Determination of the photon spectrum in an intense fission neutron beam

Neutrons are always accompanied by photons, especially if generated by nuclear fission. In a fast neutron beam, filters of lead can change the neutron-to-photon ratio, but only at the expense of a considerable loss in the fast neutron flux, and always with a residue of photons. Hence, medical and technical applications require some knowledge about the energy distribution of both, neutrons and photons. The predominance and high dose rate of fast neutrons, however, does not allow for a straight-forward measurement of the photon spectrum in the beam by use of an energy-resolving detector. Therefore, the photons were scattered by a rod of tungsten at an angle of 10.4 degree and subsequently corrected for the loss of energy by use of the Compton formula. The spectra were recorded mainly by a 5"-NaI(Tl)-Detector. The scattering experiments served to validate the MCNPX simulation. Using a lead filter with 3.5 cm thickness, the mean energy of the photon component is 2.4MeV and the flux is about 3.7 x 10 super(8)/(s super(-1) cm super(-2)). The methods agree well up to photon energy of 3MeV; above the experimental results become qualitative. Additionally, the MCNPX simulation of the spectrum at the entrance of the beam tube was combined with a transmission calculation through the beam tube and filters. This method avoided the influence of the detector and delivered the most detailed results.