publications

2025

1. 

   Potential absence of observed π² linear-chain structures in 14O via 10C(α,α) resonant scattering

   J. Bishop, A. Hollands, Tz. Kokolova, and 14 more authors

   Phys. Rev. C, Nov 2025

   DOI

2. 

   Application of a molecular model to band-head states in 9B

   A D Brooks, J Bishop, Tz Kokalova, and 2 more authors

   Journal of Physics G: Nuclear and Particle Physics, Aug 2025

   Abs DOI

   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.

3. 

   Elastic scattering in the 12N+197Au system at E_lab=70 MeV

   P. L. D. Magro, V. Guimarães, R. Linares, and 9 more authors

   Phys. Rev. C, Mar 2025

   DOI PDF

4. 

   Solution to the 9B(1/2+) state problem using the R-matrix formalism

   A. D. Brooks, J. Bishop, Tz. Kokalova, and 1 more author

   Phys. Rev. C, Feb 2025

   DOI

2024

1. 

   A Birmingham Laboratory Portrait

   Carl Wheldon, Tzany Kokalova Wheldon, Jack Bishop, and 3 more authors

   Nuclear Physics News, Feb 2024

   DOI

2. 

   Searching for particle-hole cluster bands in 8Be using the ISOLDE Solenoidal Spectrometer

   Haverson, Kristian C.Z., Smith, Robin, Gai, Moshe, and 23 more authors

   EPJ Web Conf., Feb 2024

   DOI

3. 

   Investigating nuclei produced in 9Li +11B reaction

   Sigmund, Margareta, Soić, Neven, Alcorta Moreno, Martin, and 18 more authors

   EPJ Web Conf., Feb 2024

   DOI

4. 

   Exploring the Trojan Horse Method for Nuclear Fusion Data

   Stajkowski, Dominik, Kokalova, Tzany, Wheldon, Carl, and 1 more author

   EPJ Web Conf., Feb 2024

   DOI

5. 

   Signatures of clustering accessible with a Time Projection Chamber: TexAT

   Bishop, Jack, Ahn, S., Barbui, M., and 9 more authors

   EPJ Web Conf., Feb 2024

   DOI

6. 

   Cluster structure of 3α+p states in 13N

   J. Bishop, G. V. Rogachev, S. Ahn, and 16 more authors

   Phys. Rev. C, May 2024

   DOI

7. 

   Radiative decay branching ratio of the Hoyle state

   Zifeng Luo, M. Barbui, J. Bishop, and 9 more authors

   Phys. Rev. C, Feb 2024

   DOI

2023

1. 

   Spectroscopy of 13Be through isobaric analog states in 13B

   C. Hunt, S. Ahn, J. Bishop, and 13 more authors

   Phys. Rev. C, Nov 2023

   DOI

2. 

   Search for an s-wave resonance in 7Li just above the proton-decay threshold

   N. Dronchi, J. Berkman, R. J. Charity, and 12 more authors

   Phys. Rev. C, Jun 2023

   DOI

3. 

   First Observation of the β3αp Decay of 13O via Beta-Delayed Charged-Particle Spectroscopy

   J. Bishop, G. V. Rogachev, S. Ahn, and 16 more authors

   Phys. Rev. Lett., Jun 2023

   DOI

4. 

   TexAT detector upgrade for 14O(a,p)17F cross section measurement

   C. Park, S. Ahn, S. Bae, and 21 more authors

   Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Jun 2023

   Abs DOI

   A direct cross-section measurement of the 14O(a,p)17F reaction is important to understand the light curves of x-ray bursts. The measurement will be performed using the Texas Active Target TPC version 2 (TexAT_v2). The TexAT_v2 aims at measuring lower energy protons from the reaction than the original TexAT. Newly developed silicon and CsI(Tl) detector arrays are added at the left, right and bottom of a modified field cage to increase its detection efficiency. This paper describes the overall specifications and two commissioning experiments performed at Texas A and M University.

2022

1. Horizons: nuclear astrophysics in the 2020s and beyond

   H Schatz, A D Becerril Reyes, A Best, and 162 more authors

   Journal of Physics G: Nuclear and Particle Physics, Nov 2022

   Abs DOI

   Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.

2. α-cluster structure of 18Ne

   M. Barbui, A. Volya, E. Aboud, and 15 more authors

   Phys. Rev. C, Nov 2022

   DOI

3. 

   Neutron-upscattering enhancement of the triple-alpha process

   J. Bishop, C. E. Parker, G. V. Rogachev, and 24 more authors

   Nature Communications, Apr 2022

   Abs DOI

   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).

4. Modular next generation fast-neutron detector for portal monitoring

   E. Aboud, S. Ahn, G. V. Rogachev, and 6 more authors

   Nuclear Science and Techniques, Jan 2022

   Abs DOI

   Nuclear nonproliferation is of critical importance for global security. Dangerous fissile materials including highly enriched uranium and weapons-grade plutonium are especially important to detect. Active interrogation techniques may result in much better sensitivity but are difficult with conventional portal monitors that rely on detecting thermal neutrons. Also, most conventional portal monitoring systems rely on }}^3\backslashhbox {He}}}, which has a finite and continually decreasing supply. By designing a highly segmented array of organic scintillators, we posit that we can accurately and quickly identify fissile materials, including weapons-grade plutonium and highly enriched uranium, being smuggled. We propose a new design for a fast-neutron detector that overcomes the limitations of the current generation of portal monitors. MCNP6 simulations have been performed in conjunction with the UMPBT statistical model to determine the sensitivity limitations of the proposed detector. Results suggest that the proposed detector may be }}\backslashsim 10}}times more efficient than current-generation thermal neutron detectors and may be able to positively identify a }}\backslashsim 81}} mg }}^{235}\backslashhbox {U}}}source in as little as 192 seconds utilizing active interrogation techniques.

2021

1. A position and pulse shape discriminant p-terphenyl detector module

   D.P. Scriven, G. Christian, G.V. Rogachev, and 9 more authors

   Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Jan 2021

   Abs DOI

   We present the development of a neutron detector array module made with para-terphenyl, a bright, fast, n/γ discriminating crystalline organic scintillator. The module is comprised of 2 cm × 2 cm × 2 cm p-terphenyl crystals that have been optically coupled together to create a pseudo-bar module. While only relying on two photo detectors, the module is capable of distinguishing interactions between up to eight crystals. Furthermore, the module retains the p-terphenyl’s pulse shape discrimination (PSD) capability. Together this makes the pseudo-bar module a promising position-sensitive neutron detector. Here we present characteristics of the pseudo-bar module — its timing resolution as well as its pulse shape and position discrimination capabilities, and briefly discuss future plans for utilizing an array of pseudo-bar modules in a useful neutron detector system.

2. 

   Evidence against the Efimov effect in 12C from spectroscopy and astrophysics

   J. Bishop, G. V. Rogachev, S. Ahn, and 16 more authors

   Phys. Rev. C, May 2021

   DOI
3. Direct fusion measurement of the 8B proton-halo nucleus at near-barrier energies

   J.C. Zamora, V. Guimaraes, G.V. Rogachev, and 18 more authors

   Physics Letters B, May 2021

   Abs DOI

   Direct measurements of the total fusion cross section for B8+40Ar were achieved with the active target technique. The fusion excitation function was extracted at energies near the Coulomb barrier. The cross section is well described by a coupled reaction channels calculation. The data were compared with previous B8 fusion experiments on Si28 and Ni58 targets. No evidence of striking enhancement of the total fusion cross section at near the Coulomb barrier, that was previously reported for the B8+58Ni system, was observed in these direct measurements. The present data are systematically consistent with the results for B8+28Si at higher energies and with other weakly-bound systems at near-barrier energies.

4. Precision branching-ratio measurements in 18O

   S. Pirrie, C. Wheldon, Tz. Kokalova, and 11 more authors

   The European Physical Journal A, Apr 2021

   Abs DOI

   An experiment has been performed utilising the }}^{12}}}C(}}^{7}}}Li,p)}}^{18}}}O reaction to populate high-energy states in }}^{18}}}O. Using the Munich Q3D magnetic spectrograph in conjunction with the Birmingham large-angular-coverage DSSD array, branching ratios have been measured for over fifty states in }}^{18}}}O, investigating the }}\backslashalpha }}-decay, n-decay, 2n-decay and }}\backslashgamma }}-decay branches. In tandem, Monte-Carlo techniques have been used to identify and separate features.

2020

1. Search for evidence of rotational cluster bands in 18O

   S. Pirrie, C. Wheldon, Tz. Kokalova, and 11 more authors

   Phys. Rev. C, Dec 2020

   DOI

2. 

   Almost medium-free measurement of the Hoyle state direct-decay component with a TPC

   J. Bishop, G. V. Rogachev, S. Ahn, and 14 more authors

   Phys. Rev. C, Oct 2020

   DOI
3. The Hoyle Family: The Search for Alpha-Condensate States in Light Nuclei

   R. Smith, J. Bishop, J. Hirst, and 2 more authors

   Few-Body Systems, Apr 2020

   Abs DOI

   Our present understanding of the structure of the Hoyle state in }}^{12}}}C and other near-threshold states in }}\backslashalpha }}-conjugate nuclei is reviewed in the framework of the }}\backslashalpha }}-condensate model. The }}^{12}}}C Hoyle state, in particular, is a candidate for }}\backslashalpha }}-condensation, due to its large radius and }}\backslashalpha }}-cluster structure. The predicted features of nuclear }}\backslashalpha }}-particle condensates are reviewed along with a discussion of their experimental indicators, with a focus on precision break-up measurements. Two experiments are discussed in detail, firstly concerning the break-up of }}^{12}}}C and then the decays of heavier nuclei. With more theoretical input, and increasingly complex detector setups, precision break-up measurements can, in principle, provide insight into the structures of states in }}\backslashalpha }}-conjugate nuclei. However, the commonly-held belief that the decay of a condensate state will result in N}}\backslashalpha }}-particles is challenged. We further conclude that unambiguously characterising excited states built on }}\backslashalpha }}-condensates is difficult, despite improvements in detector technology.

4. Search for the high-spin members of the α:2n:α band in 10Be

   S. Upadhyayula, G. V. Rogachev, J. Bishop, and 9 more authors

   Phys. Rev. C, Mar 2020

   DOI
5. Beta-delayed charged-particle spectroscopy using TexAT

   J. Bishop, G.V. Rogachev, S. Ahn, and 18 more authors

   Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Mar 2020

   Abs DOI

   β-delayed charged-particle emission is a sensitive probe of three-body decays in light nuclei. Time Projection Chambers (TPCs) offer a significant advantage over traditional charged-particle spectroscopy techniques due to a low-energy threshold and a high-geometric efficiency (≈4π) which are essential for use with radioactive ion beams where the beam intensities are limited. The technique for high-sensitivity spectroscopy of β-delayed charged-particle emission is shown to be possible using the Texas Active Target (TexAT) TPC in conjunction with the General Electronics for TPCs (GET) system. The benchmark case studied was that of 12N β-decay to the first α-unbound state in 12C, the Hoyle state. Half-life and branching ratio measurements are presented and are in good agreement with previous studies. The efficacy of using TPCs to study such a near-threshold state and disentangle the three-body dynamics of the decay products is demonstrated.

6. Texas Active Target (TexAT) detector for experiments with rare isotope beams

   E. Koshchiy, G.V. Rogachev, E. Pollacco, and 14 more authors

   Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Mar 2020

   Abs DOI

   The TexAT (Texas Active Target) detector is a new active-target time projection chamber (TPC) that was built at the Cyclotron Institute Texas A&M University. The detector is designed to be of general use for nuclear structure and nuclear astrophysics experiments with rare isotope beams. TexAT combines a highly segmented Time Projection Chamber (TPC) with two layers of solid state detectors. It provides high efficiency and flexibility for experiments with low intensity exotic beams, allowing for the 3D track reconstruction of the incoming and outgoing particles involved in nuclear reactions and decays.

7. Clustering in 18O - absolute determination of branching ratios via high-resolution particle spectroscopy

   S. Pirrie, C. Wheldon, Tz. Kokalova, and 11 more authors

   SciPost Phys. Proc., Mar 2020

   DOI
8. The Hoyle Family: break-up measurements to probe α-condensation in light nuclei

   Robin Smith, Jack Bishop, Joel Hirst, and 2 more authors

   SciPost Phys. Proc., Mar 2020

   DOI

2019

1. A Fast Universal Kinematic Fitting Code for Low-Energy Nuclear Physics: FUNKI_FIT

   Robin Smith and Jack Bishop

   Physics, Mar 2019

   Abs DOI

   We present an open-source kinematic fitting routine designed for low-energy nuclear physics applications. Although kinematic fitting is commonly used in high-energy particle physics, it is rarely used in low-energy nuclear physics, despite its effectiveness. A FORTRAN and ROOT C++ version of the FUNKI_FIT kinematic fitting code have been developed and published open access. The FUNKI_FIT code is universal in the sense that the constraint equations can be easily modified to suit different experimental set-ups and reactions. Two case studies for the use of this code, utilising experimental and Monte–Carlo data, are presented: (1) charged-particle spectroscopy using silicon-strip detectors; (2) charged-particle spectroscopy using active target detectors. The kinematic fitting routine provides an improvement in resolution in both cases, demonstrating, for the first time, the applicability of kinematic fitting across a range of nuclear physics applications. The ROOT macro has been developed in order to easily apply this technique in standard data analysis routines used by the nuclear physics community.

2. 

   Experimental investigation of α condensation in light nuclei

   J. Bishop, Tz. Kokalova, M. Freer, and 29 more authors

   Phys. Rev. C, Sep 2019

   DOI
3. State of the art measurements with TexAT

   J Bishop, G V Rogachev, E Aboud, and 20 more authors

   Journal of Physics: Conference Series, Aug 2019

   Abs DOI

   In the past several years, there has been a large interest of Time Projection Chambers (TPCs) for use in experimental nuclear physics. This has continued in tandem with the requirement for high efficiency detectors with low intensity radioactive ion beams. TexAT is a Active Target TPC (AT-TPC) built at Texas A&M University utilizing MICRO MEsh GASeous (MicroMegas) pads and GET electronics developed specifically for TPCs. This design combines good TPC pixelation with a surrounding shell of Si/CsI telescopes to make an extremely versatile detector capable of a wide range of different experimental techniques with only minor modifications to the electronics setup. Two recent experiments performed at the Cyclotron Institute, Texas A&M University, are detailed here demonstrating versatility beyond the usual Thick Target Inverse Kinematics (TTIK) or transfer reactions that these TPCs are more typically used for. The first, a measurement of the 12N → 12C* → 3α decay demonstrates the capabilities of TexAT as a low-energy detector operating at low pressure (20 Torr) to measure β-delayed particle decay. The second, a direct measurement of the 8B+40Ar fusion cross section shows the advantages of operating in active target mode where the target also functions as the detector gas.

4. Theoretical approaches to the 3α break-up of 12C

   Robin Smith, Jack Bishop, Carl Wheldon, and 1 more author

   Journal of Physics: Conference Series, Aug 2019

   Abs DOI

   Two recent experiments have indicated that the break-up of the 12C Hoyle state is dominated by the sequential 8Be(g.s.) + α decay channel. The rare direct 3α decay was found to contribute with a branching ratio of less than 0.047% (95% C.L.). However, the ability of experimentalists to successfully disentangle these two competing decay channels relies on accurate theoretical predictions of how they each manifest in phase space distribution of the three break-up α-particles. The following paper reviews the current theoretical approaches to calculating the break-up of the Hoyle state and introduces a semi-classical WKB approach, which adequately reproduces the results of more sophisticated calculations. It is proposed that a more accurate upper limit on this branching ratio may be obtained if these new theoretical results are taken into account when analysing experimental data.

2018

1. An improved upper limit on the direct 3α decay of the Hoyle state

   Jack Bishop, Robin Smith, Tzany Kokalova, and 4 more authors

   AIP Conference Proceedings, Nov 2018

   Abs DOI

   The structure of the Hoyle state, a near-threshold 0+ state of extreme astrophysical significance in 12C has long been investigated. An experiment was performed to measure the branching ratio for the decay of this state directly into 3 α-particles. Such a branching ratio is expected to be a good observable for whether the resonance can be described as a dilute gas of α- particles known as an α-condensate. This experiment gave the best upper limits to date for this direct decay via the improvement of the traditionally used DDΦ model to isotropic decay to the available phase space. The new DDP2 model includes three-body penetrabilities and gives a limit of < 0.026% (95% C.L.), a factor of 5 improvement over the previous experimentally obtained limit.

2. A method to determine γ branching ratios using charged particle detectors for states in 18O

   S. Pirrie, Tz. Kokalova, C. Wheldon, and 11 more authors

   AIP Conference Proceedings, Nov 2018

   Abs DOI

   A series of proposed bands in 18O with potential nuclear molecular structures, such as 14C ⊗ α or 12C ⊗ 2n ⊗  α, are being investigated. This was done through the use of the Q3D magnetic spectrograph at the Maier-Leibnitz Laboratory in Munich in conjunction with an array of double-sided silicon strip detectors (DSSDs). These detectors allowed for high resolution reconstruction of detected particles, enabling measurement of the branching ratios of the states that make up these bands. Here, a method is discussed for extracting the branching ratios of both material particles as well as γ-particles using the DSSD array to detect only charged decay fragments.

3. Optimising resistive charge-division strip detectors for low energy charged-particle spectroscopy

   R. Smith, J. Bishop, Tz. Kokalova, and 5 more authors

   Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Nov 2018

   Abs DOI

   Two novel approaches to improving the signal-to-background ratio (SBR) for silicon resistive charge-division strip detectors (RSDs), when performing low energy charged-particle spectroscopy, are presented. Firstly, the normally-unutilised rear contact of the detector was used to veto events where the charge collected by this rear face did not match the sum of the charges collected by the strips on the front. Secondly, leading edge discriminator time walk was used to determine complementary information about the hit position along a strip. Using this alongside the position extracted from the charge division allowed clearer identification of true events over background, leading to an improved SBR. These methods were tested by measuring radiation from a triple-α source and then the 12C(4He, α)ααα breakup reaction at 40 MeV beam energy. The first method was found to improve the SBR by a factor of 4.0(2). The second method gave a SBR improvement of factor of 3.7(4). When both methods are applied together, a total improvement by a factor of 5.7(3) was measured.

2017

1. New Measurement of the Direct 3α Decay from the 12C Hoyle State

   R. Smith, Tz. Kokalova, C. Wheldon, and 4 more authors

   Phys. Rev. Lett., Sep 2017

   DOI
2. High multiplicity α-particle breakup measurements to study α-condensate states

   J Bishop, Tz Kokalova, M Freer, and 29 more authors

   Journal of Physics: Conference Series, Jun 2017

   Abs DOI

   An experiment was performed to investigate α-condensate states via high α-particle multiplicity breakup. The nucleus of interest was 28Si therefore to measure multiplicity 7 particle breakup events, a highly granular detector with a high solid angle coverage was required. For this purpose, the CHIMERA and FARCOS detectors at INFN LNS were employed. Particle identification was achieved through ΔE-E energy loss. The α-particle multiplicity was measured at three beam energies to investigate different excitation regimes in 28Si. At a beam energy where the energy is sufficient to provide the 7 α-particles with enough energy to be identified using the ΔE-E method, multiplicity 7 events can be seen. Given these high multiplicity events, the particles can be reconstructed to investigate the breakup of α-condensate states. Analysing the decay paths of these states can elucidate whether the state of interest corresponds to a non-cluster, clustered or condensed state.