Accurate measurements were made of the detailed shapes of $\gamma$-ray yield curves from semithick aluminum targets over the 992-keV resonance. Detailed information from these targets was possible through the use of a new electronic device which allowed data to be taken while incrementing the target voltage to provide energy changes and simultaneously correcting for energy fluctuations in the beam. To provide pure targets, fast evaporation rates were used with pressures usually 3×${10}^{-7\mathrm{}}$ Torr or less. During runs the pressure was usually 3×${10}^{-9\mathrm{}}$ Torr or less. No contamination effects appeared. Experimental results were compared to curves computed by Monte Carlo techniques for an assumed collision spectrum in which the contributions of electrons from each shell were computed using the Born approximation. These calculated curves gave closer fits to the experimental data than previous curves for which all electrons were assumed to be free and stationary. The experimental results departed somewhat from those calculated because nonuniformities in target thickness were not completely eliminated. Some additional results on thick targets show that surface contamination can take place if ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$ gases are introduced into the vacuum system.

• Received 15 April 1964

DOI:https://doi.org/10.1103/PhysRev.135.A1429