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11 ICATPP october, 2009 Como [email protected] 1/12
Catania
Catania
Comparative measurements of the Comparative measurements of the performances of four super bialkali performances of four super bialkali
large area photomultiplierslarge area photomultipliers
Leonora Emanueleon behalf of
NEMO collaboration
11th ICATPP Conference on
Astroparticle, Particle, Space Physics, Detectors and Medical Physics Applications
(Villa Olmo, Como 5-9 October 2009)
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Hamamatsu Super BiAlkali (SBA) photomultipliers
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 Measurements performed
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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CataniaThe SBA quantum efficiency increase The SBA quantum efficiency increase
QE SBA increase @ 410 nm [%]
SBA PMT 1 37.7
SBA PMT 2 32.1
SBA PMT 3 39.7
SBA PMT 4 36.1
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.
Considering a standard QE of 25%, the measured QE improvement in the SBA complies with Hamamatsu specification Sketch of the testing facility to illuminate the whole photocathode
Comparison of the TT spectrum of a SBA and standard
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Overall characteristics of the SBA PMTsOverall characteristics of the SBA PMTs
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
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)
Resume of the overall characteristics of the 4 SBA PMTs with respect to standard
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SBA decay time measurementSBA decay time measurement
The mean dark count (DC) decay time after a short light exposure was measured for 2 SBA PMTs and 2 standard PMTs
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
SBA PMTs have a decay time of over 10 hours
Standard PMTs have a decay time of around 6 hours
Other published studies on SBA PMT confirm this behaviour
Mean dark count rate behaviour vs. time from power ON
Decay time: time required to achieve a stable minimum value of dark count rate after light exposure
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Spurious pulses definition and typical time distributionsSpurious pulses definition and typical time distributions
Time distributions of pre- and late pulses
after pulse 1 after pulse 2
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 the AP1 and AP2
Arrival time of spurious pulses with respect to main pulses
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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
The fractions of the spurious pulsesThe fractions of the spurious pulses
The ratio of the number of spurious pulses divided by the number of the true pulses was measured for each SBA PMT
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
Resume of the SBA fraction of spurious pulses compared with standard
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Type 2 after pulse time distributionType 2 after pulse time distribution
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 super bialkali photocathode did not introduce other types of impurity with respect to the standard
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
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The SBA uniformity was measured and compared to that of the standard by scanning the photocathode surface:
SBA photocathode uniformity measurementsSBA photocathode uniformity measurements
• 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 systemPicture of the dark box, movement system and a tested PMT
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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
Standard and SBA local detection efficiency Standard and SBA local detection efficiency
The SBA photocathode conserves the uniformity of the standard
Local detection efficiency along vertical axis. Each plot is normalized to its max value
Dashed line: declared active area for a 10 inch. PMT
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SummarySummary
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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• 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
How is it possible to boost the QE ?
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Method to measure after pulses Method to measure after pulses
