Ionization Loss at Relativistic Velocities in Nuclear Emulsion

The variation of grain density in the tracks of singly-charged relativistic particles traversing a nuclear emulsion has been investigated as a function of velocity in Ilford G.5 plates exposed to the cosmic radiation at 100 000 feet. Multiple scattering measurements and "blob counts" were made on long tracks of electrons, mesons, and protons with energies $\gamma$ up to 3400 rest mass units. The blob density $G_{\mathrm{pl}}$ at the Fermi plateau of ionization was found to be 1.14±0.03 times $G_{min}$, in agreement with the result previously obtained by another method. Protons and electrons show the same value of $G_{\mathrm{pl}}$. The data are compared with the theories of Halpern-Hall and of Sternheimer for AgBr, using the ionization potentials of Bakker and Segrè. Since only grains along the track were counted, the calculated energy loss is restricted to energy transfers less than an upper limit $T_0$. Calculations for two assumed values of $T_0$, 2 kev and 5 kev, fit the data equally well, yielding ratios $\frac{G_{\mathrm{pl}}}{G_{min}}$ of 1.15 and 1.14, respectively. The data are also consistent with the slow rate of rise from minimum ($\gamma \sim 4$) to plateau ($\gamma >100\mathrm{}$), which is predicted by the theory.