Comparison between in situ and ex situ gamma measurements on land 1 Comparison between in situ and ex situ gamma measurements on land areas within a decommissioning nuclear site: a case study at Dounreay. Authors: Peter D Rostron 1 , John A Heathcote 2 and Michael H Ramsey 1 1 University of Sussex, Falmer, Brighton, United Kingdom BN1 9QG. 2 Dounreay Site Restoration Limited, Dounreay, Thurso, United Kingdom KW14 7TZ Abstract. Measurements made in situ with gamma detectors and ex situ measurements of soil samples in a laboratory can have complementary roles in the assessment of radioactively contaminated land on decommissioning nuclear sites. Both in situ and ex situ methods were used to characterize 137 Cs contamination within an area at the Dounreay site in Scotland. The systematic difference (bias) between estimates of the mean activity concentration was found to be non-significant when in situ measurements were interpreted using a linear depth model, based on ex situ measurements made at two different depths. An established method of evaluating the random components of measurement uncertainty was used. The random component of analytical uncertainty in the in situ measurements, made in field conditions, was found to exceed that for the ex situ measurements, made in the controlled conditions of a laboratory. However, contamination by the target radionuclide was found to be heterogeneous over small spatial scales. This resulted in significantly higher levels of random sampling uncertainty in individual ex situ measurements. As in situ measurements are substantially less costly, a greater number of measurements can be made, which potentially reduces the uncertainty on the mean. Providing the depth profile of contaminants can be modelled with confidence, this can enable estimates of mean activity concentration over an averaging area to be made with lower overall uncertainties than are possible using ex situ methods. 1. Introduction Regulation of radioactively contaminated land on nuclear licensed sites in the UK is the responsibility of the Office for Nuclear Regulation, an agency of the Health and Safety Executive (HSE). The immediate concern is risk arising from exposure to radiation of workers and/or the general public. This risk is managed via the nuclear site licence. Dounreay’s mission is complete and the long-term ambition is to terminate the nuclear site licence. De-licensing of land areas requires the site licence holder to demonstrate that “there is no danger from ionising radiations from anything on that part of the site” (NIA, 1965). On areas that have been subject to radioactive contamination, this requires characterisation of the inventory and concentration distributions of contaminants in the ground (HSE, 2005; HSE, 2008). Both in situ methods and conventional laboratory measurements of field samples (ex situ methods) can be used for land characterisation, and both are often used at the same site. When there is a high probability of gamma-emitting radionuclides existing at or near the ground surface, then in situ gamma-ray detection by scintillation or semi-conductor detectors can offer several advantages over the ex situ analysis of soil samples. In situ measurements are taken in real time and can be interpreted immediately, substantial cost savings can be made when compared to expensive laboratory procedures, and a larger sampling mass may give a more representative picture of the distributions of radionuclides over the site area. However, conversion of the activity counts recorded by an in situ detector into activity concentration units (e.g. Bq g -1 ) requires knowledge of the depth distributions of contaminants. In contrast, ex situ measurements of samples acquired from different soil depths provides explicit information on depth distributions at discrete locations. It is usually necessary to use ex situ methods for the direct identification of alpha and beta emitters, because of the lower penetration of these types of emissions through intervening media such as soil and air. They may also be prescribed by regulatory authorities (IAEA, 1998).