1. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
Studies on High QE PMT
Tadashi Nomura
(Kyoto U.)
Contents
Motivation
Performance of H7422P-40
Application to Scintillation counter with WLSF readout
Summary
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

2. Motivation Kpnn experiment needs hermetic veto system
Signature = 2g + nothing
Major KL decay modes
KLp+p-p0 (13%)
KLpen(g) (39%)
These may fake the signal if 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
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

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: Ep~170MeV)
Required inefficiency < 10-4
Energy deposit ~ 2MeV/cm
Edeposit before Pint=10-4 ~ 40keV 6x1023[n/mol] / 104[g/mol] x 8[CH pair/n] x 1[g/cm3] x 100x10-27[cm2] x (thickness) = 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
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

4. Example: KOPIO Downstream CPV
Beam pipe liner inside pre-radiator / calorimeter
Counter should be inside vacuum, but it’s desirable to locate PMT outside  Long WaveLength-Shifting-Fiber (WLSF)  Small light yield  Need High QE PMT
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

5. High QE PMT : Hamamatsu H7422
H7422P-40
GaAsP photocathode
Sensitive area: 5mm in diameter
Metal channel dynode structure
Price ~1.8k$
QE=30-40% for Green
Measured QE using LEDs (relative to bi-alkali PMT)
Expected QE
GaAsP (H7422)
Calculated from relative QE and bi-alkali catalog value
Bi-alkali (H7415)
Peak Wavelength of WLSF
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

6. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
Basic Properties of H7422
Stable upto 1MHz
Rate dependence
Light yield: 100 p.e.
Linearity
Gain~6x106
Sensitivity map
gain drops
above 200kHz
Light yield: 1000 p.e.
Gain~106
Linearity not so good (even within 100 p.e.)
Improvement might be possible by optimizing base circuit
Structure due to focusing mesh was seen… (~10% dip)
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

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 (Edep~580keV)
70 p.e. / 0.58 MeV  120 p.e. / MeV
Results: 70 p.e (both) with High QE PMT (x 3~3.5 larger than with Bi-alkali PMT)
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

8. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
WLSF attenuation
Attenuation in WLSF alone
with LED-excited, viewed by High QE PMT
lL=6.1m
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
AS=9.0 , lS=1.0m
AL=12.9 , lL=6.1m
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

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 mm  8mm diameter
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

10. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
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
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

11. Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy
Summary (2)
Application to KOPIO DS Charged Particle Veto
In case we use 4m long WLS fiber, 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 3x (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?)
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy

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
May 26-27, 2005
Tadashi Nomura (Kyoto U), KRare05 at Frascati, Italy