The activation curve for the reaction ${\mathrm{C}}^{12}(\gamma , n){\mathrm{C}}^{11}$ was obtained with the use of both the Mark II (38-Mev) and Mark III (630-Mev) linear accelerators, covering the energy range 18-260 Mev. Stacked foils of polystyrene detectors and tantalum and copper radiators were exposed to the direct electron beam and the resulting ${\mathrm{C}}^{11}$ activity was measured with a $4\pi$ scintillation counter. The photon flux was obtained from the measured electron flux by means of the Bethe-Heitler theory of bremsstrahlung, and the photon-difference method was applied to obtain the cross-section curve. This curve was found to be similar to some previous results, but with the addition of a high-energy tail extending beyond 38 Mev. The experimental accuracy does not permit determination of the shape of this tail, but it cannot be accounted for by mesonic effects alone. The cross section has a peak value of about 8.3 millibarns at 22.5 Mev, and integrated values of 0.056±0.003 and about 0.080±0.01 Mev-barns when the integrals are carried up to 38 and 250 Mev, respectively. These numerical values were obtained from the data obtained by using a copper radiator. The corresponding results with a tantalum radiator are about 7 percent lower, indicating a possible failure of the bremsstrahlung formula for high atomic numbers.

• Received 13 December 1954

DOI:https://doi.org/10.1103/PhysRev.98.73