$β$-decay Measurements Near the $N=40$ Island of Inversion to Quantify Cooling of Accreted Neutron Star Crusts

Abstract

Understanding the thermal structure of the outer crust of accreting neutron stars is important to interpret astronomical X-ray observations. Ground-state to ground-state $β$-decay transitions of neutron-rich nuclei comprising the crust enable Urca neutrino cooling processes that affect this thermal structure. Here we constrain the ground-state to ground-state transition strengths for the decays of $^{57}$Sc, $^{57}$Ti, and $^{59}$Ti based on experimental data. The data were obtained by combining total absorption $γ$-spectroscopy data from the SuN detection system with $β$-delayed neutron emission data from the NERO detection system at Michigan State University's National Superconducting Cyclotron Laboratory. We find $\log ft=$5.8$^{+0.3}_{-0.2}$ and $\log ft=$5.34$^{+0.08}_{-0.24}$ for the decays of $^{57}$Ti and $^{59}$Ti, respectively, and find no evidence for ground-state feeding in the decay of $^{57}$Sc. The results indicate weaker transitions than predicted by theory and indicated by previous measurements, resulting in reduced efficiency of neutrino cooling in accreted neutron star crusts in systems that exhibit X-ray superbursts.

Figures (12)

• Figure 1: $P_{\rm n}$ values as functions of mass number for Sc isotopes (left) and V isotopes (right). Shown are the results from this work (red), results from previous work (black) crawfordDecayIsomericProperties2010bongDecay61VIts2020, and theoretical predictions from QRPA-fY (blue).
• Figure 2: TAS (top), SS (bottom), and multiplicity (bottom inset) measured spectra (black) together with the corresponding template fits (red) for the decay of $^{57}$Ti. The black vertical line indicates a change in the energy axis scale. Blue triangles mark the neutron separation energy of the $^{57}$V daughter.
• Figure 3: Discrete levels in $^{57}$V populated by the decay of $^{57}$Ti and associated $\gamma$-transitions used in this work. The newly identified transition is drawn in red. Dashed red lines indicate new tentative levels that have been inserted to improve the fits.
• Figure 4: TAS (top), SS (bottom), and multiplicity (bottom inset) measured spectra (black) together with the corresponding template fits (red) for the decay of $^{57}$Sc. The vertical black line marks a change in energy axis scale. Blue triangles mark the neutron separation energy of the $^{57}$Ti daughter.
• Figure 5: Discrete levels in $^{57}$Ti populated by the decay of $^{57}$Sc and associated $\gamma$-transitions used in this work. Newly identified levels and transitions are drawn in red. Dashed lines indicate tentative levels that have been inserted to improve the fits.