Dr Connor O'Shea

We demonstrate the benefits of applying advanced waveform processing to commercially available scintillators to provide neutron sensitivity. Compared to integration pulse shape discrimination (PSD), this approach can improve the performance of detectors that would otherwise be unsuitable for neutron measurements. By passing the waveforms of LaCl3(Ce) and LaBr3(Ce) scintillators through a fast Fourier transform we observe an enhancement of up to 200% in the separation of the gamma-ray, proton, and alpha-particle scintillation pulses by comparing the ratios of high-and low-frequency components of the waveforms. Using this method, we demonstrate that LaX3 detectors can function as fast neutron spectrometers through the detection of protons and alpha-particles from neutron-induced reactions with an AmBe source and monoenergetic neutrons in the 1-15.2 MeV range.

The observation of active Ti-44 in supernova remnants offers the potential to solve one of the most debated questions in modern astrophysics, the exact underlying explosion mechanism of core collapse supernovae (CCSNe). In particular, a comparison between the predicted synthesized yield of Ti-44 and the ejected flux is expected to allow for a determination of the mass cut of the star. Unfortunately, such comparisons are currently severely hindered by large uncertainties in the nuclear reactions governing the production and destruction of Ti-44. On this note, the astrophysical V-45(p, gamma)Cr-46 reaction is expected to play a decisive role. We present the first experimental information on resonant states in the V-45(p, gamma)Cr-46 reaction. Excitation energies have been measured with high precision for ten previously unknown, low-spin, proton-unbound states in Cr-46, and spin-parity-assignment constraints have been made using observed gamma-decay patterns and shell-model calculations. We find that an l = 1 resonance at 714 keV dominates the V-45(p, gamma)Cr-46 reaction for the entire temperature range of alpha-rich freezeout. Moreover, the nominal rate is found to be similar to 25-50% of previous estimates for T > 1.2 GK, indicating an increase in Ti-44 production in CCSNe of similar to 20-35%.

Lead-208 is the heaviest known doubly magic nucleus and its structure is therefore of special interest. Despite this magicity, which acts to provide a strong restorative force toward sphericity, it is known to exhibit both strong octupole correlations and some of the strongest quadrupole collectivity observed in doubly magic systems. In this Letter, we employ state-of-the-art experimental equipment to conclusively demonstrate, through four Coulomb-excitation measurements, the presence of a large, negative, spectroscopic quadrupole moment for both the vibrational octupole 3 1 − and quadrupole 2 1 + state, indicative of a preference for prolate deformation of the states. The observed quadrupole moment is discussed in the context of the expected splitting of the 3 − ⊗ 3 − two-phonon states, due to the coupling of the quadrupole and octupole motion. These results are compared with theoretical values from three different methods, which are unable to reproduce both the sign and magnitude of this deformation. Thus, in spite of its well-studied nature, Pb 208 remains a puzzle for our understanding of nuclear structure.

We have performed a detailed γ-ray spectroscopy study of the nucleus, 49Mn, using the GRETINA tracking array and FMA recoil separator. With this powerful new setup, low-spin excited states, which are most relevant for astrophysical processes, have been identified for the first time, including four proton-unbound levels, corresponding to key astrophysical resonances in the 48Cr(p,γ)49Mn reaction. Of these four levels, two were found to dominate the 48Cr(p,γ)49Mn reaction for temperatures, T = 0.2 − 1.4 GK, and uncertainties in the rate have been reduced by more than 3 orders of magnitude. Specifically, γ decays were observed from 1/2+ and 3/2− excited states at Ex = 2570.9(26) keV and 2595.8(21) keV, corresponding to an ℓ = 0 and ℓ = 1 resonance in the 48Cr + p system at Er = 482.9(84) keV and 507.9(83) keV, respectively. Present simulations of Type-I X-ray burst nucleosynthesis indicate that the newly defined 48Cr(p,γ) reaction rate is sufficiently fast to drive the flow of material towards higher masses in such environments. Consequently, despite the relatively long half life of 48Cr, we now do not expect a strong waiting point in the rp process at A = 48.

Recent models of the rapid proton (rp) capture process indicate that a competition between the 59Cu(p, gamma)60Zn and 59Cu(p, alpha)56Ni reactions may result in the formation of a nickel-copper (NiCu) cycle that traps the flux of material between 56Ni and 60Zn. Here, we report the identification of 15 proton-unbound levels in 60Zn, populated via 59Cu(d, n) transfer, which govern the rate of the 59Cu(p, gamma)60Zn reaction in XRBs. Precise excitation energies for levels in 60Zn were obtained from observed gamma decays, and spectroscopic factors were determined from angle-integrated cross sections. Incorporating these results into stellar-model calculations, we find that with experimentally constrained uncertainties a NiCu cycle in XRBs is indeed possible, though we limit its branching strength to less than 38%. While modest, such a branching has significant impact on the light curve, motivating further studies of the relevant rates. Our calculations also indicate that a significant NiCu cycle leads to an increase in the amount of odd-A nuclei in the burst ashes, which may affect Urca cooling processes in neutron star crusts.