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A low background gamma-ray spectrometer with a large well HPGe detector
Journal Pre-proof A low background gamma-ray spectrometer with a large well HPGe detector Jong-In Byun, Han-Yul Hwang, Ju-Yong Yun PII:
S0969-8043(19)30143-5
DOI:
https://doi.org/10.1016/j.apradiso.2019.108932
Reference:
ARI 108932
To appear in:
Applied Radiation and Isotopes
Received Date: 16 February 2019 Revised Date:
24 September 2019
Accepted Date: 9 October 2019
Please cite this article as: Byun, J.-I., Hwang, H.-Y., Yun, J.-Y., A low background gamma-ray spectrometer with a large well HPGe detector, Applied Radiation and Isotopes (2019), doi: https:// doi.org/10.1016/j.apradiso.2019.108932. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Title page Names of the authors: Jong-In Byun, Han-Yull Hwang, Ju-Yong Yun Title: A Low Background Gamma-ray Spectrometer with a Large Well HPGe Detector Affiliation(s) and address(es) of the author(s): Jong-In Byun, Korea Institute of Nuclear and Safety, 62 Gwahak-ro Yuseong-gu, 34142 Daejeon, Republic of Korea Han-Yull Hwang, Mokwon University, 88 Doanbuk-ro Seo-gu, 35349 Daejeon, Republic of Korea Ju-Yong Yun, Korea Institute of Nuclear and Safety, 62 Gwahak-ro Yuseong-gu, 34142 Daejeon, Republic of Korea E-mail address of the corresponding author: [email protected]
A Low Background Gamma-ray Spectrometer with a Large Well HPGe Detector Jong-In Byun1, *, Han-Yul Hwang2, Ju-Yong Yun1 1
Korea Institute of Nuclear Safety 2
Mokwon University
Abstract A low background gamma-ray spectrometer was established in a ground-based laboratory. It consists of a large well (diameter: 28 mm, depth: 40 mm) HPGe detector and an anti-cosmic shielding system. The photon background of the present system was measured with and without the anti-cosmic mode, and compared with each other, and the detection capability with the detection efficiency calibration was estimated. The background with passive and active shields for the energy region between 40 keV and 3000 keV was reduced by 78% compared to one with just passive shield. The full energy peak efficiency for cylindrical samples with three different heights and the minimum detectable activity of the present system were tabulated for main radionuclides to be measured in environmental monitoring. Keywords: Gamma-ray measurement, Low background, Anti-coincidence, Cosmic-ray, HPGe
1. Introduction In gamma-ray measurements using an HPGe (High Purity Germanium) detector, the ability to detect gamma-ray emitting radionuclides can be improved by reducing the measurement system background or by increasing detection efficiency. In practice, the background counts can increase as detection efficiency increases under the same measurement conditions. Therefore, identifying and lowering factors contributing to background is important to minimizing it and thus improve detection capability. The photon background of the measurement system is mainly caused by building materials surrounding the measurement system, detector shielding materials, detector components, radionuclides in the air, and cosmic rays. In general, most of this background can be reduced using passive shields, made of materials such as lead, copper, nitrogen gas or low-background materials for detector components and shielding. On the other hand, cosmic-ray induced background can be increased by thick shielding because it is mainly produced by collisions of muons with materials of high atomic number surrounding the detector, creating secondary particles such as electrons, positrons, neutrons and protons. The intensity of cosmicray contribution to the background can be significantly decreased in deep underground laboratories
(Brodzinski et al. 1988; Bourlat et al. 1994; Arnold et al. 2002; Lindstrom 2017). At ground level, active shields with anti-coincidence electronics and a guard detector for cosmic-ray detection can be used to reduce the cosmic-ray induced background. Anti-cosmic shielding systems have been previously developed by other researchers, and effective background reduction was accomplished (Reeves et al. 1988; Shizuma et al. 1992; Pointurier et al. 1996; Laurec et al. 1996; Beda et al. 2000; Semkow et al. 2002; Byun et al. 2003). As one of the factors affecting the detection capability of the measurement system, detection efficiency can be increased using a solid angle depending on the detector crystal’s size or the measurement arrangement of the gamma-ray source and detector. In order to increase the solid angle, the well-type HPGe detector was developed and effectively used for small amounts of samples. In general, well-type HPGe detectors have a well approximately 16 mm in diameter and 40 mm in depth. Recently, the welldiameter was increased to 28 mm due to improvements in instrument manufacturing technology, which measurable sample amount can be increased. This study was conducted to improve the detection capability of a ground level gamma-ray measurement system by combining a low background shielding system and a large well-type HPGe detector. The low background gamma-ray measurement system was provided with passive and active shields, and the full energy peak efficiency of the HPGe detector was calibrated using a standard mixed gamma-ray source. In this paper, the proposed measurement system is introduced and its background level and detection capability are discussed with the minimum detectable activity (MDA) for the main radionuclides encountered in environmental monitoring.
2. Methods and materials 2.1 An anti-cosmic shielding system with a well-type HPGe detector The present gamma-ray measurement system was installed in the laboratory on the first floor of a three-story building in Daejeon, Korea. Outdoor and indoor backgrounds that measured above about 6 MeV were 0.0165 and 0.0124 counts cm-2 s-1 on the basis of the BC-408 (BICRON series) plastic scintillation detector’s surface area, which had dimensions of 700 mm × 800 mm × 50 mm, respectively. As a result, cosmic muon intensity was reduced by about 25% by the building material in which the system was installed. An electrically cooled SAGe (Small Anode Germanium, GSW275L, Mirion Inc.) well detector with a typical energy resolution of 2.2 keV at 1332 keV was used as the main detector for gamma-ray measurement. It has a crystal volume of 275 cm3 and a well with a diameter of 28 mm and a depth of 40 mm. In order to minimize the intrinsic background, the detector components were made of low-background materials with < 1 ppb of U and Th, and the detector crystal was mounted on a U-style cryostat away from the preamplifier and the electrical cooler.
The anti-cosmic shielding system was established with passive and active shields as shown in Fig. 1(a). Five 4 mm-thick borated-silicon plates (SWX-238, Shieldwerx Inc.), which had a boron weight percent of 27.6% and a density of 1.64 g/cm3, were placed with a macroscopic thermal neutron cross section of 18.9 cm-1 between each plastic scintillation detector and lead shield. The boron plates were used to physically buffer the plastic detectors and shielding, and to further limit the introduction of thermal neutrons produced by the interaction of fast neutrons and plastic detectors into the lead shielding. The lead shield consisted of a 50 mm-thick interior lead with <5 Bq/kg of 210Pb and an 100 mm-thick exterior lead with < 100 Bq/kg of
210
Pb. To reduce fluorescence X-rays, 4 mm-thick oxygen free high conductivity (OFHC)
copper plates were placed inside the lead shield. N2 gas also was injected at 50 ml/min inside the shielding system with a volume of 0.03 m3 to reduce the contributions of background photons by Rn progenies during gamma-ray measurement. In addition, the innermost part of the shielding was sealed with 3 mm-thick plexiglass plates. This was done for two purposes: 1) to minimize the consumption of the N2 gas injected to reduce the contributions of background photons by Rn progenies during gamma-ray measurement; and 2) to maintain cleanliness inside the system. One side of the shielding system has two 5 mm diameter holes, to inject nitrogen gas and draining the first internal air. After opening and closing the shielding door, high-pressure nitrogen gas with a volume of 0.03 m3 was first injected into the shielding displace the air inside the shielding and fill the system with nitrogen gas. The gas outlet was then blocked, and nitrogen was continuously injected at the rate of 50 ml/min. The injected nitrogen was naturally released into the fine gaps of the front door. The active shield consisted of five BC-408 (BICRON series) plastic scintillation detectors with 50 mm thickness, and the dimensions were 800 mm × 800 mm for the top, 700 mm × 800 mm for the three sides and 700 × 600 mm for one side. Fig. 1(b) shows an anti-coincidence electronics diagram for anticosmic gamma-ray measurement. To avoid random coincidences due to gamma-rays from natural radionuclides in the building materials and indoor air, such as 40K, Th- and U-series, the energy threshold for the plastic scintillation detector as an active shield was conservatively set to about 7.5 MeV. The threshold was adjusted at the input of constant fraction discriminator (CFD) for the plastic scintillation detectors. Under these conditions, the total count rates for the plastic scintillation detectors were approximately 70 s-1 for the top and 30 s-1 for one side of the measurement system. The pulses from the plastic scintillators and HPGe detector were amplified and converted to fast logic signals at each CFD. The logic output signal of the plastic detector leads to the ‘START’ input and the one for the HPGe detector leads to the ‘STOP’ input of the time-to-amplitude converter (TAC). A coincidence signal is generated when two input signals are within the preset time interval. The TAC
output signal as a stretched linear pulse was converted to a gate signal with a pulse width of 90 μs extended by a delay/gate generator, and the gate signal inputs to the gate of an analog-to-digital converter (ADC). The linear energy pulse (i.e., cosmic-ray induced ones) from the HPGe detector is vetoed by the anti-coincidence mode of the ADC if it is covered by the gate signal time. Background photons with and without the anti-cosmic mode were measured for 100,000 seconds to evaluate the background reduction rate. The Genie2K (Version 3.1, Mirion Inc.) software was used for the data acquisition.
2.2 Full energy peak efficiency calibration and MDA estimation In order to evaluate the full energy peak efficiency of the well HPGe detector, a cylindrical plastic (polyethylene) bottle with an inner diameter of 24 mm and a wall thickness of 1.5 mm was prepared and filled to heights of 10 mm, 20 mm and 30 mm with a certified gamma-ray source (Eckert & Ziegler Inc.) in a 1 M HCl solution containing 51
keV), Cr (320.1 keV), keV) and
88
113
241
Am (59.5 keV),
109
Cd (88.0 keV),
85
Sn (391.7 keV), Sr (514.0 keV),
137
57
Co (122.1 keV),
139
Ce (165.9
60
Cs (661.7 keV), Co (1173.2 keV, 1332.5
Y (898.0 keV, 1836.1 keV). The standard uncertainty of each prepared calibration source
ranges from 0.97% to 1.55%. Here, the maximum sample height was set to 30 mm, to place the sample within the Ge crystal since there is a gap of around 6 mm between the well endcap and the Ge crystal. The source was placed inside the well of the detector, and gamma-rays were measured for 50,000 seconds, for during which the standard statistical uncertainties for all radionuclides is less than 0.7%. In general, true coincidence summing (TCS) can occur with a well type detector because of its higher detection efficiency. The true coincidence summing correction factor (CFTCS) was determined using Monte Carlo simulation, and the total efficiency obtained with the MCNPX (Version 2.7.0) code (Venkataraman et al. 2005; Byun 2018). Information about the detector material and geometry for modeling the detector was provided by the manufacturer. The thickness of dead layers inside the well of the detector crystal used in this study were 0.08 mm for the side and 0.055 mm for the bottom. Fig. 2 shows the geometrical model and the pulse height tally ‘F8:P’ with the default physics option in the MCNPX code was used to simulate the full energy peak and total efficiency (Pelowitz 2011). To validate the modeling, efficiencies obtained from simulations and experiments for 51
Cr,
113
241
Am,
109
Cd,
57
Co,
Sn, 85Sr, and 137Cs, which can ignore the TCS for the detector used in this study, were compared
with each other. The CFTCS was calculated for
60
Co, and
88
Y using the total efficiency obtained in the
simulation, and following the general formula (Debertin et al. 1988; Montgomery et al. 1995)
=1−∑
=
(1)
is the gamma-ray count rate without TCS, is the gamma-ray count rate with TCS, is the where total photon energy lines coinciding (‘within the resolving time’) with the gamma-ray of interest, is the emission fraction of -th photon coinciding with gamma-ray of interest, and is total efficiency for the -th photon coinciding with the gamma-ray of interest. The gamma-ray energy and emission probability, the electron capture probability, internal conversion coefficients, and K-shell X-ray fluorescence needed for were referenced using the DDEP Recommended Data (LNHB National Becquerel, updated on Oct 20 2017). 139Ce in the calibration source can also significantly produce the true coincidence summing by X-rays (33.2 keV, 37.8 keV and 38.8 keV) and gamma-rays (165.9 keV) due to the thin dead layer of the detector crystal. Therefore, the thicknesses of both the dead layers on the bottom and the side of the detector well should be optimized even though the simulation for 59.5 keV of 241Am can be validated. Optimization requires the specific collimator and photon sources to be placed inside the detector well. In this study, instead of optimization, the integral net counts of the energy range between full energy peaks of 165.9 keV and 204.7 keV were used to obtain the full energy peak counts of 165.9 keV from 139Ce without losing the net counts due to the TCS. With the calibration source used in this study, the Compton continuum of the region between 165.9 keV and 204.7 keV is enough steady to estimate the background baseline, and no interfered full energy peaks over the energy range is measured in the gamma-ray energy spectra. The compared spectra with and without TCS for the same radionuclides used in this study can be found in another article (Byun 2018). Fig. 3 shows the full energy peak and true coincidence summing peaks of 165.86 keV (139Ce) with the regression line of the Compton continuum in the measured gamma-ray energy spectrum for 10 mm height calibration source. The full energy peak efficiency for 165.9 keV calculated by this way was also compared with the simulated one. The MDA of the present system and full energy peak efficiency for 241Am, 212Pb, 214Pb, 131I, 208Tl, 134Cs, 214
Bi,
137
Cs,
228
60
Ac,
Co and
40
K, as the main gamma-ray emitters in environmental monitoring were
tabulated. The MDA using the Currie method (Currie 1968) in a condition of
=
= 1.645 can be
estimated as
%&' = * +
()
,
-
× ./ =
0.1 23.45 √78 * +, - 9:
× ./
where ;< is detection limit, => is background counts,
(2)
is absolute photopeak efficiency, ?@ is
gamma-ray emission probability, % is sample volume, A is measurement time in a seconds, ./ is efficiency correction factor such as self-attenuation, true coincidence summing. => was determined as the integral counts covering a region of interest (ROI) width of 2.5×FWHM for the full energy peaks.
3. Results and discussion
Fig. 4 shows a comparison of the background spectra with and without anti-comics mode. Cosmic-ray induced isomers with longer half-lives than the coincidence time interval are observed in the spectra. Details of those common background components and their production processes, by interaction between cosmic rays and surrounding materials, are well summarized elsewhere (Sánchez et al. 1994; Heusser 1996; Núñez-Lagos et al. 1996; Cannizzaro 1997; Michael 2003). Integral counting rates within an energy range between 40 keV to 3000 keV with and without anti-cosmic mode were 1.51 s-1 and 0.337 s-1, respectively. The integral count rates per 100 cm3 Ge volume with the anti-cosmic mode was 0.123 s-1 which is 8 times lower than about 1 s-1 for a general p-type HPGe detector system, with a 120 cm3 Ge volume and a relative efficiency of 30%, with typical cylindrical lead shielding. The background was reduced 0.02 s-1 by N2 gas purging inside the detector chamber and the cosmic-ray induced background was suppressed by a rate of 1.17 s-1. The full energy peak count rates for natural and cosmic-ray induced radionuclides for 100,000 seconds measurement time under the anti-cosmic mode and N2 purging conditions are as shown in Table 1. The count rates are normalized to a Ge crystal mass of 1.46 kg. The most apparently suppressed were the annihilation peaks produced by secondary radiation positrons due to colliding muons and high-Z materials, by a factor of 9.6 with the active shielding. Table 2 shows the full energy peak efficiency and the combined standard uncertainties measured for cylindrical sources with a diameter of 24 mm and different heights of 10 mm, 20 mm, and 30 mm. As mentioned in section 2, the detection efficiency for 165.86 keV from
139
Ce was determined using the
integral net counts of the energy range between full energy peaks of 165.9 keV and 204.7 keV, and the results agreed with the simulation results within 3%. In contrast, the estimated efficiencies obtained by simulation and measured with the CFTCS correction for 165.86 keV showed a maximum difference of 24% for the calibration sources with heights of 10 mm, 20 mm, and 30 mm. This indicates that the efficiency of approximately 30 keV X-rays from 165 keV is very sensitive, with dead layers of about 0.055 mm and 0.08 mm. Therefore, further studies on the optimization of dead layer thicknesses are required for radionuclides with X-rays. This can be implemented using small sized collimators placed inside the detector well for the tests on the bottom and side of the detector well. The full energy peak efficiencies determined by measurement and simulation for 59.5 keV, 88.0 keV, 122.1 keV, 320.1 keV, 391.7 keV, 514.0 keV and 661.7 keV also agreed with each other within 5%, and this demonstrated the validity of the
MCNPX modeling for those energy ranges. The full energy peak efficiency for 898.04 keV, 1173.23 keV, 1332.5 keV and 1836.07 keV were determined using the CFTCS obtained with Eq. (1), with the simulated total efficiency (
). The standard uncertainty of CFTCS was estimated considering the standard
uncertainties for the simulation and nuclear data. The uncertainty of simulation was also considered for statistical uncertainty from simulation results, sensitivity analysis results using the uncertainty of detector geometry, and systematical uncertainty from a difference between experimental and simulated ones. For 88
Y and
60
Co, the relative statistical uncertainties were less than 1% with the photon source’s particle
number of 5×108. As a sensitivity analysis result for the dead layer’s thickness with a standard uncertainty of ±5%, there were differences less than 1% between the CFTCSs for maximum and minimum thickness. The relative systematic uncertainty was conservatively estimated 5% considering the relative deviation between experimental and simulated efficiencies for 59.5 keV, 88.0 keV, 122.1 keV, 320.1 keV, 391.7 keV, 514.0 keV and 661.7 keV to ignore TCS. The combined standard uncertainty of efficiency corrected with TCS for 898.04 keV and 1836.07 keV of 88Y, and 1173.23 keV and 1332.5 keV of of 60Co was estimated as an around 5%. There was a maximum 5.3% difference between the simulated efficiency and the measured one with TCS correction. The accuracy may be improved by optimizing the detector geometry. The efficiency curves for each source were also obtained by log(Energy)log(Efficiency) least squared fit as shown in Fig. 5. Table 3 shows the MDA values of this system for the major radionuclides used in environmental radioactivity monitoring. One can assume the MDA with a specific sample amount of samples, and the measurement time for similar types of detectors using the information presented in Table 3. For example, if a sample of 13.6 g with a height of 30 mm is measured for 100,000 seconds using the present gamma-ray spectrometer, then the MDA without any efficiency corrections such as self-attenuation or TCS corrections can be calculated as
%&' ijk
h
= m.
l4.0 4n×m.o5m× l.4 p× mm,mmm q
= 0.185 mBq/g.
(3)
For an HPGe detector (GC3018, Mirion Inc.) with a relative efficiency of 30% with typical cylindrical lead shielding, the MDA is calculated as
%&' ijk
h
= m.m
1n.3 n1×m.o5m×
l p× mm,mmm q
= 0.420 mBq/g,
(4)
when a sample of 113 g with a diameter of 60 mm and a height of 40 mm on the detector endcap is measured for 100,000 s. Here, the detection efficiency used in Eq. (4) was obtained using
137
Cs in a
mixed source (Eckert & Ziegler Inc.) contained in a cylindrical plastic bottle with a wall thickness of 1.5 mm. The uncertainties for
and ?@ were applied to calculate the vw in Eq. (2) for the MDA. This
shows that the detection capability of the present system is more than twice that of a conventional gamma-ray spectrometer with similar mass efficiency ( × %) due to its significantly lower background, despite sample amounts that were about 8 times smaller. However, as shown in Table 3, it should be noted that the detection capability of gamma-ray energy lines of radionuclides such as 583.2 keV (208Tl), 604.7 keV (134Cs), 609.3 keV (214Bi) and 1173.2 keV (60Co) can be significantly decreased due to strong TCS. This result does not mean that the large well HPGe detector has a lower MDA than the HPGe detector with 30% relative efficiency in any measurement condition. In practice, if there is no limitation on sample amount, and the
× % can be much higher than one of the SAGe detectors with a lower TCS.
Therefore, depending on the measurement condition and purpose, the appropriate detector type should be considered.
4. Conclusion. A ground-based low background gamma-ray measurement system with a large well HPGe detector consisting of low background materials was successfully implemented using passive and active shields. The background count rate with passive shielding for 40~3000 keV was suppressed by 78% with the anticosmic mode. The integral count rates per 100 cm3 Ge volume was up to 8 times lower than the general gamma-ray measurement system with a conventional p-type HPGe detector system and a relative efficiency of 30%. The MDA of the system for
137
Cs was also decreased by more than 50%, a sample
amount corresponding to about 12% of the general one. The detection capability of the present system was also significantly improved due to the larger well volume of the detector, higher detection efficiency, and lower background. The MDAs and true coincidence summing correction factors tabulated in this paper can be useful for estimating minimum detectable activity, to establish a the gamma-ray measurement system with higher detection capability. The present equipment is also especially useful for measuring small amounts of samples with low level radioactivity.
Acknowledgement
This work was supported by the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KoFONS), granted financial resource from the Nuclear Safety and Security Commission (NSSC), Republic of Korea. [No.1305012-0517-GC100]
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Montgomery, D.M., Montgomery, G.A., 1995. A method for assessing and correcting coincidence summing effects for germanium detector efficiency calibrations. Journal of Radioanalytical and Nuclear Chemistry 193(5) 71-79. Núñez-Lagos, R., Vitro, A., 1996. Shielding and background reduction. Applied Radiation and Isotopies 47 1011-1021. Pelowitz, D.B., 2011. MCNPX User’s Manual. Version 2.7.0, Los Alamos National Laboratory, LA-CP11-00438. Pointurier, F., Laurec, J., Blanchard,. X., Adam, A., 1996. Cosmic-ray induced background reduction by means of an anticoincidence shield. Applied Radiation and Isotopes 47(9) 1043-1048. Reeves, J.H., Arthur, R.J., 1988. Anticosmic-shielded ultralow-background germanium detector systems for analysis of bulk environmental samples. Journal of Radioanalytical and Nuclear Chemistry 124(2) 435-447. Sánchez, F., Navarro, E., Ferrero, J.L., Baeza, A., Barrigón, J.M., 1994. Study of the background components for a Ge(HP) detector in environmental radioactivity measurements. Nuclear Instruments and Methods in Physics Research A 339 297-301. Semkow, T.M., Parekh, P.P., Schwenker, C.D., Khan, A.J., Bari, A., Colaresi, J.F., Tench, O.K., David, G., Guryn, W., 2002. Low-background gamma spectrometry for environemental radioactivity. Applied Radiation and Isotopes 57 213-223. Shizuma, K., Fukami, K., Iwatani, K., Hasai, H., 1992. Low-background shielding of Ge detectors for the measurement of residual 152Eu radioactivity induced by neutrons from the Hiroshima atomic bomb. Nuclear Instruments and Methods in Physics Research B 66 459-464. Venkataraman, R., Croft, S., Russ, W.R., 2005. Calculation of peak-to-total ratios for high purity germanium detectors using Monte-Carlo modeling. Journal of Radioanalytical and Nuclear Chemistry 264 183-191.
Tables Table 1. Full energy peak count rates normalized to the Ge mass (1.46 kg) for natural and cosmic-ray induced radionuclides with the passive and active shielding mode. Nuclides 210 Pb 234 Th 73m Ge 75m Ge 77m Ge 71m Ge 226 Ra, 235U 212 Pb 214 Pb 214 Pb Annihilation 208 Tl 214 Bi 228 Ac 40 K 214 Bi 208 Tl Integral * Detection limit (
Energy (keV) 46.5 63.3 66.8 139.5 159.5 198.3 186 238.6 295.2 351.9 511 583.2 609.3 911.2 1460.8 1764.5 2614.5 40 - 3000 ) with a 95% confidence level
Count rates (counts min-1 kg-1) <* 0.0336 < 0.0366 0.112 0.0460 0.0288 0.0322 0.0392 0.0904 0.0321 0.0473 0.116 0.0230 0.0444 0.0213 0.0104 < 0.0119 0.0184 13.8
Table 2. Absolute experimental full energy peak efficiency versus energy with different heights of the calibration sources. Absolute experimental full energy peak efficiency (k=1) 30 mm height: 13.65 10 mm height: 4.5 cm3 20 mm height: 9.0 cm3 cm3 0.620±0.006 0.571±0.006 0.521±0.005 241 59.54 Am (0.637) (0.590) (0.536) 0.696±0.007 0.637±0.007 0.586±0.006 109 88.03 Cd (0.707) (0.657) (0.596) 0.707±0.010 0.655±0.009 0.601±0.008 57 122.06 Co (0.709) (0.655) (0.592) 0.626±0.007 0.571±0.006 0.513±0.006 139 165.86 Ce (0.638) (0.587) (0.528) 0.375±0.004 0.338±0.004 0.308±0.004 51 320.08 Cr (0.380) (0.346) (0.311) 0.313±0.003 0.284±0.003 0.260±0.003 113 Sn 391.7 (0.318) (0.289) (0.259) 0.243±0.003 0.229±0.003 0.201±0.003 85 514.0 Sr (0.251) (0.228) (0.204) 0.204±0.003 0.185±0.003 0.169±0.003 137 Cs 661.66 (0.203) (0.184) (0.166) 0.154±0.008 0.139±0.007 0.125±0.007 88 898.04 Y (0.158) (0.144) (0.129) 0.122±0.006 0.110±0.006 0.0991±0.0051 60 1173.23 Co (0.127) (0.116) (0.104) 0.109±0.006 0.0984±0.0050 0.0891±0.0046 60 Co 1332.5 (0.114) (0.104) (0.0937) 0.0828±0.0043 0.0750±0.0039 0.0674±0.0035 88 1836.07 Y (0.0868) (0.0790) (0.0711) Simulated full energy peak efficiency for the modeling validation in parentheses in each column Energy (keV)
Nuclid es
Table 3. MDAs with a 95% confidence level for several natural and artificial radionuclides in a 1 M HCl sample solution after a measurement time of 100,000 seconds. Nuclides 241
Am Pb 214 Pb 131 I 208 Tl 134 Cs 214 Bi 137 Cs 228 Ac 60 Co 40 K 214 Bi 212
Energy (keV) 59.5 238.6 351.9 364.5 583.2 604.7 609.3 661.7 911.2 1173.2 1460.8 1764.5
10 mm height MDA (mBq) CFTCS 3.11 1.00 4.79 1.00 6.70 1.01 1.94 0.97 6.52 2.24 5.97 2.66 13.9 1.99 2.09 1.00 11.9 1.14 4.64 1.87 34.1 1.00 22.0 0.98
20 mm height MDA (mBq) CFTCS 3.38 1.00 5.27 1.00 7.35 1.01 2.15 0.98 6.75 2.13 5.97 2.44 14.4 1.90 2.30 1.00 13.0 1.13 4.92 1.79 38.0 1.00 24.7 0.99
30 mm height MDA (mBq) CFTCS 3.70 1.00 5.82 1.00 8.11 1.01 2.38 0.98 6.97 1.98 5.95 2.19 15.1 1.79 2.52 1.00 14.3 1.12 5.19 1.70 42.0 1.00 27.4 0.99
The MDAs were calculated considering true coincidence summing correction, but not background contribution from the Compton continuum or full energy peak interference. Decay correction during measurement time was considered only for 131I.
Figures
②
①
③ ①
①
④
⑤
⑥
(a)
(b)
Fig. 1. (a) Drawing of the gamma-ray measurement system with the anti-cosmic shield and (b) block diagram of anticoincidence counting system; ①Plastic scintillator (5 ea), ②Borated-silicon plate (5 ea), ③Lead (interior 50 mm-thick with <5 Bq/kg of 210Pb and exterior 100 mm-thick with <100 Bq/kg of 210Pb), ④4 mm-thick copper + 3 mm-thick plexiglass (inside copper), ⑤HPGe detector, ⑥Electrical cooler
Copper shield
Lead shield
Ge crystal
Gamma source
SAGe detector
Fig. 2. Geometrical model of the well type SAGe (Small Anode Germanium) detector with the shield and a cylindrical bottle gamma-ray source using the MCNPX code
Fig. 3. Full energy peak and true coincidence summing peaks of 165.86 keV (139Ce) with the regression line of the Compton continuum in the measured gamma-ray energy spectrum for 10 mm height calibration source
500
300
350
400
450
500
511 keV, Annihilation peak
40~3000 keV
1000
250
351.9 keV, 214 Pb
300
200
198.3 keV, 71m Ge
200
550
600
609 keV, 214 Bi
400
100
159.5 keV, 77m Ge
150
278.3 keV, 63* Cu 295.2 keV, 214 Pb
Energy (keV)
583 keV, 208Tl
100
0
(b)
(a)
100
238.6 keV, 212 Pb
300
200
100
0 50
66.8 keV, 73m Ge
139.5 keV, 75m Ge
mm and heights of 10 mm, 20 mm and 30 mm.
650
Fig. 5. Absolute full energy peak efficiencies for cylindrical water sources with an inner diameter of 24
Active and passive shields, (b) Passive shield.
Fig. 4. Background spectra obtained during 100,000 seconds with and without anti-cosmic mode; (a)
Counts
Highlight
A ground-based low background gamma-ray measurement system was established with passive and active shields. A large well HPGe detector with low background materials was used as a main detector of anticosmic background measurement system. The background level and detection capability of the present system were estimated comparing a conventional HPGe detector system.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0969-8043(19)30143-5
DOI:
https://doi.org/10.1016/j.apradiso.2019.108932
Reference:
ARI 108932
To appear in:
Applied Radiation and Isotopes
Received Date: 16 February 2019 Revised Date:
24 September 2019
Accepted Date: 9 October 2019
Please cite this article as: Byun, J.-I., Hwang, H.-Y., Yun, J.-Y., A low background gamma-ray spectrometer with a large well HPGe detector, Applied Radiation and Isotopes (2019), doi: https:// doi.org/10.1016/j.apradiso.2019.108932. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Title page Names of the authors: Jong-In Byun, Han-Yull Hwang, Ju-Yong Yun Title: A Low Background Gamma-ray Spectrometer with a Large Well HPGe Detector Affiliation(s) and address(es) of the author(s): Jong-In Byun, Korea Institute of Nuclear and Safety, 62 Gwahak-ro Yuseong-gu, 34142 Daejeon, Republic of Korea Han-Yull Hwang, Mokwon University, 88 Doanbuk-ro Seo-gu, 35349 Daejeon, Republic of Korea Ju-Yong Yun, Korea Institute of Nuclear and Safety, 62 Gwahak-ro Yuseong-gu, 34142 Daejeon, Republic of Korea E-mail address of the corresponding author: [email protected]
A Low Background Gamma-ray Spectrometer with a Large Well HPGe Detector Jong-In Byun1, *, Han-Yul Hwang2, Ju-Yong Yun1 1
Korea Institute of Nuclear Safety 2
Mokwon University
Abstract A low background gamma-ray spectrometer was established in a ground-based laboratory. It consists of a large well (diameter: 28 mm, depth: 40 mm) HPGe detector and an anti-cosmic shielding system. The photon background of the present system was measured with and without the anti-cosmic mode, and compared with each other, and the detection capability with the detection efficiency calibration was estimated. The background with passive and active shields for the energy region between 40 keV and 3000 keV was reduced by 78% compared to one with just passive shield. The full energy peak efficiency for cylindrical samples with three different heights and the minimum detectable activity of the present system were tabulated for main radionuclides to be measured in environmental monitoring. Keywords: Gamma-ray measurement, Low background, Anti-coincidence, Cosmic-ray, HPGe
1. Introduction In gamma-ray measurements using an HPGe (High Purity Germanium) detector, the ability to detect gamma-ray emitting radionuclides can be improved by reducing the measurement system background or by increasing detection efficiency. In practice, the background counts can increase as detection efficiency increases under the same measurement conditions. Therefore, identifying and lowering factors contributing to background is important to minimizing it and thus improve detection capability. The photon background of the measurement system is mainly caused by building materials surrounding the measurement system, detector shielding materials, detector components, radionuclides in the air, and cosmic rays. In general, most of this background can be reduced using passive shields, made of materials such as lead, copper, nitrogen gas or low-background materials for detector components and shielding. On the other hand, cosmic-ray induced background can be increased by thick shielding because it is mainly produced by collisions of muons with materials of high atomic number surrounding the detector, creating secondary particles such as electrons, positrons, neutrons and protons. The intensity of cosmicray contribution to the background can be significantly decreased in deep underground laboratories
(Brodzinski et al. 1988; Bourlat et al. 1994; Arnold et al. 2002; Lindstrom 2017). At ground level, active shields with anti-coincidence electronics and a guard detector for cosmic-ray detection can be used to reduce the cosmic-ray induced background. Anti-cosmic shielding systems have been previously developed by other researchers, and effective background reduction was accomplished (Reeves et al. 1988; Shizuma et al. 1992; Pointurier et al. 1996; Laurec et al. 1996; Beda et al. 2000; Semkow et al. 2002; Byun et al. 2003). As one of the factors affecting the detection capability of the measurement system, detection efficiency can be increased using a solid angle depending on the detector crystal’s size or the measurement arrangement of the gamma-ray source and detector. In order to increase the solid angle, the well-type HPGe detector was developed and effectively used for small amounts of samples. In general, well-type HPGe detectors have a well approximately 16 mm in diameter and 40 mm in depth. Recently, the welldiameter was increased to 28 mm due to improvements in instrument manufacturing technology, which measurable sample amount can be increased. This study was conducted to improve the detection capability of a ground level gamma-ray measurement system by combining a low background shielding system and a large well-type HPGe detector. The low background gamma-ray measurement system was provided with passive and active shields, and the full energy peak efficiency of the HPGe detector was calibrated using a standard mixed gamma-ray source. In this paper, the proposed measurement system is introduced and its background level and detection capability are discussed with the minimum detectable activity (MDA) for the main radionuclides encountered in environmental monitoring.
2. Methods and materials 2.1 An anti-cosmic shielding system with a well-type HPGe detector The present gamma-ray measurement system was installed in the laboratory on the first floor of a three-story building in Daejeon, Korea. Outdoor and indoor backgrounds that measured above about 6 MeV were 0.0165 and 0.0124 counts cm-2 s-1 on the basis of the BC-408 (BICRON series) plastic scintillation detector’s surface area, which had dimensions of 700 mm × 800 mm × 50 mm, respectively. As a result, cosmic muon intensity was reduced by about 25% by the building material in which the system was installed. An electrically cooled SAGe (Small Anode Germanium, GSW275L, Mirion Inc.) well detector with a typical energy resolution of 2.2 keV at 1332 keV was used as the main detector for gamma-ray measurement. It has a crystal volume of 275 cm3 and a well with a diameter of 28 mm and a depth of 40 mm. In order to minimize the intrinsic background, the detector components were made of low-background materials with < 1 ppb of U and Th, and the detector crystal was mounted on a U-style cryostat away from the preamplifier and the electrical cooler.
The anti-cosmic shielding system was established with passive and active shields as shown in Fig. 1(a). Five 4 mm-thick borated-silicon plates (SWX-238, Shieldwerx Inc.), which had a boron weight percent of 27.6% and a density of 1.64 g/cm3, were placed with a macroscopic thermal neutron cross section of 18.9 cm-1 between each plastic scintillation detector and lead shield. The boron plates were used to physically buffer the plastic detectors and shielding, and to further limit the introduction of thermal neutrons produced by the interaction of fast neutrons and plastic detectors into the lead shielding. The lead shield consisted of a 50 mm-thick interior lead with <5 Bq/kg of 210Pb and an 100 mm-thick exterior lead with < 100 Bq/kg of
210
Pb. To reduce fluorescence X-rays, 4 mm-thick oxygen free high conductivity (OFHC)
copper plates were placed inside the lead shield. N2 gas also was injected at 50 ml/min inside the shielding system with a volume of 0.03 m3 to reduce the contributions of background photons by Rn progenies during gamma-ray measurement. In addition, the innermost part of the shielding was sealed with 3 mm-thick plexiglass plates. This was done for two purposes: 1) to minimize the consumption of the N2 gas injected to reduce the contributions of background photons by Rn progenies during gamma-ray measurement; and 2) to maintain cleanliness inside the system. One side of the shielding system has two 5 mm diameter holes, to inject nitrogen gas and draining the first internal air. After opening and closing the shielding door, high-pressure nitrogen gas with a volume of 0.03 m3 was first injected into the shielding displace the air inside the shielding and fill the system with nitrogen gas. The gas outlet was then blocked, and nitrogen was continuously injected at the rate of 50 ml/min. The injected nitrogen was naturally released into the fine gaps of the front door. The active shield consisted of five BC-408 (BICRON series) plastic scintillation detectors with 50 mm thickness, and the dimensions were 800 mm × 800 mm for the top, 700 mm × 800 mm for the three sides and 700 × 600 mm for one side. Fig. 1(b) shows an anti-coincidence electronics diagram for anticosmic gamma-ray measurement. To avoid random coincidences due to gamma-rays from natural radionuclides in the building materials and indoor air, such as 40K, Th- and U-series, the energy threshold for the plastic scintillation detector as an active shield was conservatively set to about 7.5 MeV. The threshold was adjusted at the input of constant fraction discriminator (CFD) for the plastic scintillation detectors. Under these conditions, the total count rates for the plastic scintillation detectors were approximately 70 s-1 for the top and 30 s-1 for one side of the measurement system. The pulses from the plastic scintillators and HPGe detector were amplified and converted to fast logic signals at each CFD. The logic output signal of the plastic detector leads to the ‘START’ input and the one for the HPGe detector leads to the ‘STOP’ input of the time-to-amplitude converter (TAC). A coincidence signal is generated when two input signals are within the preset time interval. The TAC
output signal as a stretched linear pulse was converted to a gate signal with a pulse width of 90 μs extended by a delay/gate generator, and the gate signal inputs to the gate of an analog-to-digital converter (ADC). The linear energy pulse (i.e., cosmic-ray induced ones) from the HPGe detector is vetoed by the anti-coincidence mode of the ADC if it is covered by the gate signal time. Background photons with and without the anti-cosmic mode were measured for 100,000 seconds to evaluate the background reduction rate. The Genie2K (Version 3.1, Mirion Inc.) software was used for the data acquisition.
2.2 Full energy peak efficiency calibration and MDA estimation In order to evaluate the full energy peak efficiency of the well HPGe detector, a cylindrical plastic (polyethylene) bottle with an inner diameter of 24 mm and a wall thickness of 1.5 mm was prepared and filled to heights of 10 mm, 20 mm and 30 mm with a certified gamma-ray source (Eckert & Ziegler Inc.) in a 1 M HCl solution containing 51
keV), Cr (320.1 keV), keV) and
88
113
241
Am (59.5 keV),
109
Cd (88.0 keV),
85
Sn (391.7 keV), Sr (514.0 keV),
137
57
Co (122.1 keV),
139
Ce (165.9
60
Cs (661.7 keV), Co (1173.2 keV, 1332.5
Y (898.0 keV, 1836.1 keV). The standard uncertainty of each prepared calibration source
ranges from 0.97% to 1.55%. Here, the maximum sample height was set to 30 mm, to place the sample within the Ge crystal since there is a gap of around 6 mm between the well endcap and the Ge crystal. The source was placed inside the well of the detector, and gamma-rays were measured for 50,000 seconds, for during which the standard statistical uncertainties for all radionuclides is less than 0.7%. In general, true coincidence summing (TCS) can occur with a well type detector because of its higher detection efficiency. The true coincidence summing correction factor (CFTCS) was determined using Monte Carlo simulation, and the total efficiency obtained with the MCNPX (Version 2.7.0) code (Venkataraman et al. 2005; Byun 2018). Information about the detector material and geometry for modeling the detector was provided by the manufacturer. The thickness of dead layers inside the well of the detector crystal used in this study were 0.08 mm for the side and 0.055 mm for the bottom. Fig. 2 shows the geometrical model and the pulse height tally ‘F8:P’ with the default physics option in the MCNPX code was used to simulate the full energy peak and total efficiency (Pelowitz 2011). To validate the modeling, efficiencies obtained from simulations and experiments for 51
Cr,
113
241
Am,
109
Cd,
57
Co,
Sn, 85Sr, and 137Cs, which can ignore the TCS for the detector used in this study, were compared
with each other. The CFTCS was calculated for
60
Co, and
88
Y using the total efficiency obtained in the
simulation, and following the general formula (Debertin et al. 1988; Montgomery et al. 1995)
=1−∑
=
(1)
is the gamma-ray count rate without TCS, is the gamma-ray count rate with TCS, is the where total photon energy lines coinciding (‘within the resolving time’) with the gamma-ray of interest, is the emission fraction of -th photon coinciding with gamma-ray of interest, and is total efficiency for the -th photon coinciding with the gamma-ray of interest. The gamma-ray energy and emission probability, the electron capture probability, internal conversion coefficients, and K-shell X-ray fluorescence needed for were referenced using the DDEP Recommended Data (LNHB National Becquerel, updated on Oct 20 2017). 139Ce in the calibration source can also significantly produce the true coincidence summing by X-rays (33.2 keV, 37.8 keV and 38.8 keV) and gamma-rays (165.9 keV) due to the thin dead layer of the detector crystal. Therefore, the thicknesses of both the dead layers on the bottom and the side of the detector well should be optimized even though the simulation for 59.5 keV of 241Am can be validated. Optimization requires the specific collimator and photon sources to be placed inside the detector well. In this study, instead of optimization, the integral net counts of the energy range between full energy peaks of 165.9 keV and 204.7 keV were used to obtain the full energy peak counts of 165.9 keV from 139Ce without losing the net counts due to the TCS. With the calibration source used in this study, the Compton continuum of the region between 165.9 keV and 204.7 keV is enough steady to estimate the background baseline, and no interfered full energy peaks over the energy range is measured in the gamma-ray energy spectra. The compared spectra with and without TCS for the same radionuclides used in this study can be found in another article (Byun 2018). Fig. 3 shows the full energy peak and true coincidence summing peaks of 165.86 keV (139Ce) with the regression line of the Compton continuum in the measured gamma-ray energy spectrum for 10 mm height calibration source. The full energy peak efficiency for 165.9 keV calculated by this way was also compared with the simulated one. The MDA of the present system and full energy peak efficiency for 241Am, 212Pb, 214Pb, 131I, 208Tl, 134Cs, 214
Bi,
137
Cs,
228
60
Ac,
Co and
40
K, as the main gamma-ray emitters in environmental monitoring were
tabulated. The MDA using the Currie method (Currie 1968) in a condition of
=
= 1.645 can be
estimated as
%&' = * +
()
,
-
× ./ =
0.1 23.45 √78 * +, - 9:
× ./
where ;< is detection limit, => is background counts,
(2)
is absolute photopeak efficiency, ?@ is
gamma-ray emission probability, % is sample volume, A is measurement time in a seconds, ./ is efficiency correction factor such as self-attenuation, true coincidence summing. => was determined as the integral counts covering a region of interest (ROI) width of 2.5×FWHM for the full energy peaks.
3. Results and discussion
Fig. 4 shows a comparison of the background spectra with and without anti-comics mode. Cosmic-ray induced isomers with longer half-lives than the coincidence time interval are observed in the spectra. Details of those common background components and their production processes, by interaction between cosmic rays and surrounding materials, are well summarized elsewhere (Sánchez et al. 1994; Heusser 1996; Núñez-Lagos et al. 1996; Cannizzaro 1997; Michael 2003). Integral counting rates within an energy range between 40 keV to 3000 keV with and without anti-cosmic mode were 1.51 s-1 and 0.337 s-1, respectively. The integral count rates per 100 cm3 Ge volume with the anti-cosmic mode was 0.123 s-1 which is 8 times lower than about 1 s-1 for a general p-type HPGe detector system, with a 120 cm3 Ge volume and a relative efficiency of 30%, with typical cylindrical lead shielding. The background was reduced 0.02 s-1 by N2 gas purging inside the detector chamber and the cosmic-ray induced background was suppressed by a rate of 1.17 s-1. The full energy peak count rates for natural and cosmic-ray induced radionuclides for 100,000 seconds measurement time under the anti-cosmic mode and N2 purging conditions are as shown in Table 1. The count rates are normalized to a Ge crystal mass of 1.46 kg. The most apparently suppressed were the annihilation peaks produced by secondary radiation positrons due to colliding muons and high-Z materials, by a factor of 9.6 with the active shielding. Table 2 shows the full energy peak efficiency and the combined standard uncertainties measured for cylindrical sources with a diameter of 24 mm and different heights of 10 mm, 20 mm, and 30 mm. As mentioned in section 2, the detection efficiency for 165.86 keV from
139
Ce was determined using the
integral net counts of the energy range between full energy peaks of 165.9 keV and 204.7 keV, and the results agreed with the simulation results within 3%. In contrast, the estimated efficiencies obtained by simulation and measured with the CFTCS correction for 165.86 keV showed a maximum difference of 24% for the calibration sources with heights of 10 mm, 20 mm, and 30 mm. This indicates that the efficiency of approximately 30 keV X-rays from 165 keV is very sensitive, with dead layers of about 0.055 mm and 0.08 mm. Therefore, further studies on the optimization of dead layer thicknesses are required for radionuclides with X-rays. This can be implemented using small sized collimators placed inside the detector well for the tests on the bottom and side of the detector well. The full energy peak efficiencies determined by measurement and simulation for 59.5 keV, 88.0 keV, 122.1 keV, 320.1 keV, 391.7 keV, 514.0 keV and 661.7 keV also agreed with each other within 5%, and this demonstrated the validity of the
MCNPX modeling for those energy ranges. The full energy peak efficiency for 898.04 keV, 1173.23 keV, 1332.5 keV and 1836.07 keV were determined using the CFTCS obtained with Eq. (1), with the simulated total efficiency (
). The standard uncertainty of CFTCS was estimated considering the standard
uncertainties for the simulation and nuclear data. The uncertainty of simulation was also considered for statistical uncertainty from simulation results, sensitivity analysis results using the uncertainty of detector geometry, and systematical uncertainty from a difference between experimental and simulated ones. For 88
Y and
60
Co, the relative statistical uncertainties were less than 1% with the photon source’s particle
number of 5×108. As a sensitivity analysis result for the dead layer’s thickness with a standard uncertainty of ±5%, there were differences less than 1% between the CFTCSs for maximum and minimum thickness. The relative systematic uncertainty was conservatively estimated 5% considering the relative deviation between experimental and simulated efficiencies for 59.5 keV, 88.0 keV, 122.1 keV, 320.1 keV, 391.7 keV, 514.0 keV and 661.7 keV to ignore TCS. The combined standard uncertainty of efficiency corrected with TCS for 898.04 keV and 1836.07 keV of 88Y, and 1173.23 keV and 1332.5 keV of of 60Co was estimated as an around 5%. There was a maximum 5.3% difference between the simulated efficiency and the measured one with TCS correction. The accuracy may be improved by optimizing the detector geometry. The efficiency curves for each source were also obtained by log(Energy)log(Efficiency) least squared fit as shown in Fig. 5. Table 3 shows the MDA values of this system for the major radionuclides used in environmental radioactivity monitoring. One can assume the MDA with a specific sample amount of samples, and the measurement time for similar types of detectors using the information presented in Table 3. For example, if a sample of 13.6 g with a height of 30 mm is measured for 100,000 seconds using the present gamma-ray spectrometer, then the MDA without any efficiency corrections such as self-attenuation or TCS corrections can be calculated as
%&' ijk
h
= m.
l4.0 4n×m.o5m× l.4 p× mm,mmm q
= 0.185 mBq/g.
(3)
For an HPGe detector (GC3018, Mirion Inc.) with a relative efficiency of 30% with typical cylindrical lead shielding, the MDA is calculated as
%&' ijk
h
= m.m
1n.3 n1×m.o5m×
l p× mm,mmm q
= 0.420 mBq/g,
(4)
when a sample of 113 g with a diameter of 60 mm and a height of 40 mm on the detector endcap is measured for 100,000 s. Here, the detection efficiency used in Eq. (4) was obtained using
137
Cs in a
mixed source (Eckert & Ziegler Inc.) contained in a cylindrical plastic bottle with a wall thickness of 1.5 mm. The uncertainties for
and ?@ were applied to calculate the vw in Eq. (2) for the MDA. This
shows that the detection capability of the present system is more than twice that of a conventional gamma-ray spectrometer with similar mass efficiency ( × %) due to its significantly lower background, despite sample amounts that were about 8 times smaller. However, as shown in Table 3, it should be noted that the detection capability of gamma-ray energy lines of radionuclides such as 583.2 keV (208Tl), 604.7 keV (134Cs), 609.3 keV (214Bi) and 1173.2 keV (60Co) can be significantly decreased due to strong TCS. This result does not mean that the large well HPGe detector has a lower MDA than the HPGe detector with 30% relative efficiency in any measurement condition. In practice, if there is no limitation on sample amount, and the
× % can be much higher than one of the SAGe detectors with a lower TCS.
Therefore, depending on the measurement condition and purpose, the appropriate detector type should be considered.
4. Conclusion. A ground-based low background gamma-ray measurement system with a large well HPGe detector consisting of low background materials was successfully implemented using passive and active shields. The background count rate with passive shielding for 40~3000 keV was suppressed by 78% with the anticosmic mode. The integral count rates per 100 cm3 Ge volume was up to 8 times lower than the general gamma-ray measurement system with a conventional p-type HPGe detector system and a relative efficiency of 30%. The MDA of the system for
137
Cs was also decreased by more than 50%, a sample
amount corresponding to about 12% of the general one. The detection capability of the present system was also significantly improved due to the larger well volume of the detector, higher detection efficiency, and lower background. The MDAs and true coincidence summing correction factors tabulated in this paper can be useful for estimating minimum detectable activity, to establish a the gamma-ray measurement system with higher detection capability. The present equipment is also especially useful for measuring small amounts of samples with low level radioactivity.
Acknowledgement
This work was supported by the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KoFONS), granted financial resource from the Nuclear Safety and Security Commission (NSSC), Republic of Korea. [No.1305012-0517-GC100]
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Tables Table 1. Full energy peak count rates normalized to the Ge mass (1.46 kg) for natural and cosmic-ray induced radionuclides with the passive and active shielding mode. Nuclides 210 Pb 234 Th 73m Ge 75m Ge 77m Ge 71m Ge 226 Ra, 235U 212 Pb 214 Pb 214 Pb Annihilation 208 Tl 214 Bi 228 Ac 40 K 214 Bi 208 Tl Integral * Detection limit (
Energy (keV) 46.5 63.3 66.8 139.5 159.5 198.3 186 238.6 295.2 351.9 511 583.2 609.3 911.2 1460.8 1764.5 2614.5 40 - 3000 ) with a 95% confidence level
Count rates (counts min-1 kg-1) <* 0.0336 < 0.0366 0.112 0.0460 0.0288 0.0322 0.0392 0.0904 0.0321 0.0473 0.116 0.0230 0.0444 0.0213 0.0104 < 0.0119 0.0184 13.8
Table 2. Absolute experimental full energy peak efficiency versus energy with different heights of the calibration sources. Absolute experimental full energy peak efficiency (k=1) 30 mm height: 13.65 10 mm height: 4.5 cm3 20 mm height: 9.0 cm3 cm3 0.620±0.006 0.571±0.006 0.521±0.005 241 59.54 Am (0.637) (0.590) (0.536) 0.696±0.007 0.637±0.007 0.586±0.006 109 88.03 Cd (0.707) (0.657) (0.596) 0.707±0.010 0.655±0.009 0.601±0.008 57 122.06 Co (0.709) (0.655) (0.592) 0.626±0.007 0.571±0.006 0.513±0.006 139 165.86 Ce (0.638) (0.587) (0.528) 0.375±0.004 0.338±0.004 0.308±0.004 51 320.08 Cr (0.380) (0.346) (0.311) 0.313±0.003 0.284±0.003 0.260±0.003 113 Sn 391.7 (0.318) (0.289) (0.259) 0.243±0.003 0.229±0.003 0.201±0.003 85 514.0 Sr (0.251) (0.228) (0.204) 0.204±0.003 0.185±0.003 0.169±0.003 137 Cs 661.66 (0.203) (0.184) (0.166) 0.154±0.008 0.139±0.007 0.125±0.007 88 898.04 Y (0.158) (0.144) (0.129) 0.122±0.006 0.110±0.006 0.0991±0.0051 60 1173.23 Co (0.127) (0.116) (0.104) 0.109±0.006 0.0984±0.0050 0.0891±0.0046 60 Co 1332.5 (0.114) (0.104) (0.0937) 0.0828±0.0043 0.0750±0.0039 0.0674±0.0035 88 1836.07 Y (0.0868) (0.0790) (0.0711) Simulated full energy peak efficiency for the modeling validation in parentheses in each column Energy (keV)
Nuclid es
Table 3. MDAs with a 95% confidence level for several natural and artificial radionuclides in a 1 M HCl sample solution after a measurement time of 100,000 seconds. Nuclides 241
Am Pb 214 Pb 131 I 208 Tl 134 Cs 214 Bi 137 Cs 228 Ac 60 Co 40 K 214 Bi 212
Energy (keV) 59.5 238.6 351.9 364.5 583.2 604.7 609.3 661.7 911.2 1173.2 1460.8 1764.5
10 mm height MDA (mBq) CFTCS 3.11 1.00 4.79 1.00 6.70 1.01 1.94 0.97 6.52 2.24 5.97 2.66 13.9 1.99 2.09 1.00 11.9 1.14 4.64 1.87 34.1 1.00 22.0 0.98
20 mm height MDA (mBq) CFTCS 3.38 1.00 5.27 1.00 7.35 1.01 2.15 0.98 6.75 2.13 5.97 2.44 14.4 1.90 2.30 1.00 13.0 1.13 4.92 1.79 38.0 1.00 24.7 0.99
30 mm height MDA (mBq) CFTCS 3.70 1.00 5.82 1.00 8.11 1.01 2.38 0.98 6.97 1.98 5.95 2.19 15.1 1.79 2.52 1.00 14.3 1.12 5.19 1.70 42.0 1.00 27.4 0.99
The MDAs were calculated considering true coincidence summing correction, but not background contribution from the Compton continuum or full energy peak interference. Decay correction during measurement time was considered only for 131I.
Figures
②
①
③ ①
①
④
⑤
⑥
(a)
(b)
Fig. 1. (a) Drawing of the gamma-ray measurement system with the anti-cosmic shield and (b) block diagram of anticoincidence counting system; ①Plastic scintillator (5 ea), ②Borated-silicon plate (5 ea), ③Lead (interior 50 mm-thick with <5 Bq/kg of 210Pb and exterior 100 mm-thick with <100 Bq/kg of 210Pb), ④4 mm-thick copper + 3 mm-thick plexiglass (inside copper), ⑤HPGe detector, ⑥Electrical cooler
Copper shield
Lead shield
Ge crystal
Gamma source
SAGe detector
Fig. 2. Geometrical model of the well type SAGe (Small Anode Germanium) detector with the shield and a cylindrical bottle gamma-ray source using the MCNPX code
Fig. 3. Full energy peak and true coincidence summing peaks of 165.86 keV (139Ce) with the regression line of the Compton continuum in the measured gamma-ray energy spectrum for 10 mm height calibration source
500
300
350
400
450
500
511 keV, Annihilation peak
40~3000 keV
1000
250
351.9 keV, 214 Pb
300
200
198.3 keV, 71m Ge
200
550
600
609 keV, 214 Bi
400
100
159.5 keV, 77m Ge
150
278.3 keV, 63* Cu 295.2 keV, 214 Pb
Energy (keV)
583 keV, 208Tl
100
0
(b)
(a)
100
238.6 keV, 212 Pb
300
200
100
0 50
66.8 keV, 73m Ge
139.5 keV, 75m Ge
mm and heights of 10 mm, 20 mm and 30 mm.
650
Fig. 5. Absolute full energy peak efficiencies for cylindrical water sources with an inner diameter of 24
Active and passive shields, (b) Passive shield.
Fig. 4. Background spectra obtained during 100,000 seconds with and without anti-cosmic mode; (a)
Counts
Highlight
A ground-based low background gamma-ray measurement system was established with passive and active shields. A large well HPGe detector with low background materials was used as a main detector of anticosmic background measurement system. The background level and detection capability of the present system were estimated comparing a conventional HPGe detector system.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: