Preliminary study of a target-moderator assembly for a linac-based neutron source

Abstract

The report concerns a design of a future pulsed neutron source at an electron linac. A massive target is irradiated with an electron beam and the neutrons are generated mainly by collisions of the bremsstrahlung photons. A first step of the work, related to the optimization of the target materials and geometry using numerical simulations, is presented. The Monte Carlo FLUKA and MCNP codes are used. The water-cooled tantalum target is investigated: 0.41 cm Ta slices separated with 0.15 cm H2O layers. Two different sizes of the cylindrical target are assumed: 5 or 2.5 cm in diameter. The 1 GeV and 1.5 GeV electron beams are tested. The outgoing neutron angular-energy spectra are presented. The angular space 0°÷180° is divided in 10-degree intervals. The neutron emission in the direction perpendicular to the originated electron beam has been observed in the particularly narrow (2°) interval. The FLUKA results of a comparison of the neutron currents in main directions (0° – forward, 90° – perpendicular, 180° – backward) are as follows. For the 5 cm target the distribution is quite uniform: at 1 GeV input electrons – n(180°)/n(0°) = 1.00, n(90°)/n(0°) = 1.04, and at 1.5 GeV electrons – n(180°)/n(0°) = 0.91, n(90°)/n(0°) = 1.00. For the 2.5 cm target the relative neutron current in the perpendicular direction is significantly higher: at 1 GeV electrons – n(180°)/n(0°) = 1.02, n(90°)/n(0°) = 1.53, and at 1.5 GeV electrons – n(180°)/n(0°) = 0.93, n(90°)/n(0°) = 1.45. In the cases when the FLUKA and MCNP simulation results can be compared, a high similarity of the neutron energy distributions is stated although a possible discrepancy of the values reaches 20 %. Spectra of the accompanying radiation (photons, electrons, positrons, protons, charged pions) have been also obtained. The angular distributions of photons, electrons, and positrons are strongly peaked up towards the beam direction. Their emission at 90° is significantly lower, which means a decrease of the background in this direction. The energy deposition in the target is estimated on a simplified model with no cooling system. About 63 % of energy is then stored in the space at ca. 20÷40 % target length along the initial electron beam axis.