In measuring β-radioactivity of thick sources, one has to take in account the phenomenon of self-weakening in the source. The present paper shows if the choice of the distance between source and counter on one hand, and of the chemical nature of the support on the other hand, are well chosen, it is possible to find a self-weakening correction curve that is exponential. The values of the necessary parameters are given for three radioisotopes of different β-energies: phosphorus-32, calcium-45 and carbon-14.
