Alpha  autoradiograph of a star-shaped hot particle found in soils

near the AGR at Sellafield (1993)

The AGR was an experimental nuclear reactor which was closed down in the early 1980's. It was quite close to my main air and deposition experimental site so I thought it might be a good spot to take samples from. The experimental procedure involved taking soil samples to a depth of 2 mm and preparing the sample for alpha autoradiography i.e. ensuring the sample is dry and then simply exposing the soil to the cellulose nitrate film and leaving them in intimate contact with each other. After the nitrate film is etched (and if you are lucky) you will see wonderful imagery of shapes or blots under the microscope.

Red areas on autoradiograph show the cellulose nitrate medium as intact i.e. no damage from alpha particles. Yellowish-white areas, particularly the 'star' show intense track damage from alpha particles which indicates a hot particle is present. Track damage from tip to tip is approximately 1.25 mm and the white area within the star is where the cellulose nitrate detection medium has been 'completely burnt-out' or over-exposed to alpha emissions. This area is proportional to the size of the hot particle.

When I first started to look for these types of particles at Sellafield I was told they would be quite rare and difficult to find. In any event I managed to find and characterise quite a few, particularly around the main road close to the Windscale Piles. Bearing in mind the initial mass of sample containing many millions of particles of which perhaps one or two may be hot particles I was very pleased with my 'hit' rate for finding and isolating hot particles of plutonium and/or americium.

This particular particle was characterised for gamma and alpha-emitters and this one particle if it was possible to isolate and weigh the hotspot material itself probably contributed the bulk of radioactivity for the total sample.