Development of a quasi-monoenergetic 6 MeV Gamma Facility at NASA Goddard Space Flight Center

Abstract

The 6MeV Gamma Facility has been developed at NASA Goddard Space Flight Center (GSFC) to allow in-house characterization and testing of a wide range of gamma-ray instruments such as pixelated CdZnTe detectors for planetary science and Compton and pair-production imaging telescopes for astrophysics. The 6MeV Gamma Facility utilizes a circulating flow of water irradiated by 14MeV neutrons to produce gamma rays via neutron capture on oxygen (16O(n,p)16N→16O⁎→16O+γ). The facility provides a low cost, in-house source of 2.742, 6.129 and 7.117MeV gamma rays, near the lower energy range of most accelerators and well above the 2.614 MeV line from the 228Th decay chain, the highest energy gamma ray available from a natural radionuclide. The 7.13s half-life of the 16N decay allows the water to be irradiated on one side of a large granite block and pumped to the opposite side to decay. Separating the irradiation and decay regions allows for shielding material, the granite block, to be placed between them, thus reducing the low-energy gamma-ray continuum. Comparison between high purity germanium (HPGe) spectra from the facility and a manufactured source, 238Pu/13C, shows that the low-energy continuum from the facility is reduced by a factor ∼30 and the gamma-ray rate is ∼100 times higher at 6.129MeV.