The detection of a particle as hot depends unon the number of silver grains exposed in the film, and the determining factor in such &@ measuremant 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.e, Flux = KA,(R)V (R)G(R)E Kk = constant of proportionality Am(R)= activity concentration which may vary with size V(R) = volume (4/3913) G(R) = geometry factor (inverse square law for spherical particles and isotropic radia- BE 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 Yen particles detected will decrease from 100 to 0. This variation is to ve expected since the experimental methods of detecting, the process of radioactive decay, and the exposure of developing centers of a filn 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 effectis present. Because 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 po/mg. It was found with artificial hot Sr?°clg particles that the point of 50 per cent detection efficiency was at about 10 microns for a specific activity of 0.005 po/mg. Two considerations make these figures cornarae ble. The fission products have a considerably higher mean energy than sr 6 and therefore proportionately less will be registered by this fiin. The sr70Cl2 crystals wera very flat and would have a much smaller geometry correction than spherical particles. The flux will then in. crease much faster with particle size, so that a smaller particle -1i11 be detected as hot. The fallout particles in the range under consi ‘era- tion are essentially spherical. Therefore, the effect under consi teration may result in a variation in detection efficiency from 109 per cant as the particle size decreases, which would have to be corrected for. 39 SECRET — RESTRICTED DATA