Speaker
Description
Nuclear mass measurements give access to nuclear binding energies, which reflect the sum of interactions among its constituent nucleons. Certain double differences of neighboring nuclei binding energies provide $\delta$V$_{pn}$, the average interaction strength of the last one or two protons with the last one or two neutrons [1].
We will present highly-accurate mass measurements of the nuclides $^{69}$As, $^{70,71}$Se, and $^{71}$Br, produced via fragmentation of a $^{124}$Xe primary beam at the Fragment Separator (FRS) at GSI. These measurements were performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS Ion Catcher with an unprecedented mass resolving power of almost 1,000,000 [2]. Such high resolving power is the only way to achieve accurate results and resolve overlapping peaks of short-lived exotic nuclei, whose total number of accumulated events is always limited.
We obtained relative mass uncertainties at the level of 10$^{-7}$ with only up to 20 events for $^{69}$As (first direct mass measurement), $^{71}$Se and $^{71}$Br, and at the level of 10$^{-8}$ with less than 500 events for $^{70}$Se. All our results are in good agreement with the 2020 Atomic Mass Evaluation. We measured also the mass of the stable molecule $^{14}$N$^{15}$N$^{40}$Ar with a relative mass accuracy of $\delta$m/m = 1.7 $\times$ 10$^{−8}$, the best yet achieved with an MR-TOF-MS.
Our results show that the measured re-strengthening of the proton-neutron interaction ($\delta$V$_{pn}$) for odd-odd nuclei along the N = Z line above Z = 29 (recently extended to Z = 37) is hardly evident at the N − Z = 2 line, and not evident at the N − Z = 4 line. Nevertheless, detailed structure of $\delta$V$_{pn}$ along the N − Z = 2 and N − Z = 4 lines, confirmed by our mass measurements, may provide a hint regarding the ongoing $\approx$ 500 keV discrepancy in the mass value of the nuclide $^{70}$Br, which prevents including it in the world average of Ft value for super-allowed 0$^{+}$ $\rightarrow$ 0$^{+}$ $\beta$-decays. The reported work sets the stage for mass measurements with the FRS Ion Catcher of nuclei at and beyond the N = Z line in the same region of the nuclear chart, including the nuclide $^{70}$Br.
We will further present results of a systematic search for deviations from the expected $\delta$V$_{pn}$ trends throughout the nuclear chart, which may indicate that some mass evaluations in the Atomic Mass Evaluation are not correct and even some mass measurements are questionable.
We will also present an analysis of mass measurements via higher-order mass-difference indicators, which may disentangle the Wigner term (p-n correlations) from the Symmetry term (mean-field) [3]. This analysis is performed for both N $\approx$ Z nuclei and neutron-rich N$_{val}$ $\approx$ Z$_{val}$ (equal proton and neutron numbers above doubly-magic cores) nuclei, for which $\delta$V$_{pn}$ is apparently enhanced, perhaps hinting emergent symmetries in these nuclei that are similar to those of N $\approx$ Z nuclei [4].
We plan to apply these analyses to our upcoming mass measurements of neutron-rich nuclei around N$_{val}$ $\approx$ Z$_{val}$, which are planned and proposed via spontaneous fission at the FRS Ion Catcher at GSI, and via multi-neutron-transfer reactions at JYFLTRAP at the University of Jyväskylä.
References:
[1] P. Van Isacker et al., Phys. Rev. Lett. 74, 4607 (1995)
[2] I. Mardor et al., Phys. Rev. C 103, 034319 (2021)
[3] W. Satula et al., Phys. Lett. B 407, 103 (1997)
[4] R. B. Cakirli et al., Phys. Rev. C 82, 061304(R) (2010)