Constraining Nuclear Level Densities for Insight into Ultradense Matter

Type-I X-ray bursts on the surfaces of accreting neutron stars provide a unique window into the nature of matter at ultrahigh densities. One avenue for understanding dense matter properties could be X-ray burst model-observation comparisons. However, uncertain nuclear physics inputs limit the viability of this approach. A particularly key reaction is proton-capture gamma-emission, (p,γ), on the nucleus copper-59 (⁵⁹Cu). Depending on the rate of this ⁵⁹Cu(p,γ) reaction, energy generation during the X-ray burst may briefly stall, or not, impacting the shape of the burst's X-ray emission over time known as the light curve [1]. The abundance of mass 59 nuclides on the neutron star surface is also impacted, which can alter predictions of the outer neutron star thermal structure [2].

At present, ⁵⁹Cu(p,γ) cannot be measured directly in the laboratory due to the short lifetime of ⁵⁹Cu and challenges in radioactive ion beam production. Instead, theoretical calculations are required. A statistical approach known as the Hauser-Feshbach formalism has generally been used for this reaction. However, this assumes a substantial number of nuclear levels are available to be populated when a proton fuses with ⁵⁹Cu to make zinc-60 (⁶⁰Zn). Ohio University graduate student Doug Soltesz led the first measurement of the ⁶⁰Zn nuclear level density. The measurement, performed at the Edwards Accelerator Laboratory at Ohio University, involved impinging a 10 MeV beam of helium-3 (³He) onto a nickel-58 (⁵⁸Ni) target and detecting the spectrum of neutrons evaporated from this reaction. The neutron energy spectrum was related to the ⁶⁰Zn level density, taking advantage of the fact that evaporation populates final levels statistically. More neutrons detected with a given energy means more levels are present at the corresponding excitation energy in ⁶⁰Zn.

Our results show that the ⁶⁰Zn level density is on the boundary of Hauser-Feshbach validity for the excitation energy region of interest for X-ray bursts (see Figure 1 left panel). This means that measurements of the quantum mechanical spin of the relevant levels, or just the spin distribution, are now necessary in order to determine how many astrophysically-relevant levels are present (see Figure 1 right panel). Our results also show an interesting plateau in the nuclear level density in the ⁶⁰Zn excitation energy region of interest, contrary to the exponential increase expected from nuclear theory. This is the second observation of such a plateau, which is not presently understood theoretically.

References:

[1] Meisel, Merz, & Medvid. Astrophys. J., 872, 84, 2019.  (See March 2019 JINA Newsletter)

[2] Meisel & Deibel. Astrophys. J., 837, 73, 2017.