Design and feasibility study of a compact neutron source for extraterrestrial geochronology applications

The ⁴⁰Ar/³⁹Ar radiometric dating technique is an attractive option for future extraterrestrial geochronology applications. However, in situ ⁴⁰Ar/³⁹Ar radiometric dating on Mars presents unique challenges to the design of a device capable of achieving sufficient precision on geological samples obtained on the Martian surface. For this application, a fast neutron source with a low thermal neutron flux is ideal for inducing the ³⁹K(n,p)³⁹Ar reaction with few competing reactions that require age-correction factors. This paper explores the design of a neutron emitting device specifically for in situ geochronological applications on extraterrestrial surfaces. We have determined that a feasible design is likely to include a ²⁵²Cf spontaneous fission source shielded by polyethylene layered with a strong thermal neutron absorber. Although boosting options-induced fission sources, (α,n)-are available, they do not provide sufficient neutron multiplicity to justify the increased mass of the device. Furthermore, shielding the rover from the neutron source will likely comprise the largest fractional mass of the device, which will be reduced by shielding only a small solid angle of the source. While we have determined that it is possible to design such a neutron source, there will also be other instrumentation competing for a mass fraction of the Rover instrument payload. This may make it difficult to design a device that achieves the required mass and fluence limitations for a future mission. However, this work provides a realistic path forward in determining a future workable design.