Resolution improvement in CdTe gamma detectors using pulse-shape discrimination

NUCLEAR

INSTRUMENTS

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(I975) 591-595; ©

NORTH-HOLLAND

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R E S O L U T I O N I M P R O V E M E N T IN CdTe G A M M A D E T E C T O R S U S I N G PULSE-SHAPE DISCRIMINATION L. T. J O N E S a n d P. B. W O O L L A M

Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloueestershire, England Received 11 N o v e m b e r 1974 Pulse-shape discrimination h a s been used to improve significantly the resolution o f C d T e g a m m a detectors• By selecting those pulses with risetimes less t h a n l'00 us, the full width at half m a x i m u m (fwhm) resolution at 661 keV m a y be reduced to

1 4 k e V at r o o m temperatures a n d 8 keV at 0°C. T h e resulting peaks have a full width at tenth m a x i m u m (fwtm) less t h a n twice the f w h m a n d m a y readily be identified a n d integrated u s i n g s t a n d a r d spectrum-analysis techniques.

1. Introduction There are a number of areas on nuclear power stations where an "in-situ" analysis of gammaemitting isotopes is necessary. Usually a full width at half maximum (fwhm) resolution better than 15 keV is required in order to separate gamma lines emitted from a range of activation and fission products; for some applications this resolution must be attainable in relatively high photon fluxes. In addition, the physical constraints of the plant are often such that the overall d~mens~ons of the gamma spectrometer must be considerably smaller than would normally be acceptable in the laboratory. A typical power-station application is the analysis of photon fluxes in heat exchangers where access between the tubes may be restricted to devices only 2-3 cm in diameter. Cadmium telluride is potentially a suitable material for this application; at present, it can only be manufactured in small volumes, but in high-flux environments this may be a positive advantage. Since CdTe is capable of room-temperature operation, no bulky cooling systems are required and the necessary physical dimensions may reasonably be attained. Unfortunately, the fwhm resolutions and peak-to-Compton ratios, previously reported ~'2), are at least a factor of two worse than the values required for such power-station applications. In addition, the considerable low-energy tailing, generally observed in CdTe spectra, makes quantitative peak-area extraction very difficult. However, CdTe has already been used 3) as a room-temperature spectrometer for the identification and assay of nuclear fuels where its resolution made it superior to scintillation detectors. The present work discusses a technique in which the resolution obtainable from a CdTe detector may be improved significantly by selection of certain portions of the risetime distribution. The results presented here show a factor of-three improvement in the fwhm

resolution at 661 keV at room temperature and demonstrate that the use of pulse shape discrimination (PSD) allows a full width at tenth maximum (fwtm) resolution less than twice the fwhm to be achieved.

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2. Pulse-risetime considerations

Fig. 1 shows the spectra generally obtained from I37Cs and 6°C0 using a CdTe detector. The fwhm resolutions, 43 keV and 70keV, respectively, are similar to those reported previously l'2). These spectra were recorded at room temperatures, using 2/~s amplifier time constants and 200 V bias. The present detector* was 30 mm 3 of halogen-doped CdTe with platinum contacts. Whilst the t37Cs peak is reasonably Gaussian, the 6°Co peaks exhibit considerable low-energy tailing. This tailing is attributable both to charge trapping, which reduces the magnitude of the slow*

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rising pulses and to degradation o f pulse height occurring when the detector risetime becomes comparable with the amplifier time constants*). Both of these effects are proportional to photon energy and are largely masked at ~37Cs energies by the electronic resolution which, because of the high (400 nA) leakage current, is very similar to the detector resolution. The electronic resolution due to detector leakage current I is proportionaP) to \/(B), where t is the amplifier time constant. For a fixed bias, and hence leakage current, the resolution may, therefore, be improved by reducing the amplifier time constants. However, this increases the low-energy tailing because a larger proportion of detector pulses now have risetimes comparable with the amplifier time constant. Alternatiyely, the electronic resolution may be improved by reducing the bias and hence the leakage current. This, however, results in an increase in detector risetime leading to worsened tailing because of both trapping and timeconstant effects. The leakage current and electronic resolution may also be reduced by cooling the CdTe detector, but for a given temperature it is evident that best resolution will be obtained using a normal bias, short time constants and rejection of those pulses which have risetimes which lead to tailing. The risetime of a detector pulse may be identified with the charge transit time across the crystal. In CdTe the electron mobility, 1000cm2/(Vs), is ten times higher than the hole mobility, and this results in large variations in risetime with gamma-interaction position through the material. A very simplified model, which assumes uniform field and point interaction, shows that interactions close to the negative contact give a risetime of 50 ns for a 1 mm thick crystal with 200 V bias. For an event close to the positive contact the risetime is limited by the hole transit time across the detector and, for the same conditions, the risetime is then 500 ns. Since the carrier lifetimes in CdTe are of the order of microseconds, the probability of significant

charge trapping in the latter case is apparent. Interactions in the centre of the detector give risetimes between these limits. However, there is a band close to the negative contact of one tenth the crystal width in which the risetimes are approximately constant, since holes generated in this region travelling to the negative contact have the same transit times as electrons travelling ten times as far in the opposite direction. In practice, the gamma ray produces ionisation in the detector via an intermediate electron whose range at energies of interest can be greater than the crystal width. Thus, only events in which this electron looses all its energy in the surface region will have short risetimes.

3. Pulse-shape discrimination From the previous discussion, it is apparent that CdTe resolution may be improved by accepting only those pulses with short risetimes, in order to study this effect, the pulse-risetime distribution was recorded using the system shown in fig. 2. The pulse-shape analyser was the Ortec model 458. The output from the analyser was non-linear with risetime because of the _@~~!_ "

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distortions produced by the intrinsic risetime (70 ns) and finite delay line (700 ns) of the delay-line-shaped amplifier. Pulse-risetime spectra, corrected for these effects, are shown in fig. 3 for the full energy range and for the photopeak of laTcs. This shape is typical of the spectrum obtained from a range of source energies and appears, within the uncertainties, to be independent of bias polarity. For the present purpose, it is important that the pulse-risetime spectrum is independent of energy: the similarity of the two sets of data in fig. 3 confirms this point. Fig. 4 shows the spectra obtained from a 137Cs source for some particular CdTe risetimes. These were recorded using 0 5 #s amplifier shaping constants to reduce electronic noise, and the worsening in lowenergy tailing in the case where no pulse-shape discrimination (PSD) was used may be seen by comparison with fig. 1 Selecting only those pulses with risetimes less than 100 ns produces a photopeak whose high-energy edge corresponds to that of the noPSD spectrum. As the window is moved to longer risetimes, a shift in peak position occurs, accompanied 4000

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by a degradation in resolution and, consequently, peak-to-Compton ratio. The spectrum recorded at less than 100 ns risetimes clearly shows the cadmium X-ray escape peak at 24 keV below the full-energy line. As the risetimes lengthen and resolution is degraded, the escape peak is no longer resolved from the main peak, leading to a sudden worsening in fwtm resolution The contribution of time-constant effects to the variation in peak position with 10-90 % risetime may be seen from fig. 5. Here the theoretical peak shift ¢) as a function of risetime is plotted for 0.5 #s time constants and compared with peak shifts observed with

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a variable-risetime pulser. Also shown in fig. 5 is the variation in 137Cs peak position with risetime. The agreement between the theoretical time-constant effect and the pulser measurements is good, the difference between this and the photopeak shift is due to chargetrapping effects. Charge trapping has less effect than the time constant on peak shift at this 0.5 #s setting: even at the longest risetime considered the two are only just equal. At a risetime of 500 ns the peak shift due to charge trapping is 4 %, which corresponds to a mean hole lifetime in CdTe of about 15 #s. The degradation of resolution and peak-to-Compton ratio with increasing risetime is shown in fig. 6. Here the parameters are plotted against an upper risetime discrimination level, using a 137Cs source. For risetimes less than 100 ns the fwhm resolution was 14 keV, the fwtm resolution 27 keV and the peak-to-Compton ratio 2.9. When all risetimes up to 500 ns are included, these values have worsened by a factor of 2 and with no PSD the resolution becomes a factor of 3 worse. 4. Application to the radium spectrum In order to test the ability of the CdTe spectrometer to resolve and integrate closely spaced gamma lines (such as might be found under typical power-station conditions), a spectrum was recorded using a 226Ra source. Only those risetimes less than 100 ns were accepted and the amplifier time constants were again 0.5/~s. The detector countrate was 20 kHz, the exposure rate being about 2 rad/h.

The resulting spectrum is shown in fig. 7. Analysis of the data was performed using the computer code 6) SAMPO which determines, from a few well-defined lines, the peak shape as a function of energy, and subsequently searches through the whole spectrum to extract weaker lines with similar shapes. A total of 23 lines were qualitatively identified with energies '~ 10-3

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within 1 keV of the accepted 7) values. Integrated peak areas were compared with tabulated intensities 7) to produce the total-system efficiency variation shown in fig. 8. It is notable that the efficiency function is exponential rather than the power law usually associated with germanium detectors. 5. Operation at reduced temperatures T h e 2 2 6 R a spectrum discussed in section 4 showed system fwhm resolutions varying from 13 keV at 295 keV to 17 keV at 1765 keV. The pulser resolution was 12 keV, due primarily to leakage-current noise generated at ambient temperatures of 25-30oC. Thus a significant improvement in resolution should be obtained if the leakage current were reduced by cooling the detector. Because of dimension restrictions in the expected applications of the device, it is not practicable to consider large temperature drops, but fortunately the wide band gap in CdTe makes it rather sensitive to temperature changes, so that cooling to 0oC proves useful. Fig. 4 shows the spectrum obtained from 137Cs using an optimum bias of 100 V and a 100 ns risetime limit. This resulted in a leakage current at 0oC of 30 nA and a pulser resolution of 7 keV. The photopeak resolution improved from 14 keV to 8 keV and the peak-to-Compton ratio increased to 6. For nuclearpower applications, this cooled system appears to be very promising, although some further work is needed to improve the time stability of the system at this lower temperature. Recently 8), resolutions of 12 keV have been obtained at 137Cs energies using CdTe with M.I.S. contacts

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operated at high fields and cooled to - 2 0 o C . Peak shapes and pulser resolution quoted there suggest that the use of PSD could also produce a considerable improvement in the resolution of that system. 6. Conclusions Pulse-shape discrimination shows considerable advantages in improving the resolution obtainable from CdTe detectors. It has been shown that resolutions of 14 keV at room temperatures and 8 keV at 0oC may be achieved at 661 keV by selecting only those pulses with risetimes less than 100 ns. The resulting peak shapes have fwtm less than twice the fwhm and can readily be identified and integrated using standard spectrumanalysis codes. This paper is published by permission of the Central Electricity Generating Board. References 1) K. Zanio, W. Akutagawa and H. Montano, IEEE Trans. Nucl. Sci. 19 (1972) 257. 2) R. O. Bell and F. V. Wald, IEEE Trans. Nucl. Sci. 19 (1972) 334. z) W. Higinbottom, K. Zanio and W. Akutagawa, IEEE Trans. Nucl. Sci. 20 (1973) 510. 4) E. Baldinger and N. Frantzen, Electronics Electron Phys. 8 (1956) 256. 5) E. Kowalski, Nuclear electronics (Springer-Verlag, Berlin,

1970). 6) j. T. Routti and S. G. Prussin, Nucl. Instr. and Meth. 72 (1969) 125. 7) j. K. Dickens, ORNL-TM-3509, 1971. s) p. Eichinger, N. Halder and J. Kemmer, Nucl. Instr. and Meth. 117 (1974) 305.