Mass measurements of fission and projectile fragments, produced via 238 U and 124 Xe primary beams, have been performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the Fragment Separator (FRS) Ion Catcher with a mass resolving power (FWHM) of up to 410 000 and an uncertainty of down to 6 × 10 −8. The nuclides were produced and separated in flight with the fragment separator FRS at 300 to 1000 MeV/u and thermalized in a cryogenic stopping cell. The data-analysis procedure was developed to determine with highest accuracy the mass values and the corresponding uncertainties for the most challenging conditions: down to a few events in a spectrum and overlapping distributions, which can be distinguished from a single peak only by a broader peak shape. With this procedure, the resolution of low-lying isomers is increased by a factor of up to 3 compared to standard data analysis. The ground-state masses of 31 short-lived nuclides of 15 different elements with half-lives of down to 17.9 ms and count rates as low as 11 events per nuclide were determined. This is the first direct mass measurement for seven nuclides. The excitation energies and the isomer-to-groundstate ratios of six isomeric states with excitation energies of as little as 280 keV were measured. For nuclides with known mass values, the average relative deviation from the literature values is (4.5 ± 5.3) × 10 −8. The measured two-neutron separation energies and their slopes near and at the N = 126 and Z = 82 shell closures indicate a strong element-dependent binding energy of the first neutron above the closed proton shell Z = 82. The experimental results deviate strongly from the theoretical predictions, especially for N = 126 and N = 127.

Isochronous mass spectrometry has been applied to neutron-deficient 58Ni projectile fragments at the HIRFL-CSR facility in Lanzhou, China. Masses of a series of short-lived T(z)=-3/2 nuclides including 41Ti, 45Cr, 49Fe, and 53Ni have been measured with a precision of 20-40 keV. The new data enable us to test for the first time the isobaric multiplet mass equation (IMME) in fp-shell nuclei. We observe that the IMME is inconsistent with the generally accepted quadratic form for the A=53, T=3/2 quartet. We perform full space shell model calculations and compare them with the new experimental results.

Mass excesses of short-lived A=2Z-1 nuclei (63)Ge, (65)As, (67)Se, and (71)Kr have been directly measured to be -46,921(37), -46,937(85), -46,580(67), and -46,320(141)  keV, respectively. The deduced proton separation energy of -90(85)  keV for (65)As shows that this nucleus is only slightly proton unbound. X-ray burst model calculations with the new mass excess of (65)As suggest that the majority of the reaction flow passes through (64)Ge via proton capture, indicating that (64)Ge is not a significant rp-process waiting point.

No abstract