Air Activation by an Electron Synchrotron

Abstract

The activation of 13N and 15O in air of the room housing the INS electron synchrotron was estimated. The total induced activity of 13N and 15O in the room was measured with an NaI(T1) scintillator to be 119.7 and 47.0 muCi, respectively, immediately after the operation of the electron synchrotron at 725-MeV electron energy and 176 W beam power. The operation was long enough for the steady-state concentrations of 13N and 15O to be reached. The generation of 13N and 15O was also evaluated, with only about 20% underestimation using a model calculation. In this calculation the emitted photon flux and distribution were calculated by the Monte-Carlo code, EGS, for electromagnetic cascade showers, and these photon distributions were multiplied by the (gamma, n) photoneutron reaction cross sections to calculate the production of 13N and 15O. From this analysis, it is clear that the usual method of estimating the photon spectrum at an electron accelerator, by using the forward bremsstrahlung spectrum of a thin target, is quite inaccurate when electrons are incident on a bulk structure or impinge on a magnet at a small angle of incidence. In such a case, one must consider that the photons are emitted mainly by backscattering, and have a considerably softer spectrum than the thin-target bremsstrahlung.