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Evaluation of Monte Carlo-based calibrations of HPGe detectors for in situ gamma-ray spectrometry
Umeå University, Faculty of Medicine, Department of Radiation Sciences, Radiation Physics. (Swedish Defence Research Agency, FOI CBRN Defence and Security, SE-901 82 Umeå, Sweden)
Swedish Defence Research Agency, FOI CBRN Defence and Security, SE-901 82 Umeå, Sweden.
Swedish Defence Research Agency, FOI CBRN Defence and Security, SE-901 82 Umeå, Sweden.
Swedish Defence Research Agency, FOI CBRN Defence and Security, SE-901 82 Umeå, Sweden.
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2009 (English)In: Journal of Environmental Radioactivity, ISSN 0265-931X, E-ISSN 1879-1700, Vol. 100, no 11, 935-940 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this work was to evaluate the use of Monte Carlo-based calibrations for in situ gamma-ray spectrometry. We have performed in situ measurements at five different sites in Sweden using HPGe detectors to determine ground deposition activity levels of (137)Cs from the 1986 Chernobyl accident. Monte Carlo-calculated efficiency calibration factors were compared with corresponding values calculated using a more traditional semi-empirical method. In addition, results for the activity ground deposition were also compared with activity densities found in soil samples. In order to facilitate meaningful comparisons between the different types of results, the combined standard uncertainty of in situ measurements was assessed for both calibration methods. Good agreement, both between the two calibration methods, and between in situ measurements and soil samples, was found at all five sites. Uncertainties in in situ measurements for the given measurement conditions, about 20 years after the fallout occurred, were found to be in the range 15-20% (with a coverage factor k=1, i.e. with a confidence interval of about 68%).

Place, publisher, year, edition, pages
2009. Vol. 100, no 11, 935-940 p.
Keyword [en]
Germanium detector, HPGe, uncertainty, gamma spectrometry, in situ measurements, Monte Carlo, GUM
National Category
Physical Sciences
URN: urn:nbn:se:umu:diva-26103DOI: 10.1016/j.jenvrad.2009.06.006PubMedID: 19604609OAI: diva2:236581
Available from: 2009-09-24 Created: 2009-09-24 Last updated: 2011-12-07Bibliographically approved
In thesis
1. Improving accuracy of in situ gamma-ray spectrometry
Open this publication in new window or tab >>Improving accuracy of in situ gamma-ray spectrometry
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Gamma-ray spectrometry measurements performed on site, or “in situ”, is a widely used and powerful method that can be employed both to identify and quantify ground deposited radionuclides. The purpose of this thesis is to improve the calibration of high purity germanium (HPGe) detectors for in situ measurements, and calculate the combined uncertainty and potential systematic effects.

An improved semi-empirical calibration method is presented, based on a novel expression for the intrinsic detector efficiency that includes both the energy and angular response of the detector. A three-layer model for the description of the depth distribution of the radionuclide and the soil density is proposed. The combined uncertainty of intrinsic detector efficiency calibrations and in situ measurements according to the proposed method was estimated. The uncertainty in the intrinsic detector efficiency was found to be 5.1 and 8.1% (coverage factor k=1, i.e. for a confidence interval of about 68%), for the two detectors calibrated. These numbers were, however, at a later stage reduced to 3.7 and 4.2%, using a revised expression for the intrinsic detector efficiency.

For in situ measurements, the combined standard uncertainty was found to be 15-20% (k=1), based on the original expression for the intrinsic detector efficiency. Monte Carlo models of the two detectors were created and Monte Carlo calculated values for intrinsic detector efficiency were compared with experimental data. As a discrepancy was found, a thorough investigation of the detector response was performed. Scanning of the detector surface with a collimated 59.5 keV photon beam revealed the detector response to be highly irregular over the detector surface. It was concluded that the efficiency deficit of the detector could most likely be attributed to an increase in dead layer thickness compared with manufacturer supplied data. The thickness of the dead layer was estimated to be 1.5-1.9 mm, whereas the nominal value was 0.7 mm. Radiographs of the detectors were produced that provided valuable information about the physical dimensions of the germanium crystal, as well as its actual location within the detector housing.

The Monte Carlo models were employed to calculate in situ measurement efficiencies for measurements of 137Cs deposition from the Chernobyl fallout. Results from the Monte Carlo simulations were compared both with the semi-empirical method and with soil sample data, and satisfactory agreement was confirmed. It was then proceeded to employ the Monte Carlo model to calculate the effect on in situ measurement results by two influencing parameters: ground curvature and activity in trees. Neither of these parameters was found to influence the result by more than about 25%. This deviation is comparable with the measurement uncertainty, and should not deter from measurements in such terrain.

Place, publisher, year, edition, pages
Umeå: Radiofysik, 2008. 47 p.
Umeå University medical dissertations, ISSN 0346-6612 ; 1201
gamma-ray spectrometry, in situ measurement, HPGe detector, 137Cs, Monte Carlo, efficiency calibration, measurement uncertainty
National Category
Radiology, Nuclear Medicine and Medical Imaging
urn:nbn:se:umu:diva-1805 (URN)978-91-7264-621-6 (ISBN)
Public defence
2008-09-19, sal 244, by 7, NUS, Umeå, 09:00 (English)
Available from: 2008-09-03 Created: 2008-09-03 Last updated: 2010-01-22Bibliographically approved

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