOF ……………………….
Maurizio, any preliminary answer possible?
Finally, there are funding problems for the TOF systems. This issue is well known to MICE
management and will not be addressed in this report other than to say that the experiment should
have a risk-mitigation plan in place in the event that funding for the TOF systems is not provided
by INFN.
A possible solution has emerged to TOF funding problems and is being negotiated with the INFN
management. We hope to update positively this sentence within a short delay.
TOF (answers mostly edited by M. Bonesini)
Findings
The upstream Time of Flight (TOF) system for MICE has two main functions: 1) to perform pionmuon identification in the momentum range between 100 and 400 MeV/c; and 2) to provide the
necessary timing accuracy to measure longitudinal emittance in MICE. In addition the TOF
system must also determine the muon arrival time relative to the RF phase. A 70 ps TOF time
resolution seems to be sufficient to carry out these goals, but no simulation has been performed that
gives a precise assessment of the bias that this introduces in the emittance and cooling
measurements.
Please refer to comments made in the executive summary, where the needs of simulation and
analysis are discussed.
The TOF design presented in this review can achieve the above-mentioned goals (an intrinsic
resolution of 50 ps has been measured in the test beam at Frascati) provided that
1) adequate magnetic shielding can be found for the chosen R4998 PMTs. (A solution for PMT
magnetic shielding was shown and does appear to reduce the field at the PMTs to an acceptable
level. However, the solution has not been engineered in the sense that the final mechanical solution
has not been developed nor have the force calculation been done); 2) a time-walk correction can be
made, either by the use of ADC information or through a constant fraction discriminator; 3) the
laser calibration procedure can be implemented (no detailed specification has been presented); and
4) sufficient manpower can be found to carry out testing, construction, simulation and analysis.
All these aspects are being followed through, compatibly with our serious manpower limitations.
A conceptual design for magnetic shielding of the PMT’s was presented. Engineering related
issues including design details and integration with the Spectrometer Solenoid iron shielding have
not yet been addressed. Access to TOF0 and the CKOV is an issue that will require additional
work. The mechanical design for the mounting frame that holds the PMT’s and the overall support
structure were not presented in detail. The designs are not expected to be complicated, but some
engineering effort will be required.
There has been good progress on that after the review. A detailed design of the TOF I shield does
exist now. See last chapte. Access issues are being discussed within the TB.
No final selection for the readout electronics was given, however, there are a number of
commercial solutions the meet the TOF needs.
NB BTF is the Frascati Beam Test Facilty
Comments
The absolute timing measurement for the TOF relative to the RF phase is a difficult problem and
we can not tell from the presentations or support documentation whether or not the proposed
calibration system will address this problem to sufficient precision. (In addition the MICE
experiment needs to define how the RF phase will be measured to the required accuracy. This is
not currently a responsibility of the TOF/PID group).
Please refer to comments made in the executive summary, where a scoping meeting is proposed.
A light pulse to both PID detectors can measure their relative calibration, but does not necessarily
give sufficient control of the absolute timing relative to the RF. For example, the issue of
temperature effects changing the PID cable delays is a serious one. To first order it will cancel out
between the TOF detectors, but perhaps not relative to the RF. Recall that HARP saw a 50ps drift
per degree in the TOF.
Engineering designs of the PMT mounting scheme and shielding for TOF1 must be developed soon
since LBNL will begin the Spectrometer Solenoid iron shielding design early in 2007. Areas of
responsibility do not seem to be clear.
Louvain is completing the design of the global shielding cages and Geneva will construct them.
Shielding of individual PMT is also foreseen in Milano.
The support stand concept shown in the TRD (probably for TOF0) appears to be constructed with
Unistrut and is likely inadequate. A welded steel frame designed to have the necessary stiffness
would be preferred.
See answer to recommendation 6 below.
While other mechanical aspects such as details of the mounting scheme for the PMT’s was not
shown, these designs should be straightforward.
The possibility of moving TOF0 as close as possible to the upstream quad and moving the CKOV
downstream close to the shielding wall will likely improve access.
Discussions are in progress with the technical board.
Potential x-rays backgrounds, to TOF1 in particular, present a potentially serious problem and this
needs to be understood as soon as possible.
Rikard Sandstrom is producing an answer to this.
Recommendations
1. There needs to be further manpower effort invested in the simulation of the TOF system
into GEANT4 for the G4beamline and G4MICE programs, and a proper integration of the
particle identification in the analysis procedures.
2. A proper analysis of the bias introduced in the emittance measurement and the measurement
of cooling in MICE introduced by the TOF resolution and the particle misidentification
needs to be carried out.
3. It is recommended that the particle identification (PID) performance of the TOF be
quantified in terms of probability distribution function for pions and muons as a function of
momentum. This will allow the PID to be determined in number of sigma separation as a
function of momentum.
TOF studies with G4BEAM exists and are documented, though aging by now. TOF is
implemented in G4MICE but little simulation and analysis work has been done and
compared with G4BEAM results, that should in turn be updated. Finding the necessary
person-power to push this ahead is a challenge. Any commitment from those presently
involved in construction or analysis of test beam data would increase the risk not to match
the construction deadlines.
4. In the current design, access to part of the TOF system is very difficult. It is recommended
that gap be opened between the TOF and the Cherenkov systems so that maintenance be
possible.
We agree on this point and are stimulating the TB to proceed to a last revision and freezing
of the assignment of the relevant spaces in the experimental area.
5. The concept for the magnetic shields for TOF1 needs a detailed engineering design and
analysis in order to verify that the required level of shielding is obtained and that the
concept can be integrated into the primary magnetic shield for the tracker solenoid.
Integration issues with the solenoid shield (connection, added weight, areas of responsibility
etc.) should be addressed before the next collaboration meeting.
More generally, plans are such that both global and local shielding is possible. Depending on the
results in Milano of timing performance vs magnetic field, a final decision will be taken before
construction actually starts.
6. The TOF support frames should be designed with the appropriate stiffness, preferably using
welded structural steel.
7. TOF0 and TOF1 should be completed (as funds/resources allow) as specified in the TDR.
(TOF2 is probably secondary, although this was not explicitly studied by this review).
8. Start production as soon as possible
9. In lieu of direct measurements on a 201 MHz cavity, the MICE G4 framework should be
used to study x-ray background effects on TOF performance. Some data exist from the
MuCool 201 MHz cavity prototype and worst-case scenarios should be investigated.
True. But this cannot be tackled directly by the PID group.
10. Some thought needs to go into how the systematic effect of material scattering on the timing
measurement will be estimated. It is a major contribution to the total error in the RF phase
measurement.
See answer to recommendation 9.
CKOV (Answers edited by L. Cremaldi)
Findings
The simulation results indicate that the actual PID performance within MICE is less important than
the timing measurement itself, however the combination of the TOF and CKOV provides excellent
PID separation over the momentum range (200 to 350 MeV/c) that will be explored by MICE. The
redundancy provided by the combination of TOF + CKOV is extremely useful. The two section
aerogel (index: 1.07 and 1.12) CKOV design provides good mu/pi/e separation in the range of
interest.
The mirror construction procedure as presented is fairly complicated, uses non-standard fabrication
techniques, and will most likely require significant development for successful completion. The
tolerances listed in the TRD for the mirror are likely not to be achievable; however, the tolerances
appear to be more restrictive than necessary.
The most appropriate choice is being identified.
The cost to mirror the Lexan surfaces appears to be beyond the project budget.
Any comment, Lucien?
A cantilevered support for the CKOV was discussed for ease of access. RAL will supply the
design, integration and fabrication of the support (per Paul Drumm).
Comments
The current mirror design presents a variety of fabrication challenges. Coupled with the cost of
mirroring the surfaces, the present design presents a large risk and will not necessarily be as cost
efficient as other approaches (such as Tyvek). The Tyvek solution might be sufficient especially if
a thicker radiator is considered.
The mirror design does not require significant developments as they were already successfully used
for the former HARP Cherenkov detector. Consequently there is no large risk involved. The mirror
system will always have advantage over the diffuse reflector in terms of minimum bounces to the
mirror and minimum attenuation of the light. A diffuse reflector painted on flat surfaces replacing
the mirrors could be a poor man’s backup solution although it was not studied nor checked for a
full-scale device. We are reviewing the costs of mirroring at CERN and Fermilab and will make a
decision.
The calibration system and the N2 purge system have not yet been defined.
A simple pulser system has been used in PMT tests in Mississippi, which can be adequately adapted.
A blue LED is driven by a NIM 10ns pulser. The LED light is fed in to the PMT on a short length of
quartz fiber. We have other driver options. A pulser system used in HARP (Trieste) can also be
adapted to MICE.
A nitrogen purge system is not needed for the main vessel, which was not designed to be gas tight.
But it is easily and economically implemented inside the volume of the radiator box. The N2 (or dry
air) purge is a simple low-flow rate line leading in to each radiator box. We have discussed such a
line with Paul Drumm.
There are still large uncertainties regarding the rate capabilities of the PMTs that have been
proposed to be used in the system and it is very likely that the existing bases for these PMTs will
not tolerate the photon rate that will be seen in MICE. Pile-up is very likely to be a serious issue.
For example at 1.5MHz, and with a 10ns pulse length, pileup is 1.3%. However pile-up increases
rapidly with pulse length.
We now have a base design, which operates at well up to 5MHz. Transistors or JFETs are added to
the latter dynode stages to hold the voltages up. This has been verified with our pulser system. The
new bases will be produced at Fermilab in agreement with Alan Bross.
The risk analysis presented shows all risks on the project as being low which does not appear to be
realistic. Significant areas of risk include: mirror component fabrication and assembly, rate
capability of existing PMT’s, possibly inadequate resources to meet delivery schedule and
inadequate budget to complete project.
We will update the risk table. We are forming a contingency plan to deal with all manageable risks.
As in the case of the TOF group, there is no CKOV expert providing software/analysis support and
this is a serious shortcoming.
This is indeed problem. We are however drafting all possible help. Osaka ( !) set up G4 MICE
CKOVI geometry information, under Ghislain’s supervision. Digitization will be tacled next,
possibly with help from Sofia.
The experimental information is rather simple and straightforward. CKOV1 is a beamline device
where light ouput is streamed in to the data recorded for Users to cut on. In this sense everyone is
a Cherenkov expert. High and well modeled efficiencies for muons and backgrounds appear
achievable, with further work, within the time scale of the first MICE run.
Recommendations
1. The PMTs intended to be used in the MICE CKOV should be tested for their rate capability
(with an appropriate base) as soon as possible.
Done. >5MHz capability.
2. The group should verify the index of refraction of the Aerogel 1.12 and its properties with
respect to water absorption
The Matsushita aerogel is advertised as water resistant (hydrophobic). There will b other
backgrounds that will produce light below threshold as rays, interactions, etc.
In addition there is no observed signal from low energy beta P < 1MeV. This indicates an
insignificant % H2O component in the aerogel.
3.
A reliable cost estimate for the current mirror design needs to be obtained. If the cost is
prohibitive, then an alternate design should be explored and tested as soon as possible. In
view of the complexity of the mirror system and its manufacturing difficulty, it would be
beneficial to study the loss in performance of the Cherenkov system using a simpler mirror
system (i.e. either four flat or four spherical mirrors) or a system with a diffuse tyvek reflector.
The current design does not appear to be feasible within the allotted time and budget.
Simpler mirror systems (i.e. four flat or four cylindrical mirrors) have been studied at
length and the corresponding systems were found to be less efficient for the low momentum
particles. Conical surfaces are the next simplest surfaces giving a better light collection and
good particle detection efficiency. Due to the large optical apertures and aberrations
involved, there is no gain in using higher order surfaces like spherical surfaces. Using
experimental data on diffusing paints, the simulations of a Cherenkov with diffuse surfaces
have shown that the light collection efficiency is reduced by a factor 0.6 at least.
4. If the mirror option is chosen, it is recommended to look at different mirror coating options,
including the simple MgF2 option, compared to the more complex SiO2+HfO2 option that
can be done at CERN. It is not clear whether the mirror would fit in the CERN coating
facility. This should be investigated directly with the CERN expert, Andre Braem.
We have some email response from Andre Braem and are planning to meet in
February
at CERN. The light enhancing SiO2+HfO2 vs protective MgF2 is second order effect in
mirror production. Either can be added at the end of Al flashing.
What is that?
5. In the context of (3) above, the cost of Aerogel 1.12 vs. quantity should be verified
We have a bid for Aerogel 1.07 and Aerogel 1.12 panels. $47K for 52 n=1.07 tiles + 52
n=1.12 tiles.
6. Methods to reduce pile-up should be investigated.
The new 5 Mhz bases should eliminate this worry.
7. The calibration system needs to be specified in detail.
We are writing a MICE note on the calibration system. Pedestal and Single
photoelectrons are found from heavily attenuated LED pulses. This sets the
conversion scale for photoelectrons seen in the aerogel.
8. The N2 purge system needs to be specified in detail.
A nitrogen bottle pack will supply dry gas to the radiator box though a flowmeter. A
humidity monitor will be read out in slow control.
9. A detailed plan regarding how the CKOV will be assembled and shipped to RAL needs to
be developed
The Cherenkov light boxes will assembled, light tested and shipped from Mississippi to RAL.
The PMTs will be tested at Trieste and shipped to RAL directly. The aerogel radiator vessel
will be assembled in Mississippi and shipped to RAL.
10. A full check fit of all parts (fully assembled system) should be conducted at U. Miss. prior
to shipping to RAL. Any testing that can readily be carried out at U. Miss. is recommended
as well.
Yes, see 9.
Contradicts the reviewers suggestion?
11. Partial disassembly and packaging for shipment to RAL should be planned such that the
possibility of damage is minimized.
Fragile parts are shipped separately with care.
12. Consider subdividing the volume into 4 smaller volumes, separately instrumented to address
rate issue.
We reject this option due to 1) added reflection in efficiencies introduced at the partition, 2)
the aerogel light yield is somewhat diffuse and unlikely to be totally contained in one
compartment, 3) somewhat more complex design, 4) smaller volumes with separation
surfaces increase the material budget along the beam line.
13. Consider moving the CKOV downstream to reduce rate..
Being investigated. But the particle rate per unit area may not drop enough to want to deal
with increased magnetic shielding problems downstream.
14. A proper engineering drawing of the support structure for the Cherenkov systems needs to
be provided.
Completed and being reviewed. Available at ……………….?
15. A more realistic risk analysis should be carried out so that mitigation strategies for higher
risk items can be identified..
In progress.
16. A new cost analysis should be performed once the design is settled in order to identify the
possibility of a budget shortfall.
We are presently obtaining material costs and bids.
17. Extra manpower needs to be found to carry out the simulation and analysis of the
Cherenkov system, and to integrate this into the global particle identification of MICE..
We hope to apply some student/post doc manpower applied in summer ’07.
Statements & answers related to the shielding of TOFI & II (G. Grégoire)
The TOF1 and TOF2 subsystems are located very near to the upstream and downstream
ends of the tracker solenoids and are exposed to high stray magnetic fields. Thick low carbon steel
disks, initiated and already studied and presented a long time ago by the PID group, provide
important reductions of these fields. They are presently known as the Virostek shields. They are
provided with circular central holes having diameters sufficiently large to avoid beam losses by
scraping. The active areas of the TOFs must at least cover these openings while still being kept
sufficiently small to maintain the timing performances. The attached photomultipliers are thus
located near the boundaries of these circular openings. The PMTs are subjected to residual
magnetic fields still strong enough to impair their performances. It is thus essential to design
secondary local shieldings in addition to the Virostek shields.
The PID group proposes to complete the Virostek shields by attaching "magnetic cages" and
to locate TOF1 and TOF2 inside them: the magnetic cages are made attaching a closing flange,
magnetically linked to the Virostek flange by a ferromagnetic ring. The most important advantage
of the iron cages is to concentrate the magnetic field lines. The remaining field has thus essentially
a weak component transverse to the PMT axis (Figure 1)
Figure 1. Magnetic field lines in the vicinity of the downstream Virostek shield (central
hole diameter = 50 cm) and closing flange (central hole = 60 cm).
The thickness of the closing flange must be such as to avoid saturation of the material. The
computation of the field inside the iron (at z=62.5 cm from the last coil) shows that a thickness of 5
cm is adequate (Figure 2).
Figure 2. The magnetic field inside the closing flange.
The region of space downstream the closing flange is also of interest for the EmCal. The
first layer KL of EmCal is located at about z=70 cm. The minimal radial distance of the KL
photomultipliers is about 60 cm. The residual field components are represented as a function of the
distance to the beam axis in Figure 3.
Figure 3. The magnetic field components at the longitudinal position of the first KL
layer of EmCal. The abscissa is the radial distance from the beam axis.
It is seen that the field components are quite small and that a local shielding of PMTs can be
easily designed. An additional advantage of this solution (putting KL outside the cage) is that it
allows to keep EmCal compact and certainly less prone to particle leakage.
The layout for TOF1 is presented in Figure 4. The case of TOF2 is similar.
Closing flange
Virostek
shield
Beam
Linking ring
TOF1 Active area = 42 cm x 42 cm
Figure 4. 3D and exploded view of the magnetic cage for TOF1. The active area of TOF1 is
indicated by the green colored area. The incident beam is coming from the right.
The magnetic linking ring is split by a couple of openings for the signal and HV cables
serving the TOFs.
The access and maintenance of the TOFs are provided by attaching a TOF and one
sidepiece of the split ring to horizontal guides rails (not shown in Fig. 4). The TOFS can then be
shifted sideways away from the beam line for easy maintenance or repair (Fig. 5).
Figure 5. Sketch of the sideways displacement of a TOF for maintenance and repair. The
horizontal guide rails are not shown.
For the specific case of
TOF1, the central holes in both the
Virostek shield and the closing
flange have diameters of 420 mm. A
vertical cut of the magnetic cage is
represented in Figure 6.
Figure 6. YZ cut of the magnetic
cage at the position of TOF1. The
incident beam is coming from the
right.
Each of the PMTs has its own local shield made of a single 1-mm thick
mumetal sheet. The field lines around the central PMT of any layer of TOF1 is
shown in Figure 7 assuming a pure 2D symmetry of the setup.
Figure 7. Field lines in the vicinity of the central PMT of TOF1. The
horizontal axis is the longitudinal z-coordinate in centimeters measured from
the last coil, while the vertical axis is the radial distance from the beam axis.
The radial dependence of the field components along the axis of the PMT is
represented in Figure 8
Figure 8. The longitudinal magnetic component along the axis of the
central PMT of TOF1. The PMT length is about 10 cm.
and Figure 9.
Figure 9. The radial magnetic component along the axis of the central
PMT of TOF1. The PMT length is about 10 cm.
It is seen that the remaining field is now sufficiently small to avoid modifications of the
timing and gain performances of the PMTs.