We have measured the ionization loss of electrons with Lorentz factors $\gamma =2.91\mathrm{}$ and $1.6\times {10}^4<~\gamma <~{10}^5$ passing through a (100.71 ± 0.15)-μm silicon detector. Our results are in agreement with accepted theory for $\gamma =2.91\mathrm{}$ and $\gamma =1.6\mathrm{}\times {10}^4$; however, for $\gamma >~3\times {10}^4$ our results are systematically (7 ± 2)% below the theoretical value. We suggest an explanation in terms of the finite detector thickness and a relativistic effect.

• Received 31 January 1978

DOI:https://doi.org/10.1103/PhysRevLett.40.1242