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Improving accuracy of in situ gamma-ray spectrometry
Umeå University, Faculty of Medicine, Radiation Sciences, Radiation Physics.
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.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1201
Keyword [en]
gamma-ray spectrometry, in situ measurement, HPGe detector, 137Cs, Monte Carlo, efficiency calibration, measurement uncertainty
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:umu:diva-1805ISBN: 978-91-7264-621-6 (print)OAI: oai:DiVA.org:umu-1805DiVA: diva2:142038
Public defence
2008-09-19, sal 244, by 7, NUS, Umeå, 09:00 (English)
Opponent
Available from: 2008-09-03 Created: 2008-09-03 Last updated: 2010-01-22Bibliographically approved
List of papers
1. In situ gamma-ray spectrometry for environmental monitoring: a semi empirical calibration method.
Open this publication in new window or tab >>In situ gamma-ray spectrometry for environmental monitoring: a semi empirical calibration method.
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2006 (English)In: Radiation Protection Dosimetry, ISSN 0144-8420, Vol. 121, no 3, 310-316 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-16618 (URN)doi:10.1093/rpd/ncl041 (DOI)16632586 (PubMedID)
Available from: 2007-02-23 Created: 2007-02-23 Last updated: 2010-01-22Bibliographically approved
2. A detailed investigation of HPGe detector response for improved Monte Carlo efficiency calculations
Open this publication in new window or tab >>A detailed investigation of HPGe detector response for improved Monte Carlo efficiency calculations
2008 (English)In: Nuclear instruments and methods in physics research A, ISSN 0168-9002, Vol. 587, no 2/3, 304-314 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-3366 (URN)
Available from: 2008-09-03 Created: 2008-09-03Bibliographically approved
3. Uncertainty in HPGe detector calibrations for in situ gamma-ray spectrometry
Open this publication in new window or tab >>Uncertainty in HPGe detector calibrations for in situ gamma-ray spectrometry
2009 (English)In: Radiation Protection Dosimetry, ISSN 0144-8420, E-ISSN 1742-3406, Vol. 134, no 2, 122-129 p.Article in journal (Refereed) Published
Abstract [en]

Semi-empirical methods are often used for efficiency calibrations of in situ gamma-ray spectrometry measurements with high-purity germanium detectors. The intrinsic detector efficiency is experimentally determined for different photon energies and angles of incidence, and a suitable expression for the efficiency is fitted to empirical data. In this work, the combined standard uncertainty of such an efficiency function for two detectors was assessed. The uncertainties in individual efficiency measurements were found to be about 1.9 and 3.1% (with a coverage factor k = 1, i.e. with a confidence interval of about 68%) for the two detectors. The main contributions to these uncertainties were found to originate from uncertainties in source-to-detector distance, source activity and full-energy peak count rate. The standard uncertainties of the fitted functions were found to be somewhat higher than the uncertainty of individual data points, i.e. 5.2 and 8.1% (k = 1). With the introduction of a new expression for the detector efficiency, these uncertainties were reduced to 3.7 and 4.2%, i.e. with up to a factor of two. Note that this work only addresses the uncertainty in the determination of intrinsic detector efficiency.

Place, publisher, year, edition, pages
Oxford University Press, 2009
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:umu:diva-23549 (URN)10.1093/rpd/ncp079 (DOI)19429646 (PubMedID)
Available from: 2009-06-24 Created: 2009-06-24 Last updated: 2011-12-07Bibliographically approved
4. Evaluation of Monte Carlo-based calibrations of HPGe detectors for in situ gamma-ray spectrometry
Open this publication in new window or tab >>Evaluation of Monte Carlo-based calibrations of HPGe detectors for in situ gamma-ray spectrometry
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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%).

Keyword
Germanium detector, HPGe, uncertainty, gamma spectrometry, in situ measurements, Monte Carlo, GUM
National Category
Physical Sciences
Identifiers
urn:nbn:se:umu:diva-26103 (URN)10.1016/j.jenvrad.2009.06.006 (DOI)19604609 (PubMedID)
Available from: 2009-09-24 Created: 2009-09-24 Last updated: 2011-12-07Bibliographically approved
5. In situ gamma-ray spectrometry: influence of ground curvature and activity in trees
Open this publication in new window or tab >>In situ gamma-ray spectrometry: influence of ground curvature and activity in trees
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(English)Manuscript (Other academic)
Identifiers
urn:nbn:se:umu:diva-3369 (URN)
Available from: 2008-09-03 Created: 2008-09-03 Last updated: 2010-01-13Bibliographically approved

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