MASS 57, 59 URCA COOLING IN SUPERBURSTING SYSTEMS

Current astrophysical models cannot explain the observations of anomalous extendedX-ray bursts—called “superbursts”. An as yet unknown heat source was postulated as the mechanism driving these superbursts in the outer layers of the crusts of neutron stars in these X-ray transients. Urca cooling—a nuclear process by which repeated electron capture and β−-decay removes energy from the system in the form of neutrinos—may occur in the outer crust as well and further exacerbates the problem by removing heat deposited by any new heating mechanism. Estimates of the ground state to ground state β-decay transition strengths that determine the Urca cooling strength rely heavily on theoretical QRPA predictions for nuclei far from stability—nuclei which have little to no experimental data available to aid these calculations. Moreover, the limited existing experimental data are susceptible to the Pandemonium effect due to the large Q-values of the nuclei and the limited energies of the proposed level schemes. This work seeks to experimentally determine the ground state to ground state transition strength for the β-decay parents of three prominent Urca coolers in superbursting systems: 57Ti, 57Sc, and 59Ti. Nuclei were produced by projectile fragmentation and implanted into a detector system at the National Superconducting Cyclotron Laboratory (NSCL). Transition strengths were determined from the detected β-delayed γ-ray emission using the total absorption spectrometer SuN and the β-delayed neutron emission using the neutron long counter NERO. The determined ground state to ground state transition strength of 3(2)% (log f t = 5.88(18)) for the 57Ti β-decay to 57V is far below the previous experi- mental estimate of 54(3)% from an experiment using high-resolution HPGe detectors. The ground state to ground state transition strengths for the β-decays of 57Sc and 59Ti are 1(1)% (log f t = 6.02(89)) and 3(2)% (log f t = 5.80(39)), respectively—well below the QRPA pre- dicted transition strengths, corresponding to log f t values of 4.9 and 4.9. The new results lead to drastically reduced Urca cooling from these pairs, reducing the total cooling of each by 74%, 88%, and 82% respectively. The results also provide new nuclear structure infor- mation for nuclei very far from stability, with expanded level schemes for 57V and 57Ti, and the first proposed level scheme for 59V.