The detection of a particle as hot depends uxon the number of silver grains exposed in the film, and the determining factor in such a measurement is the total flux (radiation per unit area of observation per unit tine). This is a function of the activity concentration and of the particle size, shape, i.9., Px = KA,(R)V (R)GCR)E = constant of proportionality An(R)a activity concentration which may vary with size V(R) = volune (4/313) G(R) = geometry factor (inverse square law for spherical particles and isotropic radia\ @ tion) = correction factor used to express the portion of the energy distribution of beta particles to which the film is sensitive As the limit of detection is reached, the percentage of hot particles detected will decrease from 100 to 0. This variation is to be exrected since the experimental methods of detecting, the process of radioactive decay, and tne exposure of developing centers of a film are all statistical in nature. This effect will be sudden or gradual (with variation in flux), depending on the statisticaly spread, which may be quite large, It is then necessary to make a correction for the detection efficiency (percentages of hot particles detected) in the range where this effect is present. Vecause of the delay before the processing of the semples under consideration, the activity concentration may be low enough that this effect will be observed. At the time the autoradiographs were made, the activity concentration was estimated to be about 0,15 pe/mg. It was found with artificial hot Sr?°clo particles that the point of 50 per cent detection efficiency was at about 10 microns for a specific activity of 0.005 pe/mg. Teo considerations make these figures comparae ble. The fission products have a considerably higher mean energy than Sr?9 and therefore proportionately less will be registered by this film. The Sr7Oc1a erystals wera very flat and would have a much smaller geometry correction than spherical particles. The flux will then increase much faster with particle size, so that a smaller particle will be detected as hot. The fallout particles in the range under considera- tion are essentially spherical. Therefore, the effect under consideration may result in a variation in detection efficiency from 1C9 per cent as the particle size decreases, which would have to be corrected for. 39 SECRET — RESTRICTED DATA