Contamination may be dispersed across a large area , e . g . as a result of deposition from an aerial discharge , or it may be present in small pockets or “ hot spots ” resulting from localised sources such as leaks or spills . Once the historical desk study and a site walkover have been completed and likely areas of contamination identified , the land needs to be carefully monitored using detectors appropriate for the nuclides . In most cases an instrument with a thin sodium iodide crystal will be used for the initial sweep of an area unless more energetic gammas are likely to be present , in which case a thicker crystal will be required . Instrument manufacturers have developed a range of instruments that can be bolted to a trolley , mounted on the front of a vehicle or , more recently , fitted to a drone . Where contamination is detected , further sampling can be carried out , including the collection of soil samples to confirm actual contamination levels and depth . Intrusive investigations may be designed to target historical sources of contamination and potential migration pathways , but randomised sampling is necessary across the entire area if the results are to be statistically interpreted , e . g . to provide a specified level of confidence that an area is or is not contaminated .
MARSSIM ( Multi-Agency Radiation Survey and Site Investigation Manual ) is free to download , and provides detailed guidance on how to demonstrate that a site complies with radiation dose- or risk-based regulatory limits , focussing on the demonstration of compliance during the final status survey following scoping , characterisation and any necessary remedial actions . Software tools such as Visual Sampling Plan ( VSP ) are available to assist in the development of defensible sampling strategies based on sampling theory and the statistical analysis of results to support confident decision making . VSP couples site , building , and sample location visualisation capabilities with optimal sampling design and statistical analysis strategies . VSP is free to download from Pacific Northwest National Laboratory , but training costs in the region of £ 600 .