Copper-56 put on the scale

Type I X-ray bursts are explosive events that occur in binary systems consisting of a neutron star and a more traditional star, like our sun.  The neutron star accretes hydrogen and helium from the outer layers of its companion star (see Figure 1) until density and pressure increase to ignite a thermonuclear runaway that ensues a nucleosynthesis process, the so called rapid proton capture (rp) process.  These explosions, or bursts, yield a dramatic increase in X-ray output from the binary system, lasting a few seconds up to several minutes, and can be observed by X-ray telescopes like Chandra.

neutron star

Figure 1

The flow of the rp-process is particularly sensitive to nuclear properties, such as mass, of short-lived radioactive isotopes that are not naturally available on Earth.  Some of these properties are poorly known or have never been measured at all. The development of an accurate, precise model of the rp-process will allow us to extract the abundances of elements produced during these bursts and help explain the presence of some of the heavy elements in the Universe.  One key isotope, 56Cu, which has a half-life of only one tenth of a second, had never had its mass directly measured, and predictions of its fundamental nuclear property have varied significantly over the years (see Figure 2).  In a recent letter [Phys. Rev. Lett. 120 (2018) 032701], a group of researchers used the LEBIT mass spectrometer facility at the NSCL to precisely measure the mass of 56Cu.  This measurement was performed by first producing the rare isotope 56Cu at NSCL’s coupled cyclotron facility and subsequently capturing it in a Penning ion trap where its mass was measured by analyzing the motion of charged particles in electric and magnetic fields.  Using this new experimentally determined mass value, astrophysical simulations were performed and demonstrated that heavier elements were produced more quickly than previously thought, resulting in an enhancement of heavier elements in the ashes remaining on the neutron star following the X-ray burst.  High-precision mass measurements of rare isotopes have made significant contributions to the field of nuclear astrophysics, and will continue to provide key data for modelling astrophysical events that are responsible for populating the Universe with heavy elements.

mass Cu

Figure 2 – The mass value of 56Cu was based on various predictions until it was measured with LEBIT in 2018.  The error bars of the measurement are not visible at this scale.