Performance Evaluation of New Generation CdZnTe Detectors for Safeguards Applications

Transcription

1 Performance Evaluation of New Generation CdZnTe Detectors for Safeguards Applications Performance Evaluation of New Generation CdZnTe Detectors for Safeguards Applications V. Ivanov b, J. Mintcheva a, A. Berlizov a, A. Lebrun a a IAEA, Vienna, Austria b ZRF RITEC SIA, Riga, Latvia Abstract. Cadmium zinc telluride detectors (CdZnTe) have found a wide application in non-destructive assay measurements in the IAEA s verification practice. It is because of their form factor, usability, sensitivity and good spectral characteristics that they are extensively used for fresh and spent fuel attribute test measurements. Until now, the series of CdZnTe detectors utilized in the IAEA have covered the range of 5 mm 3, 20 mm 3, 60 mm 3 and 500 mm 3 of sensitive volume. Recently, new CdZnTe detectors with improved spectroscopic characteristics and significantly bigger active volume have become available, owing to advances in crystal and detector manufacturing and signal processing technologies. The distinctive feature of this new technological development is the application of a low-intensity monochromatic optical stimulation with infrared (IR) light. The use of IR illumination with a properly chosen wavelength close to the absorption edge of the CdZnTe can significantly improve the performance of the detectors. Recognizing potential benefits of these detectors in safeguards applications, the IAEA has performed an evaluation of their performance characteristics. Under evaluation were several new detectors with sensitive volumes of 500 mm 3, 1500 mm 3 and 4000 mm 3, as well as all-in-one 60 mm 3, 500 mm 3 and 1500 mm 3 integrated micro-spectrometers available from RITEC, Latvia. In addition to the standard performance characteristics, such as energy resolution, peak shape, efficiency, linearity, throughput and temperature stability, the potential use of the detectors for safeguards specific measurements, such as uranium enrichment with infinite thickness method, was of particular interest. The paper will describe the advances in the CdZnTe detector technology and present the results of their performance evaluation. 1. Introduction At present time various CdZnTe detectors of different designs and sizes are widely and successfully used for different applications due to its favourable detection properties. The current level of crystal growth technology has made it possible to grow large CdZnTe (Zn=10%) ingots with single crystal volumes more then 100 cm 3. This enabled the fabrication of larger volume detectors. But quality of single crystals is not optimal because of presence of various structural defects leading to deterioration of the charge collection and as a result, to an unsatisfactory detectors spectroscopy performance. Also, because of a poor holes transport, special techniques and detector design, allowing implementation of a single-charge collection [1], [2] should be used. There are various CdZnTe detectors of different designs and dimensions based on this technique. Coplanar grid detectors [3], [4] and pixelated detectors [6] used in some spectrometric devices [7], [8] have large sensitive volumes up to several cubic centimeters and have good spectrometric characteristics at room temperature. Other rather well known detectors employ single-charge collection are hemispherical or quasihemispherical CdZnTe detectors [8] [11]. These detectors are the simplest with regard to production and the most usable because they do not require a special electronic circuit. There are commercially available probes with CdZnTe quasi-hemispherical detectors of different volumes from 1 mm 3 to 500 mm 3 [12]. It was found that Illumination by infrared (IR) radiation with wavelengths close to the CdZnTe absorption edge greatly influences the spectrometric characteristics of quasi-hemispherical CdZnTe detectors [13], [14]. Significant improvements of spectrometric characteristic of the CdZnTe quasihemispherical detectors under influence of the correct adjusted illumination intensity and wavelength can be achieved. In the IAEA verification practice, the non-destructive assay measurements have played a significant role in the field of safeguards measurements. Gamma-spectrometry is a major method. From the authorized gamma detectors for medium resolution spectroscopy CdZnTe are the most popular in the Agency s verification practice. They have several advantages compared to other authorized gamma detectors: good energy resolution, which, in general, is better in comparison with other medium 1

2 resolution detectors at the particular energy region of interest; good spectroscopic characteristics peak-to-compton and peak-to-valley ratio; they work under room temperature and don t have any special maintenance requirements; they are light and robust. So far CdZnTe detectors have been intensively used for method F verification for fresh and spent fuel, such as uranium attribute test, plutonium attribute test, caesium attribute test, thorium attribute test. The main limitation to wider usage is their low efficiency in comparison with scintillation detectors and because of that the need of longer measurement times. In the course of using CdZnTe detectors for spent fuel measurements, Agency s inspectors have found a problem the detectors crash under low ambient temperatures around -5 C and lower. The physical explanation is that in some of detectors there are polarization effects at low temperatures. So far, for enrichment measurements the safeguards inspectors have used LaBr and NAID scintillation medium resolution detectors with appropriate software for data collection and analysis. These detectors have much higher efficiency compared to CdZnTe and all other disadvantages of scintillators. The recently introduced in our practice LaBr detectors have resolution twice as high as NAID as well as a very high price. Given new developments by Ritec, the Agency s experts in the field of NDA measurements have decided to perform precise investigation of the new detectors and spectrometers and to evaluate their potential for safeguards measurements mainly for enrichment measurements. Characteristics and descriptions of new devices based on improved CdZnTe quasi-hemispherical detectors, tests and evaluation of their possibilities for safeguards applications are presented in this paper. 2. New developments New detection probes. The quasi-hemispherical detectors with dimensions up to 20 mm 20 mm 10 mm were fabricated and used. Such detectors are rectangular with a length-width-height ratio of A A (A/2). The quasihemispherical detectors have large negative electrode on five sides and a positive dot electrode in the centre of one of the large sides. The optimal diameter d of the dot electrode is determined by the values of the electro physical characteristics of the CdZnTe crystal, the size of the detector and the achievable detector bias voltage. For improvement of the spectrometric characteristic of quasihemispherical detectors a low intensity IR stimulation using wavelengths close to the absorption edge of the CdZnTe was used. IR radiation interacts with the detector material, causing a change in the equilibrium between the population of the trapping centres and the concentration of free carriers, thereby influencing the processes of charge collection. At room temperature positive effect was obtained using wavelengths of approximately nm. Improvement can be achieved with lowintensity IR illumination at µw depending on the chosen wavelength of illumination. At lower temperatures, longer wavelengths about nm were used. CdZnTe crystals from REDLEN Technologies were used to fabricate the detectors. All probes allow installation of SMD IR LEDs near the detector. LEDs and probe s preamplifier were powered from the same power source of ± 12 V. Required LED illumination intensity was set by adjusting the LED direct leakage current. Since the LED must be supplied by V a special adjustable stabilized voltage source was used. Total consumption current of preamplifier and LED s voltage source did not exceed 20 ma. In the probes type SDP310 only one Illuminating LED was used, in the other probes types SDP1500 and SDP identical parallel connected LEDs were used. This allowed a relatively uniform illumination of the large detectors surface. The IR radiation penetrated from the side of a large negative contact through the gold electrode. The probes designed to operate over a wide temperature range have by four identical parallel connected LEDs of different wavelengths. The LED of 940 nm used at temperatures 10 C and higher, the led of 1050 nm is used at temperatures below 10 C. Illumination by a longer wavelength at room temperature does not improve the characteristics. An improvement in the spectrometric characteristics of the quasi-hemispherical detectors within a wide range of gamma-radiation was observed. For example,.an improvement in energy resolution

3 Performance Evaluation of New Generation CdZnTe Detectors for Safeguards Applications (FWHM) at 662 kev from 15.4 kev without illumination to 7.8 with illumination and at 59.6 kev from 4.1 kev without illumination to 3.4 kev with illumination by the IR LED of 940 nm was obtained with detector of 10 mm 10 mm 5 mm. 2.2 Microspectrometer µspec Another device in which can be used improved IR illuminated CdZnTe quasi-hemispherical detectors is a very compact gamma-radiation Microspectrometer µspec. The µspec is a self-sufficient device comprises charge sensitive preamplifier, digital signal processor, detector high voltage power supply, low voltage power supply and PC connection components. It communicates and is powered by the PC via micro-usb port. Microspectrometers use replaceable detector s modules with high quality CdZnTe quasi-hemispherical detector of 60 mm 3 (Model µspec60), 500 mm 3 (Model µspec500) or 1500 mm 3 (Model µspec1500). The Microspectrometer is designed on the basis of GBS MicroMCA527 [15] and is made to operate under control of MS Windows base GBS MCA166/MCA527 WinSPEC software. The WinSPEC sotware is available to download, free of charge, provides a choice of modes, MCA setting, the spectrum acquisition, display, calibration and storage. High count rate measurements by a two source method were performed for dead time corrections evaluations. Deviation about 5% of counts in the net peak area in the range up to.200 kcps was obtained. At this measurements a special setup (manually setting threshold) of the Microspectrometer s amplifier was used. 3. Evaluations ZRF Ritec SIA, Latvia delivered to the Agency the following detectors and spectrometers: 3 CdZnTe 500 mm 3 ( Ref. N: SDP 500/SN529, SDP 500/SN528, SDP 500/SN527), two of which specified for the temperature range -20 C +40 C ( Ref. N SDP 500/SN529, SDP 500/SN528), 4 CdZnTe 1500 mm 3 ( Ref. SDP 1500/SN35, SDP 1500/SN36, SDP 1500/SN37 SDP 1500/SN38 ), one for the temperature range -20 C +40 C ( Ref. N: SDP 1500/SN38 ); 2 CdZnTe 4000 mm 3 ( Ref. N: SDP 4000/SN10, SDP 4000/SN11), 1 Microspectrometer with CdZnTe 500 mm 3 ( Ref. N:µspec 500 SN:010 ) and 1 Microspectrometer with CdZnTe 1500 mm 3 ( Ref. µspec 1500 SN:011 ) for precise evaluations. Two of the detectors, have no Infrared Illumination (IR) SDP 500/SN 527 and SDP 1500/SN38. These detectors are based on specially selected crystals. The standard LaBr Agency detector and HM-5 identifinder were used as reference in the evaluation process. It was decided that for CdZnTe detectors the measurement campaign will comprise measurements of the main spectroscopic characteristics for all of them in three particular energy regions of interest. Energy resolutions (FWHM and FWTM), peak-to-compton ratio and peak-tovalley ratio and efficiency at the 662 kev line of Cs-137;energy resolutions (FWHM and FWTM) and efficiency at the 185 kev line of U-235, energy resolutions (FWHM and FWTM) and efficiency at the 122 kev line of Co-57 were measured. For those detectors, able to work under low temperatures, the change of the spectroscopic characteristics was evaluated Room temperature measurements All evaluation measurements at room temperature were performed using Inspector 2000 multi-channel analyser and Genie 2k software, produced by Canberra. The reproducibility of measurement geometry was guaranteed by using a dedicated plastic holder for the radioactive sources and detectors. The following reference radioactive sources were used: radionuclide Cs-137 with A=375 kbq (± 3%) at reference date ,radionuclide U-235 with A=880 kbq at reference date and radionuclide Co-57 with A=7,7 kbq at reference date In all measurements the source active area detector front end distance was chosen large enough so that summations corrections could be neglected. For all measurements dead time was kept below 2 %. During the evaluation process the net area of the corresponding photo peak, which is automatically determined from Genie 2k measurements was used. 3

4 The efficiency ratio was determined as a normalization to the standard Agency HM-5 NAID detector, which is equal to 1 by default. Table 1. Results from room temperature evaluations at 661,6 kev Cs-137, mid A FWHM kev FWTM kev p/v p/c Crystal volume,mm 3 Active area of the detector,mm 2 Ratio between active areas Photon attenuation coefficient, cm 2/ g Efficiency ratio Error from statistics, % SDP 4000/SN SDP 4000/SN LABR A395/ NAID 8259/ SDP 1500/SN SDP 1500/SN SDP 1500/SN SDP 1500/SN SDP 500/SN SDP 500/SN SDP 500/SN SDP Note: SDP 500 is an example for standard Agency CdZnTe 500 detector. Table 2 Results from room temperature evaluations at 185 kev Crystal U-235, N 53 FWHM, kev FWTM, kev volume,mm 3 SDP 4000/SN Active area of Ratio between Photon attenuation the active areas detector,mm 2 coefficient, cm 2/ g Efficiency ratio Error from statistics, % SDP 4000/SN LABR A395/ NAID 8259/ SDP 1500/SN SDP 1500/SN SDP 1500/SN SDP 1500/SN SDP 500/SN SDP 500/SN SDP 500/SN

5 Performance Evaluation of New Generation CdZnTe Detectors for Safeguards Applications Table 3 Results from room temperature evaluations at 122 kev Co-57 FWHM, kev FWTM, kev Active area of Crystal volume, mm 3 the detector,mm 2 Ratio beteween active areas Photon attenuation coefficient, cm 2/ g Efficiency ratio Error from statistics, % SDP 4000/SN10 SDP 4000/SN11 LABR A395/045 NAID 8259/004 SDP 1500/SN37 SDP 1500/SN35 SDP 1500/SN36 SDP 1500/SN38 SDP 500/SN527 SDP 500/SN529 SDP 500/SN Tables 1, 2 and 3 summarize the obtained data. The standard Agency periodic test for CDZT detectors is performed at 662 kev and includes determination of FWHM, FWTM, peak-to-valley and peak-to-compton ratio. Table 1 contains also representative results from such tests with CdZnTe SDP 500 detectors which are currently in use (last row). They are the biggest volume detectors which have been routinely used for years by safeguards inspectors. Comparison between these results and the results obtained for the new SDP 500 detectors, shows superior spectrometric performance of the latter with respect to all investigated characteristics. For the new detectors of bigger crystal volume, the Agency has no reference values for comparison. The results are systematically better than those in the manufacturer s declaration, most probably because of the different equipment used and condition of measurements. During the evaluation of efficiencies the following factors were taken into account: the active area and active volume of the detector and the photon attenuation coefficient in the material of the detector. Results show that for all three energy regions, the efficiency ratio increases with the increase of the active area/ active volume of the investigated detectors. In addition, for all three groups of detectors with different active area/volume, the efficiency ratio decreases with the increase of photon energy, which is connected with the energy dependence of the photon attenuation coefficient in the detector crystal, as shown in the tables. Detectors SDP1500/SN38 and SDP500/SN527, which have no IR implementation, show similar efficiencies and slightly worse resolution compared to the detectors with IR illuminated crystals. 3.2 Low temperature measurements Table 4 and 5 summarize the measured performance characteristics of the detectors specified for wider temperature range. Additional measurements were made at 0 C, -10 C, -20 C temperatures with Cs- 137 and U-235. Measurements were performed using Canberra software Genie 2k in an environmental chamber for maintaining the required temperature. The same holder was used for keeping reproducible geometry. The temperature variation has negligible influence on the holder s dimensions. 5

6 Table 4: Results for low temperature measurements at kev Cs-137, mid A t FWHM, kev FWTM, kev p/v p/c Crystal volume, mm 3 Net area of 662 kev SDP 1500/SN38 20 C C C C SDP 500/SN C C C C SDP 500/SN C C C C Table 5 Results for low temperature measurements at 185 kev U-235, N 53 t FWHM, kev FWTM, kev Efficiency ratio Error from statistics,% Crystal 3 volume,mm Net area of 185 kev Efficiency ratio Error from statistics, % SDP 1500/SN38 20 C C C C SDP 500/SN C C C C SDP 500/SN C C C C The efficiency ratios in Table 4 and Table 5 are normalized to those at room temperature. The results have shown stability of the detectors performance in a wide temperature range, even slight improvement of resolution under lower temperatures. 3.3 Enrichment measurements The new large volume CdZnTe detectors potentially represent a more convenient, better energy resolution, stability and almost equivalent detection efficiency substitute of traditional NaI(Tl) crystals in safeguards application. In this paper, the performance of the SDP4000 CdZnTe detector with regard to uranium enrichment determination was evaluated against conventional NaI(Tl)-based HM-5 instrument [15]. The applicability of the infinite thickness enrichment measurement methodology with CdZnTe detectors was examined in this study. The SDP4000 was arranged inside a 36 mm lead collimator, similar (to the extent possible) to the arrangement of a NaI(Tl) crystal inside the HM-5 instrument. Series of measurements were performed using four U 3 O 8 infinitely thick reference samples with wt%, wt%, wt% and wt% 235 U enrichment levels, in unshielded and shielded (4 mm and 8 mm stainless steel) conditions. The measurement live time was preset to 1000 sec in all measurements

7 Performance Evaluation of New Generation CdZnTe Detectors for Safeguards Applications The acquired spectra were processed using the fixed response function methodology, similar to that implemented in the NaIGEM code [16]. Several spectrum fitting examples are shown in Fig. 1. The asymmetric peak shape, which is typical of CdZnTe detectors, was described using an analytical model represented by a sum of two left-tailed Gaussians for the closely spaced unresolved full energy and X-ray escape peaks. After some experimentation, the ratio of the peak amplitudes in the fitted energy interval from 130 kev to 290 kev was fixed to 1:4. The background under the peaks included a stepped polynomial function over the whole fitted energy interval, plus exponential bumps on the left-hand side of the peaks. The latter described specific contributions from the low-angle scattered gamma-rays, an essential component of the NaIGEM model, allowing application of one-point-enrichment-calibration approach in the wide range of absorber thicknesses (typically from 0 mm to 16 mm of stainless steel). A special absorber calibration can be additionally applied, to improve accuracy of the enrichment determination for strongly shielded samples. The 235 U enrichment measurement test results for SDP4000 and HM-5 are presented in Tables 6 and 7, respectively. The both SDP4000 and HM-5 results appear unbiased in the tested ranges of the enrichments and absorber thicknesses, as indicated by the mean relative deviations between certified and measured enrichments (see mean nσ values in the last column in Tables 6 and 7). The average ratio of measurement uncertainties RSD(HM-5)/RSD(SDP4000) = 0.66 ± 0.08, evaluated on all measurement results shown in Tables 1 and 2, indicate that enrichment measurement uncertainties achievable with HM-5 and SDP4000 within the same acquisition time and under similar measurement conditions are pretty much comparable. The observed about 30% better performance of HM-5 is consistent with its better detection efficiency for kev gamma-rays (see Table 7). Figure 1. Examples of fitting of the gamma-ray spectra taken with the large volume CdZnTe detector SDP4000 from the unshielded 3.1 wt% (on the left) and the 8 mm SS shielded wt% (on the right) infinitely thick U 3 O 8 samples. Table 6. Results of the enrichment measurements using SDP4000 large volume CdZnTe detector and four infinitely thick U 3 O 8 reference samples. The last column shows differences between measured and certified enrichment levels in units of standard measurement uncertainty. Sample Certified 235 U, wt% Absorber, mm Measured Measured - Certified 235 U, wt% RSD, %, % nσ SU ± SU ± SU ± SU ± SU ±

8 SU ± SU ± SU ± SU ± SU ± SU ± SU ± Mean, nσ: -0.3 RSD, nσ: 1.8 Table 7. Results of the enrichment measurements using IAEA s standard HM-5 instrument and four infinitely thick U 3 O 8 reference samples. The last column shows differences between measured and certified enrichment levels in units of standard measurement uncertainty. Sample Certified 235 U, wt% Absorber, mm Measured Measured - Certified 235 U, wt% RSD, %, % nσ SU ± SU ± SU ± SU ± SU ± SU ± SU ± SU ± SU ± SU ± SU ± SU ± Mean, nσ: 0.1 RSD, nσ: Spectrometers The evaluation measurements of the Microspectrometer s spectroscopic characteristics have been performed by using the standard Agency WinSpec software. The results for energy resolutions (FWHM and FWTM) at 60 kev of Am-241 and energy resolutions (FWHM and FWTM) peak-to- Compton ratio and peak-to-valley ratio at 662 kev line of Cs-137 are presented in Table 8. Table 8 µspec 500, S/N:010 µspec 1500, S/N:006 Manufacturer's Manufacturer's Measured specification specification Measured Energy resolution FWHM at 662 kev 7.2 kev 7.5 kev 10.0 kev 10.5 kev Energy resolution FWHM at 662 kev 19.6 kev 19.5 kev 32.2 kev 33.5 kev Peak-to-Compton ratio at 662 kev Energy resolution FWHM at 59,6 kev 3.7 kev 4.0 kev 5.7 kev 6.3 kev

9 Performance Evaluation of New Generation CdZnTe Detectors for Safeguards Applications The results obtained have shown values, which are typical for the integrated detectors and confirmed the manufacturer s declaration. Test of the performance specifications under high count rates for both spectrometers and evaluation of the dead time corrections was performed using the two source method. Measurements were performed in standard mode of operation of WinSpec.The obtained results have shown that for integral count rates higher than 20 kcps the measurements should be performed with special setup of amplifier.for both spectrometers the linearity of the energy-channel dependence was evaluated by using sources with nuclides Am-241, Cs-137 and Co-60 to cover the energy range from 60 kev to 1332 kev. For both of them the deviation from the linear fitting is better than 0.5 %. Fig. 2. Spectra of 235 U and 152 Eu registered by the Microspectrometer with CdZnTe detector of 500 mm 3 4. Conclusions Analysis of comprehensive evaluation of newly developed detectors shows efficiencies comparable the standard Agency HM-5 Identifinder 2 and better spectrometric performance. Based on the received results the future exploration of their possibilities for routine safeguards applications will be continued. 9

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