Photonuclear Activation by 20.5-Mev Gamma Rays

The photonuclear activation cross section of elements whose ($\gamma , n$) cross sections lead to a suitable positron activity has been measured using monochromatic gamma rays from the ${\mathrm{T}}^3(p, \gamma ){\mathrm{He}}^4$ reaction. The gamma rays were monitored by a 3-in. diam by 4-in. long sodium iodide crystal and calibrated with a 4½×6 in. crystal whose response curve to the $\gamma$ rays was determined. The absorption coefficient of these photons in NaI was determined by a good geometry transmission experiment. The positron activity was determined by a coincidence detector, consisting of two 5-in. diam by 2-in. long NaI crystals set on the annihilation radiation photopeaks. This detector was calibrated against a ${\mathrm{F}}^{18}$ positron source standardized in a $2\pi$ flow counter.