A MeV tunable gamma-ray source by Compton scattering

A tunable gamma-ray source in the MeV energy range can be obtained by Compton scattering of discrete gamma rays emitted after thermal neutron capture in an in-pile target. Here we report on the principle, on the results of Monte Carlo simulations for the photon flux and on the first realization of such a tubable gamma-ray source at the ILL high flux nuclear reactor in Grenoble (France). Two strong primary gamma rays, with energies of 6.7 and 6.4 MeV, respectively, are produced (among others) by the 48Ti(n, γ)49Ti reaction. Compton scattering of these photons on a graphite target (scatterer) yields gamma rays of lower energy. Their energy is tunable between about 300 keV and 6 MeV by choosing the appropriate scattering angle. This tunable gamma-ray source could be a very useful tool to investigate photoexcitations in nuclei, which require gamma rays of several MeV. This new source might overcome the background problems present with bremsstrahlung sources, since it is a quasi-discrete and not a continuous source. Therefore, considerably less background is produced in the lower energetic part of the spectra.