Jack Bishop

This article details a comprehensive Geant4 simulation based on an analytical approach to the PrimaryGenerator class. Rather than simulating the interaction of 241Am α decay with 9Be through the in-built Geant4 physics list, this simulation uses the differential cross sections of the 9Be(α,n)12C reaction to directly produce neutrons and 12C ions while simultaneously increasing computational efficiency. The advantage of this is the ability to generate Doppler broadened 4.4 MeV and 3.2 MeV γ rays from 12C decay and hence the verification of the excited states ratio per neutron emission. Each stage of the simulation has been verified against commonly accepted results, including the mass distribution of the 241Am fission fragments. The features of the emerging neutron spectrum have been compared to commonly accepted data, confirming the validity of the simulation. The source term and the simulation template are made publicly available in conjunction with this article. Although not extensively verified, the 239PuBe neutron source has also been implemented, making it a straightforward framework for implementing further sources.

Clarification of the apparent double 1/2+ analogue states in 9B through the R-matrix formalism has allowed experimentally observed states to be fitted into rotational bands through a least squares fitting method. From such experimental fits, the moment of inertia for the three rotational bands in 9B, Kπ = 3/2−, 1/2+ and 1/2−, in addition to Coriolis decoupling parameters, a, for the 1/2+ and 1/2− bands, are extracted. Values of = 0.494 ± 0.002 MeV, = 0.378 ± 0.004 MeV and = 2.27 ± 0.04, and = 0.688 ± 0.013 MeV and = 0.186 ± 0.041 are obtained. Due to a lack of theoretical predictions for 9B, these structural parameters are compared to theoretically obtained values for analogue bands in 9Be and show remarkable agreement. Using the linear combination of nuclear orbital approach, the structures of each band-head state are examined in order to reproduce the observed properties of the respective rotational bands. It is found that without modification to the underlying 8Be substructure, these orbitals struggle to correctly generate the expected moment of inertia.

The neutron inelastic scattering of carbon-12, populating the Hoyle state, is a reaction of interest for the triple-alpha process. The inverse process (neutron upscattering) can enhance the Hoyle state’s decay rate to the bound states of 12C, effectively increasing the overall triple-alpha reaction rate. The cross section of this reaction is impossible to measure experimentally but has been determined here at astrophysically-relevant energies using detailed balance. Using a highly-collimated monoenergetic beam, here we measure neutrons incident on the Texas Active Target Time Projection Chamber (TexAT TPC) filled with CO2 gas, we measure the 3α-particles (arising from the decay of the Hoyle state following inelastic scattering) and a cross section is extracted. Here we show the neutron-upscattering enhancement is observed to be much smaller than previously expected. The importance of the neutron-upscattering enhancement may therefore not be significant aside from in very particular astrophysical sites (e.g. neutron star mergers).