Portable gamma ray spectrometry for archaeological prospection: A preliminary investigation at Silchester Roman Town

Several studies have suggested the potential value in applying gamma radiation surveys to support identification of buried archaeological features. However, the number of previous studies is very small and has yielded mixed results. The true efficacy of the technique is therefore unclear. Here, we report on an alternative survey method that uses Groundhog®, a portable gamma radiation system with spectrometric capability, to achieve high spatial density monitoring of archaeological sites. The system, which is used extensively in the nuclear industry, was used to carry out preliminary surveys at four different locations within the Silchester Roman Town. Targeting a site for which an extensive amount of archaeological data is available facilitated testing of the method on a range of known target types. Surveys were carried out along 1-m transects at an approximate walking speed of 1 m per second, resulting in the capture of one radiation measurement per square metre. Total gamma radiation, recorded in counts per second, was presented in the form of surface radiation (contour) maps and compared against existing geophysical data. Total gamma counting consists of counting gamma rays, without energy discrimination, that are spontaneously emitted by the material under investigation. The obtained counts represent the total, or gross, gamma contribution from all radionuclides, both natural background series and anthropogenic. Radiation anomalies were identified in two of the four survey sites. These anomalies correlated with features present in the geophysical data and can be attributed to a Temenos wall bounding the temple complex and an infilled clay pit. Early results suggest that this may be a complementary technique to existing geophysical methods to aid characterization of archaeological sites. However, it is believed that data quality could be significantly improved by further increasing spatial resolution. This will be explored as part of future fieldwork.