Jack Bishop – Nuclear Physicist | University of Birmingham

Data for the , , and differential cross sections are important for several different areas of nuclear physics such as understanding neutron transmutation in nuclear reactors. The TexAT time projection chamber (TPC) was used to measure the differential and angle-integrated cross sections in active-target mode. The chamber was filled with CO gas and used a quasi-monoenergetic neutron beam from the reaction at Edwards Accelerator Lab at Ohio University. A comparison between our current and previous results at overlapping energies and angles showed good agreement in angular dependence and absolute cross section. A broader angular coverage than previous results demonstrated that the integrated cross section for the reaction deviates from ENDFVIII.0 evaluations. This first instance of neutron-induced measurements with an active-target TPC demonstrates the use of this method for high-quality differential cross section data across a broad angular range, generating good statistics with a relatively low-intensity beam.

Through the application of a molecular model to band-head states in 9B, a prediction of the Coulomb energy difference relative to the 1/2+ analogue state in 9Be is calculated. Under such treatment, a positive shift in energy of 0.605 MeV is found to emerge from molecular structures defined by replication of experimental rotational-band parameters. Given the relationship between the spatial arrangement of clusters within the nucleus and the obtained Coulomb energy, it is also possible to examine nuclear structures using known Thomas–Ehrman shifts. Molecular structures built using the α–α separation, 2β, required to give Thomas–Ehrman shifts for both a 1.84 and 0.8 MeV candidate of the 9B(1/2+) analogue state (β = 1.89 fm and β = 3.18 fm respectively) are used to calculate state inertial parameters (). From these structures, it is found that a 1.84 MeV state ( = 0.407 MeV) agrees with the experimental K = 1/2+ rotational band inertial parameter ( = 0.41 ± 0.01 MeV) and a 0.8 MeV state does not ( = 0.191 MeV). Thus, due to the large α–α spacing required to lower the Coulomb energy relative to 9Be, the existence of a 0.8 MeV 9B(1/2+) analogue state is highly unlikely. Indeed, excitation energies less than ≈1.25 MeV for a 1/2+ state, corresponding to a normal Thomas–Ehrman shift, can also be ruled out.

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).