Methods for evaluating and analyzing CdTe and CdZnTe spectra

Nuclear Instruments and Methods in Physics Research A 458 (2001) 196}205

Methods for evaluating and analyzing CdTe and CdZnTe spectra Ray Gunnink , Rolf Arlt
* Fremont, CA, USA
International Atomic Energy Agency, Wagramer Strasse 5, PO Box 100, 1400 Vienna, Austria

Abstract Due to continued improvements, CdTe, CdZnTe (CZT) and other room-temperature detectors have generated renewed interest in potential applications } especially, gamma ray measurements for veri"cation of nuclear materials. However, the peaks generally have considerable tailing and therefore are di$cult to analyze accurately. Some principals of spectrum analysis are "rst summarized. This is followed by a discussion of spectrum and peak shape characteristics of some CdTe and CZT detectors we studied and a description of some methods and software analysis tools we developed to evaluate the shape characteristics of typical peaks. We conclude by illustrating the algorithms and analytical techniques we developed thus far by describing some application codes written and implemented for speci"c safeguards related measurements.  2001 Published by Elsevier Science B.V. Keywords: Room temperature semiconductor detectors; CdTe, CdZnTe Gamma spectra; Gamma peak "tting; Safeguards

1. Introduction As CdTe and CZT detectors continue to improve both in size and resolution, it is appropriate that more attention be given to the study of algorithms and methods for spectrum analysis. With the variety of detectors now available, it is also important to begin matching the performance capabilities of each detector type with speci"c applications. This paper will focus on only a few detector types that we have studied, namely, (1) small, Peltiercooled CdTe `PINa detectors, (2) small, highresolution CZT detectors, and (3) larger CZT

* Corresponding author. Tel.: #431-1-2600-21851; fax: #43-1-2600-729317. E-mail address: [email protected] (R. Arlt).

detectors, both of the `hemispherica design. These detectors were obtained by the IAEA from commercial sources. Data were taken using a variety of sources available at the agency.

2. Basic methods and algorithms for analyzing peaks If we consider an ideal, noiseless detector system, then all pulses from the `full-energya absorption of gamma rays of a given energy will appear in one channel of a spectrum, as in Fig. 1, rather than as

 Some commercial detector sources were Petersburg Nuclear Physics Institute (Russia } PIN type), RITEC (Latvia) and eV Products (US).

0168-9002/01/$ - see front matter  2001 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 0 3 6 - 6

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channel-by-channel continuum or background level from the lower boundary point to the higher:



G K B "b #(b !b ) (y !b )/ y G L K L H G I HL IL

Fig. 1. Instrument and detector e!ects disperse the sharp intrinsic characteristics of the peak width, tailing and background.

a broadened distribution. Energy degrading e!ects, such as small-angle Compton scattering and incomplete charge collection, inevitably produce pulses of lower energy. The result is a discontinuity or step in the background continuum at the peak. This ideal peak shape cannot be realized because several sources of energy dispersion are always present in gamma-ray detector systems. The principal sources are electronic noise, statistical processes in the detector associated with the conversion of gamma-ray energy into electron}hole pairs, and various charge-loss processes in the detector. They largely determine the shape of the peak observed and the underlying background continuum, illustrated in Fig. 1. The `full-energya peaks in a spectrum invariably are also superimposed on a `backgrounda continuum. This continuum is primarily due to Compton scattering of higher energy gamma rays, to Bremsstrahlung radiation, and to room background. To obtain the `neta channel counts in a region of interest, a channel-by-channel subtraction process is usually used to remove the background. The net-count intensity is then analyzed to yield peak positions and areas by a simple or more accurate, but complex, peak "tting process. To begin the background removal process, the energy boundaries of a peak or peak grouping are "rst determined, where the two continuum intensities (background levels) are measured. We use the following explicit function [1] to interpolate the



(1)

where B is the computed background at channel i, G y the spectrum count of channel i, b the previously G G determined background level at channel i from the "rst iteration, b the background level of the lowL energy side of the peak, and b the background K level of the high-energy side of the peak. This expression works equally well for single peaks and complex peak groupings. Furthermore, it produces the expected form of the continuum response when various detector and system processes disperse the ideal detection responses. Therefore, the experimental equivalent of the complementary error function approach is used in some procedures [2]. Sometimes the form of the equation must be modi"ed to account for a signi"cant slope in the background in the peak region. A number of algorithms have been developed to describe and "t the peaks of a gamma-ray spectrum. Many have been evaluated and summarized in a report by Helmer and Lee [3]. The developers of these algorithms invariably recognize that a Gaussian function describes the principal component of the peak shape. They also agree that tailing must be taken into account, particularly on the low-energy side, but have proposed di!erent methods to treat this portion of the peak shape. The results reported here generally use the algorithm we developed years ago for the GAMANAL code to analyze Ge detector spectra [4]. Two other algorithms we have used in this study and compared with our own algorithm are our implementations of algorithms "rst developed for the HYPERMET [2] and SAMPO [5] codes. All three of these algorithms combine a central Gaussian function with a low-energy exponential tail. They di!er in the method used to terminate the rising exponential tail as the calculation approaches the peak centroid. HYPERMET uses the complementary error function, which perhaps, most closely represents the model of the dispersion processes illustrated in Fig. 1. The SAMPO algorithm is discontinuous, "tting the tailing portion of the peak

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with an exponential function and the rest with a Gaussian function. The two are joined at a point where the slope and the height match. Some comparisons of results of "tting for the three methods are given later. Our (GAMANAL) algorithm contains the necessary central Gaussian component and also a `short-terma (and, if necessary, a `long-terma) tailing component, as expressed in Eq. (2): y "y exp[a(x !x)]#¹(x ) (2) G G G where y is the net counts in channel x , y the peak G G height at the peak position, x, a the peak width parameter, and ¹(x ) the tailing function, expressed G by Eq. (3): ¹(x)"[A exp(Bx)#C exp(Dx)][1!exp(Ex)]d (3) where x"x }x, A exp(B x) accounts for short term tailing and C exp(D x) accounts for long-term tailing. The parameters A and C relate to the `tailing heighta, B and D are the two tailing decay constants (slopes) and E is a truncation parameter that usually has a value of 0.4 for Ge spectra and 0.6 for CZT spectra. The "nal term, d, reduces the e!ect of T (x) to zero at the peak position x (d"1 for x (x and 0 for x 'x thereby limiting the tailing G G contributions only to the low-energy side of a peak). It is important to note that some of the variables enter linearly in the equations (such as y, A, and C), whereas others are in one of the exponent terms. The peak resolution parameter, a, is related to the peak width (p and FWHM) by the equation R a"!1/2p"!2.7726/(FWHM). (4) R Since the equations involved in "tting a peak are nonlinear, the "tting process is usually iterative. If several peaks overlap, the "tting process treats the net counts, y as a summation of the contribution of G each peak, as follows: L y" y . (5) G GH H Using Eq. (2) to describe the shape of each peak, j, in the cluster, the value of each independent vari-

able is determined so that the above function, when evaluated channel by channel, most closely approximates the original net data values, y . A `best "ta is G declared when chi-square is minimum. The leastsquares iterative procedure used in this evaluation process requires that the equations representing the data are linear. To linearize the equations, a "rstorder Taylor's series expansion of the function is performed about the variables that do not appear linearly in Eq. (2), a procedure also known as the Gauss}Seidel or Newton}Raphson method. From Eqs. (2) and (3), we see that y is related to paraGH meters of several peaks, designated here as p by the I following: y "f (p , p , p ,2, p ). (6) GH    I By retaining only the "rst term of the Taylor's expansion of this equation, we derive L yij!f (p , p , p ,2, p)" (df/dp )p (7)    I I I H where y is the contribution of the jth peak to GH channel i, p the best estimate in the kth peak I parameter, and p the change required in the best I estimate of the kth parameter to obtain a better "t to the data. Since Eq. (7) is linear in form, it can be solved by the normal method of least squares. However, because some of the answers are only incremental improvements to previously determined values, these answers are only used to update the variables and the calculations are then repeated. This process continues until convergence is reached, as determined by the magnitude of the change in each variable. The accuracy of the analysis greatly improves when the values of some variables are known, or can be predetermined and therefore treated as constants in the "tting process. 3. Characteristics of CdTe and CZT detector spectra For many years, NaI scintillation and germanium (Ge) semiconductor detectors have been the principal detectors used for gamma ray measurements and a vast knowledge and experience base

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Fig. 2. Comparison of uranium spectra taken with NaI, CZT, and Ge detectors.

has been gathered for these detector systems. NaI detectors are relatively inexpensive and easy to use but have quite a poor energy resolution compared to Ge detectors. However, the usefulness of Ge detectors in many applications is compromised by the need for liquid nitrogen temperatures to cool them. The characteristics of CdTe, CZT and some other room temperature systems generally fall between the performance extremes of NaI and Ge detectors. Typical spectra of a uranium sample taken with each of the three detector systems are shown in Fig. 2. Distinctively di!erent methods are required to analyze NaI and Ge detector spectra. Because of poor resolution, NaI spectrum analysis methods usually rely either on simple integration techniques or on more sophisticated response "tting techniques. In contrast, Ge spectrum analysis methods have evolved much farther and generally include peak search, "tting, and interpretation components. What then, is the most appropriate approach for analyzing CdTe and CZT spectra? Our studies show that many of the techniques used for analyzing Ge detector spectra can be used to analyze CdTe and CZT spectra. The most obvious changes in CZT peak shapes, compared to Ge peaks, are larger full-width at half-maximums (FWHM) and much greater low-side tailing. (Some CZT detectors may also exhibit high-side tailing [6]). Because the peak width and tailing are so

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pronounced, it is usually not necessary to use both `shorta and `longa tailing terms of Eq. (3) when "tting CZT peaks. When these di!erences are taken into account, many of the algorithms and software tools used for Ge spectrum analysis can be modi"ed and applied to the analysis of CdTe and CZT spectra. The observed energy resolution of gamma-ray peaks is generally due to electronic noise (e.g. diode leakage current and ampli"er noise), statistical #uctuations associated with charge production processes in the detector, charge trapping, and ballistic defects, due to low drift velocity of the carriers. For Ge detector systems, the "rst two e!ects dominate so that the peak width (p ) may be expressed as  p"p#p (8)    where p is the total peak width at half-maximum,  p the contributions due to the system `noise,'' and  p the detector contributions related to the statist ical process of electron}hole production. FWHM the full-width at half-maximum of the peak, as given in Eq. (4). Because p , is a constant for a given  spectrum and p is directly related to the energy  [1], Eq. (8) can be written as p"k1 #k2 E (9)  where k2 is related to the Fano factor. This factor for CZT detectors has been measured to be 0.089$0.005, a value not very di!erent from the value for Ge and Si detectors [7]. However, these measurements were made at low gamma-ray energies, using small detectors cooled to !403C and therefore do not represent the characteristics of detectors in routine use. We "rst note that because CdTe and CZT detectors are usually operated at room temperature, the noise term exceeds 1 keV. Furthermore, the low-energy tailing is quite pronounced in all the CdTe and CZT detector spectra. This is due to the ballistic de"cit, trapping and poor collection of all the charge within the integration time constant. The tailing characteristics (decay constant and height) are probably dependent on several factors, such as detector material and type of construction, size, bias potential, and gamma-ray energy. Two X-ray `escapea peaks are also sometimes observable, particularly in small detectors or at

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Fig. 3. PkCheck "t of Co spectrum peaks for a 500 mm CZT detector using three di!erent "tting algorithms.

very low energies. They will appear at 23 and 27 keV below any intense peak, corresponding to the escape of Cd and Te X-rays following gammaray interactions occurring near a surface of the detector. Very high-energy gamma-rays may also produce double and single 511 keV `escapea peaks. 4. Application codes PkCheck is a code we originally developed to evaluate several peak-shape characteristics of Ge detector spectra. It was designed to determine several peak parameters such as the position, area, FWHM, the low-side tailing height and exponential decay constant (slope), and the extent of pulsepileup or other distortions on the high side. We have now modi"ed the code to evaluate CdTe and CZT spectra as well. The code displays peak "tting pro"les as shown in Figs. 3 and 4 and also reports several peak shape characteristics. Many characteristics can be readily assessed by studying the visual displays. For example, a perfect "t produces data in a residual plot that is within about $2 standard deviations and shows no signi"cant structure. (Although this statement is true in principal, in practice, we know that the "t is very dependent on the counting statistics of the data. That is, good "ts are easy to attain when the peak counts are low but almost impossible to attain when a very high number of counts are present in a peak.) The plots of residuals in Fig. 3 for the GAMANAL and HYPERMET algorithms closely meet the ideal "t. However, the residuals from the SAMPO algo-

Fig. 4. PkCheck "t of Cs peak. Note the high-side distortion due to pile-up.

rithm signi"cantly exceed $2 standard deviations and show pronounced structure where the tail and Gaussian functions join. Other characteristics that can be visually observed include the extent of tailing and sometimes escape peaks and high-side pulse pileup (see Fig. 4). More useful than visual displays are printed results of the peak-"tting process, such as the FWHM values of peaks and the tailing intensities, reported as a percentage of the total area. The `FWHMa parameter in this discussion refers to the width of the Gaussian function only. The total peak width will be greater due to the large tailing that is usually present. Although Eq. (8) indicates that the (FWHM) should be linearly related to the gamma-ray energy,

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Fig. 5. FWHM versus energy plots for various CdTe and CZT detectors.

there are several reasons why linear plots of FWHM versus energy appear to be equally good. First, the tailing is so intense that it becomes di$cult to separate the Gaussian portion of the peak from the tailing part, particularly at higher energies (see Fig. 4). The resulting co-variance of some of the "tting parameters causes large uncertainties in the calculated results, such as the FWHM. Furthermore, the FWHM is algorithm-dependent. Second, the `noisea ("rst term in Eq. (8)) of CdTe and CZT detectors is typically much larger than the energydependent term in the equation involving the Fano factor. Third, judging by the di!ering slopes in Fig. 5, we conclude that there are more than two prominent line-broadening processes that can dominate the FWHM versus energy relationship. This should not be surprising, considering the complex gamma-ray interaction and charge collection processes that occur in these detectors. Low-energy gamma-ray interactions are localized near the surface of the detector and therefore may have very similar charge collection histories and produce pulses of uniform shape. However, higher energy gamma rays interact more homogeneously throughout the volume of the detector. This may result in further broadening of the peaks, due to di!erent charge collection properties in di!erent regions of the detector. It has been shown that when these ballistic and trapping e!ects are large and non-uniform throughout the volume of a detector, the FWHM versus energy relationship should be nearly linear [8].

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As expected, small and cooled CdTe detectors have much better resolution characteristics than larger room-temperature CZT detectors. However, because it is so di$cult to accurately measure the FWHM of peaks, the linear relationships shown in Fig. 5 should only be regarded as trends. Indeed, a non-linear plot resulted from a careful FWHM versus energy study of low-energy peaks in spectra taken with a Peltier-cooled CdTe detector. The PkCheck code is also useful for measuring the characteristics and extent of tailing, both as a function of the energy and of detector type. Although the tailing extends well below the peak position, we nonetheless generally treat it as `short-terma tailing and ignore the `longa tailing term in Eq. (3). The principal tailing characteristics that are measured are the tailing decay constant (slope) and height, and the percent of the total peak area contained in the tailing portion. It should be noted that the HYPERMET algorithm, as expected, ascribes considerably more area to the tailing portion of the peak than given by the other two algorithms. In the example illustrated in Fig. 3, the HYPERMET algorithm places nearly 60% of the peak area in the tail whereas the tailing portion reported by the other two algorithms is under 30%. Peltier-cooled PIN detectors have the least tailing, with only about 10% of the peak area in the tail at low energies, (e.g. at 122 keV peak). The tailing increases with energy for these detectors and is about 30% at 1 MeV. The room temperature detectors we tested generally start at 20 to 30% tailing at low energy and may increase to more than 50% of the peak at higher energies, as illustrated in Fig. 4. Obviously, the "tting process becomes more di$cult for these high-tailing peaks resulting also in a loss of analysis accuracy. MGAU is a code we developed several years ago to determine the isotopic enrichment of uranium samples from Ge detector gamma ray spectra [9]. This code has been widely used at the IAEA and many other facilities for several years. The primary energy region analyzed by this code is from 89 to 100 keV, shown in Fig. 6. This region has several overlapping gamma- and X-rays and therefore requires the high resolution usually associated with small germanium detectors. However, the code has been modi"ed to analyze the spectra obtained with

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Peltier-cooled CdTe PIN-type detectors. These detectors typically have a resolution of about a factor of two worse than Ge detectors. Nonetheless, U enrichment results within a few percent of the

declared values can be obtained, as illustrated in Table 1. CsRatio is a computer program we developed to determine the Cs/Cs ratio in spent fuel assemblies. This measurement is useful for determining the burnup of spent fuel assemblies. This program analyzes several peaks found in three energy regions, as shown in Figs. 7 and 8. Table 2 lists the isotopes, peak energies and other parameters used in the analysis. A small, high-resolution CZT detector placed in a highly collimated shield is used to probe isolated fuel bundles from the side. Due to the high-gamma-ray intensity, good measurements can be made in a few minutes. Figs. 8a}c illustrate the peaks and the "tting of the data found in the three energy regions. The 662 keV peak (Fig. 8b) is usually the most intense peak in aged fuel assemblies and is therefore the "rst region analyzed. The peak position, intensity, resolution and tailing parameters are all

Fig. 6. MGAU analysis of the 89}100 keV region of a uranium sample. (PIN detector).

Table 1 Comparison of measured enrichments with declared values using the MGAU code No. of meas.

Decl. U%

Meas. U%

Std. dev. (1p)

Meas. error

14 7 28 14

0.71 4.46 19.8 90.0

0.73 4.52 19.5 87.7

7.2% 1.6% 1.3% 1.8%

8.3% 0.9% 0.7% 2.7%

Fig. 7. CZT spectrum of spent fuel element.

Table 2 Peak regions and sources used in CsRatio analyses Region 1

Region 2

Region 3

Energy

Source

Energy

Source

Energy

Source

622.1 634.5 649.5 661.6

Ru/Rh Esc. Pk Esc. Pk Cs

563.2 569.3 577.6 582.6 604.7

Cs Cs Esc. Pk Esc. Pk Cs

765.8 768.7 773.7 795.8 801.9

Nb Esc. Pk Esc. Pk Cs Cs

t



Cs"754.2 days, t



Cs"11049 days.

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Fig. 8. CsRatio "t of (a) 605 keV region, (b) 662 keV region and (c) 796 keV region.

Fig. 9. NaIGEM analysis of the 130 to 300 keV region of a NaI detector spectrum.

determined when "tting this region. The tailing parameters thus determined are held constant when "tting the two remaining energy regions. A resolution versus energy curve is used to calculate the peak width for each region so that this parameter is also "xed when "tting these regions. The measured intensities and branching probabilities of the two Cs peaks at 605 and 796 keV (see Figs. 8a and c) are used to de"ne the relative e$ciency in the 600}800 keV region. The "nal Cs/Cs ratio is then calculated, with the help of known nuclear constants. The program has been used successfully in a number of safeguard applications [10]. NaIGEM was developed to determine the U enrichment from NaI detector spectra of uranium samples [11]. The method is based on the

Fig. 10. Analysis of a CZT spectrum of 4.46% enriched U using NaIGEM.

`enrichment metera technique. However, instead of using integration windows, the new method "ts computed response pro"les to the observed data in the 186 keV region, as illustrated in Fig. 9. Analyses accurate to 1% can be made when the method is applied properly. Since even large CZT detectors have a resolution than is signi"cantly better than NaI detectors, a preliminary study was made to see if the NaIGEM enrichment method could be used with CZT detectors. After writing a special version of NaIGEM, measurements were made of the CBNM set of uranium enrichment standards using an IAEA, 1500 mm detector. Fig. 10 shows a "t of the data using the CZT modi"ed version of NaIGEM to analyze a 4.46% enriched uranium standard.

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Table 3 NaIGEM results obtained by analyzing CZT spectra of CBNM uranium standards Decl. (%)

FWHM (keV)

Time (sec)

#/! Meas. (%)

Di!. (%)

0.31 0.708 1.94 2.95 4.46

7.6 7.7 7.6 7.8 7.5

1020 780 3000 1020 2040

0.36 (6.0) 0.727 (4.0) 1.947 (1.2) 2.987 (1.5) Calibration

16 2.7 0.4 1.2

Fig. 12. MGA "t of the 100 keV region of a plutonium spectrum.

were within a few percent of the declared values of the CBNM standards, and the other isotopes could be determined to about 5}10%. We believe that with some improvement in the detector characteristics and a better understanding and de"nition of some of the peak shape parameters, the results can be improved. Fig. 11. MGA code "t of the peaks of four isotopes in the 152 keV region of a plutonium spectrum.

5. Conclusions Table 3 summarizes the results of the series of measurements taken with the CdZnTe detector. MGA is a code used to determine the relative abundance of the plutonium isotopes in a sample [1]. We modi"ed our version of this code to make a preliminary assessment of what might be achievable when analyzing CdTe spectra. A Peltiercooled CdTe detector was used to obtain lowenergy spectra of the four CBNM plutonium standards. Figs. 11 and 12 are plots of the 152 and the 100 keV region of a spectrum of the 84% Pu standard. Although the 100 keV regions is very valuable when analyzing germanium detector spectra, it is not known, presently, how useful it is, compared to other regions, when analyzing CdTe spectra. Nonetheless, our initial analysis attempts gave results for the Pu isotopic abundance that

Germanium spectrum analysis algorithms can be used to analyze CdTe and CZT spectra. However, they must be modi"ed, particularly to account for the extensive tailing observed in the peaks of these spectra. The FWHM and tailing characteristics are very dependent on the detector type and the energy. Because of poor peak shapes, it is di$cult to obtain highly accurate analyses. Therefore, at this present stage of development, the detectors are best reserved for applications where it is impossible or inconvenient to use germanium detectors, such as in spent fuel ponds or when there are severe space restrictions, facility locations where liquid nitrogen is not available, or in unattended measurement systems. We have shown that for some such cases, adequate peak processing methods may now be available.

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Acknowledgements We are grateful to Ms. E. Gryshchuk and Mr. I. Hartley for performing some of the spectrum measurements. A part of this work (development of CsRatio) was supported by POTAS under task US A.931.

[7]

[8]

References [9] [1] R. Gunnink, MGA: a gamma ray spectrum analysis code for determining plutonium isotopic abundances, Report UCRL-LR-103220, Vol. 1, Lawrence Livermore National Laboratory, Livermore, CA, USA, 1990. [2] G.W. Phillips, K.W. Marlow, Nucl. Instr. and Meth. 153 (1976) 449. [3] R.G. Helmer, M.A. Lee, Nucl. Instr. and Meth. 178 (1980) 499. [4] R. Gunnink, J.B. Niday, Computerized quantitative analysis by gamma-ray spectrometry, Vol. I. description of the GAMANAL program. Report UCRL-51061, Lawrence Livermore National Laboratory, Livermore, CA, USA, 1972. [5] J.T. Routti, S.G. Prussin, Nucl. Instr. and Meth. 72 (1969) 125. [6] T.H. Prettyman, T. Marks Jr., D.G. Pelowitz, M.K. Smith, Response function analysis techniques for coplanar grid CdZnTe detectors. Presented at the 40th Annual Meeting

[10]

[11]

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of the Institute of Nuclear Materials Management, Phoenix, Arizona, Los Alamos National Laboratory document LA-UR-99-3890, July 25}29, 1999. R.H. Redus, J.A. Pantazis, A.S. Huber, V.T. Jordanov, J.F. Butler, B. Apotovsky, Mat. Res. Soc. Symp. Proc. 487 (1998) 101. M. Hage-Ali, P. Si!ert, CdTe nuclear detectors and applications, in: T.E. Schlessinger, R.B. James, Semiconductors for room temperature nuclear detector applications, James (Eds.), Semiconductor and Semimetals, Vol. 43, Acedemic Press, New York, 1995. R. Gunnink, W.D. Ruhter, P. Miller, J. Goerten, M. Swinhoe, H. Wagner, V. Verplancke, M. Bickel, S. Abousahl, MGA-U: a new analysis code for measuring U-235 enrichments in arbitrary samples, Proceedings of the IAEA Symposium on International Safeguards, Vienna, Austria, 1994. M. Aparo, J. Arenas Carrasco, R. Arlt, V. Bytchkov, K. Esmailpour, O. Heinonen, A. Hiermann, Development and implementation of compact gamma spectrometers for spent fuel measurements, Proceedings of the 21st Annual. ESARDA Symp. European Safeguards Research and Development Association, Luxembourg, Sevilla, Spain, May 4}6, 1999. R. Gunnink, R. Arlt, R. Bernt, New Ge and NaI analysis methods for measuring U enrichments. Proceeding of the 19th Annual ESARDA Symposium on European Safeguards Research and Development Association, Luxembourg, ESARDA Joint Research Centre, Ispra, Italy, May 13}15, 1997.

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