Studies on High QE PMT

1
Studies on High QE PMT

• Tadashi Nomura
• (Kyoto U.)
• Contents
• Motivation
• Performance of H7422P-40
• Application to Scintillation counter with WLSF
  readout
• Summary

2
Motivation

• K?pnn experiment needs hermetic veto system

Signature 2g nothing

• Major KL decay modes
• KL?pp-p0 (13)
• KL?pen(g) (39)
• These may fake the signalif charged particles
  are missed
• Cause of inefficiency
• p-p?p0n (all neutrals and lost)
• e annihilated with materials
• Detection before these interactions (i.e. with
  low energy deposition)reduces the inefficiency

Veto detectors surrounding Decay Region
3
Simple Consideration of Inefficiency

• In case of loss due to p-p?p0n reaction
• Cross section s(CH),max 100mb (at the resonance
  peak Ep170MeV)
• Required inefficiency lt 10-4
• Energy deposit 2MeV/cm
• Edeposit before Pint10-4 40keV 6x1023n/mol
  / 104g/mol x 8CH pair/n x 1g/cm3 x
  100x10-27cm2 x (thickness) 10-4 ?thickness
  200mm, Energy deposit 40keV
• (Note Resultant p0 can be detected by photon
  veto detectors and thus inefficiency might be
  smaller)
• How many photoelectrons per 40keV can we obtain?
  
• High Quantum Efficiency (QE) PMT desired

4
Example KOPIO Downstream CPV

• Beam pipe liner inside pre-radiator / calorimeter
• Counter should be inside vacuum, but its
  desirable to locate PMT outside ? Long
  WaveLength-Shifting-Fiber (WLSF) ? Small light
  yield ? Need High QE PMT

5
High QE PMT Hamamatsu H7422

• GaAsP photocathode
• Sensitive area 5mm in diameter
• Metal channel dynode structure
• Price 1.8k

Measured QE using LEDs (relative to bi-alkali
PMT)
Expected QE
GaAsP (H7422)
Calculated fromrelative QE andbi-alkali catalog
value
Bi-alkali (H7415)
Peak Wavelength of WLSF
6
Basic Properties of H7422
Light yield 100 p.e.
Rate dependence
Linearity
Gain6x106
Sensitivity map
Light yield 1000 p.e.
Gain106
Linearity not so good(even within 100 p.e.)
Structure due to focusing mesh was seen (10
dip)
Improvement might be possible by optimizing
base circuit
7
High QE PMT Scintillator WLSF

• Test configuration
• PMT H7422P-40
• Scintillator EJ-212 (ELJEN) 3mm-thick, 1m-long
• WLSF Y11(200) (Kuraray) 1mm-diameter
• Machined groove, 1cm-pitch, bundle 7 fibers
• Wrapped by Aluminized mylar

90Sr (Edep580keV)
70 p.e. / 0.58 MeV? 120 p.e. / MeV
Results 70 p.e (both) with High QE PMT (x 33.5
larger than with Bi-alkali PMT)
8
WLSF attenuation

• Attenuation in WLSF alone
• with LED-excited, viewed by High QE PMT
• lL6.1m
• AL/AS ratio decreases if measured by bi-alkali
  PMT(longer wavelength longer attenuation)
• Attenuation in Scinti WLSF
• Consistent well with WLSF alone
• In case of 4m long WLSF (1m in Scinti 3m
  outside vacuum),Light yield will be 46 of our
  test result? 120 x 0.46 x 40x10-3 2.2 p.e. /
  40keV

AS9.0 , lS1.0m AL12.9 , lL6.1m
9
Further Effort to Increase Light Yield

• Use thicker WLS fiber
• Better acceptance of primary scintillation
  lights
• 1.0mm ? 1.5mm diameter
• 30 increase expected (by our measurement)
• Use clear fiber to transport light
• Longer attenuation
• 1m WLSF in Scintillator 3m clear fiber (outside
  vacuum)
• 50 improvement expected Connection 90 x
  attenuation 75 ? 68 cf. 46 attenuation for
  WLSF

Need large area PMT cathode to read a bundle
of 7 fibers 5mm ? 8mm diameter
10
Summary (1)

• High Quantum Efficiency PMT
• Hamamatsu H7422P-40 (GaAsP photocathode)
• Basic properties
• QE 30-40 for green light 3 times larger
  than bi-alkali PMT
• Linearity not so good if we use default base
  circuit
• Rate capability stable upto 1MHz for 100 p.e.
  light
• Application to Scintillator WLSF
• 120 p.e. / MeV (sum of both end) with 1m-long
  test counter 3 times larger than bi-alkali
  PMT, as expected

11
Summary (2)

• Application to KOPIO DS Charged Particle Veto
• In case we use 4m long WLS fiber, 2.2 p.e. /
  40keV (sum of both end) will be expected
• Threshold of 3 p.e. in both end, for example (?6
  p.e. in total) inefficiency will be
  3x10-4 (without help by backup photon vetoes)
• Effort to increase light yield
• 1mm ? 1.5mm diameter fiber ( x 1.3 expected )
• Use clear fiber to transport long distance
• Change reflection material (Aluminized mylar ?
  Al evaporation?)

12
Further High QE PMT issues

• Large sensitive area desired
• In order to use thicker fibers
• Need negotiation with the vendor (Hamamatsu)
• Linearity might be improved
• Not so good with default base circuit
• Optimize base circuit
• Life time of photocathode?
• Degradation of GaAsP ?
• Long-term test is planned