In this paper has been showed that on the base on the model of interaction between the particles fluxes emitted from gravity centre of spherical source and the plane which is in a distance r from this centre is possible to derive the dependences between the area under photometric curve and the radioisotope concentration or particle diameter. There has been showed also the experimental verification of these dependences by use as the “point” sources the spherical particles of ion-exchange resin with isotope141Ce.