Estimating and accounting for 137Cs source burial through In-situ gamma spectrometry in salt marsh environments

The use of in-situ gamma ray spectrometry provides a means of rapidly estimating environmental radioactivity inventories. However, one of the principal limitations of this technique has been the influence of variations in vertical activity distribution on the observed photon fluence. This paper demonstrates that the quantification of the forward scattered ratio of the spectrum (Q) can be used to: (i) estimate the mean mass depth (β) of the vertical activity distribution within sediment profiles, and (ii) provide a calibration correction coefficient for in-situ gamma spectrometry in environments which exhibit variable and non-exponential activity distributions, such as salt marshes around the Irish Sea. This paper presents a successful application of a spectrally derived calibration correction coefficient to in-situ spectra from a salt marsh at Caerlaverock, Dumfries, SW Scotland, improving the correlation between soil core and in-situ derived activity estimates from r2 = 0.097 (uncorrected) to r2 = 0.801 (corrected). The scope for extending this approach to a wider range of environments, to airborne gamma spectrometry and to measurements of sedimentation rates is considered.