Precise mass measurements may help decode X-ray bursts

Researchers at the Michigan State University (MSU) National Superconducting Cyclotron Laboratory (NSCL) have made precise mass measurements of four proton-rich nuclei, ⁶⁸Se, ⁷⁰Se, ⁷¹Br and an excited state of ⁷⁰Br. The results may make it easier to understand type I X-ray bursts, the most common stellar explosions in the galaxy.

These bursts occur in the hot and dense environment that arises when a neutron star accretes matter from a companion star in a binary system. In these circumstances, rapid burning of hydrogen and helium occurs through a series of proton captures and beta decays known as the rp process, releasing an energy of 10³²–10³³ J in the form of X-rays in a burst 10–100 s long. Generally the capture-decay sequence happens in a matter of seconds or less, but “waiting points” occur at the proton dripline, where the protons become too weakly bound and the slower beta-decays intervene.

One of the major waiting points involves ⁶⁸Se, which has 34 neutrons and 34 protons, and closely related nuclei. The lifetimes of these nuclei influence the light curve of the X-ray burst as well as the final mix of elements created in the burst process. The lifetimes of the waiting points in turn depend critically on the masses of the nuclei involved, which also influence the possibility for double-proton capture that can bypass the beta-decay process and hence the waiting point.

The experiment at NSCL, conducted by Josh Savory and colleagues, used the Low Energy Beam and Ion Trap facility, LEBIT, for the mass measurements of the four nuclei. The nuclides themselves were produced by projectile fragmentation of a 150 MeV/u primary ⁷⁸Kr beam and separated in flight by the A1900 separator. LEBIT takes isotope beams travelling at roughly half the speed of light and then slows and stops the isotopes for highly accurate mass measurements via Penning-trap mass spectrometry.

The experiment was able to reach uncertainties as low as 0.5 keV for ⁶⁸Se to 15 keV for ^70mBr, with up to 100 times improvement in precision (for ⁷¹Br) in comparison with previous measurements. The team then used the new measurements as input to calculations of the rp process and found an increase in the effective lifetime of ⁶⁸Se, together with more precise information on the luminosity of a type I X-ray burst and on the elements produced.