Abstract
Abstract A 410 MeV/u 238 U projectile beam was used to create cadmium isotopes via abrasion-fission in a beryllium target placed at the entrance of the in-flight separator FRS at GSI. The fission fragments were separated by the FRS and injected into the isochronous storage ring ESR for mass measurements. Isochronous Mass Spectrometry (IMS) was performed under two different experimental conditions, with and without B ρ -tagging at the high-resolution central focal plane of the FRS. In the experiment with B ρ -tagging the magnetic rigidity of the injected fragments was determined with an accuracy of 2 ⋅ 10 − 4 . A new method of data analysis, which uses a correlation matrix for the combined data set from both experiments, has provided experimental mass values of 25 rare isotopes for the first time. The high sensitivity and selectivity of the method have given access to nuclides detected with a rate of a few atoms per week. In this letter we present for the 129,130,131 Cd isotopes mass values directly measured for the first time. The experimental mass values of cadmium as well as for tellurium and tin isotopes show a pronounced shell effect towards and at N = 82 . Shell quenching cannot be deduced from a single new mass value, nor by a better agreement with a theoretical model which explicitly takes into account a quenching feature. This is in agreement with the conclusion from γ -ray spectroscopy and confirms modern shell-model calculations.
Highlights
Accurate mass measurements over a range of isotopes reflect details of the evolution of nuclear structure and stability as well as the energy levels and spatial distributions of the bound nu-cleons [1]
The liquid-drop parameters have been deduced from a fit to the tabulated values of the Atomic Mass Evaluation 2012 (AME12) [42]
A new method of data analysis using the correlation matrix for the combined data of both types of experiment provided 25 new mass values in the range of Ge to Ce ( A = 86–154) even for isotopes measured with a rate of a few atoms per week [33]
Summary
A first microscopic explanation of the observed shell structure and the corresponding magic numbers [2,3] of neutrons and protons, at which the nuclei have larger binding energies, provided the basic understanding of nuclear properties. Nuclear structure properties can strongly influence the synthesis of elements in stars In this context, it was realized that the occurrence of a discrepant abundance trough in r-process calculations [17] could be cured by using a mass model with a quenched shell gap far from stability [18,19,20]. In mass models with a quenched shell-gap such as the modified extended Thomas–Fermi model (ETFSI-Q) [19] the deformation is greatly reduced and the ‘saddle point behavior’ in the two-neutron separation energies disappears. The present Isochronous Mass Spectrometry (IMS) of the 129,130,131Cd isotopes and the previous Penning trap mass measurements for the tin [23] and tellurium [24] isotopes yield direct information on the shell effects
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