To what extent do field conditions affect gamma dose rate determination using portable gamma spectrometry?

Martin, L. , Duval, M. and Arnold, L. J. (2024) To what extent do field conditions affect gamma dose rate determination using portable gamma spectrometry? Radiation Physics and Chemistry, 216, 111365. (doi: 10.1016/j.radphyschem.2023.111365)

Abstract

Field gamma spectrometry is a widely used approach for determining in situ gamma dose rates in dosimetric (i.e., electron spin resonance and luminescence) dating applications. In comparison with laboratory-based determinations, in situ radioactivity measurements typically provide more representative gamma dose rate evaluations for heterogeneous sedimentary environments. However, it is often not possible to perform in situ gamma spectrometry measurements under carefully controlled conditions that are directly comparable to those originally used for equipment calibration. In this study, we use Geant4 Monte Carlo simulations to model gamma spectrometry measurements under a range of field conditions, and examine the relative impacts of the following parameters on dose rate determination using the threshold calibration approach: (i) geometry and depth of the measurement hole in which the probe is inserted, (ii) nature of the sediment or rock materials and their water content, (iii) geometry of the radiation environment surrounding the measurement hole, i.e. closed and partially closed sites (e.g., caves, trenches) versus open-air sites (e.g. plain field excavations, cliff or cutting exposures). Our results show that some differences in calibration and field measurement configurations can significantly bias in situ gamma dose rate determinations. Variations in the depth of probe holes can result in underestimations of infinite matrix gamma dose rates by 5% for a 30 cm-deep hole to 58% for measurements made against sediment surfaces (i.e., 2π geometry). Use of hole shapes that do not match those of the probe can lead to underestimations of infinite matrix dose rates by up to 4%, with these biasing effects becoming more significant for shallow holes. External gamma radiation originating from, and backscattered against, structures in the surrounding environment can contribute significantly to gamma dose rates measured using shallow probe holes. The nature of the mineral materials can have a small effect on the measured gamma dose rate (equivalent to infinite matrix dose rate biases of a few percent), mostly due to differences in the density of different materials. Measurements performed in materials with high water contents can be affected by small gamma dose rate overestimations due to differences in water attenuation factors of centimetre-scale objects such as gamma spectrometer probes and those of relevance for dating smaller objects such as sediment grains. These problems can be resolved by using specific correction factors, by including additional uncertainties during dose rate determination, or by performing in situ measurements at different depths for the same location.

• Item Type: Articles
• Additional Information: The Spanish Ramón y Cajal Fellowship RYC 2018-025221-I granted to MD is funded by MCIN/AEI/10.13039/501100011033 and by ‘ESF Investing in your future’. This work is part of Grant PID 2021-123092NB-C22 funded by MCIN/AEI/10.13039/501100011033/‘ERDF A way of making Europe’.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Martin, Dr Loic
• Authors: Martin, L., Duval, M., and Arnold, L. J.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Radiation Physics and Chemistry
• Publisher: Elsevier
• ISSN: 0969-806X
• ISSN (Online): 1879-0895
• Published Online: 06 November 2023
• Copyright Holders: Copyright © 2023 The Authors
• First Published: First published in Radiation Physics and Chemistry 216:111365
• Publisher Policy: Reproduced under a Creative Commons License