Excited states in certain atomic nuclei possess an unusual structure, where the dominant degrees of freedom are those of $\alpha$ clusters rather than individual nucleons. It has been proposed that the diffuse $3\alpha$ system of the ${}^{12}\mathrm{C}$ Hoyle state may behave like a Bose-Einstein condensate, where the $\alpha$ clusters maintain their bosonic identities. By measuring the decay of the Hoyle state into three $\alpha$ particles, we obtained an upper limit for the rare direct $3\alpha$ decay branch of 0.047%. This value is now at a level comparable with theoretical predictions and could be a sensitive probe of the structure of this state.

Positron emission particle tracking (PEPT) is a technique which allows the high-resolution, three-dimensional imaging of particulate and multiphase systems, including systems which are large, dense, and/or optically opaque, and thus difficult to study using other methodologies. In this work, we bring together researchers from the world’s foremost PEPT facilities not only to give a balanced and detailed overview and review of the technique but, for the first time, provide a rigorous, direct, quantitative assessment of the relative strengths and weaknesses of all contemporary PEPT methodologies. We provide detailed explanations of the methodologies explored, including also interactive code examples allowing the reader to actively explore, edit and apply the algorithms discussed. The suite of benchmarking tests performed and described within the document is made available in an open-source repository for future researchers.

Improvements in Radio-Isotope IDentification (RIID) algorithms have seen a resurgence in interest with the increased accessibility of machine learning models. Convolutional Neural Network (CNN)-based models have been developed to identify arbitrary mixtures of unstable nuclides from gamma spectra. In service of this, methods for the simulation and pre-processing of training data were also developed. The implementation of 1D multi-class, multi-label CNNs demonstrated good generalisation to real spectra with poor statistics and significant gain shifts. It is also shown that even basic CNN architectures prove reliable for RIID under the challenging conditions of heavy shielding and close source geometries, and may be extended to generalised solutions for pragmatic RIID.

The mixing of di-neutron components in ${}^8\mathrm{He}$ is studied. The ground state of ${}^8\mathrm{He}$ corresponds to the sub-closed-shell configuration of the spin-orbit favored orbits ($p_{3/2}$) for the neutrons, and the shell-model-like configuration is considered to be dominant. However, ${}^8\mathrm{He}$ is a drip-line nucleus with a two-neutron separation energy of 2.1 MeV, and the di-neutron configuration, which is a characteristic feature of the weakly bound systems, mixes in the ground state. The motion of four neutrons around the ${}^4\mathrm{He}$ core is solved by superposing the wave functions of antisymmetrized molecular dynamics. The mixing of di-neutron configurations is estimated by coupling the THSR (Tohsaki-Horiuchi-Schuck-Röpke) wave function, which successfully describes the α-condensed state of light nuclei.

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).