Timing response of CdTe detectors

Nuclear Instruments and Methods in Physics Research A326 (1993) 319-324 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A '

Timing response of CdTe detectors G. Baldazzi a, D. Bollini b, F. Casali M. Rossi b and J.B . Stephen

b,

P. Chirco

b,

A. Donati ', W. DUSi d, G. Landini `,

Dipartimento di Fisica, Uniuersità di Bologna, Via Irnerio, 46-40126 Bologna, Italy n Dipartimento di Fisica ed INFN, sez. di Bologna, Bologna, Italy ` Istituto TESRE/CNR, Via de' Castagnoli 1, 40126 Bologna, Italy d Istituto TESRE/CNR ed INFN, Bologna, Italy

Semiconductor CdTe detectors are gaming wide acceptance in many applications where X- and -y-ray measurements are necessary, such as in astrophysical research, medical imaging and industrial radiography . Good timing response is critical both in applications like positron emission tomography, where fast coincidence capabilities are required, and in single photon counting when a high counting rate is needed . The typical configuration employed, where the direction of the impinging radiation beam is parallel to the collecting electric field, has one well known drawback an increase m active layer, necessary in order to reach a satisfactory absorption efficiency for the detection of high energy photons, leads to a longer transport path for the charge carriers generated . As a consequence, there is a degradation in energy resolution and a broadening in time response. In the present paper, measurements of the timing response for an unusual configuration of CdTe detectors are presented. In this configuration, which we call a PTF (planar transverse field) detector, the collecting electric field and hence the transport direction of carriers is transverse to the direction of the incoming photons and so detection thickness and transport length are independent . In this way the absorption layer can be increased without impairing the timing performance. The measurements described herein have been performed using a PTF detector having dimensions of 2.5 x 2.5 x 20 mm 3, m order to have a good efficiency for annihilation -y-ray photons.

1 . Introduction Of all the semiconductor materials useful for Xand -y-ray detection, the importance of cadmium telluride (CdTe) is becoming more and more enhanced due to its improved performance both as a photon counter and as a spectrometer . The main physical features that make it preferable to Ge or Si for specific applications are its capability to work at room temperature (thanks to its high bandgap (1 .47 eV), which implies a low leakage current) and good absorption efficiency (due to its high average atomic number (4852) and density (6 .06 g/cm 3)) . Furthermore, this material can be easily worked thus allowing the production of microdetectors having small dimensions and/or more convenient geometry . The low mobility of charge carriers, particularly of the holes, and the phenomenon of charge trapping which is still present in spite of the relative improvements in lattice quality of crystals, in purification and in stoichiometric balance, are the limiting factors for both the energy resolution (that is anyhow of the order of 1% at 511 keV when detectors are used as spectrometers) and timing properties .

One way to reduce these disadvantages has been found by adopting an unusual configuration which we call the PTF (planar transverse field) detector (fig . la) in which the applied collecting electric field is transverse to the direction of travel of the impinging photons [1]. The properties of CdTe detectors constructed following this configuration, allowing thick absorption dimensions and reduced collection path, are very interesting, especially for those applications which require the use of detector arrays with small pixel dimensions and/or detection of high energy photons and for those applications where good time resolution is required. In this work we have studied the timing properties of CdTe detectors manufactured by Eurorad so as to evaluate the distribution of charge collection time and the time resolution of a system formed by two CdTe detectors working in coincidence . In order to investigate the possibility of the use of this type of detector in applications such as nuclear medicine (positron emission tomography) and astrophysics (telescopes for gamma astronomy), a set of experimental tests has been carried out using 511 keV annihilation photons emitted by a "Na radioisotope source .

0168-9002/93/$06 00 © 1993 - Elsevier Science Publishers B.V . All rights reserved

VII . IONIZATION-BASED

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G. Baldazzi et al. / Timing response of CdTe detectors

2. Detector configuration and analytical method Generally speaking, to date detectors were used in a standard configuration that we call PPF (planar parallel field), where the direction of the collecting electric field is parallel to the direction of the incoming radiation (fig. lb). In this way, an increase in the detector thickness, useful in order to reach a higher absorption efficiency, implies a corresponding increase of charge collection path and importance of charge trapping, leading to a degradation of both timing properties and energy resolution . On the contrary, for detectors employed in the PTF configuration and with a well collimated narrow beam, the collection path and absorption efficiency are independent, as the collection path is related to the cross sectional dimension of the detector, while absorption efficiency depends mainly on the thickness.

2.5 mm

Moreover, it is known that the charge collection time depends strongly on the point of interaction within the body of the detector. Due to the low mobility of holes B, h, the least favourable condition occurs when the electron-hole pair production takes place near the positive electrode, where the holes must transverse all the detector before their collection on the negative electrode. In this case the collection time is : Tsiow

=

d

d

Uh

AhE

- =

d2

_

k,hv,

where d is the distance between the electrodes, Vh the bias voltage and r h the speed of the holes. The derivation of eq . (1) requires the use of the well known relationship : t'h

° Ah E,

which is valid for electric fields up to several kV/cm. On the contrary, the most favourable case occurs when the collection time of electrons and holes are equal; in this case we have : ,rfast

X, -

Lh

-

d -xm il ,

-

xmd

I2 hVh

which occurs when the primary interaction takes place at a distance xm-d

PTF detector

PPF detector

Fig. 1. Two different arrangements for planar geometry CdTe detectors: (top) planar transverse field (PTF) detector with the collecting electric field perpendicular to the direction of incoming photons and (bottom) planar parallel field (PPF) with the field parallel to the photon direction.

(4)

Ah ue+uh

from the negative electrode. The existence of a point of interaction which gives an optimum performance as regards the timing response introduces a noticeable difference between the behaviour of PPF and PTF detectors; while in the former the length of the collection path depends on the distribution of the points of interaction within the crystal body (which follow the exponential law of absorption) and there is no way to bypass this through the experimental setup, in the PTF configuration this distance depends on the incidence point of the primary radiation on the detector entrance surface. Thus it is possible to select, through the use of a collimator over the entrance window, a region of the detector centred on the point x m and, as a consequence, to allow access to the detector only those events that will have the shortest collection time and hence the best timing response . The advantage induced by this selection is noticeable as the charge collection efficiency 77(x) has a value given by Hecht's equation : r1(x)=LeII-expl d

+d

L

1 _ exp

d-x)J Ae -

h )1 '

321

G . Baldazzi et al. / Timing response of CdTe detectors

where A e and Ah are the mean free path of the carriers in the crystal before trapping, which has a maximum where x =x m. Therefore, those pulses produced in the neighbourhood of xm are less involved in charge trapping and as such are the most useful from the point of view of obtaining a good spectroscopic performance. In this way the selection of pulses made possible through the use of a collimator over a PTF detector allows an optimization in both the timing and spectroscopic properties . It is interesting to note that the ratio Tslow

Il'e + 11 h

? fast

/1 h

is a measure of the intrinsic time spread of the detector due to the mobility of the carriers in the material, and this is independent of the dimensions of the detector. The lower the ratio, the better is the time response of the detector with the limiting value of 2 being reached when Fr e = la h . For currently available CdTe crystals this ratio is about 11, which is worse than the value for a cooled detector like Ge (for which the ratio is about 3) or Si (3 .8), but better than the value for Hg12, another well known room temperature semiconductor detector, for which the ratio is about 26 . 3. Experimental setup To characterize the timing properties of solid state detectors two parameters are generally used : the distri-

CdTe Prearnp D et Power Dias Supply

Timing Filter AnVVier

Cœgant Fraction

1t

bution of charge collection time and the time resolution for a system of two detectors operating in coincidence mode . The former gives the value of the intrinsic timing response of the detector and is numerically expressed by its FWHM, while the latter is the distribution of coincidence delays in a twin detector system operating in coincidence mode, also quoted in terms of its FWHM [2]. For these measurements a source of simultaneous events that must be detected by two different chains is required . For this reason a 22 Na source, which emits two 511 keV photons deriving from the annihilation of a positron, was chosen . This radionuclide was placed in a lead collimating system together with the CdTe detectors in order to guarantee good alignment between the source and the detector . 3.1 . Distribution of charge collection time

In order to measure this parameter, a system comprising a Nal(Tl)/PMT and a PTF CdTe detector operating in coincidence mode was employed (sec fig. 2) . The signal produced by the Nal(TI) detector was used as a reference in order to evaluate the distribution in time response of the signals from the CdTe detector [3]. A single channel analyzer (SCA) selected an energy window centered on 511 keV in order to ensure that a coincidence signal was generated only when a pair of 511 keV photons interact, one in each detector . The CdTe interface between the detector and the

Gate&Delay Generates

Coincidence

C.S.F . CdTe Detector -Pb Colhmator -~ NaI(Tl) -~ Scmtillator

Photomultipler -~

Radioisotope Source 22 Na

c~-

Preamp Fower H.V . PM Supply

Gate

Start Stop

Preamplifier -

Gate &Delay Generator

TPHC

Input

ADC

A

M A C

Tm* Filter Amplifier

Fig. 2. The block diagram of a system used to measure the charge collection time distribution of CdTe detectors. VII. IONIZATION-BASED

322

C. Baldazzi et al / Timing response of CdTe detectors

electronic chain was achieved using a dedicated circuit that provided the detector bias voltage and linked the detector output with a charge sensitive preamplifier (CSP). A constant fraction (CF) discrimination unit was used to process the amplifier output pulses in order to guarantee an accurate pulse timing analysis and to reduce critical phenomena such as time walk . A coincidence unit provided a gate to an analog-to-digital conversion (ADC) unit and drove a CAMAC Lam signal for data acquisition . The gate and delay generator in coincidence output adjusted the gate signal to admit the TPHC ouput pulse for correct conversion . For technical reasons, the delay needed in order to shift the position of the distribution in the time scale was introduced in the Nal(Tl) detector branch rather than m the CdTe branch . This required the start and

stop signals to be reversed with respect to an ordinary situation (that would be characterized by Nal(TI), that is quicker, as the start signal and CdTe as the stop); so lower values on the time scale correspond to longer time delays . This arrangement does not affect the goal of this set of measurements because only the spread of these delays, and not its absolute value, is of interest . The calibration of the time scale was performed by introducing known delays between the start and stop signals in the TPHC . From eqs. (1) and (3) it is clear that the collection time is strictly dependent on the bias voltage Vi, applied to the detector . For this reason a set of measurements of this time was performed for different values of Vt, and the results are reported in fig. 3, for both the 10 x 10x5 mm ; PPF and the 2.5X2 .5 X20 mm3

100 -r 90 80 + 70 +

50 + 40

t (a)

30 +

50

i

I

75

100

r125

i

+

150

175

200

225

250

300

Bias Voltage (V) 350 300 250

v

200 150 ~ 100 50 50

75

100

125

Bias Voltage (ns)

150

170

Fig. 3. The FWHM charge collection time distribution vs bias voltage for (a) a 2.5 x 2.5 x 20 mm' PTF detector and (b) a 10 x 10x5 mm 3 PPF detector .

323

C. Baldazzi et al. / Timing response of CdTe detectors Constaid

n_

Fraction

Caixidence

Statt TP S

OP

ADC

A M A

Constant Fraction

Delay

Fig. 4 The block diagram of the electronic arrangement used to measure the time resolution between a pair of identical CdTe detectors.

PTF detectors. These data show clearly the improvement in the collecting time for PTF detectors with respect to the PPF configuration, due to the reduced collection path . It should be noted that the settings of the measuring equipment were maintained constant over the entire bias voltage range in order to guarantee comparable experimental conditions . However, measurements performed with the settings optimized for each value of the bias voltage allow better results to be obtained ; indeed the best value of 45 ± 1 ns was reached for a bias voltage of 200 V.

ns was found, while for the 2.5 x 2.5 x 20 mm 3 PTF this value is 75 .0 ± 2.5 ns (see fig. 5). Assuming that the twin detectors contribute in an independent way to this spread, we can obtain for a single detector a time resolution of 310 ns for the PPF and 53 ns for the PTF. 4. Conclusions The data reported was obtained with the aim of investigating the timing capabilities of detectors used

3 .2 . Time resolution Events

1800 7

The measurement of time resolution for two CdTe Tim e Resolu tion 1 detectors operating in coincidence mode requires an CdTe Detector electronic setup similar to that shown in fig. 2, but the 2 5.2 5x20Nal(TI) detector is now replaced by another CdTe (fig . 4). In this way, the two branches are nearly equal, with 1î0a the slight difference of an adjustable delay circuit inserted in one. The pulses coming from the TPHC produce a distribution of time delays ; the FWHM of this distribution represents a measure of the overall eo0 timing uncertainty in the measurement system . This parameter is very important for the applications cited above, as it represents the width of the timing window for event detection in a coincidence system . The measurements on both the PPF and the PTF i 35 detector were performed for a bias voltage V,, = 150 V, Delay (ps) which is the nominal bias of the detectors. For the PTF Fig 5. The time resolution of a 2.5 X 2.5 X 20 mm3 10 x 10 x 5 mm 3 PPF detector a FWHM of 438.0 + 2.5 detector pair coincidence system for a 150 V bias voltage. VII . IONIZATION-BASED

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G. Baldazzi et al / Timing response of CdTe detectors

in the new PTF configuration and are part of a wider study of this kind of detector [1,4] . The results are very interesting, as they are comparable to those reported in the literature [5] for detectors made from the same material but especially constructed so as to be able to work at higher bias voltages than used here . For this experimental setup a uniform irradiation over the entire surface of the detectors was used and so the results obtained represent an average of all possible timing responses. We can remark however, that from a theoretical evaluation of PTF detectors, it is possible to identify a very small region where an optimized timing and spectroscopic response is achievable . Moreover, it is reasonable to predict from these results that the development of CdTe PTF detectors with a special technology, for example having a diodelike structure, which could allow the application of much higher bias voltages, will permit a remarkable improvement in timing capability, which could itself lead to the employment of these detectors in applications such as PET where a very fast response is imperative .

Acknowledgements This work was partially supported by funding from the INFN (Istituto Nazionale di Fisica Nucleare) at the Department of Physics, University of Bologna, under the SOSGAD Project, from the MURST (Ministero per l'Università e la Ricerca Scientifica e Tecnologica) and from the CNR (Consiglio Nazionale delle Ricerche).

References [1] W. Dusi et al . presented at 7th Int. Workshop on Room Temperature Semiconductor X and y-Ray Detectors, Ravello, Italy, 1991 . [2] G.F . Knoll, Radiation Detection and Measurement, 2nd ed. (Wiley, New York, 1989). [3] G Bertolini and A. Coche, Semiconductor Detectors (North-Holland, Amsterdam, 1968). [41 F. Casali et al ., IEEE Trans . Nucl . Sci. NS-39(4) (1992) 598. [5] E. Frederick et al ., IEEE Trans. Nucl . Sci. NS-34(1) (1987) 354.