Neutron energy spectra inside a PET cyclotron vault room

Abstract

Neutron energy spectra were measured at three locations inside the vault room of a PET cyclotron. Measured neutron fields were produced during 17.2 MeV protons and 8.6 MeV deuterons colliding with Faraday cup, and 8.6 MeV deuterons colliding with 14N. Measurements were performed using a Bonner sphere spectrometer whose active 6LiI(Eu) scintillator was replaced by two pairs of thermoluminscent dosimeters. Modified neutron spectrometer was calibrated using bare 252Cf and D 2O moderated 252Cf neutron sources. Thermoluminiscent dosimeters were calibrated using γ-rays from 137Cs and 60Co sources. From this calibration, a single factor was derived that allow us to obtain the thermal neutron net signal used to unfold neutron spectra. MCNP 4A code was used to calculate neutron spectra at those sites were measurements were taken. For MCNP calculations a source term for protons colliding with graphite was calculated using a semiempirical model. Due the lack of source term for neutrons produced during 8.6 MeV deuterons nuclear reactions, with carbon and 14N, several models were tried. From experimental and calculated results a systematic behavior of neutron spectra was observed regardless the primary neutron source. This behavior was a peak indicating the presence of evaporation neutrons due to the large amount of iron in cyclotron.