The quality of megavoltage photon beams measured by the ratio of photoactivation and neutron capture yields

The activities of products of two photonuclear reactions (113In( γ , γ ’) 113m In and 115In( γ , γ ’) 115m In) and neutron capture (115In(n, γ ) 116m In) were measured. The measurements were made to verify that obtained activities (as well as some of their ratios) change sufficiently with the change of the energy, so that the activation of the indium could be recommended as a sensitive test for the control of the endpoint energy (and beam quality) of bremsstrahlung beams in an energy region usually used in radiation therapy. Mikrotron photon beam was used for photoactivation, and the source of neutrons was the mikrotron tungsten target itself, at energies higher than threshold for ( γ , n) reaction. At lower energies, neutrons were produced in Be target inserted in photon beam. It was obtained that sensitive index of endpoint energy variation can be obtained in a broad energy region from 5 MeV to 23 MeV by different combinations of saturation activities of three reaction products. The described measurements can be used to develop a very sensitive and simple method in a routine quality control of photon beams of megavoltage devices used in radiotherapy.