Tuning the intrinsic radiation of Lutetium based scintillators through selective neutron activation

Lutetium Oxyorthosilicate (LSO) is an attractive scintillator for positron emission tomography (PET) imaging, due to its relatively high light output, short decay time, and high stopping power. LSO has a 2.6% natural abundance of Lu-176, a long-lived (3.78×1010 years) radioisotope that generates an intrinsic background. This low background rate has been capitalized on to perform PET quality checks and measure transmission data. In this work, LSO was exposed to the neutrons produced by the O-18(p, n)F-18 reaction in a Siemens Eclipse Cyclotron to selectively alter the intrinsic background rate. The cyclotron was run for a total of 1827 microamp-hours over two weeks, with mean proton energy of 11 MeV. Ten LSO pixels (4×4×20 mm3) were placed in locations selected for a desired neutron energy range. The range of neutron energies produced permitted the Lu-175(n,γ)Lu-176 and Lu-176(n,γ)Lu-177 reactions. Three of the pixels were placed behind polyethylene to further moderate the neutrons. The pixel's scintillation properties were measured to determine if they had been damaged by the irradiation. Gamma ray spectroscopy was performed for several weeks following the irradiation to evaluate the transmutations engendered. Despite an increase in trapping centers, scintillation properties were not changed, and the expected Lu-177 activation occurred.