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11th ICATPP Conference on
Astroparticle, Particle, Space Physics, Detectors and
Medical Physics Applications
Catania
Catania
(Villa Olmo, Como 5-9 October 2009)
Comparative measurements of the
performances of four super bialkali
large area photomultipliers
Leonora Emanuele
on behalf of
NEMO collaboration
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1/12
Hamamatsu Super BiAlkali (SBA) photomultipliers
Catania
Hamamatsu has realized a Super BiAlkali large area PMT (10 inch. 10 stages ) on the
basis of the R7081 standard PMT ( standard QE ≈ 25% @ 400 nm)
SBA photocathode has a quantum efficiency (QE) ≈ 35% @ 400 nm
Standard and SBA PMTs have the same mechanical design and internal structures.
Difference only in the photocathode
Dimensions of R7081 (Hamamatsu datasheet)
Quantum efficiency of a SBA and a standard R7081 (Hamamatsu internal communication)
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Measurements performed
Catania
The performances of 4 SBA PMTs were measured and compared with those
measured on a batch of 72 standard R7081 PMTs.
Main measurements conditions:
• Room temperature and atmospheric pressure
• Attenuated laser source ( s.p.e. conditions, 410nm, 60 ps width, 15 KHz)
• All PMTs at the same gain condition ( 5 x 107 )
• All PMTs powered by ISEG active base PH7081 sel.
• Whole photocathode illuminated
Measurements performed :
- increase of the quantum efficiency of the SBA PMTs with respect to standard
PMTs
- super bialkali secondary effects:
dark count rate / decay time
time and charge resolutions
fraction of spurious pulses
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The SBA quantum efficiency increase
Catania
The quantum efficiency increase with respect to standard PMT was measured by
the comparison of the detection efficiency for each SBA PMT
detection efficiency = n° of PMT detected events / n° of emitted laser pulses
Detected events calculated on the transit time
spectrum acquired with 1/3 s.p.e. threshold.
Laser pulses emitted was measured by a PMT monitor.
Comparison of the TT spectrum of a SBA and standard
QE SBA increase @ 410 nm [%]
Sketch of the testing facility to illuminate the whole photocathode
11 ICATPP october, 2009 Como
SBA PMT 1
37.7
SBA PMT 2
32.1
SBA PMT 3
39.7
SBA PMT 4
36.1
Considering a standard QE of 25%,
the measured QE improvement in the SBA
complies with Hamamatsu specification
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Overall characteristics of the SBA PMTs
Catania
The overall characteristics were measured for each SBA PMT and compared with
those of the standard
• Dark count rate (threshold 1/3 s.p.e)
• Transit Time Spread ( threshold 1/3 s.p.e. ) acquired by TAC FAST (25 ps per
channel)
• P/V ratio and sigma charge resolution, acquired by QDC Silena (0.17 pC per
channel)
Volt @gain
5E7 [V]
DC rate
[Hz]
P/V
RES
sigma [%]
TTS FWHM
[ns]
Average 72
std. PMTs
1655
1388
3.5
32
2.8
SBA PMT 1
1760
4805
2.8
27
2.8
SBA PMT 2
1678
2003
2.5
41
3.1
SBA PMT 3
1760
3265
3.0
33
2.7
SBA PMT 4
1696
1598
3.4
28
2.6
Resume of the overall characteristics of the 4 SBA PMTs with respect to standard
The SBA Time and Charge resolution were not worse than standard
The SBA dark count rate is higher than standard PMTs (SBA mean value = 2900 Hz)
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SBA decay time measurement
Catania
The mean dark count (DC) decay time after a short light exposure was measured
for 2 SBA PMTs and 2 standard PMTs
Decay time: time required to achieve a
stable minimum value of dark count rate
after light exposure
Test procedure:
•Tested PMTs were exposed to the
ambient light of a neon lamp
(3 seconds) without power supply
•After 1 hour of darkness, powered
ON at nominal voltage
•The behaviour of the DC rate from
the instant of power ON was
measured
Mean dark count rate behaviour vs. time from power ON
SBA PMTs have a decay time of over 10 hours
Other published studies on SBA
PMT confirm this behaviour
Standard PMTs have a decay time of around 6 hours
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Spurious pulses definition and typical time distributions
Catania
Spurious pulses are noise pulses time-correlated with the main PMT response
-Pre-pulses (P.P.), appear 10-80 ns before the main pulse.
Due to direct photo-effect on the first dynode.
- Late pulse (L.P.), appear 10-80 ns after the main pulse, in
the place of it. Due to photoelectron backscattering on the
first dynode.
-Type 1 after pulse (AP1), appear 10-80 ns after the main
pulse. Due to luminous reaction on the electrodes.
- Type 2 after pulse (AP2), arrive in the 80ns-16us after the
main. Due to the ionization of the residual gas in the PMT.
Time distributions of pre- and late pulses
after pulse 1
after pulse 2
Arrival time of spurious pulses with respect to main pulses
Time distributions of the AP1 and AP2
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The fractions of the spurious pulses
Catania
The ratio of the number of spurious pulses divided by the number of the true pulses
was measured for each SBA PMT
Pre- P. [%]
Late P. [%]
After P.1 [%]
After P.2 [%]
Average 72 STD
PMT
0.02
5.4
1.1
4.4
SBA PMT 1
0.03
5.8
1.9
15.2
SBA PMT 2
0.02
6.4
1.2
8.4
SBA PMT 3
0.01
5.7
1.6
11.3
SBA PMT 4
0.01
5.8
1.3
8.7
Resume of the SBA fraction of spurious pulses compared with standard
Comparison with standard PMTs :
The fraction of pre- and late pulse were not worse
The fraction of type 1 after pulses was increased
The fraction of type 2 after pulses was considerably increased
(mean SBA AP2 fraction = 11% )
This effect is confirmed by published studies on other SBA PMT
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Type 2 after pulse time distribution
Catania
The delay time of after pulse 2 from main pulse is correlated to atomic mass of residual
gas  information about impurity inside the PMT
The mean time distribution of the SBA
PMTs was compared to the mean
distribution of the standard PMTs
• The shapes of the distributions
were the same
• The fractions of events within the
peaks with respect to the number of
total events did not vary
Comparison of the AP2 mean time distributions for SBA and standard
The super bialkali photocathode did not introduce other types of impurity
with respect to the standard
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SBA photocathode uniformity measurements
Catania
The SBA uniformity was measured and compared to that of the standard by
scanning the photocathode surface:
• Dark box (2 x 1.7 x 1.5) m
• Light source movement system, controlled by LabView code
• 410 nm Laser Source pulsed in s.p.e. condition
• 5mm light spot diameter, normal to PMT surface
• Light monitor PMT, to measure the number
of laser pulses emitted
Sketch of the light source movement system
Picture of the dark box, movement system and a tested PMT
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Standard and SBA local detection efficiency
Catania
The photon detection efficiency (PDE) was measured
on 324 points uniformly distributed on the
photocathode for a SBA and a standard PMT
Measure of the ratio of the detected events (1/3 s.p.e.
threshold) on the number of emitted light pulses
Local detection
efficiency along
vertical axis. Each
plot is normalized to
its max value
Dashed line: declared active area for a 10 inch. PMT
The SBA photocathode conserves the uniformity of the standard
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Summary
Catania
The performances of 4 new large area PMTs (10 inch.) with super bialkali
photocathodes was measured and compared with 72 standard PMTs R7081:
• The quantum efficiency increase with respect to standard PMT was between
32% - 40% (@ 410 nm)
• The time and charge resolutions were not worse than that of the standard
• The dark count rate was higher than standard
(SBA mean value ≈ 2900 Hz, standard mean value ≈ 1400 Hz )
• The decay time was around 10 hours, higher than that of the standard (6 hours)
• The fraction of type 2 after pulses was al most 3 times greater than that of the
standard (SBA mean AP2 fraction = 11% , standard = 4% )
• The super bialkali photocathode did not introduce other types of impurity inside the
PMT with respect to standard
• The SBA photocathode uniformity was not worse that that of the standard PMT
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Catania
The end …thank you !
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Catania
additional slides
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How is it possible to boost the QE ?
Catania
• Use of highly purified materials for the photo cathode
(change from 99.999 to 99.9999 or even of higher purity )
( will provide less scattering length for e- , low
recombination probability)
• Tuning of the photo cathode thickness
• Tuning of the material composition
• Tuning of the anti-reflective layer
• Tuning of the Cs layer thickness
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Method to measure after pulses
Catania
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