A lower limit to the inelastic nuclear photon scattering cross section of ${\mathrm{Rh}}^{103}$ was determined as function of energy. Rhodium foils were exposed to a bremsstrahlung beam and the yield of ${\mathrm{Rh}}^{103}(\gamma , {\gamma }^{\prime }){\mathrm{Rh}}^{103m}$ was measured as a function of the upper limit of the spectrum. Activities were counted with a windowless $4\pi$ proportional counter and ${\mathrm{Ta}}^{181}(\gamma , n){\mathrm{Ta}}^{180}$ was used as a monitor above 8 Mev, adopting the improved results of the Sasketchewan group. It was ascertained that the fast neutron contamination of the beam contributed but a small fraction of ${\mathrm{Rh}}^{103m}$ activity. The cross section, obtained from the yield curve by customary methods, is found to go through a peak of the order of 10 mb at about 13 Mev and to be appreciable below the ${\mathrm{Rh}}^{103}(\gamma , n){\mathrm{Rh}}^{102}$ threshold (9.35 Mev). The observed location of the peak is considerably lower than would be expected for ${\mathrm{Rh}}^{103}(\gamma , n)$ from the empirical $A$-dependence of ($\gamma , n$) cross sections, and is also at variance with the observations of Katz and Cameron on ${\mathrm{Au}}^{197}(\gamma , {\gamma }^{\prime }){\mathrm{Au}}^{197}$, but appears plausible on theoretical grounds as does the magnitude of the cross section.

• Received 19 January 1953

DOI:https://doi.org/10.1103/PhysRev.90.439