Evaluation of efficiency calibration techniques of hpge detector for radioactivity measurements in environmental samples

Tiểu ban C Ghi đo bức xạ, An toàn bức xạ và Quan trắc môi trường Section C Radiation measurement, Radiation safety and Environmental 225 ĐÁNH GIÁ MỘT SỐ KỸ THUẬT HIỆU CHUẨN HIỆU SUẤT CỦA ĐẦU DÒ HPGE T[.]

ĐÁNH GIÁ MỘT SỐ KỸ THUẬT HIỆU CHUẨN HIỆU SUẤT CỦA ĐẦU DÒ HPGE TRONG PHÉP ĐO HOẠT ĐỘ PHÓNG XẠ MẪU MÔI TRƯỜNG

EVALUATION OF EFFICIENCY CALIBRATION TECHNIQUES

OF HPGE DETECTOR FOR RADIOACTIVITY MEASUREMENTS

IN ENVIRONMENTAL SAMPLES

HO VAN DOANH 1 *, DOAN THI THANH NHAN 1 , VO VAN TIEN 1 , NGUYEN VAN HOAI NAM 1 ,

DAO VAN HOANG 1 , HO MANH DUNG 1 , PHAN BAO QUOC HIEU 2 , TRAN QUOC DUONG 2

1

Center for Nuclear Technologies, 217 Nguyen Trai Street, TP.HCM, Vietnam

2

Dalat Nuclear Research Institute, 01 Nguyen Tu Luc, Dalat, Vietnam

*

E-mail: hovandoanh@gmail.com

Tóm tắt: Để duy trì hạng mục được công nhận đối với phép thử dùng hệ phổ kế gamma trong phòng thí nghiệm của chúng

tôi Đường cong hiệu suất của đầu dò HPGe đối với mẫu môi trường dạng hình học Marinelli đã được xác định bởi nguyên

lý chuyển đổi hiệu suất thông qua phần mềm thương mại ANGLE3 và mô phỏng Monte Carlo bằng MCNP5 và GEANT4 Hiệu suất thu được từ các phép đo thực nghiệm đối với nguồn chuẩn được sử dụng như là hiệu suất tham khảo trong phần khai báo của phần mềm ANGLE3 cũng như dùng cho việc hiệu chỉnh code mô phỏng của MCNP5 và GEANT4 Kết quả hiệu suất của mẫu đất trong hộp Marinelli đạt được từ MCNP5 và GEANT4 được so sánh với hiệu suất đạt được từ phần mềm ANGLE3 Hơn nữa, độ đúng của hiệu suất đầu dò đạt được trong nghiên cứu này đã được đánh giá thông qua phép đo hoạt độ phóng xạ của các hạt nhân 40 K, 137 Cs, 232 Th và 226 Ra trong mẫu chuẩn (IAEA-375)

Từ khóa: Đồng vị phóng xạ tự nhiên, đầu dò HPGe, hiệu suất, hộp dựng mẫu Marinelli

Abstract: To keep the accredited category for the gamma spectrometry test in our laboratory, the efficiency curves of a

HPGe detector for soil sample in Marinelli geometry were determined by “efficiency transfer” principle via the ANGLE3 commercial software and Monte Carlo simulations via MCNP5 and GEANT4 The efficiency obtained from experimental measurement of point standard sources was used as a reference efficiency curve for the input of ANGLE3 software as well

as for correction of simulation codes of MCNP5 and GEANT4 The results of the efficiency for soil sample in Marinelli beaker obtained from the MCNP5 and GEANT4 were compared with the efficiency obtained from the ANGLE3 software Moreover, the trueness of the detector efficiency obtained in this work was evaluated by the radioactivity measurement of

40 K, 137 Cs, 232 Th and 226 Ra in a reference material (IAEA-375)

Keywords: Natural radionuclides, HPGe detector, efficiency, Marinelli beaker

1 INTRODUCTION

For measurement of voluminous samples as environmental radioactivity, their dimensions and elemental components in the sample have to be considered for detector efficiency calibrations, as well as the self-attenuation of gamma-rays Typically, the calibration of detector efficiency needs standard samples

or standard sources which contain known radionuclides with a known quantity of radioactivity The standards are required same size, density, elemental composition and multiple gamma-emitting radionuclides as the sample From a practical point of view, there are some difficulties for preparation of standard according to the requirements or for cases where the conduct of experimental measurement is impossible or very difficult such as samples of non-standard geometry and for large sized samples or standards was not easily available These difficulties can be solved by using Monte Carlo simulation such

as MCNP and GEANT4 [1] It is a powerful tool to simulate the detector response and applicable to a variety of matrices and sample geometries Thus the simulation codes have been used for detector efficiency calibration However, these simulation codes need to be verified in terms of the accuracy Therefore, the ANGLE3 software have been applied in this study for verifying the efficiency results from simulation codes of MCNP5 and GEANT4 [2-4] ANGLE3 software is commercial computer program which performs the detector efficiency calculations by solid angle method [5] The accuracy of the software for efficiency calculation was verified in appreciation The software can be applied to practically all situations encountered in gamma-laboratory practice: point, disc, cylindrical or Marinelli samples, small and large, of any matrix composition No standards are required, but a start-up “reference efficiency curve” should be obtained by measuring a calibrated source at some reference counting geometry

2 EXPERIMENTAL

The detector used in this experiment is a coaxial HPGe detector It has relative efficiency of 40% and resolution of 2.0 keV at 1.33 MeV The ratio of Peak to Compton is 59:1 It is a horizontal detector with

volume of crystal is 200 cm3 All detector dimensions are taken from the manufacturer provided documents

and these are summarized in table 1 The measurements of reference efficiency curve were carried out

using a set of point standard sources emitting gamma rays over the energy range of 50–1500 keV,

by Eckert & Ziegler manufacture The sources have the same cylinder size with a diameter of 5 mm, a height of 3.18 mm covered by an Acrylic layer The outer cover has the diameter of 25.4 mm and the height of 6.35 mm Figure 1 shows the geometry and dimensions of these standard sources

Figure 1 Dimension of point standard source

In order to calculate the efficiency of a HPGe detector for soil sample in Marinelli geometry by

ANGLE3, MCNP5 and GEANT4, the following should be known: (1) reference efficiency curve, usually

obtained by counting calibrated sources at reference geometry and covering gamma-energies in the region

of interest; (2) Geometrical and compositional data about the source, detector, and all intercepting layers

(for the latter e.g source container and holder, detector end-cap and housing, dead layers, etc.) and (3)

Gamma-attenuation coefficients for all materials involved These parameters were shown in table 1,

figure 2 and figure 3

Figure 2 Dimension of detector Figure 3 Dimension of Marinelli Beaker (mm)

Table 1: Detector dimension of detector, Marinelli Beaker

and Components of Matrix Sample (presentative of soil)

E Nominal Radius 5 Marinelli inner side thickness 2.0

3 RESULTS AND DISCUSSION

In oder to evaluate the reliability of simulation codes, the results of detector efficiency for point standard sources obtained by MCNP5 and GEANT4 were compared with the detector efficiency obtained

by experimental measurements Figure 4 shows the results obtained using the MCNP, GEANT4 and experimental measurement for point standard sources at distances of 50 mm, 100 mm and 150 mm The efficiency results at distance 100 mm were compared in Table 2 The efficiency ratios of MCNP5 to experimental measurement vary between 0.90 and 1.04 according to gamma energy peaks The average of ratios was 0.97 with relative standard deviation of 5% The efficiency ratios of GEANT4 to experimental measurement (Exp.) vary between 0.91 and 1.04 according to gamma energy peaks The average of ratios was 0.99 with relative standard deviation of 4% The results indicated that efficiency for point source obtained from MCNP and GEANT4 are in line with the efficiency obtained from experimental measurements There is a clear possibility that MCNP5 and GEANT4 simulation codes were created correctly, thereby it can be used for calculations of detector efficiency of soil sample in marinelli beaker

Table 2: The result of detector efficiency for point standard sources

at distance of 100 mm obtained by experimental measurements and simulation of MCNP and GEANT4

MCNP5 /Exp

GEANT4 /Exp

Figure 4 The result of efficiency curves for point standard sources

at distance 50 mm, 100 mm and 150 mm

The result of detector efficiency for soil sample in marinelli beaker (Marinelli efficiency) obtained by ANGLE3 software and simulation of MCNP5 and GEANT4 were presented in table 3 and figure 5 The efficiency ratios of MCNP5 to ANGLE3 vary between 0.90 and 1.01 according to gamma energy peaks It would be appear that most values obtained from MCNP5 are less than the values obtained from ANGLE3 software The average of ratios was 0.95 with relative standard deviation of 4.6% The efficiency ratios of GEANT4 to ANGLE3 vary between 0.95 and 1.36 according to gamma energy peaks The average of ratios was 1.04 with relative standard deviation of 11% There seems is a tendency to differ between GEANT4 and ANGLE3 at low energy region (59.5 keV and 81 keV) In general, the results indicated that Marinelli efficiency values obtained from MCNP and GEANT4 are in good agreement with the values obtained ANGLE3

Table 3: The result of detector efficiency for soil sample in marinelli beaker

obtained by ANGLE3 software and simulation of MCNP5 and GEANT4

ENERGY

MCNP5/

ANGLE3

GEANT4/ ANGLE3

Figure 5 Efficiency curves of detector for soil sample in marinelli beaker

obtained by ANGLE3 software and simulation of MCNP and GEANT4

The result of detector efficiency for soil sample in marinelli beaker obtained by GEANT4 were used

and 232Th

Table 4: The measurement results of some radionuclides in IAEA-375

Radionuclide

MV/

CV Radioactivity

(Bq/Kg)

Uncertainty (Bq/Kg)

Radioactivity (Bq/Kg)

Uncertainty (Bq/Kg)

* Radioactivity in reference standard material was corrected decay factors at measurement time June

23, 2021

4 CONCLUSION

The efficiency curve of point standard sources obtained from MCNP and GEANT4 are in line with the efficiency obtained from experimental measurements In addition, the efficiency results for the soil sample in Marinelli geometry obtained by MCNP5 and GEANT4 are in good agreement with the efficiency results obtained ANGLE3 Moreover, the trueness of efficiency values obtained in this work was validated through the gamma measurement of IAEA reference material (IAEA-375) Therefore, there is a clear possibility that MCNP5 and GEANT4 simulation codes produced in this study are useful for further calculations of detector efficiency in various sample geometries The results of simulation codes and detector efficiency obtained in this work have contributed to keep the accredited category for the gamma spectrometry test in our laboratory

ACKNOWLEDGEMENTS

The Ministry of Science and Technology (MOST) of Vietnam through a ministerial project coded ĐTCB.12/20TTHN is gratefully acknowledged

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

A NG L E

MC NP

G E A NT4

Energy (keV)

REFERENCES

[1] Geant4 Collaboration, Geant4 Users guide for application deverlopers, Version geant4.10.00, 2013

[2] Radu D., Transfer of detector efficiency calibration from a point source to other geometries using ETNA software,

Romanian Reports in Physics, 2010

[3] Loan T.T.H., Mô phỏng Monte Carlo một số bài toán trong vật lý hạt nhân, Luận án tiến sĩ, Trường Đại học Khoa học

Tự nhiên Tp.HCM, 2009

[4] P.B.Q Hiếu, P.N Sơn, H.V Doanh, Chuẩn hóa hiệu suất ghi toàn phần và hấp thụ quang điện cho đầu dò HPGe

bằng mô phỏng Monte Carlo, Hội nghị Khoa học và Công nghệ Hạt nhân lần thứ 11, 2015

[5] ANGLE3 Semiconductor Detector Efficiency Calculations, User guide, Ortec.