The relation between grain density in photographic tracks and particle energy has been derived theoretically assuming that the probability that a grain be developed is given by $P=c[1-\exp [-b{\left(\frac{\mathrm{dE}}{\mathrm{dR}}\right)}^{\frac{1}{2}}]$. Using a single pair of parameters $b$ and $c$ we can represent with this equation all experimental curves so far published for alpha-particles, tritons, protons, $\pi$ and $\mu$ mesons (University of Bristol). The agreement between experiments and theory is very satisfactory.

• Received 17 September 1948

DOI:https://doi.org/10.1103/PhysRev.75.279