Status of BGO-avalanche photodiode detectors for spectroscopic and timing measurements

Nuclear Instruments and Methods in Physics Research A278 (1989) 585-597 North-Holland, Amsterdam

585

STATUS OF BGO-AVALANCHE PHOTODIODE DETECTORS FOR SPECTROSCOPIC AND TIMING MEASUREMENTS Roger LECOMTE, Charles MARTEL and Christian CARRIER Department of Nuclear Medicine and Radiobiology, Faculty of Medicine, University of Sherbrooke, Sherbrooke, QC, Canada JI H 5N4

Received 17 October 1988 Silicon avalanche photodiodes (APD) can be used as photodetectors in combination with scintillation crystals for the detection of radiation . These devices have the same inherent advantages as other semiconductor photodetectors, but they show significantly superior performance due to their internal gain . In particular, APD based detectors have adequate timing performance to be used in applications requiring coincidence detection, such as positron emission tomography (PET). In this work, we report on the systematic evaluation of a sample of 20 dual BGO-APD detector modules recently introduced for the PET application by RCA Electro-Optics . Results show that a typical energy resolution of the order of 20% for the 662 keV -y-ray from 137CS is easily achieved over a broad range of operating bias. When operated at the manufacturer's recommended bias, a typical timing resolution of the order of 15 ns FWHM (for 511 keV) is obtained in coincidence with a fast plastic detector . If the bias is increased up to the APD maximum operational gain, a timing accuracy in the range of 9 to 13 ns FWHM is consistently achieved . The performance of the detectors was correlated with the characteristics of the APDs used as photodetectors in order to improve the selection criteria of the APDs and to predict their optimal operating conditions . 1. Introduction In the past few years, there has been a growing interest in the use of solid state photodiodes as the readout devices for scintillation detectors. Although the combination of a scintillator and a photodiode as a radiation detector was pioneered over two decades ago [1,2], current applications in high energy physics and medical imaging (e .g. positron emission tomography or "PET"), coupled to the advent of new large area photodiodes with improved characteristics, has triggered this renewed interest in photodiode readout. Photodiodes have a number of distinct advantages over conventional photomultiplier tubes: a higher quantum efficiency in the emission range of many scintillators, compactness and shape versatility, insensitivity to magnetic fields, simple biasing and low power requirements, and in the case of photodiodes with no internal gain, a proven short-term and long-term stability. On the negative side, large area photodiodes usually have a high capacitance as a result of a very thin depletion layer; they exhibit substantial leakage currents which contribute to the noise ; and the signal observed when coupled to a scintillator is usually quite small, which implies that very low noise preamplifiers must be used . For applications in electromagnetic calorimeters where the interaction of highly energetic particles in the scintillator produces signals of several thousand electrons, silicon p-i-n photodiodes operated at room tem0168-9002/89/$03 .50 C Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

perature yield good results. Typical values of 70-100 pF/cm2 and 1-3 nA/cm2 have consistently been reported for the capacitance and leakage current of recent p-i-n photodiodes [3-8], which translate into rms noise levels of 450-750 electrons when connected to a low noise charge sensitive preamplifier with an FET input stage and a shaping amplifier having a time constant in the microsecond range. A minimum input signal of much more than one thousand electrons is required to produce a detectable signal above the noise in these conditions ; depending on the type of scintillator and particular detector setup, the threshold runs from below 1 MeV to a few MeV of deposited energy in the crystal. For nuclear spectroscopic application with incident radiation below 1 MeV, the photon yield of the scintillator and the light collection efficiency by the photodiode become critical factors. CsI(TI) scintillators, having a high scintillation yield (- 50000 photons/MeV) and a good match to the spectral sensitivity of silicon photodiodes (emission maximum at 550 nm), have been reported to produce the best results [8-10] . However, there are applications where a high stopping power scintillator such as BGO is mandatory. Due to the very low light output of BGO, even highly optimized detector setups yield poor results with the photodiode at ambiant temperature . Cooling is then essential to reduce the photodiode dark noise and possibly the thermal (Johnson) noise in the preamplifier input FET. State-of-the-art resolution data have been obtained by Derenzo at -150'C [11] . Silicon photodiodes have

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R. Lecomte et al. / Status of BGO-avalanche photodiode detectors

been proposed as position sensing devices in conjunction with photomultiplier tubes for high resolution PET detection systems based on BGO scintillators [12,13]. Another semiconductor, mercuric iodide (HgI 2 ), can be used as a photodetector for scintillation crystals at room temperature [14-17]. Having a large bandgap (2 .13 eV) and good electron transport properties, large area HgI 2 photodiodes with very low leakage currents (< 100 pA) and a depletion layer thick enough to have small capacitance can be manufactured. In combination with BGO crystals, energy resolutions in the 18-20% range have been measured for 511 keV gamma rays [15,16] and a coincidence timing resolution of 105 ns FWHM was reported [14] . However, problems still have to be solved to achieve good transparent entrance electrodes with long term reliability, and the timing performance is not adequate for most coincidence measurements. More serious candidates as replacement of photomultiplier tubes in scintillation detection applications are silicon avalanche photodiodes (APD) having internal gains ranging from several tens to a few hundreds . Amplification in an APD is obtained by means of impact ionization within a narrow region of very high field (>_ 10 5 V/cm) [18] . APDs are intrinsically very fast (response time in the nanosecond range) but also noisier than unity gain photodetectors as a result of dark current multiplication and excess noise arising from the avalanche process. We note, also, that the avalanche gain is temperature dependent, due to the temperature dependence of the electron and hole ionization rates [18] ; therefore, APDs do not have the inherent temperature stability of unity gain devices such as p-i-n photodiodes and they must be operated with the temperature stabilized within 1 or 2'C to obtain reproducible results . Currently, two classes of APD designed for scintillation detection applications are commercially available : the "beveled" type [19,20] has the structure of a diffused p-n junction and must be operated at very high voltage ( >_ 1000 V) in order to achieve the field necessary for avalanche multiplication . Large area devices coupled to 1 in. x 1 in. diameter Nal(TI) scintillators yield resolutions similar to a PMT. With a BGO scintillator of the same size, resolutions of 40% and 24% were obtained for the 662 keV y-ray at room temperature and 0'C, respectively. The second type of APD, tailed "reach-through", have a different structure which will be described in more detail in the following section. Due to the very high uniformity required in the high field region where avalanche takes place, the size of reach-through APDs has been limited to about 25 mm2 up to now. These small area APDs have dimensions adequate for the application in PET. They have been shown to yield energy resolutions similar to or better than PMTS [21,22] and they are the only solid state photodetectors having demonstrated coinci-

dente timing capabilities below 100 ns FWHM when used with scintillators [22-241. A detector module based on reach-through APDs was specifically designed for use in high resolution PET and was recently released commercially by RCA Electro-Optics [23] . This article reports on the experimental investigation of a sample of 20 of these modules. We shall first summarize the properties of the reach-through APDs used in the modules and briefly discuss the noise characteristics of the APD detector-preamplifier combination with special reference to the timing requirements . We will then describe the tests carried out to evaluate the performance of the APD detectors. The measurements were performed with two objectives in mind : first, to determine the operating conditions for optimal performance, and second, to refine the selection criteria for APDs used in the scintillation detection application . 2. Reach-through APD characteristics The properties of reach-through APD have been extensively described elsewhere [181 and will only be briefly summarized here. These devices have the p +-m-pn + structure shown in fig. 1, in which the four layer thicknesses are typically < 1 g m, 150 p m, - 15 p m and < 10 pm, respectively . In APDs designed for scintillation counting, the thickness of the p+ layer through which the light enters is reduced to less than 0.2 p,m in order to enhance the quantum efficiency in the 400-600 nm range. As is shown in fig. 2, the APD spectral response is well matched to the emission spectrum of BGO scintillators, having a measured quantum efficiency of the order of 60% at 480 nm (measured in air) . In normal operation, the "p" and "2r "-layers are fully depleted up to the p + contact and the electrical

-ZX104

t 4---140,um--l

Fig. 1. Structure of reach-through APD and electric field distribution under normal operating conditions . (Reproduced with permission from RCA.)

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors to

100 UV ENHANCED APD

80 60

.H Z

Q u

STANDARD APD

Û Z

Z

-i

U

F Z Z

n r 10 r

Id

1

0

Q

Q

r r

.4

5-20 PMT v

n

CC Q

2

; r 5 300

100 ~

Y

r

20

a

looo

i

40

587

10

Soo

400

1i 200

300

400

500

600

BIAS VOLTAGE (Volts)

Fig. 4 . APD internal gain as a function of bias voltage for devices operated at room temperature .

600

WAVELENGTH (nm) Fig . 2. Quantum efficiency of a blue enhanced APD compared with a typical S-20 photocathode . The dashed curve is the characteristic emission spectrum of BGO at room temperature (from Weber and Monchamp, J. Appl . Phys . 44 (1973) 5495) .

field profile is as shown in fig. 1 . Photons entering through the p + contact are absorbed in or near it at short wavelengths or within the wide m-layers at longer wavelengths . The photo-generated electrons are rapidly swept to the high field (- 3 x 10 5 V/cm) region around the p-n junction where they undergo avalanche multiplication through impact ionization . The holes generated in the multiplication process are then swept back across the ir-region to the p + contact where the resulting current can be collected. Although relatively low (- 2 x 10 4 V/cm), the field in the drift region is still adequate to yield collection times typically less than 5 ns . One advantage of the fully depleted structure of reach-through APDs is their low junction capacitance. Fig . 3 shows the device capacitance as a function of

loo APDU144-2

reverse bias voltage . At low bias, a very small region around the junction is depleted and the capacitance is very high . As the voltage is increased, the avalanche region becomes fully depleted and the depletion layer rapidly expands up to the p + -layer ; concurrently, the capacitance drops steeply and levels off to a constant value as the device gets fully depleted . With the APDs used in the modules, the device capacitance is typically 12 pF throughout the useful operating range (250-600 V) . The leakage current in APDs has a surface component which is not multiplied and which normally is the dominant contribution to the dark current, and a bulk component that is multiplied . The steep increase in leakage current at avalanche breakdown is usually employed to determine the breakdown voltage of the APD . Characteristic gain-voltage curves for APDs operated at room temperature are shown in fig . 4 . The rapid drop in gain below 250 V indicates that the avalanche region is not completely depleted and that there is no field in the drift region . At voltages above about 300 V, where the diode is fully depleted, it reaches its normal operating mode with gains ranging from = 20 up to a maximum value of 150-500 where breakdown occurs . The APDs used in the modules were designed to be operated at a nominal gain of 75, but they have been systematically used at much higher gains, up to the breakdown voltage, without any observed detrimental effects . The temperature coefficient of the operating voltage for constant gain is 3 .0 V/ ° C .

3. Noise analysis 100

200

300

400

500

600

BIAS VOLTAGE (Volts)

Fig . 3 . Typical APD capacitance as a function of reverse bias voltage for a 3 mm X 3 mm active area device packaged in an RCA C30994 detector module .

The noise behavior of an APD-preamplifier combination is, in most respects, similar to that of a semiconductor detector ; the reader is thus referred to standard treatments of the subject for a detailed analysis [25,26] . In a previous work, the rms noise (in electrons) referred

58 8

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors

to the input of a charge sensitive preamplifier connected to a filter was shown to be given by (ref . [24], eq . (12)): Ne

10000

1

n 0

PRFAMP . :

0 PF 10 PF 20 pF

#108-2 : 790 v 49; V BGO signa1 --

Cf Sn( TA)

q I F(TA) 1

~q I F(TA) I

I(iA+ID)TA

+ eÂCT ( 1 + TA 2 c )i 17

where: TA) F(TA) Sn(

SHAPING TIME (nsec)

rms noise voltage measured at the filter output, transfer function of the filter . For a conventional RC-CR filter : F(T)

TA/T 1 + ( TA/'r )

Fig. 5 Noise referred to preamplifier input as a function of filter time constant for different capacitive loads and APD operating voltages . The dashed curve represents the attenua tion of the signal from BGO due to the filter time constant (in arbitrary units) .

z°

filter time constant, preamplifier feedback capacitance, total input capacitance (APD, feedback, stray), preamplifier noise current spectral density, APD noise current spectral density, noise voltage spectral density, time constant corresponding to the 1/f corner frequency. In eq . (1), the first term represents the parallel noise contribution arising from the input transistor noise current i A and the detector noise current ?D, which for avalanche photodiodes is given by [18] : TA Cf CT iA i t, eA Tc

iô=2q ( Is +I b MzF),

(2)

where IS is the surface component of the dark current, I b is the bulk component undergoing multiplication, M is the average APD gain and F is the excess noise factor of the avalanche multiplication given by [27] : F= kM + (1 - k)(2 - 1/M), where k is the effective ratio of the ionization coefficients for holes and electrons. Typically, for the 3 X 3 mmz active area devices used in this work, the surface dark current IS is less than 200 nA and the bulk dark current Ib is in the range of 100 to 300 pA . As is evident from eqs. (2) and (3), the avalanche multiplication is a noisy process dependent on the gain and the ratio of ionization coefficients, k. Due to this excess noise, the bulk component I b of the dark current always has a higher contribution to the noise than the surface component IS when the APD is operated at relatively high gain, as is the case in the scintillation detecting application . Typical values of k for silicon are - 0.01 to 0.04, but recent improvements in the manufacturing process have made possible the production of low-k devices (k-- 0.006) having a lower excess noise at

high gain . The APDs used in the detector modules investigated in this work were of the low-k type . The noise current i D in these APDs is typically 10 to 100 times higher than in silicon p-i-n photodiodes . As a consequence, for standard spectroscopic applications, a very low noise preamplifier is not needed since the noise current i A of any relatively good FET will always be small as compare to the APD noise current. The second term in eq . (1) comprises two components : the series noise arising from thermal noise in the APD and preamplifier series resistances which will be the dominant contribution to the noise for short shaping time constants, and the 1/f noise whose contribution is independent of the filter shaping time constant . Both these contributions to the noise can be minimized by keeping the total input capacitance CT as low as possible. In a preamplifier recently developed especially for use with APD detectors, this was achieved by employing a very low capacitance MOSFET as the input transistor and by carefully designing the input stage to eliminate parasitic capacitances [24] . Fig. 5 shows the resultant noise (referred to the preamplifier input) as a function of shaping time constant. The APD dark noise current and the capacitance associated noise define a minimum well below 100 ns shaping time which is appropriate for coincidence timing measurements with the precision required in PET. For comparison purposes, it will be instructive to estimate the equivalent noise charge, ENQ, or number of primary photoelectrons required to give a signalto-noise ratio of unity in an APD coupled to a scintillator. This is simply expresses as : Ne 1 ENQ = M f(Tç/TA)

(5)

where f(Tç/TA) represents the signal attenuation due to the filter for a scintillator having a decay time constant

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors This function is plotted in fig. 5 as a function of the filter time constant for BGO (TS = 300 ns) . One can extract from this curve and the noise curves a maximum signal-to-noise ratio in the range of 200 to 500 ns shaping time which will be preferred for best energy resolution . It is worth noting that this is an order of magnitude faster than for detectors based on photodiodes with no internal gain . The energy resolution of an APD detector can be expressed as [22] : Ts .

~ E

)z F-1 =2 .355M+ ~(N

z

11/2

N+ N

where N is the effective number of primary photoelectrons generated in the APD . The first term is the electronic noise contribution, the second term is the excess noise in the signal due to avalanche multiplication and the third term is the statistical contribution including a systematic distribution due to the intrinsic resolution of the scintillator (a > 1). As long as the signal is high relative to the noise, the electronic and excess noise contributions do not significantly affect the energy resolution which is mostly determined by the crystal intrinsic resolution [22] . Therefore, the spectroscopic performance of APD detectors is expected to be fairly stable over a broad range of operating voltages . For timing with constant fraction discrimination, the noise-induced rms uncertainty in the triggering time, OT, can be related to the signal characteristics as [28] : (YT

=

aV VI+f 2 1J/t,

where a rms noise at the input of the discriminator,

58 9

f

constant fraction attenuation factor, signal amplitude at the input of the discriminator, signal rise time . tr With BGO scintillators, a tradeoff between signal amplitude and pulse rise time must be made since for short shaping times the signal is drastically reduced . Operating the APD at higher gain partly compensates for the drop in signal up to the point where the APD noise begins to increase at a higher rate than the signal. Since this maximum operational gain occurs near breakdown and since APDs show dissimilar characteristics in this region, the optimum conditions for timing have to be determined individually for every detector . V

4. Materials and methods A sample of 20 detector modules of the RCA C30994 type [23] was tested . A schematic diagram of the module is shown in fig. 6 . The module comprises two discrete detector channels in one hermetically sealed package. Each detector consists of a 3 mm x 5 mm x 20 mm BGO scintillator cemented to a 4 mm x 4 Trim APD having a 3 mm x 3 mm active area. The coupled surface of the crystal is beveled at an angle of 35 ° to accommodate the APD chip and to improve scintillation light collection from the crystal. Scintillators are wrapped in aluminium foil and glued to the bottom lid of the package with epoxy . The modules can be stacked in a two-dimensional array and the two detectors within the same module can be operated together or independently. The manufacturer supplied each module with data for breakdown voltage, operating voltage at gain 75 and FWHM resolution of the 137 Cs 662 keV photo-

-PHOTODIODE 2 - CASE - -~- PHOTODIODE 2 _ _ 9"9- n /c - PHOTODIODE 1 ~- n/c PHOTODIODE 1

POSITIVE

LEAD

NEGATIVE LEAD NEGATIVE LEAD POSITIVE LEAD

38

20 .0 33 " 0

One surface of the BGO crystal is slanted as shown. The package wall material is kovar

Dimensions in millimeters .

Fig. 6 . Schematic diagram of the RCA C30994 BGO-APD detector module. (Reproduced with permission from RCA .)

590

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors

peak for both detectors within the module . The tested modules were manufactured at different times over a year period, and utilized BGO crystals from at least two manufacturers . The APDs were also from different lots, fabricated separately. The detectors in the same module usually have been matched so that the APD voltages for gain 75 are within ± 20 V, and the output signals at gain 75 are within 10% ; however, those characteristics were not guaranteed as specifications by the manufacturer [29] . In the present study, modules manufactured since 6 to 18 months were evaluated. The two detectors in a module were used independently with one preamplifier per APD. This is motivated by the requirements for high spatial resolution in the PET application and also by noise considerations and the need to operate each detector at its optimum point for timing . The following characteristics were measured for the 40 detectors : (1) Leakage current, (2) Signal amplitude, (3) Dark noise, (4) Equivalent noise charge, (5) Energy resolution . (6) Timing resolution . The current-voltage characteristics of all APDs were drawn to identify anomalous features and to determine the avalanche breakdown voltage. All other measurements were performed using a prototype charge sensitive preamplifier (CSP) circuit implemented on PC board [241 and standard NIM electronics . A block diagram of the setup is shown in fig. 7. The signal amplitude was estimated from the position of the 662 keV -y-ray photoelectric peak on the multichannel analyser (MCA), calibrated in equivalent electrons referred to the preamplifier input. The gain-voltage characteristics of the APDs were extracted from these meassurements, assuming a gain of 75 at the manufacturer recommended bias. The noise measurements were taken with the help of a

Fluke 8920A True rms voltmeter connected at the output of the slow (SA: TA = 300 ns) and fast (TFA : TA = 20 ns) filter amplifiers . The APD dark noise current was evaluated by subtracting quadratically from the measurements at 300 ns the noise contribution from the preamplifier with the APD detector replaced by an equivalent capacitance. The total noise and the relative contributions to the noise from the APD and the preamplifier were also evaluated in the fast channel by the same technique. The energy resolution measurements were carried out to determine the optimum operating voltage and the range of stable performance. A t37CS source (662 keV) was used for these measurements . Finally, timing measurements were taken in reference to a fast plastic detector and with constant fraction discrimination (CFD) on both channels . A filter time constant of 20 ns was used, as this was found to be a good compromise between a fast signal rise time and an acceptable signal-to-noise ratio. An energy window of 50% was set on the 511 keV peak using a single channel analyser (SCA) to gate the time spectrum . The APD gain, leakage current and dark noise, as well as the energy resolution of the 511 keV photoelectric peak were noted at the optimum voltage for coincidence timing . All measurements were performed in an uncontrolled temperature environment but the detector temperature was carefully monitored throughout . The temperature varied between extremes of 20 and 26'C, but most measurements were taken between 22 and 24'C. No corrections have been applied for temperature variations between measurements . 5. Results Out of the 40 tested detectors (2 detectors per module), one was found totally inoperative (no. 170-2) and two others (nos . 150-1 and 159-1) had significantly larger energy resolution than when tested by the manufacturer, and were very poor for timing . All other detectors showed proper operation in reasonably good agreement with the data supplied by RCA. Results of the systematic investigation are summarized in table 1 and will be detailed in the following. 5.1 . Leakage current

Fig. 7. Setup used in the measurements . CSP: charge sensitive preamplifier ; SA : shaping amplifier; SCA : single channel analyser ; TFA: timing filter amplifier; CFD: constant fraction discriminator; TAC: time-to-amplitude converter; RMS: rms voltmeter.

A plot of the current-voltage characteristics of a few representative APDs is shown in fig. 8. The relative height of the curves in the range of normal operation (- 250 V to below breakdown) is indicative of the surface component of the leakage current. At the RCA recommended voltage, the leakage current varied from 120 to 280 nA with an average of 186 nA, consistent with the rated value of 200 nA [23] . Most APDs (36 out of 40) were found to have a breakdown voltage between

R. Lecomte et al. / Status of BGO-avalanchephotodiode detectors

591

Table 1 Summary of measured characteristics for the APD detectors operated at the RCA recommended voltage and at the conditions of best spectroscopic and timing performance . Measurements were taken at an average temperature of 23 ° C.

RCA recommended operation Gain a) Output signal Leakage current APD dark noise Equivalent noise charge b) Energy resolution (662 keV) Timing resolution')

Mean value (standard deviation)

Range of measured values

Typical and extreme specifications

Units

75 133 (20) 186 (42) 0.86 (0.36) 40 (15) 21 (3) 15 (2)

90 -180 120 -280 0.27- 1.5 20 - 70 17 .5 - 30 12 .3 - 17 .7

Spectroscopic operation Gain Leakage current APD dark noise Equivalent noise charge b) Energy resolution (662 keV)

40 (12) 164 (38) 0.45 (0 .16) 43 (13) 20 (2)

23 - 75 101 -238 0.23- 0.90 26 - 65 16 - 27

nA pA/Hz 1 / 2 rms electrons

Timing operation Gain Leakage current APD dark noise Equivalent noise charge d> Energy resolution (511 keV) Timing resolution c)

200 (45) 208 (44) 2.8 (1 .2) 100 (25) 34 (5) 10 .5 (1 .1)

105 149 1.0 60 24 8.7

nA pA/Hz 1 /2 rms electrons

75 110 (90-) 200 <1 .4

electrons/keV nA pA/Hzl i2 rms electrons

20 (15-26) < 20

ns FWHM

-350 -334 - 5.9 -180 - 47 - 14.7

ns FWHM

Nominal. Measured using a 300 ns shaping time filter . °) Using 511 keV y-ray from 22 Na, in reference to a fast plastic detector . d) Measured using a 20 ns shaping time filter . a) b)

500 and 600 V. The match for breakdown between the two APDs in the same module was found to be rather good with a maximum difference of 25 V in two modules but an average of less than 10 V for the sample of 20 modules. 5.2. Signal amplitude The distribution of the signal amplitude from the APD detectors measured at a gain of 75 is shown in fig.

u

12 N OC

à 300

s Z

9. A factor of 2 is observed between the minimum and the maximum output signals. The detector with the lowest signal falls sharply within the minimum specification of 60000 electrons for the 137 Cs 662 keV -y-ray, while the mean of 88 000 electrons over the sample of 39 detectors is slightly higher than the quoted typical value of 75 000 electrons [23] . From these figures, the signal in the APD is found to be generated by an average of 1 .8

200

10

O V

8

G

64~

100

200 300 400 BIAS VOLTAGE (Volts)

500

Fig. 8. Leakage currents as a function of reverse bias voltage for four representative APDs operated at room temperature .

40

50

60

70

80

90

100

110

120

APD SIGNAL (x1000 electrons)

130

140

Fig. 9. Distribution of signal amplitude from APD detectors at a gain of 75 .

59 2

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors

âa ô Y

a0 0 a Q

0.1

T

10

20

_

30

50 70 100 APD GAIN (M)

200

300

500

Fig. 10 . Dark noise as a function of APD internal gain. primary photoelectrons per keV of deposited energy in the BGO scintillator. The best spectroscopic performance is obtained at a somewhat lower gain producing a signal of the order of 45-50000 electrons on the average. For the timing measurements, the APD gain was pushed up to the limit in order to fulfill the requirements for large signals. Depending on the device, signal amplitudes between 100000 and 400000 electrons, corresponding to gains of 105 up to nearly 350, respectively, are obtained ; however, most devices reach their optimal timing performance at a gain in the range of 150-250 . With both detectors in the module operated at the RCA recommended bias, 15 out of 19 modules were found to have the output signal of one detector within 10% of the other. In the four modules where the difference in output signal exceeded 10%, the differences generally became smaller when the detectors were operated at lower bias but increased significantly with the bias voltage. 5 .3. A PD dark noise

The dark noise and the gain of an APD have a very similar behavior as a function of bias voltage. In fact, as can be seen from fig. 10, the dark noise is nearly linear with the APD gain over the range of normal operation (M = 20 to --- 150) . This was expected from eq. (2), assuming that the contribution from the surface component of the leakage current is negligible. At higher gain, the excess noise factor, and eventually, the rate of spurious avalanche pulses triggered by thermally generated carriers contribute to the more rapid increase of the dark noise as a function of the APD gain . A few APDs (--- 13% of the devices) showed a higher noise slope as displayed by the curve labeled no . 167-2 in fig. 10 . These devices have important gain nonuniformities in their multiplication region . They usually operate properly at low gain (M < 75), but as the average gain is increased, small areas of the device reach avalanche breakdown and contribute significantly to the total noise while the rest of the APD remains operative.

Fig. 11 shows the distribution of the dark noise for the detectors operated at a gain of 75 and at the optimum points for energy resolution and timing . At a gain of 75, one APD (no. 170-2, which was previously identified as inoperative) is observed to significantly exceed the maximum noise figure of 1 .4 pA/ Hz used as a selection criterion by the manufacturer [231 . The broad dispersion of the dark noise at the optimum point for timing results from the wide variation in operating gain at that point and from the differences in APD characteristics near breakdown. 5.4 . Equivalent noise charge

The equivalent noise charge of four typical APDs is plotted as a function of the APD gain in fig. 12 for 12

(a)

10-

.4 fW W

0

.6

Optimum bias for energy

.8

10 -

(b)

1 .0

1.2

1.4

Recommended

1.6 bias

8

O W

m Z 1 .0 10

(c)

2

3

1 .2

1 .4

Optimum bias

4

for

1.6

timing

n n i 5

APD DARK NOISE (pA/Hz")

6

7

Fig. 11 . Distributions of APD dark noise measured : (a) at the optimum bias for energy resolution ; (b) at the recommended gain of 75 ; and (c) at the optimum bias for timing resolution.

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors

593

1500-

(a) #144-2 (b) # 12B-2 (C) # 134-1 (d) # 167-2

APDu160-1

Z âx

(500V, M=75) 1000

U

I

â

Z O

140 keV

500

,

V 20 1 10

20

30

50

70

100

200 300

500

APD GAIN (M)

50

100

150

200

CHANNEL NUMBER

Fig. 12 . Equivalent noise charge (ENQ) as a function of APD internal gain for the shaping times used in timing and energy measurements .

Fig. 13 . Pulse height spectrum of the 140 keV -y-ray from 99'Tc for an APD detector operated at the bias of minimum ENQ. The peak-to-valley ratio is over 4.

shaping times of 300 ns and 20 ns . Since the measurement of the equivalent noise charge includes the noise sources due to the preamplifier in addition to the APD noise, the region of minimum ENQ represents the optimum operating point of the APD-preamplifier combination . In this respect, the recommended operating gain of 75 appears to be more adequately selected for spectroscopic applications . The few nonuniform devices of which APD no . 167-2 is representative have their minimum ENQ at a much lower gain as a consequence of their higher dark noise at high gain . The minimum achievable ENQ shows large variations from one APD to the other. At the 300 ns shaping time, the average of 40 ± 15 rms electrons for the sample of detectors tested in this study compares favourably to the best reported value of-- 280 Tins electrons at 2 ws for a selected p-i-n photodiode of comparable active area operated at room temperature [11] . The minimum -y-ray energy that can be detected with the present detector is therefore a factor of 7, on the average, below the best p-i-n based detectors. Using an above average device, the 140 keV -y-ray from 99'Tc can be clearly separated from the noise, as the spectrum of fig. 13 shows.

best performance with a contribution of the order of 16% on the average, for the 137Cs 662 keV peak . The electronic and multiplication noise contributions amount to 7% and 8%, respectively, on the average. Since these components, having a dependence on APD gain, have only minor contributions to the total energy resolution, the fine tuning to reach the optimum operating point is not too critical . As a rule of thumb, it was found that the best performance is achieved at a gain corresponding to the lower end of the plateau of minimum ENQ, where the excess noise in the signal is also minimized. Fig. 15 shows typical pulse height spectra obtained under two different operating conditions . (Note that the poorer resolution at the timing bias is a result of both operation at high gain and measurement of a lower energy -y-ray) . The distributions of the measured energy resolution at the recommended operating gain of 75 and at the optimum bias for spectroscopic and timing performance are shown in fig. 16 . The slightly higher average temperature at the time of measurement (=

F

3

5.5. EnerU resolution

z

The variation of spectroscopic performance over the range of operating voltage of the APD is shown in fig. 14. As expected, the energy resolution remains relatively stable over a broad range of bias ; at low bias, it is degraded as a result of the high detector capacitance and the drop in signal which results from an incomplete depletion of the APD; at high bias, the degradation in resolution results from the higher excess noise in the signal and from other unaccounted effects such as gain nonuniformity in the APD when approaching breakdown . To a large extent, the scintillator intrinsic resolution determines the detector resolution in its region of

ô

0

U z

BIAS VOLTAGE (Volts)

Fig. 14 . Energy resolution of the 662 keV -y-ray from 137Cs as a function of bias voltage for three representative detectors and one degraded device . The manufacturer's data indicated an energy resolution of 21% FWHM at 440 V for detector no . 150-1 . Below about 300 V, the APD has not reached its normal operating mode as it is not completely depleted .

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors

594

24' C) and the marked degradation of two detectors (nos . 150-1 and 159-1) mostly explain the difference between the distribution of energy resolution at gain 75 measured in this work and the one obtained from the RCA supplied data (dashed line in fig. 16).

(a) Optimum bias for energy

L-

5.6 . Timing resolution

(662 keV)

Fig. 17 shows the timing performance of typical APD detectors as a function of bias . There clearly exists a region of optimal bias where the best timing resolution is achieved . This region generally occurs at a bias of 10-30 V below the breakdown voltage but does not necessarily coincide with the maximum signal-to-noise ratio as measured at the output of the fast filter . Instabilities in APD characteristics near breakdown, it seems, are responsible for the rapid degradation of timing performance beyond some point. For this reason, estimates of the optimum operating point for timing based only on the measurement of equivalent noise charge are unreliable. Operation at a voltage 25-30 V below breakdown is generally very close to the optimum for most APDs and avoids most of the undesirable effects associated with avalanche breakdown. The distribution of the best timing results obtained for the 38 tested detectors is given in fig. 18 . The average of 10 .5 ± 1 .1 ns FWHM represents a significant improvement over the 15 +_ 2 ns FWHM measured at the manufacturer's recommended voltage where the APD is operating at lower gain . At the point of best timing, the contribution

14

16

18

20

22

24

26

28

30

(b) Recommended bias -measured (24°C) ----- specified(22°C)

C)

66.

O

tY W m

(662 keV)

Z 0

4

14

16

18

20

22

24

26

28

30

(c) Optimum bias

12

for timing

(511 keV)

0

15

20

25

ENERGY

30

~ 35

r 40

r 45

RESOLUTION (%

r 50

~ 55

FWHM)

Fig. 16 . Distribution of the measured energy resolution for APD detectors operated at : (a) the optimum bias for energy resolution ; (b) at the recommended gain of 75 ; and (c) at the optimum bias for timing resolution (511 keV) . The dashed line in (b) represents the distribution of the energy resolution data supplied by the manufacturer (see text). to the noise from the APD is from 0 .35 to 1 .8 times that of the preamplifier . The difference in timing performance of the two detectors within the same module rarely exceeds 2 ns ; however, the optimum performance is often obtained at different bias voltages (up to 40 V apart) and at gains differing by a factor of up to 2.5 .

0

100

zoo

300

CHANNEL NUMBER

400

500

Fig. 15 . Typical pulse height spectra obtained with APD detectors (a) at the optimum bias for energy resolution (662 keV from 137 Cs); (b) at the optimum bias for timing resolution (511 keV from 22 Na).

6. Discussion 6.1 . Selection criteria An important issue for the performance, and ultimately, the cost of APD detectors has been the selection

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors 30~

o " "

F 3 v

# 144-2 #159-2 #164-1 #167-2

595

30

(490 , 605V) (520, 570V) (490 , 560V) (490 , 565V)

F x 3

z O

Ô

Ô

Ô N OC

U z_

450

Soo

BIAS VOLTAGE (Volts)

550

600

Fig. 17 . Timing resolution as a function of APD bias for detectors in coincidence with a fast plastic detector (511 keV) . The recommended bias (M = 75) and the breakdown voltage of the tested detectors is indicated.

1V 10

20

30

40

SNR (electrons/rms electrons)

50

60

Fig. 20 . Energy resolution as a function of signal-to-noise ratio for the detectors operated at the point of optimum spectro scopic performance. SNR is measured in the slow channel (TA = 300 ns). A fit to the data yields : R = (9 .4 SNR-z + 0.0268) 1 / 2.

1s

tude reported in fig. 9 suggest that the statistical and, most of all, the systematic contributions to the detector resolution cannot be neglected . In fact, as fig. 20 clearly

0 ~ 10 1 u

shows, an excellent correlation exists between the energy resolution and the measured signal-to-noise ratio. Whereas the APD noise performance has some bearing

c ô m F z

5-

o

on the achieved resolution, we must conclude that the input signal to the APD, which is dependent on the scintillator light output and the quality of the APD

678910111213141516 TIMING RESOLUTION (nsec EWHM)

Fig. 18 . Distribution of the best timing resolution (511 keV, in coincidence with a plastic detector) obtained with the APDBGO detectors. of the APDs for the application in scintillation detection . Results obtained in a previous work [23] and in the present investigation (fig . 19) support the selection of

the APDs based on their dark noise current. However, the dispersion of the resolution data relative to APD

noise in fig. 19 and the large variation in signal ampli30 F x 3 e z 250

M=75 662 keV

ô 09

For timing purposes, the largest possible signal from the APD detector is desired in order to minimize the effect of the preamplifier noise in the fast channel. APDs achieving the highest gain would thus be expected to yield the best timing performance. Our results show no evidence of such a correlation if the breakdown voltage, the maximum operational APD gain or the gain at the point of best timing is considered . No clear correlation can be established either with the detector

output signal and the APD dark noise measured at the timing operation or at some reference point such as the recommended gain of 75 . Therefore, these characteris-

tics do not provide reliable selection criteria of APDs intended for timing applications . As for the spectroscopic performance, the timing resolution can be weakly correlated with the equivalent noise charge (fig . 21) and more strongly with the signal-to-noise ratio (fig . 22). Consequently, the same conclusions as above still apply regarding the selection of APD and the quality of

20

U z

15_~ 0

coupling to the crystal, is largely responsible for the variation of spectroscopic performance from one detector to the other.

20

40 60 60 80 ENQ (rms electrons)

Fig. 19 . Energy resolution as a function of equivalent noise charge for the detectors operated at the recommended bias (M = 75). ENQ is measured in the slow channel (TA = 300 ns).

detector assembly for devices used in timing applications. However, any improvements in signal amplitude or APD dark noise is likely to produce more significant improvements of the timing resolution, since the timing accuracy is directly proportional to (SNR)-1 .

In another respect, the wide dispersion of APD characteristics makes their matching within the same

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors

59 6 16

3 d z 12-

0

5 ô

10

U

z s 6

0

100

50

150

200

250

ENQ (rms electrons)

Fig. 21 . Timing resolution as a function of equivalent noise charge for detectors operated at the optimum point for timing. ENQ is measured in the fast channel (TA = 20 ns). module rather tricky . According to the manufacturer, the APDs used in the detectors tested in this work came from various small production lots which allowed matching of the gain generally within 10%. However, large differences exist in noise characteristics and resolution performance of the detectors. As production for this application gradually moves to a larger scale, a larger sample of APDs will be available. Although this will permit better matching of the two detectors within a module for spectroscopic applications, our results indicate that good matching will be difficult to achieve for timing due to the rapid variations of characteristics near breakdown. 6.2 . Future improvements

Possible improvements of APD detectors performance include the availability of APDs with better intrinsic characteristics and more complete collection of the scintillation light from the crystals . In particular, the recent production of more uniform devices reaching avalanche breakdown at higher gain [20] will result in

SNR- ' (x10-' rms electrons/eledrons)

Fig. 22 . Timing resolution as a function of the inverse of the signal-to-noise ratio for detectors operated at the optimum point for timing . SNR is measured in the fast channel (TA = 20 ns).

better timing performance due to the increased signal amplitude. However, as the results of this investigation tend to demonstrate, gain in signal amplitude resulting from an optimization of the light collection by the APD will likely yield the most significant improvements . The light output from the BGO scintillators in RCA modules is found to be a factor of 2 lower, on the average, than what was previously reported for an approximately equivalent geometry [22] . Moreover, the signal amplitude shows large variations from detector to detector . Studies carried out in this laboratory have shown that a 30% increase in light output and good reproducibility are readily possible with the present detector setup by improving the optical coupling of the APD to the scintillator and by eliminating trapped light modes from the crystal [30] . In the timing measurements, the APD noise current is the limiting factor for about 50% of the tested detectors. This confirms that some improvements are possible by cooling the detectors. From an analysis of the data, it appears that cooling would be most useful for the more noisy detectors which yielded poor timing results . For the best devices where the preamplifier noise is already dominant, relatively small improvements in timing resolution ( < 2 ns) can be expected . In any case, cooling would be justified not only by the improvements in performance resulting from a decreased APD dark noise, but also because it would permit easier detector operation and would allow devices otherwise unusable at ambiant temperature to be utilized for timing. 7. Conclusion The measurements reported here have demonstrated that BGO-APD detectors operated at room temperature and with an amplifier shaping time an order of magnitude shorter than with other unity gain solid state photodetectors yield excellent spectroscopic performance. Energy resolution of the 662 keV photopeak is 20% on the average, with a best observed value of 16%, mostly determined by the crystal intrinsic resolution . A typical minimum equivalent noise charge of only 40 rms electrons is also achieved, producing a detection threshold of the order of 100 keV (BGO) with the present detector setup. Unprecedented timing performance has been obtained when operating the APD detectors at their maximum operational gain . A timing accuracy of 10 .5 ns FWHM on average has been measured in reference to a fast plastic detector, which translates to a timing resolution of the order of 15 ns FWHM for coincidence detection of annihilation events by a pair of APD detectors. Thus, APDs are the only semiconductor photodetectors having appropriate timing capabilities

R. Lecomte et al. / Status of BGO-avalanche photodiode detectors for coincidence detection in the PET application. It must be mentioned that such performance can only be achieved using front end electronics adapted to the APD detector characteristics .

Clear correlations of the detector performance in energy resolution and timing have been established with the signal-to-noise ratio of the BGO-APD assembly . A quick determination of the detector optimum operating conditions and an estimation of its performance can

therefore be obtained from a direct measurement of this parameter. In addition, better matching of the devices appears possible through the use of the signal-to-noise ratio as selection criterion. Further improvements in spectroscopic and timing performance can be expected

from the availability of more uniform and higher gain APDs, from an optimization of the scintillation light collection by the APD and by cooling the detectors to

reduce APD dark noise. The APD detectors tested in this work are currently being monitored as part of a prototype positron tomograph for reliability and stability of performance.

Acknowledgements We wish to thank D. Gendron and P. Bergeron for their assistance in taking the measurements . Frequent discussions with A.W . Lightstone and R.J . McIntyre of

RCA Electro-Optics (Vaudreuil, Qc, Canada) were most useful and are gratefully acknowledged. This work was supported by grants from the Medical Research Council of Canada, from the Natural Sciences and Engineering Research Council of Canada and by a contract from RCA Inc. Support by Le Fonds de la Recherche en Sant6 du Qu6bec through a scholarship (R .L .) and a studentship (C .M .) is also greatly appreciated .

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59 7

[5] G. Hall, D. Robinson and 1. Siotis, Nucl. Instr. and Meth . A245 (1986) 344. [6] K. Yamamoto, Y. Fujii, Y. Kotooka and T. Katayama, Nucl . Instr. and Meth . A253 (1987) 542. [7] M. Goyot, B. Ille, P. Lebrun and J.P . Martin, Nucl . Instr. and Meth . A263 (1988) 180. [8] H. Grassman et al ., Nucl . Instr. and Meth. A234 (1985) 122. [9] E. Sakai, IEEE Trans. Nucl . Sci. NS-34 (1987) 418. [10] I. Holl, E. Lorenz and G. Mageras, IEEE Trans. Nucl . Sci. NS-35 (1988) 105. [11] S.E. Derenzo, Nucl . Instr. and Meth . 219 (1984) 117 . [12] S.E. Derenzo, IEEE Trans. Nucl . Sci. NS-31 (1984) 620. [13] Y. Yamashita, H. Uchida, T. Yamashita and T. Hayashi, IEEE Trans. Nucl. Sci. NS-31 (1984) 424. [14] J.B. Barton et al ., IEEE Trans. Nucl . Sci. NS-30 (1983) 671 . [151 J.S. Iwanczyk et al ., IEEE Trans. Nucl. Sci . NS-30 (1983)

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