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Readout of a 3 m long scintillating fiber by an avalanche photodiode
Nuclear Instruments and Methods in Physics Research A 459 (2001) 440}447
Readout of a 3 m long scintillating "ber by an avalanche photodiode 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 27 June 2000; accepted 5 August 2000
Abstract We carried out an experiment to evaluate the performance of an avalanche photodiode (APD) as a photosensor for a multiclad 3HF scintillating "ber. The scintillating "ber used was 3 m in length and 0.75 mm in diameter. The temperature of the APD was controlled in the range from !503C to room temperature. The detection e$ciency for a minimum ionizing particle traversing the "ber was measured as a function of bias voltage and temperature of the APD. Cooling the APD was essential to clearly distinguish a minimum ionizing particle from noises to exclude. Detection e$ciency of nearly 100% was achieved by the APD operated at !403C or below. 2001 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 playing important roles in some particle-physics experiments which require multi-track resolution beyond the capability of conventional multiwire gas chambers [1}3]. Well-established high-gain photosensors such as position-sensitive photomultipliers and image intensi"ers are generally used for scintillating "ber readout. However, the relatively low quantum e$ciency (425%) of their photocathodes limits the detection e$ciency per "ber. Consequently, one is forced to increase the number * Corresponding author. Tel.: #81-6-6605-2646; fax: #816-6605-2646. E-mail address: [email protected] (T. Yoshida).
of "bers along a particle trajectory in order to keep the overall tracking e$ciency high. Visible light photon counters (VLPCs), which have high quantum e$ciency near 70% and moderately high gain over 10 000, have been introduced as an improvement in the optoelectronic readout system for scintillating "bers [4,5]. E$cient readout of each individual "ber is feasible with VLPCs. However, one often hesitates to use VLPCs on a large scale, since their optimum operating temperature around 6.5 K requires a cryostat system using liquid helium. We have been testing avalanche photodiodes (APDs) as another optoelectronic solution for scintillating-"ber readout, counting on their high quantum e$ciency comparable to that of VLPCs. The signal-to-noise (S/N) ratio of an APD operated
0168-9002/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 0 4 7 - 0
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We carried out an experiment to evaluate the performance of an APD as a photosensor for light signals produced by minimum ionizing particles traversing a 3 m long scintillating "ber. The schematic diagram of the experimental setup is shown in Fig. 1. The scintillating "ber we used was a Kuraray SCSF-3HF(1500)M(non-S) multiclad 3HF "ber. The polystyrene-based scintillating core contains 1.0% of PTP as the primary #uor and 1500 ppm of 3HF as the secondary #uor. The emission spectrum ranges from 500 to 600 nm, peaking at 530 nm. The #uorescence decay time is 7 ns. It is known that this type of "ber was originally developed for applications in large-scale high-luminosity collider experiments, where high light yields, good optical transmission, and less radiation damage were required [5,9]. The scintillating "ber prepared for our measurement was 0.75 mm in outer diameter, 0.66 mm in core diameter, and 3 m in length. Both ends of the "ber were polished with alumina powder. The APD we chose was a Hamamatsu S5343 silicon APD. It is a short-wavelength enhanced
type and has high quantum e$ciency around 75}80% for the spectral emission of the 3HF "ber. The junction capacitance is 15 pF when fully depleted. The active area of 1 mm in diameter was appropriate for the thin "ber we prepared. The glass window on the standard TO-18 package of the APD was removed so that the "ber end could come closer to the active silicon surface. The temperature of the APD was controlled by a small thermoelectric cooling device (Peltier device). The APD and the cooling device were placed in a small vacuum vessel to prevent the APD from being frosted and also to improve the thermal e$ciency. When an electric current was applied to the cooling device, heat was pumped from the APD to the water-cooled copper plate at the bottom of the vessel. The APD temperature could be lowered to !553C when the maximum allowed current was applied to the cooling device. A tip of the scintillating "ber was led into the vacuum vessel through a small hole in the Lucite window of the vessel. Then the hole was hermetically sealed with epoxy glue. In the vessel, the "ber was coupled to the APD with a 0.2 mm gap between the "ber end and the active surface of the APD. No optical interface material was inserted in the gap. Another end of the "ber was terminated by a mirror made of an aluminized-polyester "lm. The scintillating "ber was excited using a Sr/ Y -source. As shown in Fig. 2, the "ber was "xed between the upper and the lower collimator. The -particles were localized on the "ber through a 0.32 mm slit in those collimators. The length of the slit was 6 mm along the "ber. Underneath the lower collimator two plastic scintillation counters were placed, the "rst one of which was 0.5 mm thick. A coincidence of those two counters produced a trigger signal for data acquisition. The ionization energy loss of a triggering -particle in the "ber core was estimated and compared with that of a minimum ionizing particle by a GEANT3-based Monte}Carlo simulation. The scintillating "ber, the collimators, and the trigger
Kuraray Co., Ltd., Methacrylic Resin Division, 2-3-10 Nihonbashi, Chuo-ku, Tokyo 103-0027, Japan. Hamamatsu Photonics K.K., 1126-1 Ichino-cho, Hamamatsu City 435-8558, Japan.
K2M-05026A, Komatsu Electronics Inc., 2597 Shinomiya, Hiratsuka-shi, Kanagawa-ken 254-8543, Japan. GEANT } Detector Description and Simulation Tool, CERN, CH-1211 Geneva 23, Switzerland.
at room temperature is not su$cient for a thin "ber [6]. In our previous study [7,8], however, cooling the APD from room temperature to !303C enhanced the avalanche gain by more than an order of magnitude without increasing the noise. As a result of such an improvement at low temperature, detection e$ciency of over 98% was achieved with a scintillating-"ber prototype array 55 cm in length and 1 mm in thickness. We are further attempting to read out longer and thinner "bers with APDs. In this paper, we report the results obtained with a 3 m long scintillating "ber. This length of "ber was chosen to have compatibility with today's large-scale collider experiments.
2. Experimental arrangements
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T. Okusawa et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 440}447
Fig. 1. The schematic diagram of the experimental setup.
Fig. 3. Distribution of ionization energy loss in the scintillating-"ber core, calculated by a Monte}Carlo simulation for (a) the triggering -particles and (b) the 560 MeV/c charged pions (" minimum ionizing particles).
Fig. 2. A cross-section of the scintillating "ber "xed between the collimators.
counters were de"ned in the simulation as they were in reality. The simulated physics processes were the ionization energy loss and the multiple scattering. The #uctuations of those processes were also simulated. For the -particles, we assumed a simple energy spectrum calculated only with the phase-space factors and the maximum kinetic energy of 2.28 MeV. Of the -particles generated in
isotropic directions on the upper collimator, only the ones that hit both the trigger counters were accepted as the triggering -particles. The minimum ionizing particles simulated for comparison were 560 MeV/c charged pions going through the 0.32 mm slit at right angles to the "ber. The results are shown in Fig. 3. The di!erence between a triggering -particle and a minimum ionizing particle is only a few per cent on average. A signal from the APD was ampli"ed by a charge-sensitive preampli"er using a Digitex HIC-1576 hybrid preampli"er chip [10] equipped with a simple CR pulse shaper on its output stage. The time constant of the pulse shaper was 165 ns. The gain of the preampli"er measured by injecting test charge was 33.6 mV/fC. The rise time of an
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output pulse of the preampli"er was 50 ns. An output pulse of the preampli"er was further processed by a fast voltage-sensitive ampli"er (Phillips Model 777), and was "nally digitized by a CAMAC peak-sensing ADC (LeCroy 2259B). A gate signal for the ADC was produced by a coincidence of the two trigger counters mentioned above.
3. Basic properties of the APD An APD is usually operated at a reverse bias near the breakdown voltage to make use of its large avalanche gain. The S/N ratio of an APD operated near the breakdown is determined by the avalanche gain and the dark current. Both of them steeply rise as the bias approaches the breakdown voltage. As a result, shot noise by the dark current becomes a dominant noise source, while the photocurrent is multiplied in proportion to the avalanche gain. The shot noise is caused as a result of statistical #uctuations in the dark current. When the avalanche gain of an APD is large enough, the dark current is dominated by the bulk dark current, which is a #ow of electrons that thermally di!use from the valence band to the conduction band in the APD and are multiplied in the same avalanche process as photoelectrons are multiplied. Fluctuations in the number of thermally di!using electrons are also multiplied in the same avalanche process. As the bias approaches the breakdown voltage, the resultant shot noise is usually enhanced more rapidly than the photocurrent. This is because the noise expected only from the #uctuations in the number of thermally di!using electrons is further ampli"ed by the #uctuations in avalanche gain itself [11]. Thus, the S/N ratio peaks at a certain bias voltage before the APD completely breaks down. The only way to improve the S/N ratio is to operate the APD at lower temperature. Lower temperature reduces the number of thermally di!using electrons in the APD. As a result, the bulk dark current and its shot noise can be kept low even though the avalanche gain becomes larger. Moreover, lower temperature reduces the number of phonons that interrupt avalanche electrons in the APD. Therefore the avalanche gain of the APD is enhanced more smoothly at lower temperature.
Fig. 4. (a) Dark current and (b) avalanche gain of the Hamamatsu S5343 APD, as a function of bias voltage. The APD is operated at temperatures as indicated.
We measured the dark current and the avalanche gain of the S5343 APD. The method of measurement is the same as described elsewhere [8]. The results obtained at #283C (room temperature) and at !503C are shown in Fig. 4 as a function of bias voltage. At !503C, the APD suddenly breaks down when the bias voltage exceeds 143.0 V. Therefore, the avalanche gain was measured up to 142.8 V at this temperature. The dark current component that steeply rises beyond 1 nA toward the breakdown is the bulk dark current discussed above. The dark current component which gradually increases below 1 nA is called the surface dark current, which is a sort of leakage current on the outer surface of the APD. The surface dark current is not temperature dependent, since it is not multiplied in the APD. By cooling the APD from room temperature to !503C, the avalanche gain is substantially enhanced without increasing the dark current. For instance, the avalanche gain can reach 2700 at !503C with a dark current kept below 1 nA, while it is only 100 at #283C even though we tolerate
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the dark current up to 2 nA. In our previous estimation [6], the shot noise at room temperature becomes as large as the signals expected from a thin "ber. Therefore, cooling of the APD is essential in order to improve the S/N ratio and to obtain good detection e$ciency.
4. Experimental results 4.1. Overview Using the apparatus described in Section 2, the APD signals produced by the triggering -particles were measured as a function of APD temperature, bias voltage, and distance from the APD to the irradiated position. The measurement was carried out primarily with the aluminized-polyester mirror on the far end of the "ber, while the mirror was sometimes removed for comparison. Some of typical preampli"er output pulses produced by the triggering -particles are shown in Fig. 5, where the pulses of the APD operated at #283C, at !203C, and at !503C are compared. The irradiated position was 2.37 m away from the
Fig. 6. Hatched histograms are pulse-height spectra of the preampli"er output signals produced by the triggering -particles. Shaded histograms are random-noise spectra. The APD temperature, the bias voltage < , the dark current I , and the ava" lanche gain M are as indicated.
APD along the "ber. The far end of the "ber was terminated by the mirror. The pulse-height spectra measured on the same conditions respectively are also shown in Fig. 6 together with noise spectra measured with random triggers. As the temperature falls, the pulses of the -particles are enlarged, while the noise is even reduced. At !503C, the signals of the -particles are clearly distinguished from the noises. 4.2. Photoelectron yields
Fig. 5. Typical preampli"er output pulses produced by the triggering -particles. The APD temperature, the bias voltage < , the dark current I , and the avalanche gain M are as " indicated.
The photoelectron yield is one of fundamental properties of a scintillating-"ber tracking detector. The average number of photoelectrons produced by a triggering -particle was extracted using the average pulse height of the preampli"er output signals, the gain of the preampli"er (33.6 mV/fC), and the avalanche gain of the APD. The APD was operated at !503C and at a bias voltage of 142.0 V during the measurement of the pulse-height spectra. The avalanche gain was 520. The results obtained with and without the mirror are shown in
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Fig. 7. The average number of photoelectrons produced by a triggering -particle versus the irradiated position. The measurements with and without the mirror are presented. The dashed curves are the "tted functions.
Fig. 7 as a function of the irradiated position. The advantage of using the mirror is obvious. When the mirror is used, the average number of photoelectrons varies little around 20 all along the "ber. When no mirror is used, the photoelectron yield is smaller and attenuates along the "ber more steeply. To extract the values such as the attenuation length of the "ber and the re#ectivity of the mirror, the data in Fig. 7 were "tted by a set of functions expressed as
x 2¸!x > (x)"> exp ! #R exp ! + x > (x)"> exp ! ,+
,
,
where > and > are photoelectron yields with + ,+ and without the mirror, respectively, x is the distance from the APD to the irradiated position, > is a common normalization factor, is the attenuation length of the "ber, R is the mirror re#ectivity, and ¸ is the "ber length ("3 m). The "tted values of , R, and > are 6.8$1.2 m, 0.71$0.08, and 17.0$0.8, respectively. The "tted functions are shown by dashed curves in Fig. 7. 4.3. Detection ezciency The detection e$ciency for a triggering -particle was extracted using pulse-height spectra as
Fig. 8. Temperature and bias voltage dependence of (a) the average signal of the triggering -particles, (b) the threshold to exclude 99.5% of random noises, and (c) the detection e$ciency for a triggering -particle. The average signal and the threshold are expressed in terms of preampli"er output pulse height.
already shown in Fig. 6. First, we de"ned in a noise spectrum a threshold by which 99.5% of random noises were excluded. The threshold thus de"ned is temperature and bias voltage dependent, as the noise itself is. The detection e$ciency of the triggering -particles was de"ned in a pulse-height spectrum as the number of pulses higher than the threshold divided by the total number of pulses. The detection e$ciency thus extracted is shown in Fig. 8 as a function of APD temperature and bias voltage, together with the average signal of the triggering -particles and the threshold de"ned above. The average signal and the threshold are expressed in terms of preampli"er output pulse height. The irradiated position was 2.37 m away from the APD. The mirror was used on the far end of the "ber. Cooling the APD can enhance the average signal without raising the threshold. This substantially improves the detection e$ciency. At !403C or
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T. Okusawa et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 440}447
Fig. 9. The maximum detection e$ciency reached at each temperature.
below, the detection e$ciency increases steadily as a function of bias voltage and reaches a plateau beyond 99%. At !203C or above, the threshold begins to rise more steeply than the average signal, before the average signal becomes large enough. As a result, the detection e$ciency declines before it forms a plateau. At !403C or below, no decline in detection e$ciency was observed though we continued the measurement up to a bias voltage only a few tenths of a volt below the breakdown voltage. As the bias voltage decreases, the threshold curves measured at a variety of temperatures converge approximately at 6 mV. The threshold in this region is largely due to the noise proper to our readout electronics including the preampli"er. The contribution from the shot noise in this region is estimated to be small ((1 mV) [6]. The maximum detection e$ciency reached at each temperature was picked out from the measurements given in Fig. 8. Fig. 9 summarizes the maximum detection e$ciency thus obtained. The detection e$ciency reaches 97% at !203C and nearly 100% at !403C or below, though it is only 50% at room temperature. To study the detection e$ciency for a smaller number of photoelectrons, we irradiated the "ber at a position 2.82 m away from the APD, which was nearly the far end of the "ber, and measured the detection e$ciency both with and without the mirror. The APD was operated at !503C. The detection e$ciency measured is shown in Fig. 10 as a function of bias voltage. The detection e$ciency
Fig. 10. Detection e$ciency for a triggering -particle versus bias voltage of the APD. The measurements with and without the mirror are presented. The irradiated position is nearly the far end of the "ber. The temperature of the APD is !503C.
curve measured with the mirror is similar to the one in Fig. 8, because the average number of photoelectrons is almost the same. When no mirror is used, no plateau can be seen. However, the detection e$ciency still reaches 99% at 142.8 V. The average number of photoelectrons produced without the mirror is 11 as shown in Fig. 7.
5. Summary Using a short-wavelength enhanced type APD, we measured the detection e$ciency for a minimum ionizing particle traversing a multiclad 3HF scintillating "ber 3 m in length and 0.75 mm in diameter. The temperature of the APD was controlled in the range from !503C to room temperature. Cooling of the APD was e!ective in separating a minimum ionizing particle clearly from noises to be excluded. As a result, detection e$ciency of nearly 100% was achieved when the APD was operated at !403C or below. In the present measurement, we used only one APD coupled to a scintillating "ber. In reality, however, we have to cope with thousands of "bers closely lined up in scintillating-"ber tracking detectors. While the APD we tested was a singlechannel type, it is technically possible to arrange this type of APDs monolithically in an array. Our future e!orts will be directed towards developing
T. Okusawa et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 440}447
a multi-channel readout system using the APD arrays. Acknowledgements We thank Y. Ishikawa and T. Inuzuka from Hamamatsu Photonics K. K. for their useful advice concerning APDs. This work was supported by the Grant-in-Aid for Scienti"c Research (B) of Japan Society for the Promotion of Science. 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.
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[3] V. Agoritsas et al., Nucl. Instr. and Meth. A 411 (1998) 17. [4] M. Atac et al., Nucl. Instr. and Meth. A 314 (1992) 56. [5] B. Abbott et al., Nucl. Instr. and Meth. A 339 (1994) 439. [6] S. Okumura, T. Okusawa, T. Yoshida, Nucl. Instr. and Meth. A 388 (1997) 235. [7] T. Yoshida et al., SCIFI97: Conference on Scintillating Fiber Detectors, AIP Conference Proceedings Vol. 450, AIP, New York, 1998, p. 157. [8] T. Okusawa, Y. Sasayama, M. Yamasaki, T. Yoshida, Nucl. Instr. and Meth. A 440 (2000) 348. [9] B. Baumbaugh et al., Nucl. Instr. and Meth. A 345 (1994) 271. [10] T. Taniguchi, Y. Fukushima, Y. Yoribayashi, IEEE Trans. Nucl. Sci. NS-36 (1989) 657. [11] J.P. Pansart, Nucl. Instr. and Meth. A 387 (1997) 186.
Readout of a 3 m long scintillating "ber by an avalanche photodiode 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 27 June 2000; accepted 5 August 2000
Abstract We carried out an experiment to evaluate the performance of an avalanche photodiode (APD) as a photosensor for a multiclad 3HF scintillating "ber. The scintillating "ber used was 3 m in length and 0.75 mm in diameter. The temperature of the APD was controlled in the range from !503C to room temperature. The detection e$ciency for a minimum ionizing particle traversing the "ber was measured as a function of bias voltage and temperature of the APD. Cooling the APD was essential to clearly distinguish a minimum ionizing particle from noises to exclude. Detection e$ciency of nearly 100% was achieved by the APD operated at !403C or below. 2001 Elsevier Science B.V. All rights reserved. PACS: 29.40.Gx; 29.40.Mc; 42.81.Cn; 85.60.Dw Keywords: Scintillating "ber; Avalanche photodiode
1. Introduction Scintillating-"ber tracking detectors are playing important roles in some particle-physics experiments which require multi-track resolution beyond the capability of conventional multiwire gas chambers [1}3]. Well-established high-gain photosensors such as position-sensitive photomultipliers and image intensi"ers are generally used for scintillating "ber readout. However, the relatively low quantum e$ciency (425%) of their photocathodes limits the detection e$ciency per "ber. Consequently, one is forced to increase the number * Corresponding author. Tel.: #81-6-6605-2646; fax: #816-6605-2646. E-mail address: [email protected] (T. Yoshida).
of "bers along a particle trajectory in order to keep the overall tracking e$ciency high. Visible light photon counters (VLPCs), which have high quantum e$ciency near 70% and moderately high gain over 10 000, have been introduced as an improvement in the optoelectronic readout system for scintillating "bers [4,5]. E$cient readout of each individual "ber is feasible with VLPCs. However, one often hesitates to use VLPCs on a large scale, since their optimum operating temperature around 6.5 K requires a cryostat system using liquid helium. We have been testing avalanche photodiodes (APDs) as another optoelectronic solution for scintillating-"ber readout, counting on their high quantum e$ciency comparable to that of VLPCs. The signal-to-noise (S/N) ratio of an APD operated
0168-9002/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 0 4 7 - 0
T. Okusawa et al. / Nuclear Instruments and Methods in Physics Research A 459 (2001) 440}447
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We carried out an experiment to evaluate the performance of an APD as a photosensor for light signals produced by minimum ionizing particles traversing a 3 m long scintillating "ber. The schematic diagram of the experimental setup is shown in Fig. 1. The scintillating "ber we used was a Kuraray SCSF-3HF(1500)M(non-S) multiclad 3HF "ber. The polystyrene-based scintillating core contains 1.0% of PTP as the primary #uor and 1500 ppm of 3HF as the secondary #uor. The emission spectrum ranges from 500 to 600 nm, peaking at 530 nm. The #uorescence decay time is 7 ns. It is known that this type of "ber was originally developed for applications in large-scale high-luminosity collider experiments, where high light yields, good optical transmission, and less radiation damage were required [5,9]. The scintillating "ber prepared for our measurement was 0.75 mm in outer diameter, 0.66 mm in core diameter, and 3 m in length. Both ends of the "ber were polished with alumina powder. The APD we chose was a Hamamatsu S5343 silicon APD. It is a short-wavelength enhanced
type and has high quantum e$ciency around 75}80% for the spectral emission of the 3HF "ber. The junction capacitance is 15 pF when fully depleted. The active area of 1 mm in diameter was appropriate for the thin "ber we prepared. The glass window on the standard TO-18 package of the APD was removed so that the "ber end could come closer to the active silicon surface. The temperature of the APD was controlled by a small thermoelectric cooling device (Peltier device). The APD and the cooling device were placed in a small vacuum vessel to prevent the APD from being frosted and also to improve the thermal e$ciency. When an electric current was applied to the cooling device, heat was pumped from the APD to the water-cooled copper plate at the bottom of the vessel. The APD temperature could be lowered to !553C when the maximum allowed current was applied to the cooling device. A tip of the scintillating "ber was led into the vacuum vessel through a small hole in the Lucite window of the vessel. Then the hole was hermetically sealed with epoxy glue. In the vessel, the "ber was coupled to the APD with a 0.2 mm gap between the "ber end and the active surface of the APD. No optical interface material was inserted in the gap. Another end of the "ber was terminated by a mirror made of an aluminized-polyester "lm. The scintillating "ber was excited using a Sr/ Y -source. As shown in Fig. 2, the "ber was "xed between the upper and the lower collimator. The -particles were localized on the "ber through a 0.32 mm slit in those collimators. The length of the slit was 6 mm along the "ber. Underneath the lower collimator two plastic scintillation counters were placed, the "rst one of which was 0.5 mm thick. A coincidence of those two counters produced a trigger signal for data acquisition. The ionization energy loss of a triggering -particle in the "ber core was estimated and compared with that of a minimum ionizing particle by a GEANT3-based Monte}Carlo simulation. The scintillating "ber, the collimators, and the trigger
Kuraray Co., Ltd., Methacrylic Resin Division, 2-3-10 Nihonbashi, Chuo-ku, Tokyo 103-0027, Japan. Hamamatsu Photonics K.K., 1126-1 Ichino-cho, Hamamatsu City 435-8558, Japan.
K2M-05026A, Komatsu Electronics Inc., 2597 Shinomiya, Hiratsuka-shi, Kanagawa-ken 254-8543, Japan. GEANT } Detector Description and Simulation Tool, CERN, CH-1211 Geneva 23, Switzerland.
at room temperature is not su$cient for a thin "ber [6]. In our previous study [7,8], however, cooling the APD from room temperature to !303C enhanced the avalanche gain by more than an order of magnitude without increasing the noise. As a result of such an improvement at low temperature, detection e$ciency of over 98% was achieved with a scintillating-"ber prototype array 55 cm in length and 1 mm in thickness. We are further attempting to read out longer and thinner "bers with APDs. In this paper, we report the results obtained with a 3 m long scintillating "ber. This length of "ber was chosen to have compatibility with today's large-scale collider experiments.
2. Experimental arrangements
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Fig. 1. The schematic diagram of the experimental setup.
Fig. 3. Distribution of ionization energy loss in the scintillating-"ber core, calculated by a Monte}Carlo simulation for (a) the triggering -particles and (b) the 560 MeV/c charged pions (" minimum ionizing particles).
Fig. 2. A cross-section of the scintillating "ber "xed between the collimators.
counters were de"ned in the simulation as they were in reality. The simulated physics processes were the ionization energy loss and the multiple scattering. The #uctuations of those processes were also simulated. For the -particles, we assumed a simple energy spectrum calculated only with the phase-space factors and the maximum kinetic energy of 2.28 MeV. Of the -particles generated in
isotropic directions on the upper collimator, only the ones that hit both the trigger counters were accepted as the triggering -particles. The minimum ionizing particles simulated for comparison were 560 MeV/c charged pions going through the 0.32 mm slit at right angles to the "ber. The results are shown in Fig. 3. The di!erence between a triggering -particle and a minimum ionizing particle is only a few per cent on average. A signal from the APD was ampli"ed by a charge-sensitive preampli"er using a Digitex HIC-1576 hybrid preampli"er chip [10] equipped with a simple CR pulse shaper on its output stage. The time constant of the pulse shaper was 165 ns. The gain of the preampli"er measured by injecting test charge was 33.6 mV/fC. The rise time of an
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output pulse of the preampli"er was 50 ns. An output pulse of the preampli"er was further processed by a fast voltage-sensitive ampli"er (Phillips Model 777), and was "nally digitized by a CAMAC peak-sensing ADC (LeCroy 2259B). A gate signal for the ADC was produced by a coincidence of the two trigger counters mentioned above.
3. Basic properties of the APD An APD is usually operated at a reverse bias near the breakdown voltage to make use of its large avalanche gain. The S/N ratio of an APD operated near the breakdown is determined by the avalanche gain and the dark current. Both of them steeply rise as the bias approaches the breakdown voltage. As a result, shot noise by the dark current becomes a dominant noise source, while the photocurrent is multiplied in proportion to the avalanche gain. The shot noise is caused as a result of statistical #uctuations in the dark current. When the avalanche gain of an APD is large enough, the dark current is dominated by the bulk dark current, which is a #ow of electrons that thermally di!use from the valence band to the conduction band in the APD and are multiplied in the same avalanche process as photoelectrons are multiplied. Fluctuations in the number of thermally di!using electrons are also multiplied in the same avalanche process. As the bias approaches the breakdown voltage, the resultant shot noise is usually enhanced more rapidly than the photocurrent. This is because the noise expected only from the #uctuations in the number of thermally di!using electrons is further ampli"ed by the #uctuations in avalanche gain itself [11]. Thus, the S/N ratio peaks at a certain bias voltage before the APD completely breaks down. The only way to improve the S/N ratio is to operate the APD at lower temperature. Lower temperature reduces the number of thermally di!using electrons in the APD. As a result, the bulk dark current and its shot noise can be kept low even though the avalanche gain becomes larger. Moreover, lower temperature reduces the number of phonons that interrupt avalanche electrons in the APD. Therefore the avalanche gain of the APD is enhanced more smoothly at lower temperature.
Fig. 4. (a) Dark current and (b) avalanche gain of the Hamamatsu S5343 APD, as a function of bias voltage. The APD is operated at temperatures as indicated.
We measured the dark current and the avalanche gain of the S5343 APD. The method of measurement is the same as described elsewhere [8]. The results obtained at #283C (room temperature) and at !503C are shown in Fig. 4 as a function of bias voltage. At !503C, the APD suddenly breaks down when the bias voltage exceeds 143.0 V. Therefore, the avalanche gain was measured up to 142.8 V at this temperature. The dark current component that steeply rises beyond 1 nA toward the breakdown is the bulk dark current discussed above. The dark current component which gradually increases below 1 nA is called the surface dark current, which is a sort of leakage current on the outer surface of the APD. The surface dark current is not temperature dependent, since it is not multiplied in the APD. By cooling the APD from room temperature to !503C, the avalanche gain is substantially enhanced without increasing the dark current. For instance, the avalanche gain can reach 2700 at !503C with a dark current kept below 1 nA, while it is only 100 at #283C even though we tolerate
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the dark current up to 2 nA. In our previous estimation [6], the shot noise at room temperature becomes as large as the signals expected from a thin "ber. Therefore, cooling of the APD is essential in order to improve the S/N ratio and to obtain good detection e$ciency.
4. Experimental results 4.1. Overview Using the apparatus described in Section 2, the APD signals produced by the triggering -particles were measured as a function of APD temperature, bias voltage, and distance from the APD to the irradiated position. The measurement was carried out primarily with the aluminized-polyester mirror on the far end of the "ber, while the mirror was sometimes removed for comparison. Some of typical preampli"er output pulses produced by the triggering -particles are shown in Fig. 5, where the pulses of the APD operated at #283C, at !203C, and at !503C are compared. The irradiated position was 2.37 m away from the
Fig. 6. Hatched histograms are pulse-height spectra of the preampli"er output signals produced by the triggering -particles. Shaded histograms are random-noise spectra. The APD temperature, the bias voltage < , the dark current I , and the ava" lanche gain M are as indicated.
APD along the "ber. The far end of the "ber was terminated by the mirror. The pulse-height spectra measured on the same conditions respectively are also shown in Fig. 6 together with noise spectra measured with random triggers. As the temperature falls, the pulses of the -particles are enlarged, while the noise is even reduced. At !503C, the signals of the -particles are clearly distinguished from the noises. 4.2. Photoelectron yields
Fig. 5. Typical preampli"er output pulses produced by the triggering -particles. The APD temperature, the bias voltage < , the dark current I , and the avalanche gain M are as " indicated.
The photoelectron yield is one of fundamental properties of a scintillating-"ber tracking detector. The average number of photoelectrons produced by a triggering -particle was extracted using the average pulse height of the preampli"er output signals, the gain of the preampli"er (33.6 mV/fC), and the avalanche gain of the APD. The APD was operated at !503C and at a bias voltage of 142.0 V during the measurement of the pulse-height spectra. The avalanche gain was 520. The results obtained with and without the mirror are shown in
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Fig. 7. The average number of photoelectrons produced by a triggering -particle versus the irradiated position. The measurements with and without the mirror are presented. The dashed curves are the "tted functions.
Fig. 7 as a function of the irradiated position. The advantage of using the mirror is obvious. When the mirror is used, the average number of photoelectrons varies little around 20 all along the "ber. When no mirror is used, the photoelectron yield is smaller and attenuates along the "ber more steeply. To extract the values such as the attenuation length of the "ber and the re#ectivity of the mirror, the data in Fig. 7 were "tted by a set of functions expressed as
x 2¸!x > (x)"> exp ! #R exp ! + x > (x)"> exp ! ,+
,
,
where > and > are photoelectron yields with + ,+ and without the mirror, respectively, x is the distance from the APD to the irradiated position, > is a common normalization factor, is the attenuation length of the "ber, R is the mirror re#ectivity, and ¸ is the "ber length ("3 m). The "tted values of , R, and > are 6.8$1.2 m, 0.71$0.08, and 17.0$0.8, respectively. The "tted functions are shown by dashed curves in Fig. 7. 4.3. Detection ezciency The detection e$ciency for a triggering -particle was extracted using pulse-height spectra as
Fig. 8. Temperature and bias voltage dependence of (a) the average signal of the triggering -particles, (b) the threshold to exclude 99.5% of random noises, and (c) the detection e$ciency for a triggering -particle. The average signal and the threshold are expressed in terms of preampli"er output pulse height.
already shown in Fig. 6. First, we de"ned in a noise spectrum a threshold by which 99.5% of random noises were excluded. The threshold thus de"ned is temperature and bias voltage dependent, as the noise itself is. The detection e$ciency of the triggering -particles was de"ned in a pulse-height spectrum as the number of pulses higher than the threshold divided by the total number of pulses. The detection e$ciency thus extracted is shown in Fig. 8 as a function of APD temperature and bias voltage, together with the average signal of the triggering -particles and the threshold de"ned above. The average signal and the threshold are expressed in terms of preampli"er output pulse height. The irradiated position was 2.37 m away from the APD. The mirror was used on the far end of the "ber. Cooling the APD can enhance the average signal without raising the threshold. This substantially improves the detection e$ciency. At !403C or
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Fig. 9. The maximum detection e$ciency reached at each temperature.
below, the detection e$ciency increases steadily as a function of bias voltage and reaches a plateau beyond 99%. At !203C or above, the threshold begins to rise more steeply than the average signal, before the average signal becomes large enough. As a result, the detection e$ciency declines before it forms a plateau. At !403C or below, no decline in detection e$ciency was observed though we continued the measurement up to a bias voltage only a few tenths of a volt below the breakdown voltage. As the bias voltage decreases, the threshold curves measured at a variety of temperatures converge approximately at 6 mV. The threshold in this region is largely due to the noise proper to our readout electronics including the preampli"er. The contribution from the shot noise in this region is estimated to be small ((1 mV) [6]. The maximum detection e$ciency reached at each temperature was picked out from the measurements given in Fig. 8. Fig. 9 summarizes the maximum detection e$ciency thus obtained. The detection e$ciency reaches 97% at !203C and nearly 100% at !403C or below, though it is only 50% at room temperature. To study the detection e$ciency for a smaller number of photoelectrons, we irradiated the "ber at a position 2.82 m away from the APD, which was nearly the far end of the "ber, and measured the detection e$ciency both with and without the mirror. The APD was operated at !503C. The detection e$ciency measured is shown in Fig. 10 as a function of bias voltage. The detection e$ciency
Fig. 10. Detection e$ciency for a triggering -particle versus bias voltage of the APD. The measurements with and without the mirror are presented. The irradiated position is nearly the far end of the "ber. The temperature of the APD is !503C.
curve measured with the mirror is similar to the one in Fig. 8, because the average number of photoelectrons is almost the same. When no mirror is used, no plateau can be seen. However, the detection e$ciency still reaches 99% at 142.8 V. The average number of photoelectrons produced without the mirror is 11 as shown in Fig. 7.
5. Summary Using a short-wavelength enhanced type APD, we measured the detection e$ciency for a minimum ionizing particle traversing a multiclad 3HF scintillating "ber 3 m in length and 0.75 mm in diameter. The temperature of the APD was controlled in the range from !503C to room temperature. Cooling of the APD was e!ective in separating a minimum ionizing particle clearly from noises to be excluded. As a result, detection e$ciency of nearly 100% was achieved when the APD was operated at !403C or below. In the present measurement, we used only one APD coupled to a scintillating "ber. In reality, however, we have to cope with thousands of "bers closely lined up in scintillating-"ber tracking detectors. While the APD we tested was a singlechannel type, it is technically possible to arrange this type of APDs monolithically in an array. Our future e!orts will be directed towards developing
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a multi-channel readout system using the APD arrays. Acknowledgements We thank Y. Ishikawa and T. Inuzuka from Hamamatsu Photonics K. K. for their useful advice concerning APDs. This work was supported by the Grant-in-Aid for Scienti"c Research (B) of Japan Society for the Promotion of Science. 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.
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[3] V. Agoritsas et al., Nucl. Instr. and Meth. A 411 (1998) 17. [4] M. Atac et al., Nucl. Instr. and Meth. A 314 (1992) 56. [5] B. Abbott et al., Nucl. Instr. and Meth. A 339 (1994) 439. [6] S. Okumura, T. Okusawa, T. Yoshida, Nucl. Instr. and Meth. A 388 (1997) 235. [7] T. Yoshida et al., SCIFI97: Conference on Scintillating Fiber Detectors, AIP Conference Proceedings Vol. 450, AIP, New York, 1998, p. 157. [8] T. Okusawa, Y. Sasayama, M. Yamasaki, T. Yoshida, Nucl. Instr. and Meth. A 440 (2000) 348. [9] B. Baumbaugh et al., Nucl. Instr. and Meth. A 345 (1994) 271. [10] T. Taniguchi, Y. Fukushima, Y. Yoribayashi, IEEE Trans. Nucl. Sci. NS-36 (1989) 657. [11] J.P. Pansart, Nucl. Instr. and Meth. A 387 (1997) 186.