2
Development and Beam Test of a Prototype Scintillating Fiber Tracker
for MICE
3
MICE Tracker Team
1
4
Abstract
5
7
1. Introduction
1.1. MICE Overview
8
The MICE experiment requires that the emittance be measured as the muon beam enters the cooling
9
channel and again as it leaves. The emittance measurement will be accomplished using two identical
6
10
solenoid spectrometers.
11
The spectrometer module consists of a 4 T superconducting solenoid of 40 cm bore instrumented
12
with five planar scintillating-fiber stations. Each station is composed of three doublet layers laid out
13
in a ‘u, v, w’ arrangement. The active area of the device is a circle of 30 cm diameter. The prototype
14
and tests provide the basis for the design of the MICE tracker.
15
1.2. Requirements to the Tracker
16
The principal requirements that must be satisfied by the MICE tracking system are:
17

High efficiency reconstruction of muon tracks in the presence of background;
18

Adequate resolution in the reconstructed track parameters to allow a measurement of emittance
19
with an absolute precision of 0.1% to be made.
20
Simulations have shown that these goals can be achieved using the baseline five-station
21
scintillating-fiber tracker. A 3D engineering model of the fiber tracker is shown in Figure 10. Each
22
station consists of three sets of fiber doublet layers mounted at 120º to one another. The fiber doublet
23
arrangement is illustrated in Figure 11. The mechanical specification of the tracker is summarized in
24
Table 2.
25
The performance of the spectrometer has been simulated in Geant4-based program using the
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nominal input beam of MICE. At the entrance of the upstream spectrometer, the nominal beam
27
momentum is 200 MeV/c with an emittance of 6.4 mm mrads. Figure 12 shows the resolution in the
28
reconstructed track parameters as a function of the transverse momentum, pT , and the longitudinal
29
momentum, pZ . The five-plane fiber tracker is very insensitive to photon backgrounds from the RF
30
cavities and maintains its excellent tracking performance at rates significantly higher (260 kHz/cm2)
31
than that expected in MICE based on current measurements.
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1.3. Beam Test of the Prototype Tracker
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The experiment, KEK-PS T585, was performed during Sept. 30th to Oct. 7th in KEK. The purpose is
34
to check the ability of construction and operation of the MICE tracker in a realistic environment, to
35
check the alignment and light yields with high-momentum pions injected, and to demonstrate
36
tracking in 1-Tesla solenoid magnetic field with low momentum muons.
2
2. The MICE Tracker
2.1. Operation Principle
3
The basis of charged particle tracking in MICE will be the production of scintillation light in 350 m
4
diameter double clad, doped polystyrene fibers. The concentration of the primary and secondary
5
dopants must be optimized to maximize the light yield while minimizing the fiber-to-fiber optical
6
cross talk.
7
Small-diameter fibers are required to reduce multiple scattering in the stations. However, reading out
8
each fiber leads to a large channel count and a significant electronics cost. The channel count will be
9
reduced by having 7 scintillating fibers read out through a single clear-fiber waveguide (see Figure
10
15). Seven-fold ganging of scintillating fibers leads to a sensitive element that is 1.63 mm across
11
(see Figure 11) and hence give a resolution of 470 m. Simulation has shown that this resolution is
12
acceptable.
13
Clear fiber, of diameter 1.05 mm, will be used to transport the light from the stations to the patch
14
panel and from the patch panel to the photo-detector. The longest clear-fiber run inside the magnet
15
bore (~2 m) will be matched to the shortest run from the patch panel to the photo detector (~2 m).
16
The total length of clear fiber will therefore be kept at ~4 m. The attenuation length of the clear fiber
17
has been measured to be 7.6 m. A total clear-fiber length of 4 m therefore corresponds to half of an
18
attenuation length. It has been estimated that the reduction in light yield by attenuation in the clear
19
fiber is acceptable.
20
The Visible Light Photon Counter (VLPC) developed for use in the D0 experiment will be used. The
21
VLPC is a low band-gap light-sensitive diode that is operated at 9 K to reduce thermal excitation and
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is ideal for use in MICE because of its large quantum efficiency (85%) and high gain (50,000). The
23
device is also insensitive to the magnetic fields in the neighborhood of the MICE spectrometer
24
solenoids and to the RF power radiated by the MICE cavities and associated power supplies and
25
RF-power distribution system. The latter was demonstrated in a dedicated series of measurements in
26
which the D0 VLPC test stand was exposed to levels of radiated RF power several times in excess of
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those expected in MICE with no detrimental effect on performance.
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2.2. Scintillating Fiber
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The passage of a charged particle through the fiber causes energy to be transferred to the primary
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dopant, para-terphenyl (pT). The peak of the scintillation light spectrum of pT is at a wavelength of
31
~350 nm. The secondary dopant, 3-hydroxflavone (3HF), absorbs this light and re-emits it at a
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wavelength of ~525 nm. The concentration of primary dopant must be high enough that sufficient
33
primary light is generated, but small enough to ensure that re-absorption of primary light in the pT is
34
small. The concentration of 3HF must be small enough to ensure negligible secondary light
35
attenuation along the length of the active fiber, but large enough that the absorption length of the
36
primary light in the 3HF is small compared to the fiber diameter. The latter condition ensures that
1
1
fiber-to-fiber cross talk is eliminated. Measurements have shown that pT and 3HF concentrations of
2
1.25% and 0.25% by weight, respectively, give sufficient primary light and an attenuation length for
3
absorption of the primary light in the 3HF of 25 m. A series of measurements of scintillator
4
properties as a function of primary and secondary dopants is planned to optimize the dopant
5
concentrations for the MICE fiber tracker. The baseline specification is given in Table 2.
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2.3. Scintillating Fiber Ribbon
7
The scintillating fibers used in the prototype were 350 m diameter Kuraray multi-clad. They used
8
the standard pT primary dopant. The prototype test was used to study the light yield versus
9
secondary dopant (3HF) concentration. Fiber with 2500, 3500, and 5000 parts per million of 3HF
10
doping were studied in this test. All fibers were first cut to length and then polished on one end so
11
that a vapor-deposited Al mirror could be applied. Although the quality of the mirrors on the fibers
12
used in our test has not been measured, the D0 experiment measured an average reflectivity of
13
approximately 90% for the fibers used in their tracker and the mirroring procedure applied here was
14
the same as for the D0 fiber.
15
The ribbons were made following the technique developed for the D0 fiber tracker. A grooved plastic
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(Delrin) mould was first fabricated (see Figure 16). The mould was measured on a coordinate
17
measuring machine and the mean groove pitch was determined to be 419 m. Our target groove
18
pitch was 420 micron (pitch/diameter = 1.2). A teflon release film (25 m) was first pressed into the
19
mould with the aid of vacuum (pump-out holes were drilled into the grooves in the mould). A tack
20
adhesive was then sprayed on the Teflon and the first layer of fibers was placed in the mould. A
21
circular stop fabricated from a plastic sheet was placed over the mould in order to form a ribbon with
22
the proper circular active aperture. After the first layer of fiber was in the mould, the spray adhesive
23
was applied to the fiber and the second layer of fiber (forming the doublet) was placed on top of the
24
first layer. A polyurethane adhesive was then spread over the fibers and finally a 25 m mylar film
25
was placed over the assembly. The assembly was then clamped under pressure during an overnight
26
adhesive cure. The resultant ribbon was removed from the mould with the release film still attached.
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The final step in the ribbon fabrication was to remove carefully the release film from the ribbon.
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2.4. Optical Connectors
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The optical signal from the tracker is piped to the VLPC system via the fiber light-guides. The light
30
guides terminate in the D0 warm-end optical connector shown in Figure 17. This is an
31
injection-molded part made of Delrin. The typical optical transmission for this connector interface is
32
approximately 98%. Light-guide fiber of 1.05 mm diameter is used while the D0 cassette uses fiber
33
of 0.965 mm diameter. This mismatch results in a light loss of approximately 15%.
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MICE Optical Connectors at Station
35
The optical connector to be used on the station is required to mate seven 350 m scintillating fibers
36
to one 1.05 mm clear fiber. The connector design is shown in Figure 17. The connector has gone
1
through 2 iterations; the ones used in the prototype stations had 18 1.05 mm holes laid out in a
2
regular pattern but the final connector has 22 1.05 mm holes drilled to take the fibers. The
3
diameter of the hole was matched to the seven scintillating fibers as shown below. The connector
4
was machined in black Delrin.
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Optical Connectors at Patch Panel
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The Optical patch panel connector has an O-ring incorporated to ensure a vacuum seal and contains
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128 fibers which give a 1 to 1 match to the VLPC cassettes. The connector map has now been
8
developed to provide ease of connection, as shown in Figure 18.
9
As in the case of the station optical connector, this connector is manufactured from Delrin and as
10
Figure 18 shows there are 6 station connectors to each patch-panel connector; although not all of the
11
channels in the station connectors are used. Figure 19 shows photographs of the connectors that have
12
been produced for the next prototype station.
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D0 Optical Connectors at VLPC Cryostat
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The optical signal from the tracker is piped to the VLPC system via the fiber waveguides. The
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waveguides terminate in the D0 warm-end optical connector shown in Figure 20. This is an
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injection-molded part made of Delrin. Shown are the 128 holes for fibers, two holes (left/right) for
17
alignment pins and two holes (up/down) for threaded inserts. The typical optical throughput for this
18
connector interface is approximately 98%. Since MICE will use waveguide fiber of 1.05 mm
19
diameter and the D0 cassette uses fiber of 0.965 mm diameter, approximately 15% of the light will
20
be lost due to this mismatch.
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2.5. Mechanical Design
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The active area of the tracker is required to be 30 cm in diameter. The five stations that make up a
23
tracker will be held in position by a carbon-fiber frame that will be supported at each end from the
24
inner surface of the magnet cryostat. To reduce multiple Coulomb scattering, the inner bore of the
25
solenoid will be evacuated.
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The stations will be constructed using carbon fiber to give a rigid structure onto which the three
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layers of 350 m scintillating-fiber will be glued. Each station body will be constructed from a
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single carbon-fiber structure that allows for the mounting of optical connectors on a flat annulus,
29
separated by a conical section from a thin, flat ring onto which the scintillating-fiber planes can be
30
bonded. The optical connectors on the station will mate seven 350 m scintillating fibers to one
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1.05 mm clear-fiber light guide. The light guides will transport the scintillation light from the
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stations to an optical patch panel that will be mounted on the end flange of the magnet cryostat.
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Since the tracker will be operated in a vacuum, the patch panel is required to form a vacuum seal to
34
the cryostat end flange.
35
2.6. Carbon Fiber Station Former
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The prototype station bodies have been constructed from three separately cured pieces of carbon
1
fiber: the support/connector flange; the spacer cone; and the scintillating-fiber support. These were
2
jigged and bonded together. However, in the final design, the aim is to produce the stations in one
3
piece. Forming tooling for the prototype station supports has been machined from an epoxy board
4
produced for carbon fiber applications. It has excellent machining qualities and good dimensional
5
stability. It is made in two halves, split on the centre-line and is dowelled and bolted together. The
6
completed tool profile was degreased and a two-part epoxy coating was spray applied. When fully
7
cured, the surface was rubbed flat with a fine wet abrasive sheet and finally polished with an
8
automotive polish producing a high gloss finish. In order to protect the station support structure,
9
carbon fiber pieces were cut to a pattern (12 per layer) and applied radially to the tool surface with
10
no overlap. Three layers in total were applied, with each new layer starting at a different position to
11
cover existing joints. Finally three carbon fiber pieces were cut in the shape of a ring and applied to
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the area of the connector flange. The assembly was then placed in a vacuum bag with another bag
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passing through the tool centre. Vacuum was applied and the assembly placed in an oven. The
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temperature was raised 0.5ºC per minute up to 80ºC and left to cure for four hours in order to
15
produce the final station support structure.
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2.7. Optical Patch Panel and Vacuum Seal
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Figure 21 shows a patch panel conceptual model. Further work will be required to mate it to the
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magnet and to ensure that a vacuum seal can be maintained. It has space for 26 connectors. As only
19
25 of these are needed, the unused opening can be used for field monitoring services. The patch
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panel will need to have additional ribs to ensure that the front cover will not deflect under vacuum.
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The dogleg design is to allow the patch panel to fit in a Z dimension of 60 mm.
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2.8. Station Numbering Scheme
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In order to simplify manufacture and to allow a common set of spares to be held, the upstream and
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downstream trackers will be as close to identical as possible. A clear nomenclature is therefore
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required to avoid confusion during design, manufacture, assembly and installation. A local
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coordinate system is also required both during design and construction but, also for use in the
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simulation and reconstruction of the tracker digitization.
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A schematic diagram of the five stations and the support structure is shown in Figure 14. The end of
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each tracker closest to the liquid-hydrogen absorber is referred to the absorber end, while the end
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closest to the optical patch-panel is the patch-panel end. The stations are numbered from 1 to 5:
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station 1 being closest to the absorber end of the tracker, station 5 being closest to the patch-panel
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end. The station number will be prefixed with U, for stations in the upstream tracker, or D for
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stations in the downstream tracker.
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2.9. Local coordinate system
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Figure 14 shows a schematic diagram of the five stations that make up one of the trackers. The z axis
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lies along the centre line of the tracker and is orientated such that the z location of station 5 is larger
1
than the z location of station 1. Hence, the z axis is parallel to the beam direction in the downstream
2
tracker and anti-parallel to the beam axis in the upstream tracker. The x and y axes are defined to
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make right-handed coordinate system. The x axis is horizontal and the y axis is vertical. The point x
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= y = z = 0 lies on the centre line of the tracker and in the centre of the singlet layer of fibers closest
5
to the absorber end of the tracker.
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2.10. Tracker Assembly
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The first step in processing the fiber ribbons is to put the individual fibers into the correct
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seven-fiber bundles. The groups of 7 will have the central fiber marked in order to facilitate this step.
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The groups of 7 are held together using black heat-shrink tubing. Once this is accomplished, the
10
fiber planes need to be aligned and then fixed to the carbon fiber support structure. This step uses the
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jigging system developed for the prototype. There will be a few minor modifications to ease the
12
assembly and a new profile machined to match the final station geometry.
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The first step is to align a fiber plane onto the vacuum chuck. This positioning need only be within
14
the bounds of the chuck’s own alignment system. After the plane is secured by vacuum it is aligned
15
using a microscope and linear stage (see Figure 22).
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The first plane is glued directly onto the carbon fiber support structure. The next two fiber planes are
17
then glued in sequence to the bottom plane. However the glue must not be allowed to form a
18
complete circle around the active area as air will be trapped forming an air pocket. Upon evacuation
19
of the tracker solenoid, expansion of this gas volume would be destructive. Therefore, extreme care
20
must be taken in order to avoid trapped air pockets.
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Fitting the fibers into the optical connectors is a time consuming job that requires a great deal of
22
dexterity and concentration. A method for checking the accuracy of the connections is essential and
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any errors must be rectified before the fitting of the connectors to the station is completed. Figure 24
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shows details of the ribbon connector assembly. The ribbon connectors are attached to the carbon
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fiber support structure before the fibers are potted. This ensures the best possible lay of the fibers by
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gently pulling the fibers. When the fibers are at the best possible position without any undue strain
27
being exerted, they are potted.
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After the fibers are assembled into the connector and checked for accuracy, they are laid out in a
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gentle sweeping curve and can then be potted. To do this they will be shortened to about 25 mm
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beyond the face of the connector to enable the fitting of a vacuum cup. When the potting adhesive is
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applied in the recess, vacuum is used to ensure that the adhesive travels the length of the fiber in the
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bore. After the adhesive is through the bore, the vacuum is removed and the recess filled with
33
additional adhesive. When the adhesive is in a stable state (i.e. does not run) the station is turned
34
over and adhesive is applied to the front face of the connector around the fibers. This is to prevent
35
them moving/vibrating as they are cut and polished.
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When the potting has cured sufficiently to allow the station to be turned over, more adhesive is
1
applied to the front face to form a solid block of material. This ensures that there is a minimum of
2
stress and vibration transferred to the fibers during machining. We anticipate that the machining
3
process will need further development of the tools and techniques to improve the polished finish
4
(although the prototype was deemed to be acceptable). The cutting/polishing is done using a
5
diamond tipped tool which should give the required finish without further polishing that might
6
degrade the flatness.
7
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3. Fiber Light-guide
3.1. Clear fiber
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The light guides for the prototype have been fabricated using 1.05 mm diameter clear fiber (Kuraray
10
CLEAR-PS, Round-type, Multi-Clad). This fiber has a 2.5% tolerance at 3 , and the thickness of
11
the cladding layer is 6 % of the diameter. Attenuation length in the clear fiber has been measured to
12
be 8 m using an Oriel silicon detector system. The light guide is terminated at the optical patch
13
panel.
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3.2. Internal Light-guide
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A light guide bundle inside the tracker volume consists of 128 clear fibers. One end is divided into 6
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groups of 20 or 22 fibers and connected to MICE connectors on the tracker station. The other end is
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connected to the patch panel. Five bundles are assigned for one station. Clear fibers are attached to
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optical connectors with epoxy glue, and the surface is then polished using a diamond fly cutter. The
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length of the light guides is adjusted to match the distance from each station to the patch panel. The
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light guides for the nearest station are 1.2 m long, and 2.3 m for the farthest station.
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3.3. External Light-guide
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A light guide between the patch panel and the VLPC cryostat contains 128 clear fibers in a bundle.
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One end of the bundle is assembled in the vacuum-tight connector to be attached to the patch panel,
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and the other end is connected to a 128-way D0 connector to fit the input of VLPC cassette. The
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length is adjusted from 2.0 m to 3.1 m to keep the total length from the tracker station constant at a
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4.3 m length. The bundles are contained in a fire-resistant flexible sleeve made of polyamide to
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provide mechanical support and to exclude light. The 0.5 m of the fibers at both ends are shielded by
28
the conduit of 16.6 mm inner diameter and 21.2 mm outer diameter (Adaplaflex PAFS21). The
29
centre section of the light guide is covered by the conduit of 21.7 mm inner diameter and 28.5 mm
30
outer diameter (Adaptaflex PAFS28).
31
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4. Readout Electronics
4.1. Overview
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MICE will use the D0 central fiber tracker (CFT) optical readout and electronics system. This
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system has been operating reliably for the D0 experiment for almost 4 years now. The photodetector
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is the visible light photon counter (VLPC) manufactured by Boeing. The VLPCs operating at 9 K
36
and will require a cryogenic system. The VLPCs are packaged into a cassette which contains 1024
1
channels. Two analog front-end boards (512 channels each) provide readout, temperature control,
2
and VLPC bias.
3
4.2. VLPC
4
The VLPC is a cryogenically operated silicon-avalanche device. The operation and development of
5
the VLPC has been discussed extensively in the literature [VLPC]. It is a descendant of the Solid
6
State Photomultiplier, an impurity-band silicon avalanche photodetector. It has undergone six design
7
iterations, specified as HISTE I - HISTE VI. HISTE VI is the version used in the D0 CFT. It is an
8
eight-element array in a 2 × 4 element geometry. Each pixel in the array has a diameter of 1 mm. The
9
HISTE VI operational parameters are given below:
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
Quantum yield > 0.8
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
Gain > 40,000
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Operating temperature 9K
13

Operating bias 6-8V
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4.3. Cryostat
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The VLPCs operate at cryogenic temperatures and a cryo-system is required. The current baseline
17
for the VLPC cryo-system is to use Gifford-McMahon (GM) cryo-coolers to maintain the 9 K
18
operating temperature for the VLPCs. The design work for this system has just started, but it is
19
believe that commercial GM coolers are a cost-effective approach to the VLPC cryo needs. Four
20
cryostats will be used in MICE, each cryostat holding two cassettes (which will read out one half of
21
one of the trackers). The cassette cold end will sit in a stagnant gaseous-helium volume which will
22
be cooled to approximately 8K by the second stage of the GM cryo-cooler. The first stage of the
23
cryo-cooler, operating at approximately 50K, will be used to remove heat from an intermediate heat
24
intercept and thus reduce the heat load to the 8K second stage.
25
specification of the VLPC cryostats is ±50 mK.
26
4.4. VLPC cassettes
27
The VLPC cassette contains 1024 channels of VLPC readout and is divided into 8 modules of 128
28
channels, each of which is interchangeable and repairable. This is illustrated in Figure 25 and Figure
29
26. Figure 25 shows the full cassette with readout boards attached. Figure 26 shows the inner
30
components of the cassette, with the readout boards and cassette body removed. Both figures clearly
31
show the 8-fold modularity of the cassette design.
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Sitting directly over each VLPC pixel is an optical fiber which brings the light from the detector to
33
the VLPC chip. Each cassette module is comprised of an optical bundle assembly, a cold-end
34
electronics assembly, and an assembly of mounted VLPC hybrids. The cold-end assembly is
35
designed to be easily removable for repair without disturbing other modules due to the high cost and
36
delicate nature of this device. Another important design requirement of this cassette concerns the
The temperature stability
1
read-out electronics: the readout electronics boards and the PC boards which act as interface to the
2
data acquisition system must be removable and replaceable without removing a cassette from the
3
cryostat. The readout electronics are discussed in detail in the following section.
4
The cassette, for purposes of discussion, is broken down into several major components. The
5
cassette is distinguished as having a “cold-end”, that portion of the cassette which lies within the
6
cryostat, and a “warm-end”, the portion of the cassette which emerges from the cryostat and is at
7
room temperature. At the cold-end, eight cold-end assemblies, each of 128 channels of VLPC
8
readout, are hung from the feed-through by the optical bundles and are surrounded with a copper cup
9
at the cold-end. Each cold-end assembly consists of sixteen 8-channel VLPC hybrid assemblies, the
10
“isotherm” or base upon which they sit, the heater resistors, a temperature-measurement resistor, the
11
cold-end flex-circuit connectors and the required springs, fasteners and hardware. Running within
12
the cassette body from top to bottom are eight 128-channel optical bundle assemblies which accept
13
light from the detector wave-guides connected to the warm-end optical connectors at the top of the
14
cassette and pipe the light to the VLPC's mounted at the cold-end (see Figure 26). The electronic
15
read-out boards are located on rails mounted to the warm-end structure and are connected
16
electrically with the cold-end assemblies via kapton flex circuits. In addition, the electronics boards
17
are connected to a backplane card and backplane support structure by card edge connectors and
18
board mount rails. The flex circuits and read-out boards are electrically and mechanically connected
19
by a high-density connector assembly.
20
The cassette body can also be broken down into cold-end and warm-end structures. The cold-end
21
structure is broken down into several sub-assemblies: namely the “feed-through assembly”, the G-10
22
walls, the heat “intercept” assemblies, and the cold-end copper cup (see Figure 26). Along the length
23
of the cold-end, two heat intercepts are integral to the cold-end cassette structure. The first is the
24
liquid nitrogen intercepts (77 Kelvin) which serves to cut off the flow of heat from the warm-end.
25
The second is the liquid helium intercept, with a name more historic than functionally descriptive,
26
which serves as an IR suppression device and terminating structure. The warm-end structure is made
27
of parallel aluminum plates spaced by spacer bars which form a protective box for the optical
28
bundles.
29
4.5. AFE Boards
30
MICE will use the D0 AFEII boards in order to read out the VLPC system. This new electronics
31
board is currently under development by D0. It will include the following:
32

Front end preamplifier with 48 bin analog pipeline
33

Commercial 8 bit 20 MHz flash ADC
34

Discriminator outputs for each channel
35

FPGA for each module of 32 channels
36

Temperature control circuitry for each cassette module (control and heater)
1

2
4.6. VME System
3
Two standard 9U VME crates are required to read out all channels in the two fiber trackers in MICE.
4
MICE plans to use a LVDS to VME readout architecture for the AFE II boards. This is different from
5
what D0 uses, but presents an easier solution for MICE A new LVDS VME receiver card is being
6
developed for this purpose. Two options are being considered. The first is to modify a board that
7
has been used at KEK. The second option is to use a LVDS receiver board designed for D0 test
8
systems. Two standard 9U VME crates are required to read out all channels in the two fiber
9
trackers. These crates will each house a BIT3 interface, 8 LVDS receiver boards (one for each AFE
VLPC bias circuitry with voltage and current read back
10
board that is read out) and a 1553 I/O interface.
11
5.
12
The prototype of the MICE tracker was constructed to check the basic performance using cosmic ray
13
and accelerator secondary beam. The beam test, KEK-PS T585, was performed in October 2005 at
14
the secondary beam line of 12-GeV proton synchrotron in High Energy Accelerator Research
15
Organization (KEK) in Japan. The purpose of the experiment is to check the ability of construction
16
of the MICE tracker, to check the alignment and light yields with high-momentum pions, and to
17
demonstrate tracking in 1-Tesla solenoid magnetic field with low momentum muons.
18
5.1. Prototype Tracker
19
It consists of 4 stations of scintillating fiber plane. Scintillating light is transported along
20
4-meter-long fiber light guides and detected by VLPC. The distances among stations are varied from
21
15 cm to 55 cm to enlarge momentum acceptance, as indicated in Figure 27. Figure 28 shows a
22
picture of the prototype tracker with clear fiber light guides.
23
5.2. Detector Solenoid
24
The prototype tracker is installed inside a superconducting solenoid magnet developed in KEK with
25
the magnetic field of 1 T in the bore. The solenoid has 0.85 m inner bore and 1.4 m depth.
26
5.3. Beam Line
27
The secondary particles such as pions with the momentum up to 3 GeV/c and muons with the
28
momentum around 0.3 GeV/c can be provided at KEK-PS pi2 beam line.
29
5.4. Beam Counters
30
The various beam counters are placed in the test beam area to measure momentum of extracted beam
31
and to identify the particle species. The time of flight of each particle is measured by two scintillator
32
counter systems with the distance of 8 m. The upstream counter (T1) consists of plastic scintillator
33
with the dimension of 5-cm height, 10-cm width and 4-cm thickness. The time-of-flight (TOF)
34
hodoscope, is located just upstream of the tracker. It consists of 2 layers of plastic scintillator planes.
35
Each layer is divided into 5 scintillator bars, which has a dimension of 8-cm height, 40-cm width and
36
4-cm thickness. Thus the TOF hodoscope covers a large area of 40 cm by 40 cm. The resolution of
Layout of the Prototype Test
1
time of flight is calibrated to be 60 ps.
2
In addition to the time-of-flight system, an aerogel Cherenkov counter is placed 35-cm downstream
3
of T1 to distinguish muons from contaminating electrons and pions. The refractive index of the
4
aerogel is 1.05 so as to reject pions with the momentum below Cherenkov threshold and also muons
5
produced in pion decay after passing T1. The block of aerogel is surrounded by 10 photo-multiplier
6
tube to collect as much as possible. Typically in 0.4 GeV/c beam, 30 photo-electrons are measured
7
for electrons, 10 photo-electrons for muons, and pions are under threshold. Pions can be rejected by
8
95% keeping efficiency for muons to be 95%.
9
A plastic scintillator disk, which signal is readout by long wavelength-shifting fiber attached on the
10
surface, is placed just upstream of the prototype tracker to select particles in the tracker acceptance
11
efficiently.
12
5.5. DAQ
13
Scintillation photons are detected by VLPC. The data are extracted from AFE along LVDS cable,
14
and stored in VLSB. The trigger is generated from a coincidence of T1, TOF hodoscope and
15
sampling clock in AFE.
16
5.6. Beam Properties
Momentum
Profile
Simulation
17
18
19
Momentum selection by TOF hodoscopes
Acceptance of the tracker
20
21
22
6.
Basic Performance of the prototype tracker
23
High energy pions are injected to the prototype tracker to calibrate its alignments and to measure
24
light yields, and the other basic parameters.
25
6.1. Alignments
26
Precise alignment among each station is checked by fitting straight track of 3 GeV/c pions. Tracking
27
is performed with the other three stations than the station which is looked at. Residual is calculated
28
by taking the distance between fitted track and the position of hit fiber above threshold at 2.5
29
photo-electrons. The alignment is found to be quite good as less than 1 mm, as shown in Figure 1.
1
2
Figure 1. Residual of linear-fitted track for 3 GeV/c pions.
1
6.2. Light yields
2
3
Figure 2. Distributions of detected photo-electrons in X view of station B for 3 GeV/c pions
4
penetrating the prototype tracker. Cross indicates DATA and histograms are Monte-Carlo
5
simulation.
6
6.3. Noise
7
Figure 3 shows typical ADC distributions of pedestal data in a VLPC channel. A fraction of fake
8
photons in pedestal data is calculated to be 0.03. This indicates that expected number of fake fiber
9
hits in the view is less than 1 hit. The noise rate is stable within 10% through the period of data
10
taking.
1
2
Figure 3. Typical ADC distributions of pedestal data in a VLPC channel.
3
6.4. Hit efficiency / Dead channels
4
A fraction of dead channels is found to be 0.4% in this experiment by counting channels which has
5
no fiber hits above threshold at 2 photo-electrons.
6
Hit efficiency is measured by tracking straight pass of 3 GeV/c pions. As shown in Table 1, hit
7
efficiency is typically 95%.
8
Table 1. Hit efficiency of each view of the prototype tracker for 3 GeV/c pions.
9
Station
View
DATA
MC
B
X
0.95
0.97
B
V
0.96
0.97
B
W
0.95
0.96
A
X
0.97
0.97
A
V
0.96
0.97
C
X
0.94
0.97
C
W
0.94
0.97
1
2
7. Momentum measurement by the tracker
7.1. Helical tracking in magnetic field
3
Systematic effect of non-uniform magnetic field
4
Expected resolution
5
7.2. Residual distributions
6
7
Figure 4. The residual distributions in helical tracking for 325 MeV/c muons in 1-Tesla magnetic
8
field.
1
2
Figure 5. The residual distributions in helical tracking in 1-Tesla magnetic field for 325 MeV/c
3
muons (MC).
1
7.3. Pz, Pt distributions
2
3
Figure 6. Distributions of measured muon momentum by TOF system (upper histograms) and the
4
prototype tracker (lower).
1
2
Figure 7. Distributions of measured transverse momentum of 400 MeV/c muons in the prototype
3
tracker.
1
2
Figure 8. Distributions of measured transverse momentum of 400 MeV/c muons (MC) in the
3
prototype tracker.
4
8.
5
Successfully constructed, operated the tracker prototype, and measured momentum distribution.
6
9.
7
UK/US/Japan
8
Conclusion
Acknowledgments
1
Figures and Tables
2
3
Figure 9. Drawing of the MICE experiment showing the upstream (left) and downstream (right)
4
spectrometers and the MICE cooling channel.
5
6
Figure 10. Engineering model of the tracker module showing the 5-station scintillating fiber tracker
7
installed in the solenoid and the optical patch panel.
8
a)
9
b)
1
2
Figure 11. Detail of arrangement of fibers in doublet layer. (a) Cross-sectional view of fiber doublet.
3
The dimensions of the fiber and fiber spacing are indicated in m. The fibers shown in red indicate
4
the seven fibers ganged for readout via a single clear fiber. (b) Layout of doublet layers in a station.
5
The angle between the fibers in the doublet layers is 120º.
6
7
c)
d)
8
9
10
11
Figure 12. 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).
Fibre Exit
Station
5
Station
4
Station
3
Station
2
Station
1
1
2
Figure 13. Schematic view of the MICE scintillating fiber tracker. The station numbering scheme is
3
indicated. Station number 1 is closest to the liquid-hydrogen absorber. Station 5 is closest to the optical
4
patch panel.
5
6
7
Figure 14. Schematic representation of the five stations that make up one of the MICE trackers and
8
the local coordinate system. The local origin is placed at the centre of the singlet plane of fibers
9
closest to the absorber end of the tracker.
10
1
2
Figure 15. Detail of the seven-fold ganging: Seven 350 m scintillating fibers (shown in red) are
3
read out through a single 1.05 mm clear fiber (shown in black).
4
5
6
Figure 16. Schematic drawing of the fiber-double layer laid in the delrin mould.
7
8
9
10
Figure 17. Left:Fibers fitted in hole (not polished). Center: The new connector (right) is the station
11
fitted half; Right:.
12
MICE optical connector at patch panel – bulk-head connector.
1
2
Figure 18. Optical Patch-Panel Layout.
3
4
Figure 19. Left: Patch-panel connector components. Right: Patch-panel connector assembled.
5
6
7
8
Figure 20. D0 warm-end optical connector.
1
2
Figure 21. Conceptual scheme for patch panel
3
4
5
a)
b)
c)
6
Figure 22. a) Vacuum Chuck on Linear Stage; b) Vacuum Chuck on Alignment Jig; and c) Alignment
7
for Vacuum Chuck
8
9
10
11
Figure 23. Left: Station holder. Right: Station holder and station
1
2
Figure 24. Left: Connectors fitted and awaiting potting. Right: Test piece showing potting
3
4
5
Figure 25. The VLPC cassette with readout electronics board attached
1
2
3
4
5
Figure 26. Inside view of the VLPC cassette with cassette body removed.
1
Table 2 Key parameters of the tracker module.
Component
Scintillating
fiber
tracker
Parameter
Value
Scintillating fiber diameter
350 m
Primary dopant, pT, concentration
1.25% (by weight)
Secondary dopant, 3HF, concentration
0.25% (by weight)
Fiber pitch
427 m
Estimated light yield per singlet (photo-electrons)
8
Number of scintillating fibers per optical readout
7
channel
Position resolution per plane
470 m
Views per station
3
Radiation length per station
0.45% X0
Stations per spectrometer
5
Station separation: 1 – 2
45 cm
Station separation: 2 – 3
35 cm
Station separation: 3 – 4
20 cm
Station separation: 4 – 5
10 cm
Sensitive volume: length
1,10 cm
Sensitive volume: diameter
30 cm
Spectrometer
Magnetic field in tracking volume
4T
solenoid
Field uniformity in tracking volume
1‰
Field stability
1%
Bore diameter
40 cm
Pressure in magnet bore
Vacuum
Tracking volume
2
3
1
2
Figure 27 Schematic view of the prototype tracker with 4 stations of scintillating fiber plane.
3
4
5
6
Figure 28. Picture of the prototype tracker with clear-fiber light guides.
1
2
3
4