Improvement of a Compact Fast Neutron Generator for Imaging Applications

Abstract

Fast neutrons are highly penetrating and exhibit macroscopic attenuation coefficients which do not strongly vary from one material to another. This makes them an attractive alternative to X-rays when imaging bulky samples, particularly when a low-Z material of interest, like water or oil, is shielded within a thick high-Z (e.g. steel) structure. A compact fast neutron generator based on deuterium-deuterium (D-D) fusion is under development at the Paul Scherrer Institute to work with techniques taking advantage of those properties. This generator was designed with fan-beam transmission imaging as its primary use and has a small emitting spot size (~2 mm) to reduce emitting spot blur. The source emits neutrons quasi mono-energetically depending on emission angle from about 2.2 to 2.8 MeV. Like other compact D-D neutron generators, the source has an intrinsic low neutron output which brings challenges of achieving reasonable exposure times for imaging applications. The neutron generation rate is mainly limited according to the maximum heat deposition rate on the ion beam target. If the temperature of the host material reaches a critical level, deuterium outgasses and neutron yield decreases. To overcome this temperature limitation, a rotating target rod was designed, implemented and tested. This upgrade increased the total neutron output by a factor of about 4.4 to ~3∙107 neutrons/s over 4π at an acceleration of roughly -100 kV and a beam current of ~1 mA. In addition to the details of the design and recent performance improvements of the rotating beam target, the latest work in characterizing the neutron generator thoroughly using COMSOL Multiphysics and MCNP6 simulations as well as experiments will be described.