In a nuclear weapon accident involving fire or conventional explosion, most of the radiation dose received by people in the immediate vicinity would result from inhalation of 239Pu. This is accompanied by the nuclide 241Am, which is much easier to determine by external counting because of the 60 keV gamma ray emission. In the event of an accident, a priority would be to identify any people who have had intakes of 239Pu which were so large that decorporation therapy should be considered. Direct measurement of lung content provides the most rapid and convenient method for assessing intakes by inhalation. A transportable system has been considered as this could be deployed close to the site of the accident and would allow rapid measurements to be made. The feasibility of a transportable 241Am-in-nose-blow and nasal swab measurement system has also been considered. This would be used to help select people for 241Am-in-lung measurements.

Clearance from the extrathoracic (ET) airways was measured in nine volunteers for up to Eve days following inhalation of 3 mu m aerodynamic diameter particles. An initial assessment of the implications of the results for dose assessment, and a discussion of the potential of nose blow sampling for internal dose monitoring, are given.

The performance of the germanium (Ge) detector body monitor that has been recently installed at NRPB Chilton is evaluated and compared with the performance of the existing NRPB scintillation detector body monitor. Calibration of a p-type Ge detector for the measurement of radionuclides distributed throughout the body which emit photons with energies >100 keV is described. An ongoing systematic calibration programme of n-type Ge detectors for the measurement of low energy (<200 keV) photon emitting radionuclides in the lung is outlined. Results for the detection sensitivity for 241Am in the right lung are presented.

A method is being developed for assessing intakes by inhalation of insoluble thorium by measurement of thoron (220Rn) in exhaled air. It has been shown that it is possible to collect the short-lived decay products of thoron, 216Po and 212Pb, electrostatically. The subsequent counting is done using a silicon surface barrier detector, which gives a high detection efficiency and good energy resolution. Experiments have been carried out using a calibrated thoron source to determine the effects of parameters such as flow rate, applied collection voltage and humidity on the collection efficiency of thoron daughters. The estimated minimum detectable activity of the prototype system for lung thorium activity is between 1 and 10 Bq for a 45 min. exhaled air collection period. With this method it will be possible to measure thorium levels which correspond to intakes of a fraction of an ALI (Annual Limit on Intake).