|Year : 2017 | Volume
| Issue : 2 | Page : 69-72
Assessment of the inactive dead layer thickness of old high-purity germanium detector: A study by Monte Carlo simulations and experimental verification
Narayani Krishnan, S Anilkumar, Amit K Verma, Rajvir Singh
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
|Date of Submission||24-Mar-2017|
|Date of Decision||18-Apr-2017|
|Date of Acceptance||25-Apr-2017|
|Date of Web Publication||13-Jul-2017|
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
High-purity germanium (HPGe) detectors are commonly used for high-resolution gamma spectrometry. A HPGe detector considered for this study was in continuous operation for the past 15 years in our laboratory. The performance of the detector system has been monitored periodically using energy and efficiency calibration data. It is observed from the experimental efficiency calibration that there is a reduction in absolute efficiency of the detector for practical geometries after long period of operation. The main reason for this reduction in efficiency is due to increase in inactive Ge dead layer thickness of the detector. A study has been carried out using Monte Carlo techniques to assess the dead layer thickness of the detector after 15 years of operation and compared with the value quoted by the manufacturer while procuring the system and find out how much the dead layer thickness of the detector has varied from the manufactured value. The study shows that dead layer thickness has increased from 0.5 to 1.028 mm.
Keywords: Dead layer, high purity germanium detector, Monte Carlo simulation, relative efficiency
|How to cite this article:|
Krishnan N, Anilkumar S, Verma AK, Singh R. Assessment of the inactive dead layer thickness of old high-purity germanium detector: A study by Monte Carlo simulations and experimental verification. Radiat Prot Environ 2017;40:69-72
|How to cite this URL:|
Krishnan N, Anilkumar S, Verma AK, Singh R. Assessment of the inactive dead layer thickness of old high-purity germanium detector: A study by Monte Carlo simulations and experimental verification. Radiat Prot Environ [serial online] 2017 [cited 2017 Aug 19];40:69-72. Available from: http://www.rpe.org.in/text.asp?2017/40/2/69/210574
| Introduction|| |
High-resolution gamma ray spectrometry technique is a popular radio analytical technique used for the assay of radionuclides in laboratories. Germanium (Ge) detectors are commonly used for the high-resolution gamma ray spectrometry applications. One of the basic operational specifications of the Ge detectors quoted by the manufacturer is the relative efficiency. In principle, it is the ratio of the absolute efficiency of the high purity Ge (HPGe) detector for counting the 1332 keV gamma ray from 60Co at 25 cm to the absolute efficiency of a 3” × 3” sodium iodide (NaI(Tl)) crystal at the same source to detector distance. It is quoted in terms of percentage. It is reported that the relative efficiency reduces marginally with time due to various factors. One of the important factors is the increase in the Ge dead layer thickness of the detector., In HPGe detectors, the active volume of the detector is the region between n + andp + contacts. These contacts may have appreciable thickness which can represent a dead layer on the surface. The dead layer is formed when n + contact is fabricated by evaporating and diffusing lithium atoms into the surface. The change in dead layer thickness occurs slowly over a period due to vacuum degradation with time. Due to this, there will be a significant reduction in the absolute efficiency of the detector for measurement geometries. Many studies have been reported in the literature on the assessment of the dead layer thickness by employing Monte Carlo simulation techniques and experimental methods. Simulation techniques are widely used for obtaining the spectral response of gamma photons in NaI(Tl) and Ge detectors. Monte Carlo based simulation techniques have been successfully used for the evaluation of detector response for gamma photons of wide energy range and for different geometries. Dimensions of the detector configuration are essential for generating the detector model for accurate simulation. The detector configuration data can be obtained from the manufacturer. Standard reference sources such as 22Na,137Cs,54Mn, and 60Co can be used for generating the experimental data for absolute detection efficiency. To minimize the effects due to coincidence summing effects sources can be placed 15 cm away along the axis of the detector. In the present work, a combination of experimental measurements and Monte Carlo simulation techniques have been used for the assessment of the Ge dead layer thickness of the 15 years old detector, which was in continuous operation in our laboratory. Instead of using many sources, only 60Co standard source was used for the experiment. The relative efficiency parameter of the detector calculated by simulation and that obtained by experimental method were used for assessing the Ge dead layer thickness.
| Materials and Methods|| |
The detector used for this study was P type coaxial horizontal HPGe (Eurisys Mesures). The relative efficiency reported by the manufacturer was 50% and dead layer thickness reported was 0.5 mm Ge in the detector. The resolution of the detector was 1.9 keV for 1.33 MeV of 60Co. The detector system was connected with PC based 8K MCA and InterWinner 4G software for the acquisition and analysis of the spectral data. The detector was shielded with 3” lead on all sides to reduce the background due to natural radioactivity. The detector system was installed in 1999. Since then, it was in continuous operation for various activity measurements. The performance evaluation of the detector system was monitored periodically by collecting energy and efficiency calibration data.
Experimental efficiency calibration
The efficiency calibration of the system was done every year for various geometries using standard reference materials. It was observed that the absolute efficiency had reduced marginally during the years. To find the extent of efficiency variation, it was decided to select one particular geometry and compare the efficiency for that geometry in the different years. For that purpose a geometry of 250 ml cylindrical container (7 cm diameter × 7.5 cm height) filled with soil medium was selected. The standard geometry was prepared by filling the container with International Atomic Energy Agency standard uranium ore reference material (RGU-1), sealed and kept for a month to attain radioactive equilibrium between 226Ra and its daughter products. This standard source was used for generating the efficiency calibration data of the detector throughout the period of operation. The spectra of the standard source were acquired for 50,000 s. The peak area of the prominent gamma lines from 226Ra and its daughters 214Pb and 214Bi were used for the efficiency calibration. The efficiency of the detector for each gamma energy (E) was calculated and fitted into semi-empirical mathematical equation like:
Where A, B, and C are coefficients obtained by fitting the energy and efficiency data. The energy versus efficiency curve was plotted. The same procedure was followed for the efficiency calibration of the system every year. It was observed from the experimental efficiency calibration data that the absolute efficiency reduced marginally over the period. To find the variation in efficiency over the period, the efficiency data obtained during three different periods (year 2000, 2008, and 2013) were compared. [Table 1] shows the comparison of the efficiency data of gamma energies of 226Ra and daughters obtained by measurements in different years. [Figure 1] shows the plot of the energy efficiency calibrations curves plotted for the respective periods. It was observed that the absolute detector efficiency had decreased in the energy region of 200–2500 keV over a period. Between the years 2000 and 2008, the reduction was less compared to the later year 2013. Around the year 2013, the vacuum degradation in the detector system was noticed. This may be the reason for the reduction in absolute efficiency of the system during the year 2013. Since the detector, standard and geometry all remain same, the reduction in the absolute efficiency is attributed due to the increase in thickness of the Ge dead layer caused by the vacuum degradation.
|Table 1: Absolute efficiency of 250 ml container with RGU during different years|
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|Figure 1: The variation of absolute detector efficiency calibration with time|
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Monte Carlo simulation
The inactive Ge dead layer thickness was estimated by comparison of Monte Carlo based simulated and experimental detector efficiency. Simulation techniques are widely used for obtaining spectral response of gamma photons in Ge detector. The detector model was developed using detector dimensions supplied by the manufacturer. The diameter of the Ge crystal was 66.9 mm; length 56.1 mm, and distance from cap was 5 mm. Dead layer thickness was 0.5 mm. Photon pulse height tally F8 was used to obtain the energy distribution of pulses (spectrum) produced by the gamma photons inside the detector. For the present work, 107particle histories were simulated for detector and sample geometry. When performing the mathematical simulation of the detector using Monte Carlo based code, to obtain the detector spectral response for the particular energy and geometry, some modification needs to be made to improve the simulation to match the experimental spectrum., The main modification involve the introduction of energy resolution of the spectral response which is the intrinsic property of the detector under consideration. It is related to the ability of the detector to distinguish between different peaks very close to each other in the energy spectrum. Their determination is of great importance when performing identification of radionuclides and while simulating detector response that approximate of the experimental spectra. Variation of the detector's active volume, statistical fluctuation in the charge carrier formation, collection, and electronic noise are the important factors which introduce a broadening of the photopeak in the energy spectrum. The spectrum obtained by the simulation of a detector using the Monte Carlo technique, when applying the pulse height distribution does not introduce the effects of broadening the photopeak. To optimize the detector response and to consider these physical effects in the simulation it is necessary to obtain experimentally derived parameters of the energy resolution of detector. It is assumed that the energy resolution parameters experimentally derived is a smoothly varying function of energy. These experimentally generated energy resolution fitting factors can be introduced in the Gaussian peak broadening procedure available in the code. Thus, Gaussian broadened pulse height spectra was obtained after simulation which is almost similar to experimentally generate shape of the spectrum from the particular detector. The detector model was validated by comparing the relative efficiency calculated by simulation with that of value supplied by manufacturer. For that, the response spectrum of 60Co source kept at a distance of 25 cm from the detector was obtained by simulation. The absolute efficiency for 1.33 MeV was calculated using the simulated spectral data. This was divided by 1.2 × 10−3, absolute efficiency of 3” × 3” NaI (Tl) detector for 1.33 MeV at 25 cm to obtain the percentage relative efficiency.
| Results and Discussion|| |
The relative efficiency thus obtained was 50.07% which is in agreement with the value 50% supplied by the manufacturer. Thus, the detector model was validated. Since the relative efficiency of HPGe detector is basically the absolute efficiency it was decided to use this parameter for finding the dead layer thickness. It is observed that the effect of the increase in dead layer of the Ge contributes to the reduction in the relative efficiency of the detector after long periods of operation. To know how much the dead layer had increased from its original thickness, the simulated spectra of 60Co at 25 cm from the detector were obtained for varying Ge dead layer thickness of 0.5, 1, 1.5, and 2 mm. The corresponding absolute efficiency of 1.33 MeV at 25 cm for varying dead layer thickness was calculated using simulated spectral data. The data are tabulated in [Table 2]. Dead layer thickness versus efficiency is plotted [Figure 2]. A relation of absolute efficiency (η) as a function of the dead layer thickness (x) of the detector was formulated as:
|Table 2: Absolute efficiency of 1.33 MeV by simulation for different dead layer thickness|
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|Figure 2: Plot of calculated efficiency of 1332 keV for various dead layer thickness|
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The absolute efficiency of the detector was calculated recently using the standard 60Co source for the energy of 1332.46 MeV at a distance of 25 cm from the detector. The absolute efficiency thus calculated experimentally was compared with the data obtained by simulation with different dead layer thickness. From the mathematical expression above, the dead layer thickness of the detector corresponding to the present experimental absolute efficiency was evaluated. The dead layer thickness of the Ge detector evaluated from our study is 1.028 ± 0.08 mm, which is much higher than the value 0.5 mm quoted by the manufacturer in 1999. This leads to reduction in absolute efficiency. The detector model was corrected by including the new value of dead layer thickness. Using the new model, the simulated spectra of 60Co at a distance of 25 cm was obtained and compared with experimental data. [Figure 3] shows the simulated spectrum after introduction of the increased dead layer thickness in the detector model and experimental response spectrum of 60Co at 25 cm taken recently. The simulated spectrum is matching well with the experimental spectrum. From this study, it is concluded that the dead layer thickness of the HPGe detector is important factor in efficiency calculations. The dead layer thickness quoted by the manufacturer does not remain same. It varies gradually due to many factors over a period, but main factor is due to vacuum degradation with time. The simulation modeling of the detector requires the correct value of dead layer thickness for efficiency calculation otherwise the efficiency using the simulated spectrum will not match with the experimental efficiency. Continuous and periodic monitoring of the system parameters are essential to notice such marginal variations which may otherwise lead to errors in activity estimation. If the efficiency of the detector system is reducing significantly then evacuation of the detector has to be done to rectify the vacuum degradation.
|Figure 3: Comparison of experimental and simulated spectrum of 60Co at 25 cm from the detector|
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The authors are thankful to Dr. K. S. Pradeepkumar, Associate Director, HS&E Group and Head, RSSD for his keen interest in this work and Shri Amar D Pant of RSSD for the help during the work.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]