The photonuclear reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(\gamma , n){}_{}{}^{11}{}_{}{}^{}\mathrm{C}$ was produced in natural carbon by means of the monochromatic gamma rays from $\mathrm{T}(p, \gamma )\mathrm{He}$ in the range of $21<\mathrm{}{E}_{\gamma }<26.7\mathrm{}$ MeV. The reaction was detected by means of the positron radioactivity of ${}_{}{}^{11}{}_{}{}^{}\mathrm{C}$ using coincidences of the annihilation gamma radiation from the positrons. The cross section was determined absolutely to an accuracy of 10%. The gamma-ray energy width or resolution varied from about 0.1 MeV at $E_{\gamma }=22\mathrm{} \mathrm{MeV} \mathrm{to} 0.2 \mathrm{MeV}$ at $E_{\gamma }=26\mathrm{}$ MeV. The peak cross section is 7.8 mb at both 22.15 and 23.0 MeV, and additional structure is observed at 25.6 MeV. The integrated cross section from 20 to 27 MeV is 36 MeV mb. Comparison is made with other reported measurements and with theoretical calculations. Some agreement is found with the deformed-nucleus calculation of Nilsson, Sawicki, and Glendenning. Also, some indication is observed of transition to excited states of ${}_{}{}^{11}{}_{}{}^{}\mathrm{C}$ suggesting two-hole-two-particle excitation in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$.

• Received 9 September 1965

DOI:https://doi.org/10.1103/PhysRev.141.1002