Cross Sections for the ${\mathrm{B}}^{11}(\gamma , {\pi }^{-}){\mathrm{C}}^{11}$ and ${\mathrm{B}}^{11}(\gamma , {\pi }^{+}){\mathrm{Be}}^{11}$ Reactions

Cross sections for the ${\mathrm{B}}^{11}(\gamma , {\pi }^{-}){\mathrm{C}}^{11}$ and ${\mathrm{B}}^{11}(\gamma , {\pi }^{+}){\mathrm{Be}}^{11}$ reactions were studied by measuring the radio-activities of the product nuclei in boron containing samples that were irradiated with bremsstrahlung of maximum energy varying from 80 to 300 MeV. In both cases activity was observed below the thresholds for the meson production reactions. These below-threshold activities were studied in some detail and are believed to be due mainly to low-energy reactions involving impurities in the target materials. Corrections were made to the yield data to remove the contributions from the low-energy reactions before the cross sections were calculated. The resulting cross sections for the production of ${\mathrm{C}}^{11}$ average about seven times those for the production of ${\mathrm{Be}}^{11}$. The cross sections for the ${\mathrm{B}}^{11}(\gamma , {\pi }^{-}){\mathrm{C}}^{11}$ reaction agree with those calculated by Laing and Moorhouse for surface production of pions. Although detailed theoretical calculations of the cross sections expected for the ${\mathrm{B}}^{11}(\gamma , {\pi }^{+}){\mathrm{Be}}^{11}$ reaction have not been made, the observed cross sections are qualitatively in agreement with those expected for surface production.