2006 IEEE Nuclear Science Symposium Conference Record
High
Speed
HPD
N05-1
for
Photon Counting
A. Fukasawa, J. Haba, A. Kageyama, H. Nakazawa, M. Suyama Abstract- We have succeeded in the development of a highspeed Hybrid-Photodetector (HPD), by using reduced electron lens and newly developed avalanche diode (AD) with very low capacitance. The HPD shows fast time response of less than 400 ps in both rise and fall times, and good timing resolution of 23 ps in sigma for single photons at full illumination on photocathode of o 8 mm, after deducting the waveform of the input light pulse. Limiting factor of the timing resolution was further investigated, and clarified to be the transit time difference of electrons in vacuum along the photocathode. The timing resolution was estimated to be less than 10 ps for the spot illumination of o 1 mm on the central part of the photocathode, where small transit time difference was expected by the simulation. This resolution is the world highest in the HPD, and matches for those of MCP-PMTs. Both GaAsP photocathode having close to 50 % quantum efficiency in visible region and bialkali photocathode having 34 % in UV region were fabricated in view of various applications. In this paper, we report the results of the evaluation including discussion about the limiting factors of the timing resolution for the new HPD. I. INTRODUCTION
The detection of extremely weak and fast optical signal is Tdesired for photodetectors in various applications, such as physics, chemistry, biomedical experiments and so on. For the detection of single photon level signal, photomultiplier tube (PMT) having a photocathode and high gain dynode-chain in vacuum is one of the most widely used detectors for long time. PMTs have high gain of on the order of 106, fast time response of a few ns, large effective area up to 20 inches, and timing resolution of a few 100 ps for single photons. In late years, other types of photodetectors have been developed, in terms of fast response with single photon detection capability. One of such detectors is an MCP-PMT [1], which incorporates a micro-channel-plate (MCP) instead of dynodes. Due to small transit time spread in the MCP, this detector Manuscript received November 17, 2006. This work was partially supported by the Grant-in-Aid for Scientific Research (A) 17204020 of the Japan Society for the Promotion of Science (JSPS). A. Fukasawa is with the Electron Tube Division, Hamamatsu Photonics K.K., Iwata-city, 438-0193, Japan (e-mail:
[email protected]) J. Haba is with the Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, 305-0801, Japan (e-mail:
junji.haba@kekjp)
A. Kageyama is with the Electron Tube Division, Hamamatsu Photonics K.K., Iwata-city 438-0193, Japan (e-mail:
[email protected]) H. Nakazawa is with the Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan
(e-mail:
[email protected])
M. Suyama is with the Electron Tube Division, Hamamatsu Photonics K.K., Iwata-city 438-0193, Japan (e-mail:
[email protected])
1-4244-0561-0/06/$20.00 ©2006 IEEE.
43
shows much higher timing resolution of 10 ps in sigma, though the maximum count-rate and the detection efficiency are limited by the MCP, and the cost is relatively high. A Multi-Pixel-Photon-Counter (MPPC or SiPM) [2] based on semiconductor technology attracts a great deal of attention in late years, because of its good timing resolution of on the order of a few 100 ps and pulse height resolution for single photons, however, its effective area is small, such as 1 mm2, with dark count rate of 100 kHz at room temperature, and dynamic range is narrow in the present circumstances. An HPD [3] incorporating a semiconductor device in a vacuum tube to receive electrons from the photocathode is also used for single photon detection. HPDs have inherently good pulse height resolution because of the mechanism of amplification, where deposited energy of electrons determines the gain in the semiconductor device. The time response is generally limited to be 10 ns by the capacitance of the avalanche diode (AD). As mentioned above, each detector has strong points, as well as weak points. Among them, we found out that the response time of the HPD could be drastically improved by using low capacitance AD. In this case, a large effective area can be realized by an electron lens focusing electrons from the photocathode to the tiny AD with low capacitance. If this is the case, the HPD will be one of the ideal photodetectors, in terms of large effective area and fast time response. Based on this concept, we have developed the high speed HPD for single photon detection. The structure of the developed HPD, and the results of the evaluation are shown below. II. STRUCTURE OF THE DEVELOPED HPD
The developed HPD consists of a photocathode, an AD, a cylindrical ceramic sidewall and a stem, as illustrated in Fig. 1. A bialkali photocathode was fabricated on a plano-concave faceplate of +8 mm to adjust the transit time of electrons to be equal over the photocathode. A highly sensitive GaAsP photocathode in visible region was also prepared, where the effective area was limited to be +3 mm to achieve enough small transit time fluctuation, because the photocathode made of crystal should be flat and adjustment of the transit time was difficult for a larger photocathode area.
III. TEST RESULT
Photocathode (Bi-Alkali) +-Plano-Concave Faceplate -
A. Photocathode Sensitivity The quantum efficiency of the photocathode as a function of wavelength was measured by a photocathode current, in response to calibrated input light. The quantum efficiency of the bialkali photocathode was 34 % at the wavelength of 350 nm, and 46 % for the GaAsP photocathode at 500 nm, as shown in Fig. 3. The potential of quantum efficiency of higher than 50 % for GaAsP photocathode is shown elsewhere [4],
Cylindrical Ceramic Electron Bombarded Gain
Avalanche Diode (AD)
Photocathode Voltage -8kV
Avalanche Gain
±
T I,
SMA Conn4etor Output
[5], [6].
AD Reverse Bias Volta2e 400V
100
XJ77.
Fig. 1. The developed HPD consists of the faceplate, the AD, the cylindrical ceramic sidewall and the stem. -8 kV is applied to the photocathode to produce electron lens in the vacuum for focusing electrons to the small AD. 400 V is applied to the AD for avalanche multiplication.
0
a, .a)
.0
The new AD with 1 mm in diameter was developed for the high speed HPD. The capacitance was estimated to be 3.4 pF, suggesting rise and fall times of 370 ps for 50 Q load. In order not to deteriorate the fast signal from the low capacitance AD, a SMA connector was hermetically mounted on the stem. The bias voltage is applied to the AD from the other pin. The photograph of potted tube is shown in Fig. 2. In response to incident photons, electrons are emitted from the photocathode at the potential of -8 kV, accelerated, and focused onto the AD of the ground potential by the electron lens inside the vacuum. Then, electrons deposit their kinetic energy in the AD and produce thousands of electron hole pairs, which is called an electron bombarded gain. The generated electrons are drifted in the AD, and further multiplied by ten to hundred times by impact ionization, called an avalanche multiplication. The total gain is the multiplication of these two gains beyond 105. The HPDs with two types of photocathode have been fabricated and evaluated.
10
E uJ
a
1L
GaAsP - Bi-Alkali
200
44
400
500 600 Wavelength (nm)
700
800
Fig. 3. The quantum efficiencies for bialkali [blue line] and GaAsP photocathode [red line] are shown as a function of wavelength. The peak quantum efficiencies are 34 % at 350 nm with bialkali and 46 % at 500 nm with GaAsP photocathode.
B. Gain As mentioned above, there are two gain stages, electron bombarded gain and avalanche gain, in the HPD. The electron bombarded gain was measured as a ratio of the input and output currents of the AD, where the avalanche gain was unity with no bias voltage to it. The electron bombarded gain is a function of the photocathode voltage, and reaches 1600 at -8 kV, as shown in Fig. 4. The avalanche gain was measured as an output current of the AD with varying bias voltage to it. The measured currents at various voltages were normalized by the current at the unity avalanche gain. The avalanche gain is a function of the bias voltage, which determines electric field in the AD, and reaches 110 at the bias voltage of 405 V, as shown in Fig. 5. The overall gain is given by the multiplication of these two gains to be 180, 000,
approximately.
Fig. 2. A rear view ol the developed HPD is shown. To ac response, a SMA connector is hermetically sealed to the stem.
300
2000
o 1500
n 0.9-1 00.8-0.9 El 0.7-0.8
0
* 0.6-0.7 * 0.5-0.6 0.4-0.5 0.3-0.4
E~
2o 1000 m
0
a
500
0.2-0.3
*0.1 -0.2 *0-0.1
0 0
2
4 6 8 Photocathode Applied Voltage (-kV)
10
Fig. 4. The electron bombarded gain is shown, as a function of the photocathode voltage. The gain is approximately 1600 at the photocathode voltage of -8 kV.
Fig. 6. The output profile by scanning (0.5 mm spotlight over the bialkali photocathode shows excellent uniformity of the HPD, as expected by the simulation. The effective area of (8 mm (shown by yellow circle) was confirmed as well.
350 Il
300
D. Pulse Height Spectrum for multi-photons The pulse height spectrum was measured for the pulsed light at several photoelectrons per pulse on average. An LED at the wavelength of 470 nm was used as a light source. -8 kV and 380 V were applied to the photocathode and the AD, respectively, to obtain the gain of 75,000. The output signal of the HPD was amplified in a charge sensitive amplifier (580K, CLEAR-PULSE) and a shaping amplifier (3100-02, CANBERRA) to be analyzed in a multi-channel analyzer (2100C/MCA, LABOLATRY EQUIPMENT). Up to 6 photoelectron-peaks are clearly identified with bialkali type, as shown in Fig. 7. The good pulse height resolution shown here is the typical performance of the HPD [3], [4].
: 250
CD
a, 200
X 150 < 100
50
0 0
100
200 300 AD Reverse Bias Voltage (V)
Fig. 5. The avalanche gain as a function of the AD voltage shows the gain of approximately 110 at 405 V. The total gain is the multiplicatiion of the electron bombarded gain and the avalanche gain, and reaches 1 80,00(
350
C. Uniformity The uniformity was measured by scanning a spotlight of +0.5 mm over the bialkali photocathode at the voltages of -8 kV and 380 V to the photocathode and the AD, respectively. Flat uniformity over the effective area of +8 mm was confirmed, as shown in Fig. 6. This result suggests the electron lens effectively focuses electrons from the photocathode to the AD, as expected by our simulation. Similar result was obtained in the GaAsP type, within 20 % of fluctuation over the effective area.
300 250 ') 200 a)
-" 150 100 50 0
500
1000
1500
2000
2500
3000
3500
4000
Output Pulse Height
Fig. 7. The pulse height spectrum of the HPD for multi photons clearly shows peaks corresponding up to 6 photoelectrons. The voltages of -8 kV and 380 V were applied to the photocathode and the AD, respectively.
F. Time response Time response was measured with Picosec-Light-Pulser (PLPI 0-040, HAMAMATSU) as a light source, which emitted short enough pulse of 30 ps in sigma, considered as impulse for the HPD, at the wavelength of 400 nm. The output signal of the HPD was directly connected to the oscilloscope (Infiniium, AGILENT), of which bandwidth is 1.5 GHz. For 45
the full illumination on thee photocathode, the rise and fall times were 360 ps and 340 p)S, respectively, as shown in Fig. 8, for bialkali photocathode tylpe. This response is determined by the capacitance of the AD c)f 3.4 pF, suggesting rise and fall times of 370 ps with the loacI resistance of 50 Q. 60 50
E 40
0) 30
>
20
0
0 10
,_____________________________
-10 ' o
3
2
4
5
Time (ns) Fig. 8. The output waveform for impulse light shows rise and fall times of 360 ps and 340 ps, respectively. Tihe load impedance of the oscilloscope was 50 Q, and the bandwidth was 1.5 GIHz.
F. Timing Resolution for The timing resolution of bialkali type for single photons was measured using PLP fior the light source and a time to amplitude converter (TAC, 457, ORTEC) for the analyzer, at d 1 -11 -A 11_ ---1 the1- voltages o0 -6 kV and and the 4()S V to the photocathode AD, respectively. The output of the HPD was amplified with two stage amplifiers (C5594-12, HAMAMATSU and HP8447F, HEWLETTE PACKARD) and fed to a constant fraction discriminator (CFD, 9307, ORTEC) to generate a stop signal for the TAC. For the start signal, the output trigger signal from the PLP was used. The results for the cases of full illumination (+8 mm) and the spot illumination of +1 mm on the photocathode are shown as green and red lines, respectively in Fig. 9. The timing resolutions of these conditions are estimated to be 41 ps and 31 ps in sigma, respectively, including jitter of the measurement system. For the calibration purpose, the waveform of the light source was measured by a streak camera (C4334-02, HAMAMATSU), of which time resolution is about 9 ps [7], high enough for this measurement. The result is also shown in Fig. 9 as blue line. The waveform of the light source is well reproduced by the red line corresponding to the result with spot size of +1 mm. This indicates the timing resolution of the HPD for the central part of the photocathode is extremely high, whereas it is deteriorated a little bit for full illumination. By deducting the pulse width of the light source, the timing resolutions for full illumination and 1mm in diameter are estimated to be 28 ps and 10 ps or less, respectively, though it is difficult to estimate the latter precisely, due to wide pulse width of the light source for this measurement. This timing resolution is matched for those of MCP-PMTs. T
A IA C lIT
-
-
The timing resolution as a function of the spot size was measured, as shown in Fig. 10, where the pulse width of the light source is deducted. For the spot size less than +4 mm, the timing resolution is estimated to less than 14 ps, and it is deteriorated to be 28 ps at full illumination (+8 mm). The simulation was carried out to estimate the transit time spread depending upon the spot size on the photocathode, as shown in Fig. 11. It is clearly seen that the transit time spread increases drastically from the illumination of +5 mm (sky blue) to +8 mm (dark blue). From the simulated results, standard deviation for each illuminated area is calculated, as shown in the blue line in Fig. 10. The simulation explains the tendency of the experimental result, suggesting that the transit time difference in vacuum over the photocathode limits the timing resolution for full illumination. The difference between experimental and simulated results in Fig. 10 is attributed to the jitter of the electronics. In the GaAsP type, the timing resolution was 38 ps for full illumination on the photocathode (+3 mm), and 28 ps for the spot size of 1 mm, after deducting the pulse width of the input light pulse. The worse timing resolution of the GaAsP type than bialkali type can be attributed to the transit time spread in the photocathode of the thickness of 1 ptm, approximately. In the case of bialkali photocathode with the thickness of the order of 10 nm, the transit time spread of electron is less than 1 ps, which is reported for a multialkali photocathode thicker than bialkali photocathode [8].
-
46
-PLP spectrum Spot of 1 mm in diameter Full ilumination on photocathode
1.2 1 ID
0.8
(D
0.6
a)
oY 0.4 0.2 0 -300
-200
-100
0 Time (ps)
100
200
300
Fig. 9. The timing resolutions for the full illumination on the photocathode (green line) and the spot size of 4 lmm (red line) show the resolutions of 41 ps and 31 ps in sigma, respectively, including the jitter of measurement system. The waveform of the light source measured by the streak camera is also shown as blue line. By deducting the waveform of the light source, the timing resolutions of full illumination and 1mm in diameter are estimated to be 28 ps and 10 ps or less, respectively.
ACKNOWLEDGMENT
30
We would like to express our thanks to K. Yamamoto, Y. Ishikawa, and M. Muramatsu of the solid-state division of Hamamatsu Photonics K.K. (HPK) for technical discussions about avalanche diodes. We also would like to express our great thanks to J. Takeuchi, T. Morita, S. Muramatsu of electron-tube division (ETD) of HPK for technical advises about this HPD. We greatly appreciate the efforts of Y. Negi, Y. Egawa and K. Shinmura of ETD of HPK for the tube manufacturing, and S. Kimura of ETD of HPK for the simulation of the electron trajectory.
-4- simulation HPD
25 cn
20 ° 15 a)
.: 10 .E 5
0 0
2
4 6 Spot Size of Incident Light (mm)
8
10
Fig. 10. The timing resolution as a function of the spot size of the incident light is shown in red line, where the input light pulse width is deducted from the results. For the spot size from 4)1 mm to 43 mm, there is no big difference of timing resolution. However, it is deteriorated for the spot size larger than 45 mm. Voltages of -8k V and 405 V were applied to the photocathode and the AD, respectively. Blue line shows the simulated and estimated data from the results shown in Fig. 11. 120
_
[2] [3]
[4]
-8mm 5mm 3mm 1mm
100 '
REFERENCES [1] Hamamatsu Photonics K.K., "R3809U-50 SERIES catalog,"
80
[5] [6]
60 a)
oy
[7] [8]
40 20
0 -20
20
40 Time (ps)
60
80
100
Fig. 11. The results of simulation for electron transit time are shown. Each peaks were adjusted to 0 ps. From 41 to 43 mm of photocathode size, the electron transit time spread is good. However, the electron transit time spread drastically deteriorates form 45 mm to 48 mm.
IV. SUMMARY We have succeeded in the development of the high speed HPD. The time response of less than 400 ps in rise and fall times was confirmed. The excellent timing resolution of less than 10 ps for the spot size of 1 mm in diameter and 23 ps for full illumination were also confirmed with the bialkali type. The timing resolution of the GaAsP type was 34 ps for full illumination. This performance is the fastest for the HPD, as far as we have researched, and matches for those of MCPPMTs. Making full use of the characteristics, the HPD can be applied for LIDAR (Light-Detection-And-Ranging), PET scanner in nuclear medical, and TCSPC (Time-Correlated-
Single-Photon-Counting).
47
T. Nobuhara et al., "Development of Multi-Pixel Photon Counters," SNIC Symposium, Stanford, California 3-6 April 2006 M. Suyama et al., "A Compact Hybrid Photodetector (HPD)," IEEE Trans. Nucl. Sci., vol. 44, No.3, pp. 985-989, June 1997 M. Suyama et al., "A Hybrid Photodetector (HPD) with a Ill-V Photocathode," IEEE Trans. Nucl. Sci., vol. 45, No.3, pp. 572-575, June 1998 M. Hayashida et al., "Development of HPDs with an 18-mm-diameter GaAsP photo cathode for the MAGIC-II project," 29th International Cosmic Ray Conference Pune (2005) M. Suyama et al., "Development of a Multipixel Hybrid Photodetector With High Quantum Efficiency and Gain," IEEE Trans. Nucl. Sci., vol. 51, No.3, pp. 1056-1059, June 2004 Hamamatsu Photonics K.K.,"C4780 catalog," K. Kinoshita et al., "Femtosecond streak tube", Rev.Sci. Instrum.," 58 (6), pp. 932-938, 1987
