performance of a multi-anode photomultiplier employing an ultra bi-alkali photo-cathode and rugged...
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Nuclear Instruments and Methods in Physics Research A 604 (2009) 168–173
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Nuclear Instruments and Methods inPhysics Research A
0168-90
doi:10.1
� Corr
E-m
journal homepage: www.elsevier.com/locate/nima
Performance of a multi-anode photomultiplier employing an ultra bi-alkaliphoto-cathode and rugged dynodes
T. Toizumi a,�, S. Inagawa a, T. Nakamori a, J. Kataoka a, Y. Tsubuku a, Y. Yatsu a, T. Shimokawabe a,N. Kawai a, T. Okada b, I. Ohtsu b
a Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japanb Hamamatsu Photonics K.K., 314-5 Shimokanzo, Toyooka Village, Iwata-gun, Shizuoka 435-8558, Japan
a r t i c l e i n f o
Available online 30 January 2009
Keywords:
Multianode photomultiplier
Ultra bi-alkali
02/$ - see front matter & 2009 Elsevier B.V. A
016/j.nima.2009.01.063
esponding author. Tel./fax: +813 5734 2388.
ail address: [email protected] (T. T
a b s t r a c t
We report on the performance testing of a multi-anode photomultiplier (MAPMT), the R8900-200-
M16MOD-UBA, newly developed by Hamamatsu Photonics K.K. Although the R8900 series offers the
great advantage of a highly sensitive surface (X80% of physical area), the quantum efficiency (Q.E.) was
relatively low (at up to 20%). This paper describes two substantial changes we have made to the R8900-
200-M16MOD-UBA: (1) improving the Q.E. to the 40% level by employing an ultra bi-alkali (UBA) photo-
cathode and (2) constructing a rugged dynode that can withstand vibration for future use in space. We
measured each pixel signal at the single photoelectron level and the signals of scintillation photons by
using a 16-pixel plastic scintillator array. Thanks to high Q.E., good energy resolution of 29.9% (FWHM)
was obtained for 59.5 keV g-rays. We also demonstrated tolerance to vibration up to 17 Grms in possible
launching vehicles.
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
Photomultipliers are standard devices for detectors employedin many fields of physics. The multi-anode photomultiplier(MAPMT) features many anodes and can detect the receivingposition of photons.
The R8900-200-M16MOD-UBA (shown in Fig. 1) has 16anodes. It is compact and requires low voltage ð�800 VÞ comparedto other PMTs ð�1500 VÞ. The R8900 series features a highlysensitive surface (X80% of physical area) [1]. We plan to useMAPMTs to detect the positions where X-ray and g-ray eventsoccur in a plastic scintillator array. These MAPMTs require highsensitivity for scintillation photons due to the low light yield ofthe plastic scintillator ð�5 photons=keVÞ. Moreover, for future usein space, the MAPMT must have rugged dynodes that canwithstand the vibration of launching vehicles. This motivated usto develop new MAPMTs equipped with a ultra bi-alkali (UBA)photo-cathode [2] and rugged dynodes for launching of the HIIArocket. The quantum efficiency (Q.E.) of UBAs is more than 40%,which is almost doubled from a usual MAPMTs employing bi-alkali (BA) cathode ð�20%Þ. Although it is usually difficult toensure ruggedness against vibration due to the complex config-
ll rights reserved.
oizumi).
uration of dynodes in the MAPMT, we have successfully developedwith Hamamatsu Photonics K.K. the ruggedness necessary tosurvive a random vibration level of 17 Grms relative to the HIIArocket profile.
2. Setup for characterization
We evaluated MAPMT performance by using twomethods: (1) the self-coincidence system and (2) scintillationphoton detection. When using the self-coincidence system,we obtained the spectra of single photoelectrons, and thenestimated the gain and temperature dependence of dark counts.When using scintillation photon detection, we obtained the 234Amspectra, and then estimated the comparative Q.E. and energyresolution.
2.1. Self-coincidence
Fig. 2 shows a schematic view of the self-coincidencesystem. In this setup, a 10 bit ADC (CLEAR-PULSE, 1125)obtains the signal with a trigger synchronizing with the LED.The clock generator (TECHNOLAND CORP.) generates 100 Hzrectangular waves to only obtain signals without noise. If thefrequency exceeds 100 Hz, noise is added to the signal. The gate
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T. Toizumi et al. / Nuclear Instruments and Methods in Physics Research A 604 (2009) 168–173 169
generator (TECHNOLAND CORP.) converts this wave into tworectangular waves: a 40 ns width wave for the LED, and a 2mswidth wave for triggering. LED intensity is attenuatedby the attenuator so that MAPMT output is at the singlephotoelectron level. The signal output charge is integratedand converted into voltage by the charge sensitive amplifier(CSA) (CLEAR-PULSE, 557), and filtered into a 1ms widthpulse by the shaping amplifier (ORTEC, 571). Given the proce-ssing with two amplifiers, the signal was delayed �1ms. There-fore, the trigger pulse was delayed 1ms when using the gategenerator.
2.2. Using the plastic scintillator
We also examined MAPMT performance by using a plasticscintillator array. To make a previous circuit the same condition
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
Top View
Fig. 1. (left) Photograph of the R8900-200-M16MOD-UBA (S/N ZB-1500) multi-
anode PMT and (right) define for the pixel number.
CSMAPMT
Delay 1 usWidth 2 us
Width 40 ns
-HV -900 V
LED
68
GaGene
Attenuator13 dB
Fig. 2. Self-coincidence system setup to obtain single photoelectron spectra
CSAMAPMT
-HV -900 V
4 x 4Plasticarray
Source241Am
Fig. 3. Setup for scintillat
from CSA, we sequentially measured the g-ray spectrum of eachpixel. Fig. 3 shows the setup for using the plastic scintillator array.
3. Single photoelectrons
3.1. Spectra of single photoelectrons
We used the self-coincidence system (shown in Fig. 2) toobtain the spectra of single photoelectrons. The spectra of singlephotoelectrons provide the information necessary to determinethe value of MAPMT gain. The pixel with the largest gain value(pixel 3) was chosen from among all pixels (S/N ZB-1499), and weused this value to determine the absolute gain and derive therelational expression of high voltage (HV) and gain. The MAPMTgain indicates a value approximating that of each pixel. We thusobtained the comparable gain for all pixels. Then the comparablegain was used to describe the ratio to pixel 3.
3.2. MAPMT gain
The value of the charge of a single photoelectron can beestimated by using test pulse input by the pulse generator (BNC,Model PB-4) instead of the MAPMT signal. We converted the testpulse ðV ¼ 102100 mVÞ with high accuracy (error p5%) into thetest charge with the capacitor ðC ¼ 10 pFÞ, given that Q ¼ C � V .The capacitance of the capacitor has an error of 10%. This chargeinstead of the MAPMT signal is the input to the CSA. We comparedthe test pulse height and single photoelectron pulse height in thespectrum. We use the spectrum for pixel 3 to estimate the gain,because pixel 3 had the largest pulse height value among all pixels
A ShapingAmp
100 Hz
Signal
Gate 10 bitADC
WindowsPC
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ShapingAMP
10 bitADC
WindowsPC
ion photon detection.
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Fig. 4. Single photoelectron spectra for all pixels ðHV ¼ �900 VÞ (since pixel 3 has the largest pulse height value among of all pixels, we used pixel 3 to estimate the gain
(S/N ZB-1499)).
T. Toizumi et al. / Nuclear Instruments and Methods in Physics Research A 604 (2009) 168–173170
(S/N ZB-1499). At �900 V, the single photoelectron peak of pixel 3 (asshown in Fig. 4) has the following charge: Q ¼ 0:262� 0:026 pC. Bydividing this value by the elementary charge, typical gain G of pixel 3at �900 V is obtained as G ¼ Q=e� ¼ ð1:64� 0:16Þ � 106. In pixel 3,the variation in gain was measured by changing the HV. We changedthe value of HV from 800 to 950 V in increments of 10 V, obtained thespectrum of a single photoelectron, and then examined each pulseheight. Fig. 5 shows the typical HV dependence of gain. The fittingfunction is G ¼ a�HVb, and the fixed parameter log10 a ¼ �23:1�0:3, b ¼ 9:91� 0:21.
3.3. Uniformity of gain
The ratio of gain can be obtained by comparing thecharge of the anode output signals of single photoelectrons,because the spectrum of a single photoelectron is the signalof the multiplied electron. By using the pulse heightof the spectra of single photoelectrons, we obtained theratio of gain for each pixel at HV ¼ �900 V. Fig. 6 showsthe ratio of gain for each pixel by using pixel 3 as a reference(100).
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0.75 0.8
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750 800 850 900 950 1000
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HV (V)-logscale
105
106
107
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-logs
cale
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Fig. 5. Typical HV dependence of gain (the fitting function is G ¼ a �HVb , and the
fixed parameter log10 a ¼ �23:1� 0:3, b ¼ 9:91� 0:21 (S/N ZB-1499)).
Fig. 6. Uniformity of gain (S/N ZB-1499) (the ratio of the gain of each pixel is
shown by using pixel 3 as a reference (100)).
0.1
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-30 -20 -10 0 10 20 30 40
coun
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ec
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dataa = 3.57, b = 0.0703
Fig. 7. Dark count at each temperature (data were fitted by the function
N ¼ N0 � expðaTÞ. Fixed parameter N0 ¼ 3:57 and a ¼ 0:07 (S/N ZB-1499)).
T. Toizumi et al. / Nuclear Instruments and Methods in Physics Research A 604 (2009) 168–173 171
3.4. Dark counts and thermal effects
In a photo-cathode, a signal (thermal excitation electron) isalways discharged at the single photoelectron level. This electronis multiplied as well as the photoelectron, and contributes to the
same spectra as single photoelectrons. We placed the MAPMT intoa temperature chamber (ESPEC CORP., MODEL SU-641) andexamined changes in the spectrum from �20 to 30 �C inincrements of 10 �C. The MAPMT is enclosed by a light-tighttemperature chamber, and only thermal excitation electrons wereoutput as signals during measurement. For setting the thresholdof a single photoelectron peak, we counted the number of thermalexcitation electrons that exceeds the threshold. Fig. 7 shows thenumber of thermal excitation electrons (dark counts) at eachtemperature. Dark count N can be indicated as a function oftemperature as expressed by N ¼ N0 � expðaTÞ, with N0 ¼ 3:57being the count (count/s) at 0 �C. This count rate is negligibleenough to conduct typical physics experiments.
4. Scintillation photon detection
4.1. Spectra of 241Am
Fig. 8 shows the spectra of 241Am irradiated with the g-rayphotons obtained with the MAPMT-R8900-200-M16MOD-UBA (S/N ZB-1499) and the plastic scintillator array. Since the light yieldof plastic is very low, when we use the standard BA-type (S/N NA-0118), the 59.5 keV photoelectric peak is unresolvable in somepixels. The UBA-type MAPMT can resolve a photoelectric peak forall pixels because the Q.E. had been greatly improved. At pixel 15,we can clearly see not only the 59.5 keV photoelectron peak butalso a lower energy peak from 241Am ð�20 keVÞ.
4.2. Energy resolution at 59.5 keV
Next we compared the UBA-type R8900-M16 (S/N ZB-1499)with a BA-type MAMT R8900-M16 (S/N NA-0118) atenergy resolution of 59.5 keV for each pixel, as measured withthe same plastic scintillator array. We found that the UBA-typehas better resolution than BA in all pixels. The best energyresolution of 29.9% (FWHM) was obtained for the UBA-typeMAPMT, while that of the standard BA-type MAPMT was 49.8%(FWHM). The average of the energy resolution at 59.5 keV is 35.9%(FWHM) for the UBA-type and 61.2% (FWHM) for the standard BA-type. We can also estimate the number of electrons yielded from59.5 keV g-rays by comparing the peak position with a singlephotoelectron signal. When the UBA-type is used, a singlephotoelectron corresponds to 1.02 photoelectron/keV. This marksa significant improvement over the standard BA-type (0.37 photo-electron/keV).
5. Vibration test
Standard MAPMTs lack improved tolerance and are not completelyreliable against vibration impact when launching a rocket. The R8900-200-M16MOD-UBA offers improved tolerance to vibration in possiblelaunching vehicles. Prior to delivering the MAPMTs to us, HamamatsuPhotonics K.K. tested ten sample units using random levels ofvibration up to 15 Grms, and a shock level of 60 G. We continueddetailed vibration testing on each MAPMT to confirm tolerance torandom vibration that is 1.5 times that of the HIIA profile anddetermine the maximum vibration that each unit can withstand. Thedynode represents the weak point relative to vibration. When thedynode is damaged, the MAPMT loses gain. Therefore, we examinedvariations in the gain caused by vibration. For vibration tests, we useda IMV i230/SA2M vibration generator. The frequency profile wasrandom vibration based on the HIIA profile as listed in Table 1 [3].Random vibration was applied to three axes (X, Y, and Z). The durationof vibration for each axis was 2 min.
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pixel 13 pixel 14 pixel 15 pixel 16
Fig. 8. 241Am source spectra by MAPMT-200-M16MOD-UBA with a plastic scintillator array (we obtained the best energy resolution of 29.9% (FWHM) at 59.5 keV. Energy
resolution by standard BA-type MAPMT was 49.8% (FWHM) under the same condition. When we use the standard BA-type, the photoelectric peak (59.5 keV) was
irresolvable in some pixels (S/N ZB-1499)).
Table 1Characteristics of the frequency profile used in the random vibration test (the
duration of vibration was 2 min (120 s)).
Frequency range (Hz) Vibration profile
202200 þ3:0 dB=oct
20022000 0:032 G2=Hz (for 7:8 Grms)
T. Toizumi et al. / Nuclear Instruments and Methods in Physics Research A 604 (2009) 168–173172
We initially tested at 12 Grms. Before and after this test, weexamined the MAPMT gain by using single photoelectron spectra.Fig. 9 shows the spectra before the vibration test (black) and after12 Grms (gray).
We then accelerated the vibration and examined the gain aftereach test. Vibration was accelerated from 5 to 17 Grms inincrements of 3 Grms. After vibration at 12 and 17 Grms, nosignificant change in signal output was noted. Therefore, the
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Fig. 9. Single photoelectron spectra (S/N ZB-1500) before vibration test (black) and after 12 Grms (gray).
T. Toizumi et al. / Nuclear Instruments and Methods in Physics Research A 604 (2009) 168–173 173
MAPMT was considered to withstand vibration even at double theconditions of the HIIA rocket profile.
6. Conclusions
We have developed a new type of MAPMT featuring UBA andrugged dynodes. It offers improved Q.E. and stiffness. Thanks tohigh Q.E. ðX40%Þ, good energy resolution of 29.9% (FWHM) wasobtained for 59.5 keV g-rays. The results of a randomvibration test showed that the MAPMT withstood the 17 Grms
profile (X2 times the HIIA profile); therefore, we confirmed thatthe MAPMT-200-M16-UBA could sufficiently withstand thevibration caused by launching the HIIA rocket.
References
[1] Y. Kawasaki, et al., Nucl. Instr. and Meth. A 564 (2006) 378.[2] Hamamatsu Photonics K.K, UBA (ultra bi-alkali) SBA (super bi-alkali) PMT
Series hhttp://www.hamamatsu.comi.[3] Public offering the small satellite/about interface for H-IIA rocket (JAXA).