in Physics Research A 388 (1997) 2355240
NUCLEAR INSTRUMENTS &METHoos IN PHYSlcS RESEARCH Sectton A
Readout of scintillating fibers by avalanche photodiodes in the normal avalanche mode
S. Okumura, T. Okusawa, T. Yoshida* Department
of Ph.vsics. Osaka [email protected]
University, 3-3-138 Sugimoto, Sumiyoshi-ku, Received
Osaka 558, Japan
Abstract We have evaluated the performance of avalanche photodiodes (APDs) as the photon detectors for scintillating fibers. The signal-to-noise ratios were estimated and experimentally measured with commercially available APDs coupled to 3HF scintillating fibers. Following those basic studies, we have built a prototype of a scintillating fiber hodoscope read out by the APDs. The performance of this prototype is also described. Throughout this work, we considered only the APDs operated in the normal avalanche mode avoiding the problematic Geiger mode operation. In the measurements, APDs were operated at room temperature without using any cooling system.
1. Introduction The plastic scintillating fiber is one of the promising materials for charged-particle tracking devices. The small fiber diameter around 0.5 mm reduces the occupancy rate. The fluorescence decay time as short as a few nanoseconds makes high rate operation possible. The most crucial technique to make practical use of scintillating fibers for tracking devices is the photon-detection method. A fiber array usually contains thousands of fibers. and the number of photons from each fiber can be as small as lo-20 depending on the fiber geometry. Thus, we need compact photon detectors with high quantum efficiency to read out each individual fiber efficiently. So far, some chose well-established photon detectors such as position-sensitive photomultipliers [l] and image intensifiers [Z], and used them successfully in their experiments. Relatively small quantum efficiencies below 25% of these photon detectors make it inevitable to sample one per several fibers along a particle trajectory or to read out several fibers along a trajectory with one photon-detection unit. There are also other groups who pursue the use of semiconductor photon detectors such as visible light photon counters (VLPCs)  and avalanche photodiodes (APDs) , counting on their high quantum efficiencies around 60-80%. Among these semiconductor detectors, VLPCs have the largest gains above 2 x 104. * Corresponding
author. Tel. and fax: + 81 6 605 2646;
e-mail: [email protected]
0168-9002/97/$17.00 Copyright PII SO168-9002(97)00073-9
But they have to be cryogenically cooled down to a temperature around 6.5 K using liquid helium vapour in order to achieve a good signal-to-noise ratio. In this paper we discuss the performance of APDs as the photon detectors for scintillating fibers, putting a special emphasis on the signal-to-noise ratio. APDs operate at temperatures between approximately - 40°C and + 50°C including room temperatures. Lower temperatures are preferred for the noise reduction, but no complicated cryogenic cooling system is necessary. For detection of extremely weak light pulses, APDs are often operated in the Geiger mode with reverse bias voltage exceeding their breakdown voltage. Although the Geiger mode operation provides a large gain up to lo’, the long dead time and the high sensitivity to the radiation damage restrict its application . On the other hand, although gains of the APDs operated in the normal avalanche mode below their breakdown voltage are typically as small as 100-300, some low-noise types are getting commercially available nowadays. Throughout this work, we considered only the avalanche mode operation instead of the problematic Geiger mode operation.
2. Consideration on the signal-to-noise 2.1.
of the APD
The maximum quantum efficiency of a standard APD is typically about 80% at an infrared wavelength near 800 nm, but the quantum efficiency decreases toward
1997 Elsevier Science B.V. All rights reserved
S. Okrtmttru rt al.
N~cl. Insn. and M&h. in Phw Rex A 388 (1997) -735-240
shorter wavelengths. It is typically [email protected]
% at 500 run. However, there are some special types of APDs which still keep large quantum efficiency over 70% in the spectral range down to 500 nm. In these APDs, the thickness of the antireflection coating on the silicon surface is adjusted to extend the spectral response to the shorter wavelengths. The spectral characteristics of these special APDs lead us to choose the 3HF-doped green scintillating fiber among many different types of fibers ranging from blue to green. The 3HF-doped scintillators  have an emission spectrum ranging from 500 to 600nm. The emission maximum is at 530 nm. This emission spectrum is still within the region of 70&X0% quantum efficiency of some A PDs. The number of photons expected at the end of the fiber depends on some geometrical and optical conditions such as the fiber diameter, the light attenuation in the fiber, the light transmittance through the optical coupling at the fiber end. the mirror reflectivity expected on another end of the fiber, and so on. From the measurements by some authors [3,6]. we can expect at least IO--20 photons for typical 3HF fibers commercially available. 70-80% of these photons produce photoelectrons in the APD.
Fig. 1. Dark current measured for a 197-70-72-520APD as a function of reverse bias voltage.
lower voltages. and the excess current over this extrapolated value is MI,,. The equivalent noise charge generated by the Johnson noise of the preamplifier resistance, ENCj (rms), is given
2.?. Estiruation of‘rhe noise The intrinsic APD noise is dominated by two sources. One is the shot noise from the APD dark current. and another the noise charge which is stored in the APD junction capacitance by the Johnson noise of the preamplifier input resistance . It is convenient to express the APD noise in equivalent noise charge (ENC) defined by the equivalent number of photoelectrons before This makes it easy to avalanche multiplication. compare the noise with photoelectron yields by incident light pulses. The shot noise from the APD dark current, ENC, (rms). is given by
where e is the electron charge, B is the measurement bandwidth in Hz. ld, is the APD surface dark current, Idb is the APD bulk dark current. M is the avalanche gain. and F is the excess noise factor. We measured the dark current of a 5 mm 4 APD (Advanced Photonix 197-7071-520 ) operated at room temperature. The result is shown in Fig. 1 as a function of reverse bias voltage. The dark current rapidly increases as the bias voltage exceeds 3200 V. This is because the bulk dark current fdb is multiplied with a rapidly increasing gain of the APD. The surface dark current I,+ is a component which is not multiplied in the APD. Thus. for a bias voltage above 2200 V. Ids is estimated by the linear extrapolation from
0.5 1 1.5 2 2.5 3 Reverse Bias Voltage (kV)
where li is Boltzmann constant, R, is the preamplifier input resistance, and T is the absolute temperature of the resistance. Cd is the APD junction capacitance. If any other parallel capacitance exists, it also have to be included in Cd. The total APD noise ENC,,, (rms) is given by ENC,,, = ;ENC,’
Table 1 summarizes the noise estimation made for two different APDs (aforementioned 197-70-72-520 and Hamamatsu S5343 ) operated at room temperature. These APDs were selected because of their spectral responses suitable for the 3HF fiber and their low-noise characteristics. In Table 1 are given also the parameters used for the estimation. Most of these parameters, except for measured Ids and Mid, of the 197-70-72-520 APD, were quoted from the catalogues or the data sheets supplied by the manufacturers. Here, Cd includes only the APDjunction capacitance, neglecting other capacitance. The temperature T is 300 K. R, and B depend on the preamplifier. Supposing that we use an existing low-noise and high-gain (30 V/PC) charge sensitive preamplifier (Digitex HIC-1576 [lo]). R, is its input impedance of 500 R. The 50 ns rise time of this preamplifier output signal suggests that the bandwidth B is 20 MHz. assuming that noises with other rise times are suppressed by appropriate filtering techniques.
S. Okumura et al. / Nucl. Instr. and Meth. in Phys. Res. A 388 (1997) 235-240
Table 1 Estimation of photoelectron-equivalent APD noises. The equivalent noise charge (ENC) is defined by the equivalent number of photoelectrons before avalanche multiplicaton. Some parameters used in the estimation are also given APD Type
Manufacturer Diameter Bias M at the bias I& MIdh
Advanced Photonix Smm 2350 v 100 44 nA 6nA 2 15pF 9
Hamamatsu lmm 145 v 100 0.2 nA 0.8 nA 4 15pF 4
C, ENC, (p.e.) ENC, (p.e.) ENC,,, (pe.)
Fig. 2. The structure of one layer. The outer diameter of a fiber is 0.5 mm, and the core diameter is 0.44 mm. We built fiber bundles with 2, 3, 4, and 5 layers.
diameter was 0.44 mm. The light trapping efficiency by total internal reflection in the fiber is calculated to be 0.0535 in one direction, using the formula 0.5 x (1 - nl/nz), where ni( = 1.42) and nz( = 1.59) are the refractive indices of the fluorinated-PMMA outermost clad and the polystyrene core, respectively. The fiber bundles were made by stacking a number of fiber layers. A layer was 4.8 mm wide containing 12 fibers 20 cm long. Fig. 2 defines the structure of one layer. Adjacent fibers were staggered not to allow gaps between them. The horizontal distance between the centers of two adjacent fibers was 0.4 mm. The number of stacked layers determined the thickness of the bundle. We built the bundles with 2, 3,4, and 5 layers. Each fiber was painted with white extra-mural absorber (EMA, Bicron BC626WF) to suppress cross-talks between fibers. The EMA played also a role as glue between fibers. The experimental arrangement is schematically shown in Fig. 3. One end of the bundle was directly coupled to
In consequence of above considerations, the APD noises (Table 1) are comparable with the signals expected from a single fiber. However. this estimation still encourages us to have a prospect of efficient signal detection by coupling an APD to a few fibers along a particle trajectory. 3. Measurements
of the signal-to-noise
We measured APD signals with several different thicknesses of fiber bundles penetrated by 90Sr l&particles. The fiber we used was a multiclad scintillating fiber doped with 1500 ppm 3HF (Kuraray SCSF-3HF 13,111). The outer diameter was 0.50 mm, and the core
Bias Voltage 10MR 2.2 IIF
Fiber Bundle Collimators
Trigger Counters Preamp. 30 V/PC v-
#To ADC Fig. 3. The schematic for the signal-to-noise
diagram of the experimental arrangement. The distance ratio measurements (the hodoscope tests).
from the (J-source position
to the APD is 12 cm (20 cm)
S. Okumuru et al. ! Nucl. Instr. and Meth. in Phyx Rex A 388 (1997) 235-240
the antireflection coating on the silicon surface of an 197-70-72-520 APD with optical grease. We did not employ the Hamamatsu S5343 because of its size smaller than the fiber bundles. The APD coupled to the fiber bundle was followed by an aforementioned HIC-1576 preamplifier. On another end of the bundle was placed a sheet of aluminium foil as a light reflector. The reflectivity of the foil was measured to be 70%. The bundle was sandwiched between two identical brass collimators 5 mm thick. The “Sr B-source was placed on top of the first collimator. The B-ray was collimated through two 3 mm r$ holes in these collimators. Two trigger counters were placed underneath the second collimator. The first trigger counter was 0.5 mm thin so that the B-particles could pass through it to reach the second one. The distance from the B-source position to the APD was 12 cm. The APD was operated in the avalanche mode with a reverse bias of 2350 V, which was 6% smaller than its breakdown voltage. The output signals from the preamplifier were shaped with a time constant of 150 ns, and were further amplified ( x 30) by a main amplifier (Phillips Model 777). The resultant signals were digitized by a peak sensing ADC (LeCroy 2259B). Gate signals for the ADC were provided by coincidences of the two trigger counters. The measured APD pulse height spectra are shown in Fig. 4 for the bundles with 2, 3. 4, and 5 layers, together with a spectrum of noise pulse heights measured with providing ADC gates at random timing. The pulse height increases linearly as the number of layers in the bundle increases. By integrating the pulse height spectrum in the range above a certain threshold, we obtain the detection efficiency for the threshold. The efficiency curve thus obtained for each number of layers is shown in Fig. 5 as a function of the threshold, together with the corresponding curve for the noises. With three layers. for instance, we achieve 90% detection efficiency with only 1% contamination with noises. A cleaner separation of signals from noises is possible with 4 or more layers.
to a scintillating
We built a six-layer fiber bundle to show the performance as a hodoscope. Each layer in this bundle had the same structure as we employed in the signal-to-noise ratio measurement (Fig. 2). The fiber length was 30 cm. We stacked six layers to guarantee high and uniform detection efficiency all over the active area of the bundle. The six-layer bundle was segmented into three identical pieces, Fig. 6 shows the cross section of this bundle. Each segment was 1.6 mm wide containing 4 fibers per layer. and was coupled to a 197-70-72-520 APD. The test method was almost the same as that for the signal-tonoise ratio measurement, except that each of the two
1 Pulse Height (V)
Fig. 4. APD pulse height spectra measured with 2-. 3-, 4-, and 5-layer bundles penetrated by 90Sr p-particles. The spectrum for noises is also shown.
Threshold of Pulse Height (V) Fig. 5. The efficiency curve For each of 2-, 3-. and 4-, and 5-layer bundles as a function of pulse height threshold. The corresponding curve for noises is also shown.
collimators was replaced with a slit type collimator 1 mm wide and 12 mm long to obtain a narrower B-beam along fibers. The position sensitivity of this fiber hodoscope was tested by collimating the beam through those two slits 1 mm wide, as shown in Fig. 6. The distance from the irradiated position to the APDs was 22 cm.
S. Okumura et al. / Nucl. [email protected]
and Meth. in Ph_vs.Rex A 388 (1997) 235-240
Fig. 7. Correlation between slit positions and hit segments. I”“l”“l~“‘I”~‘l’~“l”“l”‘~
t-l j Segment-2 Fig 6. The cross section of the fiber hodoscope. Dashed lines represent the segment boundaries. The three APDs were read out in the same way as in the signal-to-noise ratio measurement (Fig. 3). We defined in the off-line analysis a threshold for the pulse height of each APD. Each threshold value was determined so that the noise count probability could be reduced to 1%. When a segment had a pulse height larger than the threshold, we regarded the segment as a hit segment. In case that both of two neighbouring segments overcame the thresholds, only the one with larger pulse height was regarded as a hit segment. Fig. 7 shows the correlation between slit positions and hit segments determined in this way. Fig. 8 shows the detection efficiency separately for each segment as a function of the slit position. When the 1 mm slits were placed near the junction of two segments, the detection efficiency was split between two segments. The total efficiency for each slit position is indicated by a closed circle in the same figure. Most of the total efficiencies are between 97% and 100%.
5. Summary and discussion Using 0.5 mm 4 3HF fibers, we have experimentally demonstrated that the APD operated in the avalanche mode can efficiently detect photons from three or more fibers combined along a particle trajectory with keeping the noise count rate reasonably low even at room temperature. There is a possibility of further noise reduction by cooling the APD within a temperature range allowed
!3 Y 0.8 0.6 0.4 0.2
O 0 0.5
1 1.5 2 2.5 3 3.5 4 4.5 Slit Position (mm)
Fig. 8. Detection efficiency of the fiber hodoscope versus slit position. The efficiencies of Segment 1, 2, and 3 are indicated by open circles, open squares, and open triangles, respectively. The total efficiencies are indicated by closed circles. Dashed lines represent the segment boundaries. in the specifications. Lower temperatures reduce the number of electrons diffusing thermally from the valence band to the conduction band in the APD, and consequently reduce the dark current. Furthermore, lower temperatures calm down the APD crystal lattice vibration which interrupts the avalanche electrons. Therefore, the gain of the APD usually increases at lower temperatures. By these favourable effects, the signal-to-noise ratio is improved at lower temperatures. Thus, we are still encouraged to have a prospect of efficient detection of photons from each individual fiber by cooling down the APD.
Acknowledgements We thank Y. Sasayama for measuring the light reflectivity of the aluminum foil which we put on the fiber ends.
5. Okumura et al. I Nucl. Ins&. and Meth. in Ph.vs. Res. A 388 (1997) 235-240
This work was supported by the Ministry Science and Culture of Japan.
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