- Email: [email protected]
Investigation of the peak shape parameter of CdZnTe detectors
Nuclear Instruments and Methods in Physics Research A 458 (2001) 498}502
Investigation of the peak shape parameter of CdZnTe detectors I. Hartley*, R. Arlt International Atomic Energy Agency, Wagramer Strasse 5, P.O. Box 100, A-1400 Vienna, Austria
Abstract There is a need to de"ne the magnitude of the asymmetry of the peak shapes (the tailing fraction) of CdTe and CdZnTe detectors. Since this tailing parameter determines to a large extent, the performance of peak "tting programs used to extract the peak areas from gamma spectra taken for the veri"cation of nuclear material, a well-de"ned knowledge of this parameter is an important factor in such programs. The magnitude of the asymmetry of this tailing fraction was investigated for di!erent models of CdTe and CdZnTe detectors. The gamma peak analysis program PkCheck (R. Gunnink, R. Arlt, Proceedings of 11th International Workshop on room temperature Semiconductor X- and gamma-ray detectors and associated electronics, 11}15 October 1999, Vienna, Austria, Nucl. Instr. and Meth. A 485 (2001) 196, This issue) was used to determine the tailing fraction as a function of detector type, high voltage and other operational parameters. Although there are considerable individual di!erences between di!erent detector units of the same model, a general trend towards the growing of the tailing fraction with increasing detector volume was clearly observed. The lowest fractions are observed for electrically cooled planar pin CdTe detectors operated with a charge loss corrector, followed by small size hemispheric CdZnTe detectors. 2001 Elsevier Science B.V. All rights reserved. PACS: 29.30. Kv Keywords: CdZnTe; Gamma spectrometry; Peak "tting
1. Introduction The IAEA uses CdZnTe detectors in a number of safeguards veri"cation methods [1,2]. Detectors of suitable quality are now commercially available in su$cient numbers. They have also become reliably stable in their performance. This given and the need to quantify measurement results in terms of gamma energies, peak areas, and isotopic abundances has bought about a requirement to process the gamma
* Corresponding author. Tel.: #43-1-2600-21865; fax: #431-2600-29317. E-mail address: [email protected] (I. Hartley).
spectra with computer programs to extract the peak information in an accurate and reproducible manner. The performance of such programs, however, is impacted by the bad peak shapes currently often observed with room temperature semiconductor detectors. Depending on detector type and model, we observe a more or less pronounced peak tailing } mostly at the low-energy side of the peaks } caused by charge losses. It is important in the use of these detectors as quantitative data-processing tools to describe, measure and compare the magnitude of the peak tailing of di!erent detector types and to test the ability of peak "tting software products to describe and parametrize the peaks. In addition, we need to know the deviation from the ideal
0168-9002/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 0 4 0 - 8
I. Hartley, R. Arlt / Nuclear Instruments and Methods in Physics Research A 458 (2001) 498}502
499
peak shape as a function of di!erent operational parameters of the detectors, such as high voltage, shaping time and gamma energies. A concise knowledge of such information, when fed back to manufacturers of these detectors and associated software developers facilitates the development of improved detectors' properties and peak "tting software.
2. Equipment, software and data evaluation Equipment. The measurements were performed with the Mini Multi Channel Analyser (MMCA) [3]. The input signal was usually fed directly to the MMCA's internal ampli"er. When a broader range of shaping times was required, the input was channeled via a Tennelec TC-144 external ampli"er. The detectors were used with their native charge preampli"ers as delivered by the manufacturers. Software. The MMCA was controlled from a Panasonic CF-35 laptop computer. The spectra were acquired using the WinSpec data collection software [2]. They were saved on the hard disk and processed with the PkCheck software. Data evaluation. The data was processed with the gamma-ray analysis code of the computer program PkCheck (version 1.32). The procedure used by the program to describe a gamma peak with its background in a spectrum consists of the following three steps: E Determination of the boundaries of a peak or grouping of peaks E Approximation and subtraction of the background E Peak "tting of the net counts using a complex "tting process. Examples of graphical representations of this "tting process as generated by the program during analyses are shown in Figs. 1a and b. Details are given in Ref. [4]. For the peak "tting process a Gaussian function is used to describe the principal component of the peak shape. This represents the de"nition of the
Fig. 1. Graphical displays of generation of exponential tail fraction.
peak as total peak area after background subtraction. This central Gaussian part is then combined with the low-energy tail component of the peak to generate a low-energy exponential tail fraction. This represents the tailing fraction of the principal peak area and is expressed as the percent tailing of the total peak area. This fraction is then further used to apply some quantitative characterization to asymmetries of the peak from the ideal Gaussian shape. It is calculated in terms of sigma deviation from the Gaussian and is expressed as the quality of the peak "t (QFIT).
POSTER SESSION
500
I. Hartley, R. Arlt / Nuclear Instruments and Methods in Physics Research A 458 (2001) 498}502
These values are included in the results summary "les as generated by the program, examples of which are also shown in Figs. 1a and b. The following hemispheric detectors were measured: SDP310Z20 and -Z60, CZT/500 and CZT/1500 (standard and super-grade units). The CZT- models are based on CdZnTe made by eVProducts while the detectors are fabricated by RITEC [5] and at the St. Petersburg Nuclear Physics Institute. Furthermore, cap detectors (new detector-type eV products [6]) and some planar detectors were measured and assessed. The measurements were performed with Am, Co, U and Cs sources. The count rates were normally so low that any pulse pile-up e!ects could be neglected. In some measurements the high
voltage was varied to study the e!ect on peak shape. The measurement time was long enough to provide a statistical error of the peak area no greater than a few percent. After collection, the data were processed with PkCheck to calculate the tailing fraction of the prominent peaks.
3. Measurement results The results are summarized in Table 1 and Fig. 2. In comparing the tailing fraction for di!erent detector types, the data show that the lowest values for the asymmetry of the gamma peaks are observed for high-resolution pin detectors and small size hemispheric detectors (SDP310Z20's and
Table 1 Detector no.
99-04
8472/001 8472/002 8472/002 8472/002 8472/002 8472/002 9928/47 9928/18 9928/27 No -2 No -2 8531/001 8531/001 8531/001 8531/001 8531/001
Model
Type
CAP EV-01 100C SDP310/Z/20S SDP310/Z/60 SDP310/Z/60 CZT500s CZT500s CZT500s CZT500s CZT500s CZT500s CZT500 CZT500 CZT500 CZT/1500 CZT/1500 Spear S/N B530 Spear S/N B530 Spear S/N B530 Spear S/N B530 Spear S/N B530 CRADA TC 500 CRADA TC 500 CRADA TC 500 Small-pin-3 Small-pin-3 Small-pin-3 Small;-pin-2 Small;-pin-2'
RITEC CdTe RITEC CdTe CdTe Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC 5;5;5 mm eV Products 5;5;5 mm eV Products 5;5;5 mm eV Products 5;5;5 mm eV Products 5;5;5 mm eV Products EG&G ORTEC EG&G ORTEC EG&G ORTEC CdTe
CdTe
Manufacturer
Source Bias HV
Shaping
FWHM % keV/channel tailing
QFIT
Residuals STD's
Co Co Co Cs Co Co Cs Am Am Cs Co Co Co Co Cs Co Am Am Cs Cs Am Am Co Cs Co Co Co Cs
1.00 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.00 1.00 0.25 0.25 0.25 0.25 0.25 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
7.298 4.058 4.327 8.75 4.762 5.465 10.146 4.536 4.629 10.216 5.61 6.76 6.02 6.776 16.648 3.532 2.814 3.332 10.146 10.216 2.218 2.218 2.300 2.955 7.034
30 34 38 32 43 31 33 11 4 7 40 48 50 43 42 56 2 7 33 7.41 10 10 63 11 68
4.6 5.4 1.3 2.1 7.0 3.0 1.7 0.8 0.9 1.3 2.3 1.6 2.2 3.9 7.2 8.4 20.0 4.1 1.7 1.3 2.5 2.5 1.8 26.0 17.6
4 7 3 4 6 4 3 3 2 3 4 3 4 8 7 8 11 5 3 3 4 4 4 9 8
15 9
14.2 18.7
9 9
900 300 300 300 800 700 700 700 700 700 800 800 800 1800 1800 700 700 700 700 700 auto auto auto 100 100 100 100 100
2.52 2.825
I. Hartley, R. Arlt / Nuclear Instruments and Methods in Physics Research A 458 (2001) 498}502
501
Fig. 2. %Low tailing and FWHM of Co 122 keV peak for detector CZT/500-88 as generated by program PkCheck (version 1.32).
Z60's). In general, the peak tailing fraction increases with increasing detector volume. It should also be noted that there is some variation of the fraction within individual units of the same detector models. The variation of the tailing fraction with the high voltage is shown for a CZT/500s detector in Fig. 2. The results show that it was, in general, possible to describe reasonably well the peak shape of room temperature semiconductor detectors with the peak model used in PkCheck, although in some cases the tailing fraction exceeded 50%. Deviations of the measured points in the spectrum from the "tted curve were, in general, less than a few sigma (for the given measurement statistics) (Fig. 3). Since the percentage of the tailing fraction is related to the ability of a certain spectral analysis program to determine a peak's energy and area precisely and to resolve complex groups of peaks, detectors with lower tailing fractions are in general preferable. With the present status of detector tech-
nology and the materials currently available; however, the peak tailing phenomenon will continue for some time, and reliable peak analysis methods are needed to cope with this.
4. Summary The peak shapes of di!erent commercially available CdTe and CdZnTe detectors have been analyzed using the peak "tting software PkCheck. Tailing fractions of the gamma peaks were measured and compared. The results show that at the present level of detector technology signi"cant asymmetries exist, particularly on the low-energy side of the peaks. These values can reach as high as the order of 50%. But it was also demonstrated that with appropriate software tools even higher fractions of peak tailing can be handled. Nevertheless, the improvement of detector material properties with a view to
POSTER SESSION
502
I. Hartley, R. Arlt / Nuclear Instruments and Methods in Physics Research A 458 (2001) 498}502
Fig. 3. Summary of %low-side tailing for various RTSD detectors.
reducing peak asymmetries is very desirable if peak energies and intensities are to be extracted from these gamma spectra with increased accuracy.
[3]
References
[4]
[1] M. Aparo, R. Arlt, Development and safeguards use of advanced CdTe and CdZnTe detectors, INMM 39th Annual Meeting, Naples, Florida, 26}30 July 1998. [2] M. Aparo, J. Arenas Carrasco, R. Arlt, V. Bytchkov, K. Esmailpour, O. Heinonen, A. Hiermann, Development and implementation of compact gamma spectrometers for
[5]
[6]
spent fuel measurements, Proceedings of the 21st ESARDA Annual Meeting, Sevilla, Spain, 4}6 May 1999. F. Gabriel, A. Wolf, D. Proehl, R. Jainsch, K.W. Leege, R. Arlt, P. Schwalbach, B. Richter, Proceedings of the 19th ESARDA Annual Symposium, Montpellier, France, May 1997, EUR 17665 EN, ESARDA 28, p. 419. R. Gunnink, R. Arlt, Nucl. Instr. and Meth. A 458 (2001) 196, These Proceedings. V. Ivanov, P. Dorogov, A. Loutchansky, L. Alexsejeva, E. Mochaev, Further development of hemispheric CdZnTe detectors for safeguards application, Proceedings of the 21st ESARDA Annual Symposium, Sevilla, Spain, 4}6 May 1999; http://www.ritec.mt.lv; www.evproducts.com. eV Products Catalogue, August 1999.
Investigation of the peak shape parameter of CdZnTe detectors I. Hartley*, R. Arlt International Atomic Energy Agency, Wagramer Strasse 5, P.O. Box 100, A-1400 Vienna, Austria
Abstract There is a need to de"ne the magnitude of the asymmetry of the peak shapes (the tailing fraction) of CdTe and CdZnTe detectors. Since this tailing parameter determines to a large extent, the performance of peak "tting programs used to extract the peak areas from gamma spectra taken for the veri"cation of nuclear material, a well-de"ned knowledge of this parameter is an important factor in such programs. The magnitude of the asymmetry of this tailing fraction was investigated for di!erent models of CdTe and CdZnTe detectors. The gamma peak analysis program PkCheck (R. Gunnink, R. Arlt, Proceedings of 11th International Workshop on room temperature Semiconductor X- and gamma-ray detectors and associated electronics, 11}15 October 1999, Vienna, Austria, Nucl. Instr. and Meth. A 485 (2001) 196, This issue) was used to determine the tailing fraction as a function of detector type, high voltage and other operational parameters. Although there are considerable individual di!erences between di!erent detector units of the same model, a general trend towards the growing of the tailing fraction with increasing detector volume was clearly observed. The lowest fractions are observed for electrically cooled planar pin CdTe detectors operated with a charge loss corrector, followed by small size hemispheric CdZnTe detectors. 2001 Elsevier Science B.V. All rights reserved. PACS: 29.30. Kv Keywords: CdZnTe; Gamma spectrometry; Peak "tting
1. Introduction The IAEA uses CdZnTe detectors in a number of safeguards veri"cation methods [1,2]. Detectors of suitable quality are now commercially available in su$cient numbers. They have also become reliably stable in their performance. This given and the need to quantify measurement results in terms of gamma energies, peak areas, and isotopic abundances has bought about a requirement to process the gamma
* Corresponding author. Tel.: #43-1-2600-21865; fax: #431-2600-29317. E-mail address: [email protected] (I. Hartley).
spectra with computer programs to extract the peak information in an accurate and reproducible manner. The performance of such programs, however, is impacted by the bad peak shapes currently often observed with room temperature semiconductor detectors. Depending on detector type and model, we observe a more or less pronounced peak tailing } mostly at the low-energy side of the peaks } caused by charge losses. It is important in the use of these detectors as quantitative data-processing tools to describe, measure and compare the magnitude of the peak tailing of di!erent detector types and to test the ability of peak "tting software products to describe and parametrize the peaks. In addition, we need to know the deviation from the ideal
0168-9002/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 0 4 0 - 8
I. Hartley, R. Arlt / Nuclear Instruments and Methods in Physics Research A 458 (2001) 498}502
499
peak shape as a function of di!erent operational parameters of the detectors, such as high voltage, shaping time and gamma energies. A concise knowledge of such information, when fed back to manufacturers of these detectors and associated software developers facilitates the development of improved detectors' properties and peak "tting software.
2. Equipment, software and data evaluation Equipment. The measurements were performed with the Mini Multi Channel Analyser (MMCA) [3]. The input signal was usually fed directly to the MMCA's internal ampli"er. When a broader range of shaping times was required, the input was channeled via a Tennelec TC-144 external ampli"er. The detectors were used with their native charge preampli"ers as delivered by the manufacturers. Software. The MMCA was controlled from a Panasonic CF-35 laptop computer. The spectra were acquired using the WinSpec data collection software [2]. They were saved on the hard disk and processed with the PkCheck software. Data evaluation. The data was processed with the gamma-ray analysis code of the computer program PkCheck (version 1.32). The procedure used by the program to describe a gamma peak with its background in a spectrum consists of the following three steps: E Determination of the boundaries of a peak or grouping of peaks E Approximation and subtraction of the background E Peak "tting of the net counts using a complex "tting process. Examples of graphical representations of this "tting process as generated by the program during analyses are shown in Figs. 1a and b. Details are given in Ref. [4]. For the peak "tting process a Gaussian function is used to describe the principal component of the peak shape. This represents the de"nition of the
Fig. 1. Graphical displays of generation of exponential tail fraction.
peak as total peak area after background subtraction. This central Gaussian part is then combined with the low-energy tail component of the peak to generate a low-energy exponential tail fraction. This represents the tailing fraction of the principal peak area and is expressed as the percent tailing of the total peak area. This fraction is then further used to apply some quantitative characterization to asymmetries of the peak from the ideal Gaussian shape. It is calculated in terms of sigma deviation from the Gaussian and is expressed as the quality of the peak "t (QFIT).
POSTER SESSION
500
I. Hartley, R. Arlt / Nuclear Instruments and Methods in Physics Research A 458 (2001) 498}502
These values are included in the results summary "les as generated by the program, examples of which are also shown in Figs. 1a and b. The following hemispheric detectors were measured: SDP310Z20 and -Z60, CZT/500 and CZT/1500 (standard and super-grade units). The CZT- models are based on CdZnTe made by eVProducts while the detectors are fabricated by RITEC [5] and at the St. Petersburg Nuclear Physics Institute. Furthermore, cap detectors (new detector-type eV products [6]) and some planar detectors were measured and assessed. The measurements were performed with Am, Co, U and Cs sources. The count rates were normally so low that any pulse pile-up e!ects could be neglected. In some measurements the high
voltage was varied to study the e!ect on peak shape. The measurement time was long enough to provide a statistical error of the peak area no greater than a few percent. After collection, the data were processed with PkCheck to calculate the tailing fraction of the prominent peaks.
3. Measurement results The results are summarized in Table 1 and Fig. 2. In comparing the tailing fraction for di!erent detector types, the data show that the lowest values for the asymmetry of the gamma peaks are observed for high-resolution pin detectors and small size hemispheric detectors (SDP310Z20's and
Table 1 Detector no.
99-04
8472/001 8472/002 8472/002 8472/002 8472/002 8472/002 9928/47 9928/18 9928/27 No -2 No -2 8531/001 8531/001 8531/001 8531/001 8531/001
Model
Type
CAP EV-01 100C SDP310/Z/20S SDP310/Z/60 SDP310/Z/60 CZT500s CZT500s CZT500s CZT500s CZT500s CZT500s CZT500 CZT500 CZT500 CZT/1500 CZT/1500 Spear S/N B530 Spear S/N B530 Spear S/N B530 Spear S/N B530 Spear S/N B530 CRADA TC 500 CRADA TC 500 CRADA TC 500 Small-pin-3 Small-pin-3 Small-pin-3 Small;-pin-2 Small;-pin-2'
RITEC CdTe RITEC CdTe CdTe Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC Hemispheric RITEC 5;5;5 mm eV Products 5;5;5 mm eV Products 5;5;5 mm eV Products 5;5;5 mm eV Products 5;5;5 mm eV Products EG&G ORTEC EG&G ORTEC EG&G ORTEC CdTe
CdTe
Manufacturer
Source Bias HV
Shaping
FWHM % keV/channel tailing
QFIT
Residuals STD's
Co Co Co Cs Co Co Cs Am Am Cs Co Co Co Co Cs Co Am Am Cs Cs Am Am Co Cs Co Co Co Cs
1.00 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.00 1.00 0.25 0.25 0.25 0.25 0.25 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
7.298 4.058 4.327 8.75 4.762 5.465 10.146 4.536 4.629 10.216 5.61 6.76 6.02 6.776 16.648 3.532 2.814 3.332 10.146 10.216 2.218 2.218 2.300 2.955 7.034
30 34 38 32 43 31 33 11 4 7 40 48 50 43 42 56 2 7 33 7.41 10 10 63 11 68
4.6 5.4 1.3 2.1 7.0 3.0 1.7 0.8 0.9 1.3 2.3 1.6 2.2 3.9 7.2 8.4 20.0 4.1 1.7 1.3 2.5 2.5 1.8 26.0 17.6
4 7 3 4 6 4 3 3 2 3 4 3 4 8 7 8 11 5 3 3 4 4 4 9 8
15 9
14.2 18.7
9 9
900 300 300 300 800 700 700 700 700 700 800 800 800 1800 1800 700 700 700 700 700 auto auto auto 100 100 100 100 100
2.52 2.825
I. Hartley, R. Arlt / Nuclear Instruments and Methods in Physics Research A 458 (2001) 498}502
501
Fig. 2. %Low tailing and FWHM of Co 122 keV peak for detector CZT/500-88 as generated by program PkCheck (version 1.32).
Z60's). In general, the peak tailing fraction increases with increasing detector volume. It should also be noted that there is some variation of the fraction within individual units of the same detector models. The variation of the tailing fraction with the high voltage is shown for a CZT/500s detector in Fig. 2. The results show that it was, in general, possible to describe reasonably well the peak shape of room temperature semiconductor detectors with the peak model used in PkCheck, although in some cases the tailing fraction exceeded 50%. Deviations of the measured points in the spectrum from the "tted curve were, in general, less than a few sigma (for the given measurement statistics) (Fig. 3). Since the percentage of the tailing fraction is related to the ability of a certain spectral analysis program to determine a peak's energy and area precisely and to resolve complex groups of peaks, detectors with lower tailing fractions are in general preferable. With the present status of detector tech-
nology and the materials currently available; however, the peak tailing phenomenon will continue for some time, and reliable peak analysis methods are needed to cope with this.
4. Summary The peak shapes of di!erent commercially available CdTe and CdZnTe detectors have been analyzed using the peak "tting software PkCheck. Tailing fractions of the gamma peaks were measured and compared. The results show that at the present level of detector technology signi"cant asymmetries exist, particularly on the low-energy side of the peaks. These values can reach as high as the order of 50%. But it was also demonstrated that with appropriate software tools even higher fractions of peak tailing can be handled. Nevertheless, the improvement of detector material properties with a view to
POSTER SESSION
502
I. Hartley, R. Arlt / Nuclear Instruments and Methods in Physics Research A 458 (2001) 498}502
Fig. 3. Summary of %low-side tailing for various RTSD detectors.
reducing peak asymmetries is very desirable if peak energies and intensities are to be extracted from these gamma spectra with increased accuracy.
[3]
References
[4]
[1] M. Aparo, R. Arlt, Development and safeguards use of advanced CdTe and CdZnTe detectors, INMM 39th Annual Meeting, Naples, Florida, 26}30 July 1998. [2] M. Aparo, J. Arenas Carrasco, R. Arlt, V. Bytchkov, K. Esmailpour, O. Heinonen, A. Hiermann, Development and implementation of compact gamma spectrometers for
[5]
[6]
spent fuel measurements, Proceedings of the 21st ESARDA Annual Meeting, Sevilla, Spain, 4}6 May 1999. F. Gabriel, A. Wolf, D. Proehl, R. Jainsch, K.W. Leege, R. Arlt, P. Schwalbach, B. Richter, Proceedings of the 19th ESARDA Annual Symposium, Montpellier, France, May 1997, EUR 17665 EN, ESARDA 28, p. 419. R. Gunnink, R. Arlt, Nucl. Instr. and Meth. A 458 (2001) 196, These Proceedings. V. Ivanov, P. Dorogov, A. Loutchansky, L. Alexsejeva, E. Mochaev, Further development of hemispheric CdZnTe detectors for safeguards application, Proceedings of the 21st ESARDA Annual Symposium, Sevilla, Spain, 4}6 May 1999; http://www.ritec.mt.lv; www.evproducts.com. eV Products Catalogue, August 1999.