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8.3 Upstream Cherenkov detector for π-µ-e separation The upstream Cherenkov detector, 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 consists of a single fluorocarbon radiator vessel and four triangular mirror/PMT sectors for light collection. Figure 8-1. The diameter of the radiator is about 50 cm. The C6F14 cell will be 23 mm thick consisting of a 20 mm thickness of C6F14 fluorocarbon liquid plus a 3mm fused quartz window. C6F14 has been used by the SLAC SLD [Caval] and CERN DELPHI [Annas] experiments as a Cherenkov radiator. Note that Cherenkov radiation any produced in the quartz window is trapped by total internal reflection and does not reach the photomultiplier tube. Four triangular concave mirrors tilted at 53.1º to the z-axis bring the light out to four EMI9546B 8 inch photomultiplier tubes. The inside of the box is lined with a high reflectance Tyvek liner to insure full light collection. The total length of the device is 50 cm, Figure 8-2 schematic.

Figure 8-1. 3-D Visualization of Upstream Cherenkov Detector

Part Dx (mm)

Dy (mm)

R (mm)

Dz (mm)

n �c (b=1)

�i r Material

Entrance Window

- - 260 0.2 opaque

- - Mylar

Cherenkov Radiator

- - 260 20.0 1.251 36.9o 53.1o C6F14

Quartz Window

- - 260 31.8 1.46 46.8o 43.2o Quartz Window

Triangular Reflectors

330.2 base

330.2 height

- 31.8 opaque

- - Glass mirrors b x h

Exit Window

330.2 330.2 - 0.2 opaque

- - Mylar

Table 8-1. Materials in Upstream Cherenkov Detector.

.

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Figure 8-2. Side and front views of CKOV1

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 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 Error! Reference source not found. and [Aubert], [Bartlett], and [Crema].

Figure 8-3. CKOV1 light output versus momentum for muons and pions.

In Figure 8-3 we see that up to a momentum of 190 MeV/c, only the muons produce light, so pions are completely rejected, The rejection should match/complement the TOF numbers at 190 MeV/c. Above 190 MeV/c pions produce light, so one must reject particles that fall between the muon and pion bands. This leads to inefficiency but not contamination at moderately higher momenta. From simulations of CKOV1, Figure 8-4, a (10-15)% overlap in photoelectrons is estimated when 10 cm of C6F14 is used at higher beam momentum.

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Figure 8-4 CKOV1 overlap light yields for muons and pions over threshold at 240MeV/c and 280MeV/c.

Beam Momentum

Dz- C6F14 Npe-mu Npe-pi Seperation

180 MeV/c 2cm 15 0

!

"

240 MeV/c 10cm 127 79 ~3.0 �

280 MeV/c 10cm 144 109 ~2.3�

Table 8-2 CKOV1 simulated muon and pion photoelectron yields and statistical separation with C6F14 . PMT quantum efficiency and a light collection efficiency of 90% are included.

Fluorinert C6F14 is produced by 3M Co. primarily as a heat transfer liquid. Heat transfer coefficients of order 2W/in2/oC are typical. Due to it’s inert chemical nature Fluorinert are useful as inert carries and media for chemical reactions. For use as a Cherenkov radiator it has a greater than 95% optical transmittance starting in the UV with 99% in visible range (thicknesses of 1cm). C6F14 is not chlorinated and presents no danger to the atmosphere.

C6F14 B.P. Mol. Wght

ref index n

density Vapour Pressure Viscosity

51oC 340 1.251 1.6 g/ml 232 torr (STP) 0.37 cs

Table 8-3. Properties of Fluorinert C6F14

Although practically inert we must take some precautions with our application. Fluorinert’s are known to react and expand Viton elastomers over time. Buna o-rings and pump seals are advised. O2 and CO2 are tens of times more soluble in C6F14 than water and care should be taken to keep containers sealed. During operation at (24-30)oC a small vapor pressure buildup of (0.3-0.4) atmospheres can be accommodated with a simple expansion diaphragm. A C6F14 capacitive level gauge will be incorporated in to the radiator vessel. Small leaks can easily be detected with a TC cell. In Table 8-4 we summarize these properties and subsequent actions.

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C6F14 Property Action

Chemical Reacts with Viton O-rings Use Buna rubber for seals

Optical 93% @ 220nm, - 99% @ 270nm - none

Solubility Extremely low for Aluminium, Si02

Moderately high for gases, O2, CO2.

none

Keep sealed

Safety(V.P.@300oK) 0.3 atm Provide expansion vessel

Safety (leak) Inert liquid, low vapour pressure, Level gauge

Table 8-4. Properties of Fluoriniert C6F14 relating to its use in CKOV1.

The radiator vessel should be fabricated to high machine tolerance (

!

±50µm) to insure high light transmission, and reliable and leak-tight use. The optical box holding the PMTs, and mirror can be fabricated to (

!

±1mm) tolerance. The light collecting mirrors should have good flatness to

!

±25µm and overall size to

!

±1mm. The mirror mounts should be adjustable from the rear of the box and easily accommodate mirror placements of

!

±1mm . The radiator and optical box must be light tight allowing ~1pe noise or less. CKOV1 will sit on an adjustable table to align it along the beam center line. (Table 8-5)

Issue Tolerance Comment

Radiator Vessel

!

±50µm

Optical box

!

±1mm

Mirrors

!

±25µm flatness

!

±1mm size

Mirror placement

!

±1mm

CKOV1 placement

!

±1mm on beam centerline

Background Light level

Light tighted to < 1pe

RF Grounding < 2 ch pedestal noise @1/4 pc/ch Elimination of ground loops

Magnetic Shielding

< 1 gauss inside PMT shield Soft Iron+ mu-metal shields

Inside Temperature

< 30oC Cooling by LN2 flush

Table 8-5. Mechanical and E&M tolerances.

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 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, Table 8-6.

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Device Ch Device Type Availability

EMI 9546B PMT 4 ¼ pC, 10-bit ADC

Lecroy FERA 4300B

HARP experiment (yes)

HV 4 +3KV HV Supply

Fluke, Lecroy, Caen, ..

RAL (no)

LED Pulser 1 ¼ pC, 10-bit ADC

Lecroy FERA 4300B

RAL (no)

2 0-5V ADC Slow Control Data Recorder

Mississippi (yes)

HV Monitor 4 0-5V ADC Slow Control Data Recorder

RAL (no)

Table 8-6. Table Data Acquisition and Controls

A prototype Cherenkov cell with C6F14 and a five inch RCA 8854 photomultiplier tube has been used to detect a 2.3 MeV betas from a Sr90 source and cosmic ray muons. 2.2 MeV betas have a path length of about 1.0

!

±0.3 g/cm2 in C6F14 . Calculating 0.6cm @ 8pe/cm corresponds to 9

!

±3 pes. This agrees reasonably well with observations displayed in Figure 8-5(a). In the same apparatus while triggering on cosmic ray muons 8-5(b) about 25pes are observed, a number difficult to interpret due to the unknown energy spectrum and incident angles. These performances are listed in Table 8-7.

Figure 8-5 (a) Charge collected with Sr90 triggers in to 2.5 cm of C6F14 . The single photoelectron peak in shown at the lower left, channel=28. The pedestal channel =14. The Light yield is calculated to be 107/14 = 7.6 photoelectrons. (b) Cosmic triggers.

Source Sr90 betas Cosmic Muons

Photoelectrons Observed in C6F14 7.6 pe @ 0.6 cm 25 pe @ 2.0 cm

Table 8-7. Photoelectrons measured in prototype C6F14 Cherenkov radiator cells.

(b) (a)

Sr90 Triggers