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8 Detectors
8.1 Overall description, functionality and redundancy
The MICE detector system as outlined in Section 2 is described in detail in this section. The
driving design criteria are: 1) robustness, in particular of the tracking detectors, to potentially
severe background conditions in the vicinity of RF cavities and 2) redundancy in particle
identification (PID) in order to keep contamination (e, ) below 1%.
Two spectrometers of very similar design, one upstream and one downstream of the cooling
section, measure the full set of the muon emittance parameters: Each of the spectrometers
provides a high-resolution measurement of the five parameters of the muon helix in a tracker
embedded in a 4 T solenoid, as well as a precise time measurement. In addition,
muon/pion/electron identifiers (a t0 timing station and a small Cherenkov) are situated in front
of the upstream detector and muon-electron identifiers (a larger Cherenkov and an
electromagnetic EM-Calorimeter) are situated beyond the downstream spectrometer.
8.2 Scintillators for timing, trigger and upstream PID
Three time-of-flight (TOF) stations equipped with fast scintillators are foreseen. The first two
stations (TOF0 and TOF1), upstream of the cooling section and placed between quadrupoles
Q4/Q5 and Q8/Q9, will provide the basic trigger of the experiment, in coincidence with the
beam particle signal. These two stations will have precise timing (around 70 ps) and will
provide muon identification by time-of-flight (TOF) on a pathlength of about 10 m. The
second of these stations will also provide the muon timing (vis a vis the RF phase) necessary
for the measurement of the input longitudinal emittance.
The coincidence with a third scintillator station of similar nature (TOF 2), downstream of the
second measuring station, will select particles traversing the entire cooling channel. The
variation of emittance due to losses and decays will thus be distinguishable from cooling.
The TOF2 station will of course also record the muon timing for the measurement of the
output emittance.
As discussed in [Janot01], a 70 ps resolution provides both effective (99 %) rejection of beam
pions and adequate (50) precision in the measurement of the RF phase.
Other design criteria are efficiency, redundancy and quality of calibration. The design
presented here satisfies these requirements.
8.2.1 TOF stations structure
In the present baseline design the three TOF stations have dimensions of 24x24, 48x48 and
48x48 cm2 respectively. All stations have a similar design based on two planes of crossed
scintillator slabs along X,Y directions. The planes of the two largest stations (TOF 1, TOF 2)
are equipped with 8 scintillator slabs, with dimensions 48x6x2.5 cm3. Bicron BC-404 (with
1.8 ns decay constant and 1.6 m bulk light attenuation length) is the most suitable choice for
the scintillator material. The smallest plane (TOF 0) could be made of two crossed planes (XY), each of six 24x4x2.5cm3 slabs, using BC-420 plastic scintillator, which is even faster than
BC-404 but with a shorter attenuation length (1.5 ns decay constant and 1.1 m bulk light
attenuation length). Performance ranging between 50 and 90 ps intrinsic resolution has been
already published for TOF planes of similar or slightly bigger dimensions[1], [2]. A 70 ps
resolution fits quite well the requirements of this experiment.
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Each scintillator slab, after a straight PMMA lightguide, is read at the two extremes by a fast
photomultiplier. Figure 1 shows an exploded view of a scintillator bar, with the PMT holder
and the fish-tail PMMA lightguides. Scintillation counters prototypes have been assembled
in-house starting from DTF (diamond tool finish) scintillator bars from Bicron, to which
home-made PMMA light guides, after a cylindrical PMMA collar, are glued with BC600
optical cement. Wrapping and assembly can be easily realized with tolerances less than 1-2
mm for the individual elements of each TOF plane, allowing an easy mechanical mounting
Figure 8-1. Exploded view of a scintillation counter, with details showing the scintillator active area,
PMMA ligh-guides, PMMA collars and PMTs holders
The structure of the two crossed X/Y planes is shown for TOF0 in figure 2. A similar
structure is foreseen for TOF1 and TOF2. The longitudinal thickness along the beamline is
around 5 cm, due to the scintillation counters’ thickness.
Figure 8-2. X/Y planes structure for TOF0 station.
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Figures 3a) and 3b) show the CAD design for the X/Y plane for TOF0 : (a) front view and (b)
top view. For TOF1 the same CAD design are shown in figure 4. As usual the structure is
similar for TOF1 and TOF2 stations.
Figure 8-3. Y planes for TOF1 station: a) front view, b) top view. Quotes are expressed in mm
Figure 5 shows the mechanics mounting of TOF1 station, with X/Y planes and the support
structure, realized with 5 mm L-shaped anticorrodal barrettes. Adjustment for not precise
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construction of single scintillation counters will be obtained by using a pressing screws
system (shown in red in the figure). Similar structures are foreseen for TOF0 and TOF2.
Figure 8-4. Mechanical structure of TOF1. The two X,Y planes are shown, with the mechanical
support structure.
8.2.2 Readout
The analog signal from PMTs could be fed, after a RG-213 cable 1 , to an active splitter
chain that divides the signal 25% to the ADC line and 100% to a leading edge discriminator
followed by the TDC line . The time-of-flight measurement is achieved combining leadingedge time measurements (from the TDC) with pulse-height informations for time-walk
corrections (from the ADC). While TOF0, TOF1 are in a moderate (B200 gauss) magnetic
environment, due to the stray fields of quadrupoles, TOF2 is in a high magnetic environment
(B~1000-2000 Gauss), even after the adoption of a soft iron shield after the MICE solenoid.
In addition TOF0, TOF1 are in a high rate environment ( > 1-2 MHz), while
TOF2 is in a moderate rate environment (~ .3 MHz). This prompt the use of 1” Hamamatsu
R4998 photomultipliers (20 mm useful diameter, 0.7 ns rise time,160 ps transit time jitter)
with a standard mu-metal shield for TOF0 and TOF1 and possibly fine-mesh R7761 1.5”
PMT’s (27 mm useful diameter, 2.1 ns rise time, 360 ps transit time jitter) for TOF2. The use
of R4998 PMTs with an heavy soft iron multiple shielding in addition to mu-metal is under
study also for TOF2. The main caracteristics of both photomultipliers are shown in table I
1
Used instead of standard RG-58 coaxial cables, to minimize distortion
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Table 8-1. Main characteristics of the adopted PMTs
TTS (B=0 T)
Anode pulse risetime
Typical gain (B=0 T)
Typical gain (B=0.5 T)
Max ouput current
Max rate allowed (theor)
R4998 (H6553 assembly)
0.16 ns
0,7 ns
5.7 x 106
16A
267 KHz
R7761 (H8490 assembly)
0.36 ns
2.1 ns
1.0 x 107
3.0 x 106
10A
167 KHz
The main concerns is thus rate for R4998 photomultipliers and magnetic field tolerance for
R7761 photomultipliers. Rate effects for R4998 photomultipliers can be reduced by using a
modified resistive divider with a booster for the last dynodes [3], shown in figure 6. The
shielding from moderate magnetic fields (B up to 200 Gauss) can be obtained with standard
mu-metal shields (also shown in figure 6).
Figure 8-5. Modified divider for the H6533 assembly (R4998 photomultiplier). Individual power
supplies (booster) are foreseen for the last dynodes DY8, DY9, DY10. The mu-metal shielding is also
shown
Tests for the rate effect are foreseen for this kind of tube. The theoretical maximum rate is
determined by the maximum affordable output current. It is around 1.67 MHz for R4998
photomultipliers, with modified divider, and around 0.17 MHz for fine-mesh R7761
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photomultipliers with a standard divider. Figure 7 shows some preliminary results obtained
for R7761 PMTs by using an Hitachi DL3038-011 laser diode (emitting at ~635 nm) pulsed
by a fast pulser (Lecroy 9112, trailing edge~250 ps). Additional tests are foreseen for the
R4998 mod photomultiplier tube assemblies..
Figure 8-6. Figure 7. Preliminary results for R7761 photomultipliers, as a function of incoming
rates. p.e. / pulse are an arbitrary estimate (courtesy of G. Cecchet – INFN PV).
8.2.3. Detector calibration.
For the time inter-calibration of a single detector plane we propose to use cosmic rays,
with a dedicated setup for the trigger (as done in the Harp experiment 4) or to use
beam particles impinging in the overlap region of two superimposed X-Y scintillation
slabs of the same TOF station. The time monitoring of the system will be done with a
laser-based system, similar to the one presently used in the Harp experiment [5] and
shown in figure 8. Studies are under way to assess if the expensive laser system 2
used in the Harp experiment could be refurbished.
2
Modified SYLP0 by Quanta Systems srl, Milano, Italy
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Figure 8-7. Figure 8. Layout of the laser calibration system of the Harp experiment at CERN PS.
A similar layout is foreseen for the three walls of the MICE TOF system. A fast laser pulse is
injected into the scintillator bars via optical fibers, giving the TDC STOP, while the TDC START
is obtained by beam splitting the laser pulse to a fast PIN photodiode (30 ps risetime).
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8.3 Upstream Cherenkov detector for --e separation
The upstream Cherenkov detector, Figure 8-8, provides pion/muon/electron separation to
insure a clean muon beam for the MICE experiment. The purpose of the upstream Cherenkov
is to reduce backgrounds from the time of flight detector. The detector will have four cells
each 200 mm across. Each cell will be 22 mm thick consisting of a 20 mm thickness of C6F14
fluorocarbon liquid plus a 2mm quartz window. C6F14 has been used by the SLAC SLD
[Caval] and CERN DELPHI [Annas] experiments as a Cherenkov radiator. Note that
Cherenkov radiation produced in the quartz window tends to be trapped by total internal
reflection and does not reach the photomultiplier tube. The DIRC detector at SLAC's B ABAR
experiment is based on trapping Cherenkov light in quartz [4]. Four air light guides with 45º
mirrors bring the light out to four 200 mm photomultiplier tubes. The upstream Cherenkov
will be located between the two quadrupole triplets of the MICE Muon Beam Line where
stray magnetic fields are low. The magnets on the ends of the triplets are labelled Q6 and Q7
(Section 7). Two-layer shields of low carbon steel and mu-metal should suffice to protect the
tubes from the fringe fields. The total length of the device is 50 cm. The index of refraction of
C6F14 is about 1.25 and it has thresholds of 0.7, 140, and 190 MeV/c for electrons, muons, and
pions, respectively. Pulse height information is used to aid discrimination. See Figure 8-9 and
[Aubert], [Bartlett], and [Crema].
PMT
UV
WINDOW
MIRROR
CL
C6F14
Figure 8-8. Schematic of Upstream Cherenkov Detector. A single quadrant is shown.
Up to 190 MeV/c, only the muons produce light, so pions are completely rejected, Figure 8-9.
The rejection should match/complement the TOF numbers at 190 MeV/c. Above 190 MeV/c
the pions start to produce some light, so one must start to reject particles that fall between the
muon and pion peaks. This leads to inefficiency but not contamination at moderately higher
momenta. With four PMTs, a MHz beam, and a 10 nanosecond gate, one in 400 particles will
overlap. These need to be rejected. A prototype Cherenkov counter with C6F14 and a five inch
RCA 8854 photomultiplier tube has been built and used to detect cosmic ray muons.
For data acquisition, four ADC channels such as those provided by the 10-bit LeCroy FERA
4300B and four high voltage channels are required. For slow controls, one channel for
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triggering an LED light pulser, one channel for a temperature probe, one channel for a
humidity probe, and four channels to monitor the high voltages are required.
Figure 8-9. Monte Carlo simulation of 186 GeV/c Pion-Muon-Electron response (from left to right)
of the Cherenkov for 1 cm of C6F14. The number of photoelectrons is recorded.
8.4 Tracker module
8.4.1 Overview
The MICE experiment (Figure 8-10) requires that the emittance be measured as the muon
beam enters the cooling channel and again as it leaves. The emittance measurement will be
accomplished using two identical solenoidal spectrometers.
Figure 8-10. Drawing of the MICE experiment showing the upstream(left) and downstream (right)
spectrometers and the MICE cooling channel.
The baseline spectrometer module consists of a 4 T superconducting solenoid of 40 cm bore
instrumented with five planar scintillating-fibre stations. Each station is composed of three
doublet layers laid out in a ‘u, v, w’ arrangement. The active area of the device is a circle of
30 cm diameter. Details of a scintillating fiber tracker prototype and test programme are
reported in [MICENOTE90]. The prototype and tests provide the basis for the design of the
MICE tracker.
8.4.2 Specification
The principal requirements that must be satisfied by the MICE tracking system are:

High efficiency reconstruction of muon tracks in the presence of background;
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
Adequate resolution in the reconstructed track parameters to allow a
measurement of emittance with an absolute precision of 0.1% to be made.
Simulations have shown that these goals can be achieved using the baseline five-station
scintillating-fibre tracker. A 3D engineering model of the fibre tracker is shown in Figure
8-11. Each station consists of three sets of fibre doublet layers mounted at 120º to one
another. The fibre doublet arrangement is illustrated in Figure 8-12. The station-numbering
scheme is defined in Figure 8-13 and the mechanical specification of the tracker is
summarised in Table 8-2.
Figure 8-11. Engineering model of the tracker module showing the 5-station scintillating fibre
tracker installed in the solenoid and the optical patch panel.
a)
b)
Figure 8-12. Detail of arrangement of fibres in doublet layer. (a) Cross-sectional view of fibre
doublet. The dimensions of the fibre and fibre spacing are indicated in m. The fibres shown in red
indicate the seven fibres ganged for readout via a single clear fibre. (b) Layout of doublet layers in a
station. The angle between the fibres in the doublet layers is 120º.
The performance of the spectrometer has been simulated in G4MICE using the nominal input
beam defined in Section 7. At the entrance of the upstream spectrometer, the nominal beam
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momentum is 200 MeV/c with an emittance of 6.4 mm mrads. Figure 8-13 shows the
resolution in the reconstructed track parameters as a function of the transverse momentum,
pT , and the longitudinal momentum, p Z .
a)
c)
b)
d)
Figure 8-13: The resolution in pT is shown as a function of pT in (a) and as a function of pz in (b).
The resolution in pz is shown as a function of pT in (c) and as a function of pz in (d).
Station Numbering scheme
In order to simplify manufacture and to allow a common set of spares to be held, the upstream
and downstream trackers will be as close to identical as possible. A clear nomenclature is
therefore required to avoid confusion during design, manufacture, assembly and installation.
A local coordinate system is also required both during design and construction but, also for
use in the simulation and reconstruction of the tracker digitisations.
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Table 8-2. Key parameters of the tracker module.
Component
Parameter
Value
Scintillating fibre tracker
Scintillating fibre diameter
Primary dopant, pT, concentration
Secondary dopant, 3HF, concentration
Fibre pitch
Estimated light yield per singlet (photoelectrons)
Number of scintillating fibres per optical
readout channel
Position resolution per plane
Views per station
Radiation length per station
Stations per spectrometer
Station separation: 1 – 2
Station separation: 2 – 3
Station separation: 3 – 4
Station separation: 4 – 5
Sensitive volume: length
Sensitive volume: diameter
Magnetic field in tracking volume
Field uniformity in tracking volume
Field stability
Bore diameter
Pressure in magnet bore
350 m
1.25% (by weight)
0.25% (by weight)
427 m
8
Tracking volume
Spectrometer solenoid
7
470 m
3
0.45% X0
5
45 cm
35 cm
20 cm
10 cm
1,10 cm
30 cm
4T
1‰
1%
40 cm
Vacuum
A schematic diagram of the five stations and the support structure is shown in Figure 8-14.
The end of each tracker closest to the liquid-hydrogen absorber is referred to the absorber
end, while the end closest to the optical patch-panel is the patch-panel end. The stations are
numbered from 1 to 5: station 1 being closest to the absorber end of the tracker, station 5
being closest to the patch-panel end. The station number will be prefixed with U, for stations
in the upstream tracker, or D for stations in the downstream tracker.
Fibre Exit
Station
5
Station
4
Station
3
Station
2
Station
1
Figure 8-14. Schematic view of the MICE scintillating fibre tracker. The station numbering scheme
is indicated. Station number 1 is closest to the liquid-hydrogen absorber. Station 5 is closest to the
optical patch panel.
Local coordinate system
Figure 8-15 shows a schematic diagram of the five stations that make up one of the trackers.
The z axis lies along the centre line of the tracker and is orientated such that the z location of
station 5 is larger than the z location of station 1. Hence, the z axis is parallel to the beam
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direction in the downstream tracker and anti-parallel to the beam axis in the upstream tracker.
The x and y axes are defined to make right-handed coordinate system. The x axis is horizontal
and the y axis is vertical. The point x = y = z = 0 lies on the centre line of the tracker and in
the centre of the singlet layer of fibres closest to the absorber end of the tracker.
Figure 8-15. Schematic representation of the five stations that make up one of the MICE trackers
and the local coordinate system. The local origin is placed at the centre of the singlet plane of fibres
closest to the absorber end of the tracker.
8.4.3 Scintillating fibre tracker Details
Operating principle
The basis of charged particle tracking in MICE will be the production of scintillation light in
350 m diameter double clad, doped polystyrene fibres. The concentration of the primary and
secondary dopants must be optimised to maximise the light yield while minimising the fibreto-fibre optical cross talk. The passage of a charged particle through the fibre causes energy to
be transferred to the primary dopant, para-terphenyl (pT). The peak of the scintillation light
spectrum of pT is at a wavelength of ~350 nm. The secondary dopant, 3-hydroxflavone
(3HF), absorbs this light and re-emits it at a wavelength of ~525 nm. The concentration of
primary dopant must be high enough that sufficient primary light is generated, but small
enough to ensure that re-absorption of primary light in the pT is small. The concentration of
3HF must be small enough to ensure negligible secondary light attenuation along the length
of the active fibre, but large enough that the absorption length of the primary light in the 3HF
is small compared to the fibre diameter. The latter condition ensures that fibre-to-fibre cross
talk is eliminated. Measurements have shown that pT and 3HF concentrations of 1.25% and
0.25% by weight, respectively, give sufficient primary light and an attenuation length for
absorption of the primary light in the 3HF of 25 m. A series of measurements of scintillator
properties as a function of primary and secondary dopants is planned to optimise the dopant
concentrations for the MICE fibre tracker. The baseline specification is given in Table 8-2.
Small-diameter fibres are required to reduce multiple scattering in the stations. However,
reading out each fibre leads to a large channel count and a significant electronics cost. The
channel count will be reduced by having 7 scintillating fibres read out through a single clearfibre waveguide (see Figure 8-16). Seven-fold ganging of scintillating fibres leads to a
sensitive element that is 1.63 mm across (see Figure 8-12) and hence give a resolution of
470 m. Simulation has shown that this resolution is acceptable.
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Clear fibre, of diameter 1.05 mm, will be used to transport the light from the stations to the
patch panel and from the patch panel to the photo-detector. The longest clear-fibre run inside
the magnet bore (~1.5 m) will be matched to the shortest run from the patch panel to the
photodetector (~0.5 m). The total length of clear fibre will therefore be kept at or below ~3 m.
The attenuation length of the clear fibre has been measured to be 7.6 m. A total clear-fibre
length of 3 m therefore corresponds to 40% of an attenuation length. It has been estimated
that the reduction in light yield by attenuation in the clear fibre is acceptable.
The Visible Light Photon Counter (VLPC) developed for use in the D0 experiment will be
used. The VLPC is a low band-gap light-sensitive diode that is operated at 9 K to reduce
thermal excitation and is ideal for use in MICE because of its large quantum efficiency (85%)
and high gain (50,000). The device is also insensitive to the magnetic fields in the
neighbourhood of the MICE spectrometer solenoids and to the RF power radiated by the
MICE cavities and associated power supplies and RF-power distribution system. The latter
was demonstrated in a dedicated series of measurements in which the D0 VLPC test stand
was exposed to levels of radiated RF power several times in excess of those expected in
MICE with no detrimental effect on performance.
Figure 8-16. Detail of the seven-fold ganging: Seven 350 m scintillating fibres (shown in red) are
read out through a single 1.05 mm clear fibre (shown in black).
Scintillating fibre ribbon
The scintillating fibres used in the prototype were 350 m diameter Kuraray multi-clad. They
used the standard pT primary dopant. The prototype test was used to study the light yield
versus secondary dopant (3HF) concentration. Fibre with 2500, 3500, and 5000 parts per
million of 3HF doping were studied in this test. All fibres were first cut to length and then
polished on one end so that a vapour-deposited Al mirror could be applied. Although the
quality of the mirrors on the fibres used in our test has not been measured, the D0 experiment
measured an average reflectivity of approximately 90% for the fibres used in their tracker and
the mirrroring procedure applied here was the same as for the D0 fibre.
The ribbons were made following the technique developed for the D0 fibre tracker. A plastic
(Delrin) grooved mould was first fabricated (see Figure 8-17). The mould was measured on a
coordinate measuring machine and the mean groove pitch was determined to be 419 m. Our
target groove pitch was 420 micron (pitch/diameter = 1.2). A teflon release film (25 m) was
first pressed into the mould with the aid of vacuum (pump-out holes were drilled into the
grooves in the mould). A tack adhesive was then sprayed on the teflon and the first layer of
fibres was placed in the mould. A circlular stop fabricated from a plastic sheet was placed
over the mould in order to form a ribbon with the proper circular active aperture. After the
first layer of fibre was in the mould, the spray adhesive was applied to the fibre and the
second layer of fibre (forming the doublet) was placed on top of the first layer. A
polyurethane adhesive was then spread over the fibres and finally a 25 m mylar film was
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placed over the assembly. The assembly was then clamped under pressure during an
overnight adhesive cure. The resultant ribbon was removed from the mould with the release
film still attached. The final step in the ribbon fabrication was to remove carefully the release
film from the ribbon.
Figure 8-17. Schematic drawing of the fibre-double layer laid in the delrin mould.
Mechanical design
The active area of the tracker is required to be 30 cm in diameter. The five stations that make
up a tracker will be held in position by a carbon-fibre frame that will be supported at each end
from the inner surface of the magnet cryostat. To reduce multiple Coulomb scattering, the
inner bore of the solenoid will be evacuated.
The stations will be constructed using carbon fibre to give a rigid structure onto which the
three layers of 350 m scintillating-fibre will be glued. Each station body will be constructed
from a single carbon-fibre structure that allows for the mounting of optical connectors on a
flat annulus, separated by a conical section from a thin, flat ring onto which the scintillatingfibre planes can be bonded. The optical connectors on the station will mate seven 350 m
scintillating fibres to one 1.05 mm clear-fibre light guide. The light guides will transport the
scintillation light from the stations to an optical patch panel that will be mounted on the end
flange of the magnet cryostat. Since the tracker will be operated in a vacuum, the patch panel
is required to form a vacuum seal to the cryostat end flange.
8.4.4 Carbon-fibre station former
The prototype station bodies have been constructed from three separately cured pieces of
carbon fibre: the support/connector flange; the spacer cone; and the scintillating-fibre support.
These were jigged and bonded together. However, in the final design, the aim is to produce
the stations in one piece. Forming tooling for the prototype station supports has been
machined from an epoxy board produced for carbon fibre applications. It has excellent
machining qualities and good dimensional stability. It is made in two halves, split on the
centre-line and is dowelled and bolted together. The completed tool profile was degreased and
a two-part epoxy coating was spray applied. When fully cured, the surface was rubbed flat
with a fine wet abrasive sheet and finally polished with an automotive polish producing a high
gloss finish. In order to protect the station support structure, carbon fibre pieces were cut to a
pattern (12 per layer) and applied radially to the tool surface with no overlap. Three layers in
total were applied, with each new layer starting at a different position to cover existing joints.
Finally three carbon fibre pieces were cut in the shape of a ring and applied to the area of the
connector flange. The assembly was then placed in a vacuum bag with another bag passing
through the tool centre. Vacuum was applied and the assembly placed in an oven. The
temperature was raised 0.5ºC per minute up to 80ºC and left to cure for four hours in order to
produce the final station support structure.
Optical connectors
The optical signal from the tracker is piped to the VLPC system via the fibre light-guides. The
light guides terminate in the D0 warm-end optical connector shown in Figure 8-18. This is an
injection-moulded part made of delrin. The typical optical transmission for this connector
interface is approximately 98%. Light-guide fibre of 1.05 mm diameter is used while the D0
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cassette uses fibre of 0.965 mm diameter. This mismatch results in a light loss of
approximately 15%.
MICE optical connectors at station
The optical connector to be used on the station is required to mate seven 350 m scintillating
fibres to one 1.05 mm clear fibre. The connector design is shown in Figure 8-21. The
connector has gone through 2 iterations; the ones used in the prototype stations had 18
1.05 mm holes laid out in a regular pattern but the final connector has 22 1.05 mm holes
drilled to take the fibres. The hole diameter was matched to the seven scintillating fibres as
shown below. The connector was machined in black delrin.
Figure 8-18. Left:Fibres fitted in hole (not polished). Center: The new connector (right) is the
station fitted half; Right:. MICE optical connector at patch panel – bulk-head connector.
Figure 8-19. Optical Patch-Panel Layout.
The Optical patch panel connector has an O-ring incorporated to ensure a vacuum seal and
contains 128 fibres which gives a 1 to 1 match to the VLPC cassettes. The connector map has
now been developed to provide ease of connection, as shown in Figure 8-19.
As in the case of the station optical connector, this connector is manufactured from Delrin and
as Figure 8-19 shows there are 6 station connectors to each patch-panel connector; although
not
all
of
the
channels
in
the
station
connectors
are
used.
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Figure
8-20 shows photographs of the connectors that have been produced for the next prototype
station.
D0 optical connectors at VLPC cryostat
The optical signal from the tracker is piped to the VLPC system via the fiber waveguides. The
waveguides terminate in the D0 warm-end optical connector shown in Figure 8-21. This is an
injection-molded part made of Delrin. Shown are the 128 holes for fibers, two holes
(left/right) for alignment pins and two holes (up/down) for threaded inserts. The typical
optical throughput for this connector interface is approximately 98%. Since MICE will use
waveguide fiber of 1.05 mm diameter and the D0 cassette uses fiber of 0.965 mm diameter,
approximately 15% of the light will be lost due to this mismatch.
Figure
8-20. Left: Patch-panel connector components. Right: Patch-panel connector assembled.
Figure 8-21. D0 warm-end optical connector.
Optical patch-panel and vacuum seal
Figure 8-22 shows a patch panel conceptual model. Further work will be required to mate it to
the magnet and to ensure that a vacuum seal can be maintained. It has space for 26
connectors. As only 25 of these are needed, the unused opening can be used for field
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monitoring services. The patch panel will need to have additional ribs to ensure that the front
cover will not deflect under vacuum. The dogleg design is to allow the patch panel to fit in a
Z dimension of 60 mm.
Clear-fibre light guides
The light guides for the prototype have been fabricated using 1.05 mm diameter clear fiber
(Kuraray CLEAR-PS, Round-type, Multi-Clad). This fiber has a 2.5% tolerance at 3 , and
the thickness of the cladding layer is 6 % of the diameter. Attenuation length in the clear fiber
has been measured to be 8 m using an Oriel silicon detector system. The light guide is
terminated at the optical patch panel.
Figure 8-22. Conceptual scheme for patch panel
Light guides inside the tracker volume
A light guide bundle inside the tracker volume consists of 110 clear fibers. One end is divided
into 5 groups of 22 fibers and connected to MICE-connectors on the tracker station. The other
end is connected to the patch panel. Five bundles are assigned for one station. Clear fibers are
attached to optical connectors with epoxy glue, and the surface is then polished using a
diamond fly cutter. The length of the light guides is adjusted to match the distance from each
station to the patch panel. The light guides for the nearest station are 1 m long, and 2.3 m for
the farthest station.
Light guides between optical patch-panel and VLPC cryostat
A light guide between the patch panel and the VLPC cryostat contains 110 long clear fibers in
a bundle. One end of the bundle is assembled in the vacuum-tight connector to be attached to
the patch panel, and the other end is connected to a 128-way D0-type connector to fit the
input of VLPC cassette. The length is adjusted from 1.7 m to 3 m to keep the total length from
the SciFi station constant at a 4 m length. The bundles are contained in a fire-resistant flexible
sleeve made of polyamide to provide mechanical support and to exclude light. The 0.5 m of
the fibers at both ends are shielded by the conduit of 16.6 mm inner diameter and 21.2 mm
outer diameter (Adaplaflex PRFS21). The centre section of the light guide is covered by the
conduit of 21.7 mm inner diameter and 28.5 mm outer diameter (Adaptaflex PRCS28).
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Tracker assembly
The first step in processing the fiber ribbons is to put the individual fibers into the correct
seven-fiber bundles. The groups of 7 will have the central fiber marked in order to facilitate
this step. The groups of 7 are held together using black heat-shrink tubing. Once this is
accomplished, the fiber planes need to be aligned and then fixed to the carbon fiber support
structure. This step uses the jigging system developed for the prototype. There will be a few
minor modifications to ease the assembly and a new profile machined to match the final
station geometry.
The first step is to align a fibre plane onto the vacuum chuck. This positioning need only be
within the bounds of the chuck’s own alignment system. After the plane is secured by vacuum
it is aligned using a microscope and linear stage (see Figure 8-23).
a)
b)
c)
Figure 8-23. a) Vacuum Chuck on Linear Stage; b) Vacuum Chuck on Alignment Jig; and c)
Alignment for Vacuum Chuck
The vacuum chuck assembly is located onto the linear stage by precision dowels and when
the alignment is locked these same dowels locate the assembly complete with fibre plane onto
the assembly jig. The assembly jig is in two main parts, the vacuum chuck holder shown
above and the station holder shown below (Figure 8-24). The station holder, as the name
implies, fixes the carbon fibre station in an aligned orientation in relation to the three guide
shafts. Once the station shell is in the holder it remains there until all three planes are glued in
place. As the guide shafts are at 120 radial spacing, this is used to give the radial alignment
from plane to plane.
Figure 8-24. Left: Station holder. Right: Station holder and station
The first plane is glued directly onto the carbon fiber support structure. The next two fiber
planes are then glued in sequence to the bottom plane. However the glue must not be allowed
to form a complete circle around the active area as air will be trapped forming an air pocket.
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Upon evacuation of the tracker solenoid, expansion of this gas volume would be destructive.
Therefore, extreme care must be taken in order to avoid trapped air pockets.
Fitting the fibers into the optical connectors is a time consuming job that requires a great deal
of dexterity and concentration. A method for checking the accuracy of the connections is
essential and any errors must be rectified before the fitting of the connectors to the station is
completed. Figure 8-25 shows details of the ribbon connector assembly. The ribbon
connectors are attached to the carbon fiber support structure before the fibres are potted. This
ensures the best possible lay of the fibres by gently pulling the fibres. When the fibres are at
the best possible position without any undue strain being exerted, they are potted.
Figure 8-25. Left: Connectors fitted and awaiting potting. Right: Test piece showing potting
After the fibres are assembled into the connector and checked for accuracy, they are laid out
in a gentle sweeping curve and can then be potted. To do this they will be shortened to about
25 mm beyond the face of the connector to enable the fitting of a vacuum cup. When the
potting adhesive is applied in the recess, vacuum is used to ensure that the adhesive travels
the length of the fibre in the bore. After the adhesive is through the bore, the vacuum is
removed and the recess filled with additional adhesive. When the adhesive is in a stable state
(i.e. does not run) the station is turned over and adhesive is applied to the front face of the
connector around the fibres. This is to prevent them moving/vibrating as they are cut and
polished.
When the potting has cured sufficiently to allow the station to be turned over, more adhesive
is applied to the front face to form a solid block of material. This ensures that there is a
minimum of stress and vibration transferred to the fibres during machining. We anticipate that
the machining process will need further development of the tools and techniques to improve
the polished finish (although the prototype was deemed to be acceptable). The
cutting/polishing is done using a diamond tipped tool which should give the required finish
without further polishing that might degrade the flatness.
Readout electronics
Overview
MICE will use the D0 central fibre tracker (CFT) optical readout and electronics system. This
system has been operating reliably for the D0 experiment for almost 4 years now. The
photodetector is the visible light photon counter (VLPC) manufactured by Boeing. The
VLPCs operating at 9 K and will require a cryogenic system. The VLPCs are packaged into a
cassette which contains 1024 channels. Two analog front-end boards (512 channels each)
provide readout, temperature control, and VLPC bias.
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VLPC system
The VLPC is a cryogenically operated silicon-avalanche device. The operation and
development of which has been discussed extensively in the literature [VLPC]. It is a
descendant of the Solid State Photomultiplier, an impurity-band silicon avalanche
photodetector. It has undergone six design iterations, specified as HISTE I - HISTE VI.
HISTE VI is the version used in the D0 CFT. It is an eight-element array in a 2 × 4 element
geometry. Each pixel in the array has a diameter of 1 mm. The HISTE VI operational
parameters are given below:




Quantum yield > 0.8
Gain > 40,000
Operating temperature 9K
Operating bias 6-8V
VLPC cryostat
The VLPCs operate at cryogenic temperatures and a cryo-system is required. The current
baseline for the VLPC cryo-system is to use Gifford-McMahon (GM) cryo-coolers to
maintain the 9 K operating temperature for the VLPCs. The design work for this system has
just started, but it is believe that commercial GM coolers are a cost-effective approach to the
VLPC cryo needs. Four cryostats will be used in MICE, each cryostat holding two cassettes
(which will read out one half of one of the trackers). Two cryogens are used in this system.
Liquid helium from the control dewar will allow for VLPC operation at 9 K and liquid
nitrogen will cool an intermediate heat intercept in the VLPC cassette in order to reduce the
heat load to the liquid helium. The cassette cold-end will sit in a stagnant gaseous helium
volume. Conduction through the gas will cool the VLPCs. The temperature stability
specification of the VLPC cryostats is  50 mK.
VLPC cassettes
The VLPC cassette contains 1024 channels of VLPC readout and is divided into 8 modules of
128 channels, each of which is interchangeable and repairable. This is illustrated in Figure
8-26 and Figure 8-27. Figure 8-26 shows the full cassette with readout boards attached.
Figure 8-27 shows the inner components of the cassette, with the readout boards and cassette
body removed. Both figures clearly show the 8-fold modularity of the cassette design.
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Figure 8-26. The VLPC cassette with readout electronics board attached
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Figure 8-27. Inside view of the VLPC cassette with cassette body removed.
Sitting directly over each VLPC pixel is an optical fiber which brings the light from the
detector to the VLPC chip. Each cassette module is comprised of an optical bundle assembly,
a cold-end electronics assembly, and an assembly of mounted VLPC hybrids. The cold-end
assembly is designed to be easily removable for repair without disturbing other modules due
to the high cost and delicate nature of this device. Another important design requirement of
this cassette concerns the read-out electronics: the readout electronics boards and the PC
boards which act as interface to the data acquisition system must be removable and
replaceable without removing a cassette from the cryostat. The readout electronics are
discussed in detail in the following section.
The cassette, for purposes of discussion, is broken down into several major components. The
cassette is distinguished as having a “cold-end”, that portion of the cassette which lies within
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the cryostat, and a “warm-end”, the portion of the cassette which emerges from the cryostat
and is at room temperature. At the cold-end, eight cold-end assemblies, each of 128 channels
of VLPC readout, are hung from the feed-through by the optical bundles and are surrounded
with a copper cup at the cold-end. Each cold-end assembly consists of sixteen 8-channel
VLPC hybrid assemblies, the “isotherm” or base upon which they sit, the heater resistors, a
temperature-measurement resistor, the cold-end flex-circuit connectors and the required
springs, fasteners and hardware. Running within the cassette body from top to bottom are
eight 128-channel optical bundle assemblies which accept light from the detector wave-guides
connected to the warm-end optical connectors at the top of the cassette and pipe the light to
the VLPC's mounted at the cold-end (see Figure 8-27). The electronic read-out boards are
located on rails mounted to the warm-end structure and are connected electrically with the
cold-end assemblies via kapton flex circuits. In addition, the electronics boards are connected
to a backplane card and backplane support structure by card edge connectors and board mount
rails. The flex circuits and read-out boards are electrically and mechanically connected by a
high-density connector assembly.
The cassette body can also be broken down into cold-end and warm-end structures. The coldend structure is broken down into several sub-assemblies: namely the “feed-through
assembly”, the G-10 walls, the heat “intercept” assemblies, and the cold-end copper cup (see
Figure 8-27). Along the length of the cold-end, two heat intercepts are integral to the cold-end
cassette structure. The first is the liquid nitrogen intercept (77 Kelvin) which serves to cut off
the flow of heat from the warm-end. The second is the liquid helium intercept, with a name
more historic than functionally descriptive, which serves as an IR suppression device and
terminating structure. The warm-end structure is made of parallel aluminium plates spaced by
spacer bars which form a protective box for the optical bundles.
AFE boards
MICE will use the D0 AFEII boards in order to read out the VLPC system. This new
electronics board is currently under development by D0. It will include the following:






Front end preamplifier with 48 bin analog pipeline
Commercial 8 bit 20 MHz flash ADC
Discriminator outputs for each channel
FPGA for each module of 32 channels
Temperature control circuitry for each cassette module (control and heater)
VLPC bias circuitry with voltage and current read back
Each board will service 512 channels or half of one cassette. MICE plans to also use the D0
VME architecture for readout of the AFEs into VME. This is performed by the sequencer
module. In the case of D0, the readout is synchronized with the accelerator crossing clock.
For MICE this will not be the case. The tracker will run asynchronously being triggered by
beam particles.
VME system
Two standard 9U VME crates are required to read out all channels in the two fiber trackers in
MICE. These crates will each house a BIT3 interface, 8 D0 4 SASQ boards (one for each
AFE board that is readout) and a 1553 I/O interface.
Simulation
Details of the Monte Carlo simulation that was performed to study the fibre tracker’s
performance in a high-background environment are given in Section 3; a few salient points
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are given here. The five-plane fibre tracker is very insensitive to photon backgrounds from the
RF cavities and maintains its excellent tracking performance at rates significantly higher (260
kHz/cm2) than that expected in MICE based on current measurements [MICENote90].
8.5.1 Downstream TOF2 detector
As stated earlier, the coincidence with TOF2, downstream of the second measuring
station, will select particles traversing the entire cooling channel. The variation of
emittance due to losses and decays will thus be distinguishable from cooling.
The TOF2 station will of course also record the muon timing for the measurement of
the output emittance.
In the present design the TOF2 station has an area of 48x48 cm2 , covering beam
profile at more than 3 sigmas and is equipped with R7761 Hamamatsu fine-mesh
photomultipliers.
As all details (mechanics assembly, PMTs choice, readout, detector calibration, …)
have been reviewed in section 6.2, we refer the reader to that section for more details.
8.5.2. Tests in high magnetic field of fine-mesh PMTs
Here we stress only one point. For the downstream TOF2 detector, PMTs must
sustain an appreciable magnetic field intensity : without soft iron shield B > 1 T, with
a soft iron shield B is around .1 - .2 T.
Tests for the behaviour of PMTs in a high magnetic field B (gain as a function of B,
inclination angle  as respect to B field direction and timings properties) are
underway at the LASA laboratory, INFN Milano. A resistive dipole magnet (shown
in figure 8) has been refurbished, allowing fields up to 1.2 T with an open gap of 12
cm.
Figure 8. Refurbished dipole test magnet at LASA. The PMT under test is shown
between the coils.
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A fast laser pulse from a laser diode ( 635 nm), triggered by a pulser with a risetime
around 250 ps (Lecroy 9112) is sent to the photocathode of the PMT under test, via a
CERAM OPTEC UV 100/125 optical fiber (with a minimal dispersion ~ 15 ps/m).
Some preliminary results for gain as a function of B and and transit time, transit
time spread (TTS) are shown in figure 9 and 10 for a typical R7761 PMT.
Figure 9. Gain ratio, as a function of the magnetic field B for 0, 10, 15, 20 degrees,
for a typical R7761 PMT with standard divider (H8490-70 assembly).
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Figure 10. Transit time ratio (a) and TTS ratio as a function of magnetic field B for
=0 degrees for a typical R7761 PMT with standard divider (H8490-70 assembly).
8.5 Downstream PID (e- separation)
In a small fraction (~1%) of events, a muon decays inside the cooling section or one of the
spectrometers. The resulting electrons bias the emittance measurement considerably and must
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be rejected. Momentum distributions of electrons and muons arriving downstream of the
second spectrometer are shown in Figure 8-28.
Figure 8-28. Momentum distributions of muons and electrons downstream of the second
spectrometer [Janot01].
Kinematic cuts can reject about 80% of the decay electrons, but this level of rejection is not
sufficient to avoid a bias in the emittance measurement. Dedicated detectors are needed to
separate electrons from “good muons”. The strategy is as follows:
a) to positively identify muons by requiring low and longitudinally uniform energy
deposition in an electromagnetic EM-Calorimeter (EMCAL) at the very end of the
experiment, and
b) to reject any residual background of electrons in the muon sample defined by the EMCAL
by means of a threshold Cherenkov detector immediately upstream of the EMCAL.
Geometrical features of these particle identifiers are determined by the angular and spatial
distributions of particles at the end of the experiment. Typical transverse beam profiles for
good muons at distances of 55 cm (TOF2), 70 cm (CKOV2) and 122 cm (EMCAL)
downstream of the last solenoid, obtained from simulations, are shown in Figure 8-29 [TJR].
A geometrical aperture of order 1 m diameter is needed to take into account the defocusing of
particles in the stray magnetic field of the downstream solenoid.
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a) Radial muon distribution at the TOF2 position
beam axis.
b) Good muon x-distribution at the entrance
window of CKOV2
c) Good muon -distribution in x at the position of EMCAL
Figure 8-29. Good muon spectra (blue line) for the a) TOF2, b) CKOV2 and c) EMCAL compared to all
particles (red line). [TJR]
Geometrical features of these particle identifiers are determined by the angular and spatial
distributions of particles at the end of the experiment. The profiles in position and angle were
obtained from simulations, at realistic longitudinal positions of the downstream detectors
along the general MICE beam axis. These positions are sketched in Figure 8-30. The
geometrical aperture of all devices will match these profiles.
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Figure 8-30. Longitudinal positions for the downstream detectors relative to the tracker end
correction coil (in green). A 5-cm clearance has been assumed between detectors..
8.5.1 The electromagnetic calorimeter (EMCAL)
A state-of-the-art, high-resolution electromagnetic Pb-scintillating fibre EMCAL, of the type
built by KLOE [KLOE], is proposed. It offers adequate energy resolution to perform muon
and electron identification in the momentum range of interest for MICE.
Structure and layout
The proposed EMCAL, built by gluing 1-mm-diameter blue scintillating fibers between 0.3
mm thick grooved lead plates, has a uniform and quite symmetric Pb–scintillating-fiber
structure, with a fiber spacing of 1.35 mm. When layers are superimposed, fibers are located
at the vertices of adjacent quasi-isosceles triangles, forming a homogeneous and compact
structure with a fiber to lead volume ratio of 2 to 1. Compared with the KLOE-standard
mixture of 1 to 1, the choice is dictated by the softer muon spectrum to be detected in MICE.
The resulting composite, with a density of ~ 3.7 g cm–3 , a Moliere radius of ~ 3.5 cm and a
radiation length X0 of ~ 2.5 cm , gains considerable stiffness, and can be easily machined to
the shape required for the final assembly.
This ‘spaghetti’ design offers the possibility of fine sampling and results in optimal lateral
uniformity of the EMCAL. Fibers run mostly transversely to the particle trajectories, reducing
sampling fluctuations due to channeling, i.e., showers developing along the fibers’ direction,
an effect particularly important at the low energies of interest. Finally, the very small lead foil
thickness (~ 0.05 X0) results in a quasi-homogeneous structure and a high efficiency for
minimum ionizing particles and low energy electrons.
An EMCAL module of 72 × 72 cm of active area, 16 cm thick, consisting of ~ 180 lead and
fiber layers, can be built using the facilities of the Frascati (LNF) workshop formerly used for
the construction of the KLOE EMCAL [KLOE]. The lead and fiber planes are perpendicular
to the beam axis. The EMCAL is built in four layers, each made of 18 cells 4 × 4 cm wide and
72 cm long.
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Fabrication of components
The grooved lead foils are obtained by rolling 0.3 mm thick foils in a special shaping
machine. The drawing of the lead foil cross section where the fibers are glued is shown in
Figure 8-31.
Figure 8-31. Detail of the groved Pb foi 0.5 mm thich top compared to a 0.3 mm thick foil bottom.
Based on KLOE experience, the optical fibers will be Pol.Hi.Tech. type 044 [POL], emitting
in the blue-green region and meeting, at reasonable cost, the MICE technical specifications.
Great care will be taken to maximize the efficiency of the light collection system and to
ensure uniform photocathode illumination. KLOE experience suggests to use light guides
consisting of a tapered mixing part, where the quadrangular entrance face transforms
smoothly to its inscribed circle, plus a Winston cone concentrator [Welford], matching the
area of the EMCAL element to the sensitive area of the photocathode face. If it is possible to
shield against the residual magnetic fields, a suitable readout choice, at both ends of each cell,
is the 1-1/8 inch R1355 Hamamatsu phototube. This tube, already used in the HARP
experiment, has a transit-time spread of less than 1 ns. In this case, the area concentration
factor is ~ 2.65, with a light collection efficiency of ~ 85%. Figure 8-32 and Figure 8-33
show the housing of a voltage divider, photomultiplier and light guide and the assembly with
the fiber-lead module.
Figure 8-32. Voltage divider, PMT, and light guide housing
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Figure 8-33. EMCal Assembly
Front-end electronics
The deposited energy will be digitized by conventional ADCs, e.g., the VME CAEN 792,
already used in the HARP experiment. An unbiased cross-calibration of all EMCAL readout
elements will be obtained both from cosmic rays and test-beam muons. A rough measurement
of the impact point on the EMCAL will be derived from the ratio of pulse heights on two ends
of the fired cell. A more precise reconstruction of the impact point, with independent timing
information for trigger purposes, can be achieved by implementing a TDC readout of the first
layer of cells.
Energy and timing resolution, particle identification
The fibres have a decay constant of ~ 2.5 ns, an attenuation length of ~ 3.5 m, and a yeld of
about 80 photoelectrons per minimum ionizing particle (mip) crossing the center of one of the
40 × 40 cm EMCAL cells. Based on the response of the KLOE electromagnetic EMCAL
[Adinolfi] to electromagnetic showers of similar energies at the DAPHNE Φ-Factory, the
expected energy resolution is σE ~ 5% /E (E in GeV), fully dominated by sampling
fluctuations. The time resolution was measured in KLOE, with modules 4 m long, and is also
very good, σt = 54ps /E , giving a precise measurement of the impact point along the fiber.
The KLOE EMCAL test beam data [Antonelli] show that the distributions of total energy are
Gaussian, with almost no tails. The response of the EMCAL to electrons and photons in the
energy range 20–300 MeV is linear, independent of incident angle [Antonelli]. The signal
deposited by a minimum ionizing particle in a single cell is equivalent to that of a 30 MeV
electron or photon. In the momentum range of interest for MICE, muons mostly punch
through, whereas electrons leave essentially all their energy in the first two layers. In
combination with the downstream Cherenkov, the measurement of energy deposition and of
its longitudinal profile in the EMCAL should provide an electron rejection of 99.9%. A
preliminary simulation of the EMCal performance on muon detection and muon-electron
separation with various selection cuts has been performed. The results, based on the beam
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profile and energy spectrum generated by G4MICE (version :…) , are shown in
Figure 8-34 and
Figure 8-35. Cosmic ray tests are foreseen, together with tests of detector response to low
energy electrons, hopefully at the Beam Test Facility in the DAPHNE complex of LNF.
Figure 8-34. Muon detection efficiency at the EMCAL as a function of the muon momentum.
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Figure 8-35. Electron-muon separation in EMCAL using the baricenter method.
Construction schedule
INFN technical manpower will mould the light guides, and design and fabricate the
mechanical supports for photomultipliers and patch panels. Some substantial refurbishing and
tuning of the KLOE tooling machinery will be needed. Lead foil grooving, construction of
light guides, production of fibers, gluing of layers of lead with fibers, machining of the
assembled modules and final gluing of light guides will be contracted to external firms. The
completion of the construction project will take one year after approval of funding.
8.5.2 The Downstream Cherenkov
Further electron rejection will be provided by the downstream aerogel Cherenkov detector. In
principle, this detector should not be affected by background from the RF cavities, as x-ray
energies will rarely be high enough to produce electrons that give Cherenkov light in the
aerogel (i.e. above 2 MeV). The RF noise at 201 MHz has a skin depth of 6 m in aluminium.
However, a ferromagnetic shielding is however necessary to be protected it against the stray
magnetic field.
Description and Performance
In the momentum range of interest to MICE, aerogel (1.01 < n < 1.06) appears to be the only
adequate radiator from which to build a threshold Cherenkov blind to the passage of muons.
The choice of the appropriate index of refraction for the radiator is governed by the relative
light yields of electrons and muons, and their respective detection efficiencies assuming a
fixed detection threshold. The goal is to maximize the response to electrons while minimizing
possible contributions from higher energy muons. The cylindrical radiator is made of aerogel
with an index of refraction n = 1.03 and a total thickness of 10 cm. The diameter of the
radiator is about 80 cm. This geometrical aperture is deliberately chosen to be sufficiently
large as to avoid losses of good muons.
The upstream face of the aerogel radiator will be located at z= 500 mm from the last
correction coil and the downstream face of the Cherenkov vessel at z= 967 mm.
The useful transverse size has been evaluated on the basis of GEANT4 simulations. The
simulations take into account the presence of a thick magnetic shield between the last
correction coil and the downstream detectors (Table 8-3, Figure 8-36).
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The mechanical design of CKOV2 is based on a useful transverse size of 85 cm for its
upstream face (particle entrance) and 135 cm for its downstream face to take the muon
divergence into account. The longitudinal dimension essentially depends on the diameter of
the chosen photomultipliers.
Table 8-3. RMS beam sizes obtained by simulation (without shielding) [YT].
Detector
TOF2
CKOV2
RMS beam size
(cm)
59
67
EMCAL
93
Comment
at the upstream face
of the aerogel
Figure 8-36. Muon distributions in a plane perpendicular to the beam axis [YT] Top row: x-, px- and
x'-distributions. Bottom row: y-, py- and y' distributions. All horizontal axes are in millimeters.
The environment of the downstream detectors
Because of the large bore of the solenoid, there is a strong stray field extending far away from
the last coil and giving rise to residual fields unacceptable for some of the downstream
photomultipliers (Figure 8-37). Detailed studies have sought a geometry having the smallest
shield that can give tolerable residual field levels (less than 2 kGauss) at the sensitive parts of
the downstream detectors.
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Figure 8-37. Magnetic fields at the photocathodes of the downstream detectors without any shielding
material. The coloured dots show the field intensities at the respective detector (photocathode)
positions (Figure 8-30).
To provide shielding from the stray fields, it has been shown that a 15-cm thick soft-iron disk
with a diameter of 150 cm provides an optimal solution (smallest transverse size, reasonable
thickness). It has a circular opening 50-cm in diameter along the beam axis and is located 40
cm downstream of the last superconducting coil. The longitudinal positions of the
downstream detectors and the approximate radial distances (from the beam axis) of the
various photocathodes are summarized in Table 8-4.
Table 8-4. Positions of the photocathodes of the downstream subdetectors.
Sub-detector
Longitudinal position
z (cm)
TOF2
CKOV 2
EMCAL
45
83
114
Radial distance from beam
axis
r (cm)
26
38
36
The (z-r) maps of the stray magnetic fields are then evaluated at these positions:

at the longitudinal position corresponding to TOF2 (z=45 cm), the remaining
magnetic field is less than 1000 Gauss (Figure 8-38).
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Figure 8-38. The magnetic field as a function of the radial distance to the beam axis at the position
of TOF2 (dashed vertical line). The red(green) curve is obtained with(out) the MICE shielding.
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
at the longitudinal position corresponding to CKOV2 (z=83 cm), the remaining
magnetic field is less than 600 Gauss (Figure 8-39).
Figure 8-39. The magnetic field as a function of the radial distance to the beam axis at the position
of CKOV2 (dashed vertical line). The red(green) curve is obtained with(out) the MICE shielding.

at the longitudinal position corresponding to EMCal (z=114 cm), the remaining
magnetic field is less than 300 Gauss (Figure 8-40).
Figure 8-40. The magnetic field as a function of the radial distance from the beam axis at the
position of EMCal (dashed vertical line). The red(green) curve is obtained with(out) the MICE
shielding.
One general consequence is that the remaining fields at the various detector positions can still
be lowered with standard shields (typically 5 mm soft iron plus 1 mm thick mu-metal).
The gas-tight external box enclosing the optical system will be constructed from nickel-plated
(against rust) soft steel (for magnetic shielding). Using a cylindrical approximation for the
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shape of this vessel, the remaining inside field was computed at the position of the
photocathode (Figure 8-41).
Figure 8-41. Field lines inside an approximate Cherenkov vessel at the position of the
photomultipliers with the presence of a 15cm thick iron shield, 150 cm in diameter and with a 50 cm
aperture (Figure 8-30). (fig 8-22 shows 10 cm thick shield)
Without the mu-metal shield, the residual field is 30 Gauss at most. It is also seen that the
field lines are nearly parallel to the PM axis: their influence on the photoelectron collection
efficiency is further diminished.
Additional precautions (thin lead shielding and 10-m-thick copper cladding) may be
necessary to account for the possible x-ray and RF backgrounds.
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Detailed description
Mechanical Layout
A perspective view of a partially assembled containment vessel is shown in Figure 8-42.
Figure 8-42. Perspective view of the skeleton iron structure for the Cherenkov 2 vessel.
The vessel is constructed by welding soft-steel pieces 15 mm thick to generate a 12-side
polygonal structure. The metallic structure is then plated with nickel for protection against
corrosion. Soft steel is needed as it shields the photo-detectors from the stray field of the
solenoid. Whenever possible all welds will be checked against leaks. Four stainless steel
"bridges" protrude upstream, and are intended to lean against the thick iron shielding and to
withstand the forces generated by the spectrometer and end coil. Square optical windows
cover the inner sidewalls of the box. These windows are completely aluminized except in
front of the photocathodes. The shallow volumes on all twelve lateral sides are equipped with
Winston cones. The size of these cones allows an increase in the overall optical acceptance of
the instrument (grey elements in Figure 8-43).
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Figure 8-43. The Winston cones (grey elements) and the PM tubes at their position around the
CKOV 2 box.
Next, the Winston cones and photomultiplier tubes are covered by a double layer of shielding
(5 mm soft iron plus a 1 mm Armco layer) (Figure 8-44).
Figure 8-44. The final magnetic shielding in place ( Armco sheets in yellow).
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Beam entrance and exit windows (not shown here) will be made with 10 mm thick
honeycomb panels contained within thin aluminium skins. Details of the assembly are shown
in Figure 8-45. O-rings provide the gas tightness along the windows, the radiator box and
along the edges of the entrance and exit windows. Transverse and longitudinal views are
shown in Figure 8-46 and Figure 8-47, respectively.
Apart from two identical large iron flanges which will be subcontracted to industry, the
construction is made from much smaller pieces which can be machined with standard tooling.
Figure 8-45. Detailed drawing of the CKOV2 vessel in the vicinity of a photomultiplier with the
particle entrance window (10 mm honeycomb) at the far left and the exit window at far right. The
outlines of a Winston cone and a window are visible around and below the photomultiplier.
Figure 8-46. Longitudinal cut view of CKOV2. The tiled area at left is the aerogel radiator.
Dimensions are given in millimetres.
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Aerogel radiator
The aerogel radiator has an index of refraction n=1.03. Stacking 250 tiles of aerogel of 13 ×
13 × 10 cm provides a useful radiator size of about 80 × 80 × 10 cm. The manufacturer can
provide so-called improved “hydrophobic aerogel”, which is less prone to degradation with
humidity. The thickness of the radiator is chosen on the basis of the small photoelectron yield
with low energy electrons. It is assumed that the aerogel wall will be assembled in the lab
(outside the beam area) and maintained in a gas tight container under helium. Upon
completion of the assembly of the experiment, this closed container is fixed in place to the
honeycomb particle entrance window. The downstream wall of the container can be easily
and rapidly removed (from the downstream side) and the vessel closed with the tilted mirrors.
This last operation takes only a couple of minutes. To avoid moisture degrading the aerogel ,
during filling, the vessel will be filled with dry helium gas at atmospheric pressure (Figure
8-48).
Figure 8-47. Transversal cut view of CKOV2 (as seen by the incoming muons). The polygonal
outline in the center is a cut through the honeycomb structure supporting the 12-sided pyramidal
mirror. Dimensions are in millimeters.
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Figure 8-48. Perpsective view of the aerogel box supported by the honeycomb particle entrance
window. The polygonal ring on the left side is a reflecting wall which also serves to “close” the
aerogel box.
Light collection walls and mirrors
For reasons of availability, cost and handling, the inner surfaces of the vessel potentially hit
by light are covered by optical glass plates (7 mm thick Schott B270), aluminized everywhere
except in front of the photomultipliers. Plane mirrors, tilted at 45°, reflect light at 90° to the
optical beam axis towards the photodetectors. The mirrors will be made from aluminized 3mm thick polycarbonate (Lexan) plastic sheets. In order to keep the symmetry of the device
around the beam axis, the four mirrors are assembled in a reflecting pyramid as seen in Figure
8-49.
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Figure 8-49. The 45° reflecting pyramid is attached to the downstream face The mirrors are
supported by a lightweight honeycomb structure.
The reflectivity of the aluminised plastic sheets is taken to be about 90% (as used for the
Cherenkov detector of the HARP experiment). It is, however, possible [ABr] to benefit from
recent developments based on a multilayer of aluminium, magnesium fluoride (MgF2) and
hafnium oxide (HfO2) to reach reflectivity values as high as 96% in the visible domain of
interest (250 to 500 nm), Figure 8-50.
Figure 8-50. Reflectivity of a multilayer of Al, MgF2 and HfO2.
The performances of curved (cylindrical) mirrors was studied during the initial design phase
of this project. The goal was to improve the light collection efficiency. The results were not
better than for plane mirrors. The reasons come from the very large "object" size and large
divergence of the photons generated in the radiator. At the same time, the longitudinal size of
the detector must be kept as small as possible. The optics then serves more as an
“illumination” system than as a “focusing/imaging” or “paraxial” system. The remaining
design goal is thus to minimize the total number of reflections on the walls.
Photomultipliers
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The system uses EMI 9356 KA 20-cm (8")-diameter photomultipliers, with a standard bialkali photocathode. These very low noise PMTs are those used by a former gas Cherenkov
detector built for the HARP experiment. This PMT has the advantage of a rather large gain, 3
x106, that is well matched to the low light yield of aerogel radiators. The PMTs are mounted
in volumes separated from the radiator section. This allows easy access to the PMTs without
disturbing the dry environment of the aerogel and avoids possible damage to the mirrors.
Estimated optical performances and detection efficiency
The optical characterization of this scheme has yet to be performed (and compared with that
of the MICE proposal). As for the MICE proposal, characterisation will be done by accurate
3D optical ray tracing, taking into account realistic surface properties of the mirrors (spectral
and angular reflectivity), bulk scattering inside aerogel materials, transmittance of the window
and the typical quantum efficiency of standard photomultipliers. The overall light collection
efficiency is expected to reach about 80%. On the basis of the light collection and the
photoelectron yield, the detection efficiency for electrons depending on their momentum can
then be evaluated. This essential task will start as soon as realistic simulations of the beam
profiles for electrons and muons are available, including the influence of the shielding.
8.6 DAQ, trigger, on-line monitoring
8.6.1 Beam structure and trigger
The ISIS beam delivers a 1 ms spill of about 3000 pion bunches, each 100 ns long and
separated from its neighbours by 224 ns, with a repetition rate of few Hz. The RF power
source can be operated with a duty-factor of about 10–3. Using it in the most efficient way
delivers a flat top of 850 s at 1 Hz, during which 2600 bunches will reach the experiment.
The basic enable signal to data-taking will be provided by the start of operation of the MICE
RF cavities (Start-Of-RF, or SORF). It will last until the End-Of-RF (EORF) signal, for a
data-taking gate of typically 500 s. (Because of the high level of multipacting during the RF
transient, the detectors must be insensitive during the rise- and fall-time of the RF pulse, so
the data-taking gate is shorter than the RF pulse.) This gate will be generated by the MICE
trigger system.
The time structure of the beam is such that there is not enough time for particle-by-particle
readout. For this reason, all digitisers will be buffered during the spill and read out by the
DAQ system at the end of the data-taking gate. The storage capabilities of the digitisers may
put limits on the duration of this gate. ADC gates and TDC stops will be generated by simple
coincidences of TOF planes. The RF phase and characteristics of the RF pulse will be
recorded at the same time as the muon events.
For calibration and reference purposes, one or several SORF/EORF data-taking gates can be
generated, with beam but without RF, between the real SORF/EORF cycles.
The trigger system will be designed to provide enough flexibility to allow the detectors to run
in stand-alone mode for set-up, test and calibration purposes.
8.6.2 Working hypothesis: event rates and sizes

The maximum beam rate will be 1 particle per bunch. This is a limiting case, in
which there will often be more than one particle in a 100 ns time window.

One calibration (RF off) cycle will be taken per normal (RF on) cycle

The upstream spectrometer will have to measure out-of-acceptance and out-of-RFphase particles
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
For the purposes of evaluating the data rates, it has been assumed that only 1/4 of the
incoming particles will fall into the acceptance after the diffusing plates and will be
able to reach the downstream spectrometer.
8.6.3 Readout volumes and network structure
All the digitizers will be housed in VME crates, either 6U or 9U. As can be seen from Table
8-5, no crate exceeds the maximum sustained rate attainable on a standard VME bus.
Table 8-5. Data volume for the MICE detectors.
Subsystem
Sci-Fi
TOF
e/-ID
# VME crates
2
1 front + 1 rear
1 rear (same crate as TOF?)
Data volume
[MB/s]
≤2
≤ 0.5
Negligible
Data volume/crate
[MB/s]
≤ 0.5
≤ 0.25
—
Each crate will be equipped with a VME processor, which is a single-board computer
provided with a VME interface and a network connection. The task of the processor will be to
read data out of the digitizers via the VME bus, and to pass them over the network to the
Event Builders.
As is now commonplace, the MICE DAQ system will make use of a switched Ethernet
network for transferring data to the Event Builders and the Storage System, to synchronize
and control the DAQ processes running on several nodes and to perform on-line monitoring
tasks.
To cope with the total rate of the experiment and to allow for redundancy, the DAQ will
comprise at least 2 Event-Builder (EVB) computers. Their role will be to assemble event
fragments produced by the many VME readout processors, make consistency checks, write
assembled events to the Storage System, and serve live data to the On-Line Monitoring
System. In order to minimize the cost of data storage, the possibility of building enough
parallelism and computing power into the EVB system to perform on-the-fly data
compression will be considered and tested.
The most widespread storage solutions for scientific laboratories are rack-mount PC cases
holding several large-capacity, hot-swap E-IDE hard disks, connected to the PCI bus by
hardware-RAID controllers, with an available capacity of ~1 TB [Sanders].
The performance of such systems can be considered appropriate for the MICE data rate.
However, some care must be taken since the on-line Storage System will not only provide
space for storing runs, but will also serve data for on-line analysis work. For this reason, the
hardware will have to be tested for concurrent sequential read/write access of large files
before being considered compliant with MICE rate requirements.
MICE will be able to cool, at most, 100 /s. To achieve a statistical precision of 10–3, one has
to take data for 1000 seconds. During this time the detector will produce up to 15 GB of data.
Storing a week of data, hundreds of runs, may require several TB. The availability of a
centralized storage system at RAL would be a great help by reducing the on-line storage
system complexity and eliminating the need for a high-performance backup system.
Several workstations will be available for monitoring data, either spilled on the fly from the
Event Builders, or read directly from the Storage System. The DAQ will have its own
monitoring library, useful for checking the main working parameters and performance of the
detectors. In addition, a proper design of the off-line software can offer a powerful tool to run
the reconstruction algorithms and a detector display on a sub-sample of events taken from the
live data stream.
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201 MHz
Details
The
focus
Final
of
coilsTracker
the
Cherenkov
Cavity
in the
AFC module
Solenoid
Detector
Conceptual
Design
§8 page 48/48
The on-line cluster, including the VME processors, will use the Linux operating system.
Since no readout operations are foreseen during the spill, it is unlikely that the Linux realtime extension will be needed on the VME processors; this matter will be subject to tests.
8.7 Safety
The detector systems in MICE present no extraordinary safety risks. The upstream and
downstream detectors are all based on PMT readout and only require standard PMT HV
power supplies and mains power for electronics. There are no hazardous materials in the
detectors, no flammable gas, and no systems that require high power or large drive currents.
Standard laboratory practices will be followed.
Since the fibre tracker is a totally passive device (at least within the tracking volume), its
impact on safety regarding operation of MICE is minimal. There are no active components in
the tracking volume. The only safety issue regards vacuum. Since the tracking volume will be
evacuated in order to reduce material presented to the muon beam, the optical feed-throughs
and the patch panel itself must be rated for pressure. A prototype patch panel will be
fabricated and tested for vacuum integrity and pressure in order to certify the basic design. At
this point we do not plan to test the final assemblies at pressure.
The fibre tracker readout system only requires standard line voltage (approximately 2 kW per
cryostat). The cyrogenic system is based on a closed-cycle refrigerator and therefore presents
no cryogen or oxygen deficiency hazard.
8.8 Conclusions
A complete set of detectors, capable of performing the measurement of emittance even in the
harsh background conditions of the experiment, has been identified. The resources and
competence exist in the collaboration to design, build and operate them.
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