- Email: [email protected]
Readout of a scintillating-fiber array by avalanche photodiodes
Nuclear Instruments and Methods in Physics Research A 440 (2000) 348}354
Readout of a scintillating-"ber array by avalanche photodiodes T. Okusawa, Y. Sasayama, M. Yamasaki, T. Yoshida* Department of Physics, Faculty of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Received 21 June 1999; accepted 18 August 1999
Abstract We have evaluated the performance of avalanche photodiodes (APDs) as photosensors for scintillating-"ber tracking detectors, putting emphasis on their temperature dependence. For this purpose, a scintillating-"ber array has been built with 0.5 mm diameter "bers 55 cm long. The array is so structured that an incident particle traverses two "bers coupled to an APD. As temperature of the APD falls, the signal-to-noise (S/N) ratio rises exponentially. It even reaches 90 at !303C. The detection e$ciency exceeds 98% when the temperature decreases until the S/N ratio becomes 30. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 29.40.Gx; 29.40.Mc; 42.81.Cn; 85.60.Dw Keywords: Scintillating "ber; Avalanche photodiode
1. Introduction Scintillating-"ber tracking detectors are coming into practical use in some particle-physics experiments. They have some advantages over conventional tracking detectors such as drift chambers. Sub-millimeter diameters of typical scintillating "bers improve multi-track resolution [1}3]. The short #uorescence decay time of a few nano-seconds makes it possible to incorporate track information into fast trigger logic [4,5]. When a minimum ionizing particle traverses a scintillating "ber, the number of photons expected at the photosensor coupled to the "ber is typically as small as 10}20 [6]. To cope with such a small * Corresponding author. Tel.: #81-6-6605-2646; fax: #816-6605-2522. E-mail address: [email protected] (T. Yoshida)
amount of light, well-established high-gain photosensors such as position-sensitive photomultipliers and image intensi"ers are often chosen, but their relatively small quantum e$ciency around 25% inevitably requires increase in the number of "bers along a particle trajectory in order to guarantee good tracking e$ciency. On the other hand, visible light photon counters (VLPCs) have high quantum e$ciency over 65% and moderately high gain over 20 000 [7,8]. The VLPC ensures e$cient readout of each individual "ber. However, one often hesitates to use it because of its optimum operating temperature around 6.5 K. Thus, to pursue more convenient photosensors is still an important subject in order to re"ne the scintillating-"ber tracking detectors. We have been testing silicon avalanche photodiodes (APDs) as photosensors for scintillating "bers, counting on their high quantum e$ciency of 70}90%, in spite of their small gain around 100 at
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 9 2 5 - 0
T. Okusawa et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 348}354
room temperature. In our previous paper [9], we gave our test results obtained with 0.5 mm diameter scintillating "bers and Advanced Photonix 197-7072-520 APDs operated at room temperature. To achieve neat separation of signal from noise, an APD had to be fed photons by at least four "bers along a particle trajectory. In this paper we report on our further attempt to improve the performance of those APDs by lowering their operating temperature, emphasizing how much the signal-to-noise ratio and the detection e$ciency have been improved even with a smaller number of "bers. Lower temperature reduces the number of electrons that thermally di!use from the valence band to the conduction band in the APD. This e!ect depresses the dark current and reduces the shot noise. Furthermore, the drop in temperature enhances the avalanche gain of the APD, because the lower temperature depresses thermal lattice vibrations of the silicon crystal and decreases the number of phonons that interrupt avalanche electrons in the APD. Phonons are energy quanta of crystal lattice vibrations and can behave themselves like particles in the crystal. When electrons in the avalanche process are accelerated by the electric "eld in an APD, their average kinetic energy is limited by collisions with phonons. Thus, in an environment with fewer phonons, the avalanche electrons can have higher energy to enhance the avalanche gain. As a result of these e!ects at low temperature, the signal-to-noise ratio is expected to improve [10].
349
Fig. 1. A cross-section of the scintillating-"ber array. Dashed lines indicate segment boundaries.
The aforementioned 197-70-72-520 APD was chosen because of its spectral response suited for wavelengths below 600 nm.1 Quantum e$ciency over 70% is guaranteed in the spectral range from 500 to 600 nm where typical 3HF-doped scintillators have emission spectra. To evaluate the performance of this APD as a photosensor for scintillating "bers, we built a prototype position-
sensitive charged-particle detector using multiclad 3HF-doped scintillating "bers (Kuraray SCSF3HF(1500)M, non-S type [9]).2 The core of this "ber was made of polystyrene doped with 1.0% of p-terphenyl primary #uor and 1500 ppm of 3HF (3-hydroxy#avone). A "ber was 0.50 mm in outer diameter, 0.44 mm in core diameter, and 55 cm in length. Each "ber was coated with white EMA paint to suppress cross talk between "bers. As shown in Fig. 1, we arranged 48 "bers in an array with an e!ective thickness of two "bers for incident particles [10]. The horizontal distance between centers of two adjacent "bers was 0.40 mm, 0.04 mm of which was occupied by a region where the scintillating cores were staggered to eliminate dead space at the boundary between the "bers. The array was subdivided into six segments 1.6 mm wide each. Each segment containing eight "bers was coupled to an APD. When a particle traverses a segment at right angle to the array, the APD coupled to the segment receives photons from two "bers along the particle trajectory. The maximum thickness of the scintillating cores that the particle crosses within the segment is 0.88 mm and the minimum 0.37 mm at the segment boundary. The temperature of those six APDs was controlled by thermoelectric cooling modules (Peltier devices) in a vacuum vessel as shown in Fig. 2 [10]. The vacuum environment improved the thermal e$ciency and also prevented the APDs from being
1 The Avalanche Photodiode Catalog, Advanced Photonix, Inc., 1240 Avenida Acaso, Camarillo, CA 93012, USA.
2 Kuraray Co., Ltd., Methacrylic Resin Division, 2-3-10 Nihonbashi, Chuo-ku, Tokyo 103-0027, Japan.
2. Experimental arrangements
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Fig. 2. (a) The vacuum vessel in which APD temperature is controlled; (b) a sketch of the copper APD-holder mounted on the cooling modules; (c) a sketch of the 197-70-72-520 APD.
frosted. In the vessel, the APDs were kept in a copper APD-holder that was directly mounted on the cooling modules. The cooling modules absorbed heat from the APDs through the holder and released it to the water-cooled radiator at the bottom of the vessel. The APD temperature could be lowered to !303C with Komatsu KSM-06031A cooling modules.3 The "bers were led into the vessel segment by segment through a hole in an acrylic window of the vessel. The holes were then hermetically sealed by epoxy glue. In the vessel, the end of each segment was directly coupled onto the 5 mm diameter active surface of an APD through an acrylic ferrule. Optical grease was used for better optical contact between the APDs and the polished surfaces of the "ber ends. The other end of the "ber array was terminated by a sheet of aluminum foil to re#ect the photons; its re#ectivity was measured to be 80%. Performance of this prototype detector was evaluated using 90Sr b-particles traversing the "ber array. The "ber array was sandwiched between two slit-type collimators to localize the b-ray exposure within 0.5 mm in width; the length of the slit was
3 Komatsu Electronics Inc., 3597 Shinomiya, Hiratsuka-shi, Kanagawa-ken 254-8543, Japan.
10 mm along the "bers. Each collimator was made of brass 5 mm thick. The b-source was placed on top of the upper slit. Underneath the bottom slit were placed two plastic scintillation counters one upon another, the "rst one of which was only 0.5 mm thick. A coincidence of these two counters produced a trigger signal for data acquisition. A GEANT3-based Monte Carlo simulation4 estimates that the triggering b-particles deposit in the "ber cores merely 1.04 times more energy on average than minimum ionizing particles incident uniformly on the array at right angles. In this sense, the b-particles in the triggered events can be regarded as minimum ionizing particles. The APDs were reversely biased by applying positive voltages to their cathodes with the anodes kept at ground. Pulse components of the output charge from each APD #owed through the cathode into a low-noise and high-gain preampli"er (Digitex HIC-1576 [11])5 placed outside the vacuum vessel; DC components were cut by a
4 GEANT-Detector Description and Simulation Tool, CERN Program Library Long Write-up W5013, Application Software Group, CERN-CN Division, CH-1221 Geneva 23, Switzerland. 5 Digitex Laboratory Co., Ltd., 3-3-24 Inokuchi, Mitaka-City, Tokyo 181, Japan.
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decoupling capacitor. The gain of the preampli"er was 30 mV/fC. An output signal of the preampli"er was shaped simply by a CR di!erentiator with a time constant of 165 ns, and was further ampli"ed by a factor of seven by a main ampli"er (Phillips Model 777). An analog output signal of the main ampli"er was digitized by a CAMAC peak-sensing ADC (LeCroy 2259B). A gate signal for the ADC was generated by a coincidence of the two trigger counters mentioned above.
3. Basic properties of the APDs at low temperature An APD is usually operated near the breakdown in order to make use of its large avalanche gain. Though the bias voltage approaching the breakdown threshold enhances the avalanche gain, the shot noise also increases owing to the dark current rising rapidly toward the breakdown. Therefore, the bias voltage has to be carefully adjusted within a narrow range slightly below the breakdown threshold to make the gain large enough with the dark current kept reasonably low. Before evaluating the performance of the prototype detector, we measured some basic properties of each APD such as the dark current and the gain as a function of bias voltage and temperature. For this measurement, we replaced the scintillating"ber array with a clear "ber through which we could illuminate the APD with continuous green LED light. An electrometer was inserted to the anode of the APD to measure its output current. First, we measured the dark current with the LED turned o!. Next, we turned on the LED with constant brightness and measured the photocurrent generated in addition to the dark current. The gain at a given bias voltage was de"ned as a relative photocurrent normalized to 1.0 at 100 V. Fig. 3 shows the results obtained with the APD used at the 5th segment (Segment-5). The gain is almost #at at bias voltages below 1000 V, independent of the temperature. This implies that no noticeable avalanche occurs in this #at region and also that the quantum e$ciency varies little as a function of bias voltage [12]. Thus, the gain that we de"ned can be regarded approximately as the avalanche gain.
Fig. 3. Temperature and bias-voltage dependence of (a) the dark current and (b) the gain of the Segment-5 APD.
As shown in Fig. 3a, the onset of the breakdown becomes sharper at lower temperature. At !303C, this APD suddenly breaks down when the bias voltage exceeds 2300 V. As the APD temperature falls from #183C to !303C with a bias voltage kept at 2300 V, the gain increases by a factor of 15 with the dark current decreasing to 1 . 20 Fig. 4 shows the dark current of each APD at !303C as a function of bias voltage near the breakdown threshold. The APDs at Segment-3, -4, and -5 have similar properties since they were produced in a batch. The APDs at Segment-1 and -6 were produced in another batch. The Segment-2 APD was produced alone. If those six APDs are operated with a common bias voltage, the Segment-3 APD which has the lowest breakdown threshold limits the bias voltage to 2290 V. In this case, APDs which have higher breakdown thresholds, such as the ones at Segment-1, -2, and -6, tend to have smaller gains. For instance, the gain at Segment-1 drops to 1 of that at Segment-3 with a common bias 4 voltage of 2290 V, though the breakdown threshold
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Fig. 4. The dark current of each APD at !303C as a function of bias voltage near the breakdown threshold. The dashed line indicates 2290 V.
at Segment-1 is only a few per cent higher than that at Segment-3.
4. Performance of the prototype detector In the manner mentioned in Section 2, we irradiated the prototype detector with 90Sr b-particles through the collimators, and measured the APD signals. The distance from an irradiated position to the corresponding APD was 50 cm along the "bers. Some examples of the pulse-height spectra of the preampli"er output signals are presented in Fig. 5. They were obtained from the Segment-5 APD at #183C, !23C, and !303C, respectively with the collimators placed near the center of Segment-5. The bias voltage was 2290 V for all the temperatures. The noise spectra measured with random triggers without the b-source are also presented for comparison. Signals become larger as the avalanche gain increases at lower temperature, and the separation between signals and noises is substantially improved. The average number of photons that the APD received was estimated to be 35/event, based on the pulse-height spectra as shown in Fig. 5, the measured avalanche gain, and the quantum e$ciency of 80% given by the manufacturer for a wavelength range from 500 to 600 nm. We measured such spectra as shown in Fig. 5 for each of the six segments and extracted the parameters characterizing the performance of the detector. We summarize temperature dependence of those parameters in Fig. 6. The bias voltage was
Fig. 5. Hatched histograms are pulse height spectra of preampli"er output signals obtained with b-particles incident on Segment-5. The APD was operated with a bias voltage of 2290 V and at temperatures as indicated. Shaded histograms are noise spectra.
2290 V for all the APDs at any temperature. The average signal shown in Fig. 6a is de"ned as an average pulse height of the preampli"er output signals obtained with b-particles. The noise shown in Fig. 6b is de"ned as a standard deviation of the Gaussian distribution that "ts the noise spectrum. Fig. 6c shows the signal-to-noise (S/N) ratio de"ned as a ratio of the average signal given in Fig. 6a to the noise given in Fig. 6b. The detection e$ciency shown in Fig. 6d is de"ned as a rate of signals bigger than the threshold which reduces the noise count rate to 0.5% at each segment. Here again, APDs produced in a batch tend to behave alike. The average signals at Segment-3, -4, and -5 are relatively large as expected from the bias voltage closer to their breakdown thresholds. The S/N ratio improves by more than an order of magnitude as the temperature falls from #183C to !303C with the bias voltage kept constant. This improvement is mainly due to the average signal enhanced exponentially, though the gradual noise reduction helps it a little. Taking into account the fact that the preampli"er itself generates noises of about 1 mV (r.m.s.) for the APD junction
T. Okusawa et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 348}354
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Fig. 7. Detection e$ciency as a function of the collimator position. Open marks indicate segment-by-segment e$ciency; closed circles indicate total e$ciency. Dashed lines indicate segment boundaries.
Fig. 6. Temperature dependence of (a) the average signal, (b) the noise, (c) the signal-to-noise ratio, and (d) the detection e$ciency for each segment. The bias voltage was 2290 V for all the APDs at any temperature. See the text for details.
capacitance of 15}20 pF, no steeper noise reduction could be expected. The detection e$ciency exceeds 98% at all the segments as the APD temperature decreases. Such high e$ciency can be achieved when the S/N ratio is 30. To evaluate the uniformity of the detection e$ciency across the "ber array, we irradiated the array through the collimators moved with 0.4 mm steps. In this measurement, the APDs were kept at !303C. We lowered the bias voltage to 2240 V for the APDs at Segment-3, -4, and -5; the other APDs were operated with 2290 V as before. By this adjustment, all the APDs had S/N ratios between 30 and 50. The total e$ciency as well as the e$ciency of each segment is shown in Fig. 7 as a function of the collimator position. When the collimators are placed near a segment boundary, the e$ciency is split between the two adjacent segments, since the collimators are 0.5 mm in width. The total e$ciency is larger than 98% at almost all the collimator positions except for the edges of the array. Though an S/N ratio of 30 is su$cient to obtain high detection e$ciency over 98%, we can enhance
the S/N ratio up to 90 at each segment by cooling the APD to !303C and raising the bias voltage as close as possible to its breakdown threshold, as is the case with APDs at Segment-3, -4, and -5 in Fig. 6. This implies that the detection e$ciency is still kept high enough even with a smaller amount of light than our prototype detector yields. The "ber array can probably be a few meters long, taking into account that the attenuation length of the "ber is about 4 m. Making the array thinner without changing the length much will be another option.
5. Summary We have shown that APDs are promising photosensors for scintillating-"ber readout, using a scintillating-"ber array so designed that incident particles traverse two 0.5 mm diameter "bers 55 cm long. The S/N ratio of the APD rises exponentially as the operating temperature falls. Owing to this favorable e!ect, high detection e$ciency over 98% has been achieved for the b-particles equivalent to minimum ionizing particles. When the APD is cooled to !303C, the S/N ratio can reach 90, which is about three times as large as the value su$cient to achieve such high detection e$ciency. The APD we employed for this study is a singlechannel type. Its 5 mm diameter active surface is unnecessarily large, compared with the "ber diameter. To cope with thousands of "bers closely
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lined up in typical scintillating-"ber tracking detectors, our future e!orts will be directed to developing a compact readout system with APD arrays consisting of closely packed APDs as small as the "ber diameter.
Acknowledgements We thank Y. Shimizu from Riken-sya Co., Ltd. for his useful suggestions concerning the cooling system for APDs. This work was supported by the Grant-in-Aid for Scienti"c Research (B) of Japan Society for the Promotion of Science and the Grant-in-Aid for Fundamental Science of the Sumitomo Foundation.
References [1] E. Eskut et al., Nucl. Instr. and Meth. A 401 (1997) 7. [2] J.K. Ahn et al., Phys. Lett. B 378 (1996) 53.
[3] A. Bross et al., SCIFI97: Conference. on Scintillating Fiber Detectors, AIP Conference Proceedings, Vol. 450, 1998, p. 221. [4] V. Agoritsas et al., Nucl. Instr. and Meth. A 411 (1998) 26. [5] B. Adeva et al., Lifetime Measurement of p` p~ Atoms to Test Low Energy QCD Prediction, CERN Proposal, CERN/SPSLC 95-1, SPSLC/p284, December 15, 1994. [6] A. Cardini et al., Nucl. Instr. and Meth. A 361 (1995) 129. [7] M. Atac et al., Nucl. Instr. and Meth. A 314 (1992) 56. [8] B. Abbott et al., Nucl. Instr. and Meth. A 339 (1994) 439. [9] S. Okumura, T. Okusawa, T. Yoshida, Nucl. Instr. and Meth. A 388 (1987) 235. [10] T. Yoshida et al., SCIFI97: Conference on Scintillating Fiber Detectors, AIP Conference Proceedings, Vol. 450, 1998, p. 157. [11] T. Taniguchi, Y. Fukushima, Y. Yoribayashi, KEK Preprint 88-93, December 1988 H. [12] I. Tapan et al., Nucl. Instr. and Meth. A 388 (1997) 79.
Readout of a scintillating-"ber array by avalanche photodiodes T. Okusawa, Y. Sasayama, M. Yamasaki, T. Yoshida* Department of Physics, Faculty of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Received 21 June 1999; accepted 18 August 1999
Abstract We have evaluated the performance of avalanche photodiodes (APDs) as photosensors for scintillating-"ber tracking detectors, putting emphasis on their temperature dependence. For this purpose, a scintillating-"ber array has been built with 0.5 mm diameter "bers 55 cm long. The array is so structured that an incident particle traverses two "bers coupled to an APD. As temperature of the APD falls, the signal-to-noise (S/N) ratio rises exponentially. It even reaches 90 at !303C. The detection e$ciency exceeds 98% when the temperature decreases until the S/N ratio becomes 30. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 29.40.Gx; 29.40.Mc; 42.81.Cn; 85.60.Dw Keywords: Scintillating "ber; Avalanche photodiode
1. Introduction Scintillating-"ber tracking detectors are coming into practical use in some particle-physics experiments. They have some advantages over conventional tracking detectors such as drift chambers. Sub-millimeter diameters of typical scintillating "bers improve multi-track resolution [1}3]. The short #uorescence decay time of a few nano-seconds makes it possible to incorporate track information into fast trigger logic [4,5]. When a minimum ionizing particle traverses a scintillating "ber, the number of photons expected at the photosensor coupled to the "ber is typically as small as 10}20 [6]. To cope with such a small * Corresponding author. Tel.: #81-6-6605-2646; fax: #816-6605-2522. E-mail address: [email protected] (T. Yoshida)
amount of light, well-established high-gain photosensors such as position-sensitive photomultipliers and image intensi"ers are often chosen, but their relatively small quantum e$ciency around 25% inevitably requires increase in the number of "bers along a particle trajectory in order to guarantee good tracking e$ciency. On the other hand, visible light photon counters (VLPCs) have high quantum e$ciency over 65% and moderately high gain over 20 000 [7,8]. The VLPC ensures e$cient readout of each individual "ber. However, one often hesitates to use it because of its optimum operating temperature around 6.5 K. Thus, to pursue more convenient photosensors is still an important subject in order to re"ne the scintillating-"ber tracking detectors. We have been testing silicon avalanche photodiodes (APDs) as photosensors for scintillating "bers, counting on their high quantum e$ciency of 70}90%, in spite of their small gain around 100 at
0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 9 2 5 - 0
T. Okusawa et al. / Nuclear Instruments and Methods in Physics Research A 440 (2000) 348}354
room temperature. In our previous paper [9], we gave our test results obtained with 0.5 mm diameter scintillating "bers and Advanced Photonix 197-7072-520 APDs operated at room temperature. To achieve neat separation of signal from noise, an APD had to be fed photons by at least four "bers along a particle trajectory. In this paper we report on our further attempt to improve the performance of those APDs by lowering their operating temperature, emphasizing how much the signal-to-noise ratio and the detection e$ciency have been improved even with a smaller number of "bers. Lower temperature reduces the number of electrons that thermally di!use from the valence band to the conduction band in the APD. This e!ect depresses the dark current and reduces the shot noise. Furthermore, the drop in temperature enhances the avalanche gain of the APD, because the lower temperature depresses thermal lattice vibrations of the silicon crystal and decreases the number of phonons that interrupt avalanche electrons in the APD. Phonons are energy quanta of crystal lattice vibrations and can behave themselves like particles in the crystal. When electrons in the avalanche process are accelerated by the electric "eld in an APD, their average kinetic energy is limited by collisions with phonons. Thus, in an environment with fewer phonons, the avalanche electrons can have higher energy to enhance the avalanche gain. As a result of these e!ects at low temperature, the signal-to-noise ratio is expected to improve [10].
349
Fig. 1. A cross-section of the scintillating-"ber array. Dashed lines indicate segment boundaries.
The aforementioned 197-70-72-520 APD was chosen because of its spectral response suited for wavelengths below 600 nm.1 Quantum e$ciency over 70% is guaranteed in the spectral range from 500 to 600 nm where typical 3HF-doped scintillators have emission spectra. To evaluate the performance of this APD as a photosensor for scintillating "bers, we built a prototype position-
sensitive charged-particle detector using multiclad 3HF-doped scintillating "bers (Kuraray SCSF3HF(1500)M, non-S type [9]).2 The core of this "ber was made of polystyrene doped with 1.0% of p-terphenyl primary #uor and 1500 ppm of 3HF (3-hydroxy#avone). A "ber was 0.50 mm in outer diameter, 0.44 mm in core diameter, and 55 cm in length. Each "ber was coated with white EMA paint to suppress cross talk between "bers. As shown in Fig. 1, we arranged 48 "bers in an array with an e!ective thickness of two "bers for incident particles [10]. The horizontal distance between centers of two adjacent "bers was 0.40 mm, 0.04 mm of which was occupied by a region where the scintillating cores were staggered to eliminate dead space at the boundary between the "bers. The array was subdivided into six segments 1.6 mm wide each. Each segment containing eight "bers was coupled to an APD. When a particle traverses a segment at right angle to the array, the APD coupled to the segment receives photons from two "bers along the particle trajectory. The maximum thickness of the scintillating cores that the particle crosses within the segment is 0.88 mm and the minimum 0.37 mm at the segment boundary. The temperature of those six APDs was controlled by thermoelectric cooling modules (Peltier devices) in a vacuum vessel as shown in Fig. 2 [10]. The vacuum environment improved the thermal e$ciency and also prevented the APDs from being
1 The Avalanche Photodiode Catalog, Advanced Photonix, Inc., 1240 Avenida Acaso, Camarillo, CA 93012, USA.
2 Kuraray Co., Ltd., Methacrylic Resin Division, 2-3-10 Nihonbashi, Chuo-ku, Tokyo 103-0027, Japan.
2. Experimental arrangements
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Fig. 2. (a) The vacuum vessel in which APD temperature is controlled; (b) a sketch of the copper APD-holder mounted on the cooling modules; (c) a sketch of the 197-70-72-520 APD.
frosted. In the vessel, the APDs were kept in a copper APD-holder that was directly mounted on the cooling modules. The cooling modules absorbed heat from the APDs through the holder and released it to the water-cooled radiator at the bottom of the vessel. The APD temperature could be lowered to !303C with Komatsu KSM-06031A cooling modules.3 The "bers were led into the vessel segment by segment through a hole in an acrylic window of the vessel. The holes were then hermetically sealed by epoxy glue. In the vessel, the end of each segment was directly coupled onto the 5 mm diameter active surface of an APD through an acrylic ferrule. Optical grease was used for better optical contact between the APDs and the polished surfaces of the "ber ends. The other end of the "ber array was terminated by a sheet of aluminum foil to re#ect the photons; its re#ectivity was measured to be 80%. Performance of this prototype detector was evaluated using 90Sr b-particles traversing the "ber array. The "ber array was sandwiched between two slit-type collimators to localize the b-ray exposure within 0.5 mm in width; the length of the slit was
3 Komatsu Electronics Inc., 3597 Shinomiya, Hiratsuka-shi, Kanagawa-ken 254-8543, Japan.
10 mm along the "bers. Each collimator was made of brass 5 mm thick. The b-source was placed on top of the upper slit. Underneath the bottom slit were placed two plastic scintillation counters one upon another, the "rst one of which was only 0.5 mm thick. A coincidence of these two counters produced a trigger signal for data acquisition. A GEANT3-based Monte Carlo simulation4 estimates that the triggering b-particles deposit in the "ber cores merely 1.04 times more energy on average than minimum ionizing particles incident uniformly on the array at right angles. In this sense, the b-particles in the triggered events can be regarded as minimum ionizing particles. The APDs were reversely biased by applying positive voltages to their cathodes with the anodes kept at ground. Pulse components of the output charge from each APD #owed through the cathode into a low-noise and high-gain preampli"er (Digitex HIC-1576 [11])5 placed outside the vacuum vessel; DC components were cut by a
4 GEANT-Detector Description and Simulation Tool, CERN Program Library Long Write-up W5013, Application Software Group, CERN-CN Division, CH-1221 Geneva 23, Switzerland. 5 Digitex Laboratory Co., Ltd., 3-3-24 Inokuchi, Mitaka-City, Tokyo 181, Japan.
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decoupling capacitor. The gain of the preampli"er was 30 mV/fC. An output signal of the preampli"er was shaped simply by a CR di!erentiator with a time constant of 165 ns, and was further ampli"ed by a factor of seven by a main ampli"er (Phillips Model 777). An analog output signal of the main ampli"er was digitized by a CAMAC peak-sensing ADC (LeCroy 2259B). A gate signal for the ADC was generated by a coincidence of the two trigger counters mentioned above.
3. Basic properties of the APDs at low temperature An APD is usually operated near the breakdown in order to make use of its large avalanche gain. Though the bias voltage approaching the breakdown threshold enhances the avalanche gain, the shot noise also increases owing to the dark current rising rapidly toward the breakdown. Therefore, the bias voltage has to be carefully adjusted within a narrow range slightly below the breakdown threshold to make the gain large enough with the dark current kept reasonably low. Before evaluating the performance of the prototype detector, we measured some basic properties of each APD such as the dark current and the gain as a function of bias voltage and temperature. For this measurement, we replaced the scintillating"ber array with a clear "ber through which we could illuminate the APD with continuous green LED light. An electrometer was inserted to the anode of the APD to measure its output current. First, we measured the dark current with the LED turned o!. Next, we turned on the LED with constant brightness and measured the photocurrent generated in addition to the dark current. The gain at a given bias voltage was de"ned as a relative photocurrent normalized to 1.0 at 100 V. Fig. 3 shows the results obtained with the APD used at the 5th segment (Segment-5). The gain is almost #at at bias voltages below 1000 V, independent of the temperature. This implies that no noticeable avalanche occurs in this #at region and also that the quantum e$ciency varies little as a function of bias voltage [12]. Thus, the gain that we de"ned can be regarded approximately as the avalanche gain.
Fig. 3. Temperature and bias-voltage dependence of (a) the dark current and (b) the gain of the Segment-5 APD.
As shown in Fig. 3a, the onset of the breakdown becomes sharper at lower temperature. At !303C, this APD suddenly breaks down when the bias voltage exceeds 2300 V. As the APD temperature falls from #183C to !303C with a bias voltage kept at 2300 V, the gain increases by a factor of 15 with the dark current decreasing to 1 . 20 Fig. 4 shows the dark current of each APD at !303C as a function of bias voltage near the breakdown threshold. The APDs at Segment-3, -4, and -5 have similar properties since they were produced in a batch. The APDs at Segment-1 and -6 were produced in another batch. The Segment-2 APD was produced alone. If those six APDs are operated with a common bias voltage, the Segment-3 APD which has the lowest breakdown threshold limits the bias voltage to 2290 V. In this case, APDs which have higher breakdown thresholds, such as the ones at Segment-1, -2, and -6, tend to have smaller gains. For instance, the gain at Segment-1 drops to 1 of that at Segment-3 with a common bias 4 voltage of 2290 V, though the breakdown threshold
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Fig. 4. The dark current of each APD at !303C as a function of bias voltage near the breakdown threshold. The dashed line indicates 2290 V.
at Segment-1 is only a few per cent higher than that at Segment-3.
4. Performance of the prototype detector In the manner mentioned in Section 2, we irradiated the prototype detector with 90Sr b-particles through the collimators, and measured the APD signals. The distance from an irradiated position to the corresponding APD was 50 cm along the "bers. Some examples of the pulse-height spectra of the preampli"er output signals are presented in Fig. 5. They were obtained from the Segment-5 APD at #183C, !23C, and !303C, respectively with the collimators placed near the center of Segment-5. The bias voltage was 2290 V for all the temperatures. The noise spectra measured with random triggers without the b-source are also presented for comparison. Signals become larger as the avalanche gain increases at lower temperature, and the separation between signals and noises is substantially improved. The average number of photons that the APD received was estimated to be 35/event, based on the pulse-height spectra as shown in Fig. 5, the measured avalanche gain, and the quantum e$ciency of 80% given by the manufacturer for a wavelength range from 500 to 600 nm. We measured such spectra as shown in Fig. 5 for each of the six segments and extracted the parameters characterizing the performance of the detector. We summarize temperature dependence of those parameters in Fig. 6. The bias voltage was
Fig. 5. Hatched histograms are pulse height spectra of preampli"er output signals obtained with b-particles incident on Segment-5. The APD was operated with a bias voltage of 2290 V and at temperatures as indicated. Shaded histograms are noise spectra.
2290 V for all the APDs at any temperature. The average signal shown in Fig. 6a is de"ned as an average pulse height of the preampli"er output signals obtained with b-particles. The noise shown in Fig. 6b is de"ned as a standard deviation of the Gaussian distribution that "ts the noise spectrum. Fig. 6c shows the signal-to-noise (S/N) ratio de"ned as a ratio of the average signal given in Fig. 6a to the noise given in Fig. 6b. The detection e$ciency shown in Fig. 6d is de"ned as a rate of signals bigger than the threshold which reduces the noise count rate to 0.5% at each segment. Here again, APDs produced in a batch tend to behave alike. The average signals at Segment-3, -4, and -5 are relatively large as expected from the bias voltage closer to their breakdown thresholds. The S/N ratio improves by more than an order of magnitude as the temperature falls from #183C to !303C with the bias voltage kept constant. This improvement is mainly due to the average signal enhanced exponentially, though the gradual noise reduction helps it a little. Taking into account the fact that the preampli"er itself generates noises of about 1 mV (r.m.s.) for the APD junction
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Fig. 7. Detection e$ciency as a function of the collimator position. Open marks indicate segment-by-segment e$ciency; closed circles indicate total e$ciency. Dashed lines indicate segment boundaries.
Fig. 6. Temperature dependence of (a) the average signal, (b) the noise, (c) the signal-to-noise ratio, and (d) the detection e$ciency for each segment. The bias voltage was 2290 V for all the APDs at any temperature. See the text for details.
capacitance of 15}20 pF, no steeper noise reduction could be expected. The detection e$ciency exceeds 98% at all the segments as the APD temperature decreases. Such high e$ciency can be achieved when the S/N ratio is 30. To evaluate the uniformity of the detection e$ciency across the "ber array, we irradiated the array through the collimators moved with 0.4 mm steps. In this measurement, the APDs were kept at !303C. We lowered the bias voltage to 2240 V for the APDs at Segment-3, -4, and -5; the other APDs were operated with 2290 V as before. By this adjustment, all the APDs had S/N ratios between 30 and 50. The total e$ciency as well as the e$ciency of each segment is shown in Fig. 7 as a function of the collimator position. When the collimators are placed near a segment boundary, the e$ciency is split between the two adjacent segments, since the collimators are 0.5 mm in width. The total e$ciency is larger than 98% at almost all the collimator positions except for the edges of the array. Though an S/N ratio of 30 is su$cient to obtain high detection e$ciency over 98%, we can enhance
the S/N ratio up to 90 at each segment by cooling the APD to !303C and raising the bias voltage as close as possible to its breakdown threshold, as is the case with APDs at Segment-3, -4, and -5 in Fig. 6. This implies that the detection e$ciency is still kept high enough even with a smaller amount of light than our prototype detector yields. The "ber array can probably be a few meters long, taking into account that the attenuation length of the "ber is about 4 m. Making the array thinner without changing the length much will be another option.
5. Summary We have shown that APDs are promising photosensors for scintillating-"ber readout, using a scintillating-"ber array so designed that incident particles traverse two 0.5 mm diameter "bers 55 cm long. The S/N ratio of the APD rises exponentially as the operating temperature falls. Owing to this favorable e!ect, high detection e$ciency over 98% has been achieved for the b-particles equivalent to minimum ionizing particles. When the APD is cooled to !303C, the S/N ratio can reach 90, which is about three times as large as the value su$cient to achieve such high detection e$ciency. The APD we employed for this study is a singlechannel type. Its 5 mm diameter active surface is unnecessarily large, compared with the "ber diameter. To cope with thousands of "bers closely
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lined up in typical scintillating-"ber tracking detectors, our future e!orts will be directed to developing a compact readout system with APD arrays consisting of closely packed APDs as small as the "ber diameter.
Acknowledgements We thank Y. Shimizu from Riken-sya Co., Ltd. for his useful suggestions concerning the cooling system for APDs. This work was supported by the Grant-in-Aid for Scienti"c Research (B) of Japan Society for the Promotion of Science and the Grant-in-Aid for Fundamental Science of the Sumitomo Foundation.
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