Determination of fissile materials in highly radioactive particles using a solid-state track detector method

Highly radioactive fallout particles collected from the 19th Chinese nuclear explosion test in the atmosphere were subjected to neutron irradiation in a reactor after being sandwiched with mica detectors. Star-like fission track patterns were revealed on the etched surface of the mica detectors. Uranium or plutonium could cause such prominent track patterns within the particles. Simple chemical separation procedure was applied for uranium and plutonium and both resultant fractions were examined with similar, highly sensitive fission tracking detection. Subsequently, a representative track pattern from a black spherical particle was analyzed for the determination of fissile nuclide content by comparing the total evaluated track numbers on the basis of the numerical calculation of track densities with the employed total thermal neutron fluence. The results implied that the uranium is responsible for the main fissile nuclide remaining within a particle as unfissioned fractions and should certainly be enriched with respect to235U within such small fallout particles.