The use of mathematical calibration phantoms for measurements of radionuclides in people.

It is important to be prepared in advance for a radiological incident should one occur in the UK. Such incidents could occur as a result of a nuclear reactor incident, a fire from a facility containing radioactive material or a deliberate release of radioactive material from a Radiological Dispersal Device (RDD). RDDs can either be explosive (a ‘dirty bomb’) or a device that silently contaminates an area or group of people. Assessment of radiation doses to exposed people would in most circumstances include making measurements of the amount of the radionuclide in the body. Usually, body monitoring calibrations for such measurements are made using a physical model of the body (a “phantom”) containing a known amount of radioactive material. Such phantoms are almost always constructed for measurements on adult males. However, in an RDD incident, men, women and children of different ages and different body sizes could be exposed. Using a calibration phantom with different physical attributes to the person being measured gives rise to a larger statistical uncertainty in the measured activity. Furthermore, calibrations with physical phantoms may not have been performed for the radionuclides that might be encountered. Radionuclides that are inhaled may be distributed non-uniformly in the respiratory tract, which will affect the detector efficiency for lung calibrations. Commercially available phantoms typically use a uniform radionuclide distribution. Phantoms designed to be used in a supine position do not always provide reliable calibrations for measurements on people made in a seated geometry, which presents problems for emergency monitoring applications. Mathematical phantoms can be applied to overcome all of these limitations. A research project (VOXPOP) is under way to develop mathematical phantoms and mathematical detector systems for use with particle transport codes such as MCNPX. Research is currently being carried out to produce more realistic phantoms that represent the range of subject sizes and used for calibration of body monitoring measurements occurring hours or days after the incident. The research is applicable to both hand-held and portal monitors that can be used for screening measurements, and dedicated static and mobile body monitoring systems