This invention refers to a method for the quantitative determination of the fissile material content in small size particles present in samples such as environmental samples. According to the invention, the "fission track process", known per se, is applied to said samples, according to which the samples are sandwiched in organic sheets and then submitted to a defined thermal neutron fluence whereupon fission products of the fissile material in the sample create in the sheets fission tracks which are rendered visible by etching the sheets.

Then the size of selected particles having created such tracks is determined by means of an appropriate microscope and thereafter these visible tracks of said selected particles are compared to pre-established standard tracks obtained by the same process from particles of different stepped known size and enrichment ratio.

A method is described for determining the amount and spatial distribution of impurity actinides in fission­able deposits used for measurement of solid state track recorder fission rate and neutron fluence. For example, a ²³⁹Pu fissionable deposit is autoradiographed with an alpha particle-sensitive track recorder which is then subjected to a four hour etch at room temperature. The etch induc­tion time for the ²³⁹Pu alpha particles is just barely exceeded, so that the²³⁹Pu tracks appear as very immature dots on the surface of the solid state track recorder. The ²³⁵U tracks, on the other hand, are correspondingly etched longer than their respective shorter etch induction time and appear as much more mature, longer tracks. The amounts of ²³⁵U and ²³⁹Pu present can be calculated from the numbers of different tracks that are counted.