The possibility of improving TOP counter
Nagoya university
Yuji Enari
2002.8.31
PID meeging @ Nagoya univ
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Introduction
• mention about the p/K separation of TOP counter,
especially about effect of group velocity of the photon.
• Definition of Separation.
S=
DTOP + DTOF
st
・
Nph
S1ph = Separation of single photon
DTOP,TOF : time difference of pi/K (K-pi)
st : Time resolution
Nph : Number of detected Photo-electron
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Time difference DTOP and DTOF
TOP counter measures time difference btw p/K
TOP
TOF
DTOP + DTOF = 200ps
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Uncertainty of TOP measurement.
 If we choose to use bar-TOP, this effect is negligible.
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The chromatic effect.
The chromatic effect consists of 2 component.
a. Cherenkov angle
b. Light velocity
Determined by the reflective index
Determined by ‘Group’ index
These two contribution work oppositely.
a : b = -1 : 2 
schromatic  (1/n・Dn)
b.
Dl
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a.
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The measurement of TOP resolution
• From the Beam test result.
– With improved polishing accuracy of the quartz bar.
– This result is limited by chromatic effect (at large L).
 Estimate pi/K separability in the Belle detector
2002.8.31
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TOP difference and Nph in Belle detector
P = 3.5GeV/c, barthick = 20mm
30° 45°
DTOP : -3.6
-1.4
DTOF : 62.6 44.2
Nph : ～90
～60
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60°
14.6
36.1
～40
90°
110.0
31.3
～30
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120° 140°
14.6 2.7
36.1 40.84
～40
～65
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Expected TOP counter’s performance in Belle.
30 45 60
90
140
Number of detected
photon
S1ph
S
• Separation of single photo-electron and S
lmean = 390nm, stts = 75ps
Hard to separate p/K around qin = 45, p=3.5GeV/c region.
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How we can do?
• Increase photon statistics
– Use High Q.E. material (Ga・As or APD?)
– Use thicker quartz bar. (20mm  40mm)
• Improve time resolution.
– Use long wave length region (Dn get small)
– (and) use good photon detector(T.T.S < 50ps)
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Correlation between TTS and wave length.
P = 3.5GeV/c, in Belle system
Separation of single photo-electron
S1ph
stts = 100ps
stts = 50ps
S=3
S=5.5
 We have to use long wave length with stts < 50 ps.
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• wave length : lpeak = 490nm with high Q.E.(20%@lpeak)
time resolution : stts=50ps
barthick = 20mm
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barthick = 40mm
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Summary
• The chromatic effect make the largest uncertainty.
• To get good performance in Belle system,
–
–
–
–
Measure long wave length region.
Use good Photon detector which has TTS of 50 ps.
Need good Q.E. material.(Q.E.×C.E.>２０％?)
Workable in 1.5T magnetic field
 Photon detector R&D is very very important.
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Silicon Photo-Multiplier
2002.8.31
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What’s SiPM
• SiPM
– a kind of multi pixel
Avalanche Photo Diode.
– Operate in limited Geiger mode.
pixel size : 42 ×42 mm2
• Merit?
– Available in magnetic field.
– High Q.E. at long wave length
(very attractive for TOP counter) ,
– High gain.
– Good time resolution Etc.etc.
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Geiger-mode Avalanche photo diode
• Limited Geiger-mode in SiPM
– Under bias voltage of 10-20%
more than break down voltage.
– Geiger discharge is stopped when
bias goes down below b.d. voltage.
– Gain :
• Qpixel = Cpixel ・(Vbias – Vbeakdown)
– Cpixel  100fF  Qpixel  100pC
Vbias ～ ５０V
All pixel(576pixel) are connected by common Al strip.
 each pixel operates as a binary device,
works as an analogue detector.
Close up view of pixel
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Performance of SiPM
•
•
•
•
•
•
Pulse shape
Gain
Linearity and dynamic range.
Detection efficiency
Noise – temperature dependence.
Time resolution
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pulse height spectra
SiPM has good energy resolution as
long as 2 photons enter in same pixel.
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Detection efficiency
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Noise
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Time resolution
s ～ 50ps
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Comparison to another detector
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Summary
•
•
•
•
SiPM is very good application for TOP counter.
High Q.E. High gain, single photon sensitive.
Good timing resolution.
Workable in strong magnetic field of 1.5T.
• To be use in TOP counter…
– Size. (1mm2  20mm ×200mm?)
– Enlarge the effective area in each pixel. (e = 0.3  0.5)
– Noise
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Reference
1, B.Dolgoshein (Moscow Engineering and Physics Institute)
- An Advanced study of Silicon Photomultiplier
ICFA instrumentation Bulletin
- The silicon photomultiplier and its possible
application
transparency of 3rd BEAUNE conference
and its proceeding
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