Abstract
Direct radiometric determination of N-14 (gamma, n) N-13 air activation was achieved at a 15-MV medical linear accelerator operating in a high-energy photon mode. N-13 was identified by irradiating a gas-tight Marinelli beaker filled with nitrogen gas and later observing the 10-min half-life of the 511-keV positron-electron annihilation line using high-resolution gamma spectroscopy. Quantitative evaluation of the spectral signal yielded a N-13 production rate of 836.8 +/- 32 Bq Gy(-1) in air per 40 x 40 cm(2) field cross section at 100 cm source-surface distance.