The region of heavy calcium isotopes forms the frontier of experimental and theoretical nuclear structure research where the basic concepts of nuclear physics are put to stringent test. The recent discovery of the extremely neutron-rich nuclei around $^{60}\mathrm{Ca}$ O. B. Tarasov et al. [Phys. Rev. Lett. 121, 022501 (2018)] and the experimental determination of masses for $^{55--57}\mathrm{Ca}$ S. Michimasa et al. [Phys. Rev. Lett. 121, 022506 (2018)] provide unique information about the binding energy surface in this region. To assess the impact of these experimental discoveries on the nuclear landscape's extent, we use global mass models and statistical machine learning to make predictions, with quantified levels of certainty, for bound nuclides between Si and Ti. Using a Bayesian model averaging analysis based on Gaussian-process-based extrapolations we introduce the posterior probability ${p}_{\mathrm{ex}}$ for each nucleus to be bound to neutron emission. We find that extrapolations for drip-line locations, at which the nuclear binding ends, are consistent across the global mass models used, in spite of significant variations between their raw predictions. In particular, considering the current experimental information and current global mass models, we predict that $^{68}\mathrm{Ca}$ has an average posterior probability ${p}_{\mathrm{ex}}\ensuremath{\approx}76%$ to be bound to two-neutron emission while the nucleus $^{61}\mathrm{Ca}$ is likely to decay by emitting a neutron (${p}_{\mathrm{ex}}\ensuremath{\approx}46%$).

Neutron Drip Line in the Ca Region from Bayesian Model Averaging.

Exotic nuclei explored at in-flight separators

The discovery of the important neutron-rich nucleus _{20}^{60}Ca_{40} and seven others near the limits of nuclear stability is reported from the fragmentation of a 345  MeV/u ^{70}Zn projectile beam on ^{9}Be targets at the radioactive ion-beam factory of the RIKEN Nishina Center. The produced fragments were analyzed and unambiguously identified using the BigRIPS two-stage in-flight separator. The eight new neutron-rich nuclei discovered, ^{47}P, ^{49}S, ^{52}Cl, ^{54}Ar, ^{57}K, ^{59,60}Ca, and ^{62}Sc, are the most neutron-rich isotopes of the respective elements. In addition, one event consistent with ^{59}K was registered. The results are compared with the drip lines predicted by a variety of mass models and it is found that the models in best agreement with the observed limits of existence in the explored region tend to predict the even-mass Ca isotopes to be bound out to at least ^{70}Ca.

https://link.aps.org/accepted/10.1103/PhysRevLett.121.022501

Discovery of ^{60}Ca and Implications For the Stability of ^{70}Ca.

A new empirical parametrization is developed for calculating the fragment cross sections in projectile fragmentation reactions at high energies ($g100$ MeV/nucleon). The new parametrization, FRACS, consists of two main parts, i.e., the mass yield and the isobaric distribution, on the basis of previous parametrizations. The formalism for the mass yield is improved to describe the target and the projectile energy dependences observed in measured fragmentation cross sections. The parametrization of the isobaric distribution is also modified to reproduce recent experimental data. Furthermore, an additional term is proposed and first implemented in the FRACS parametrization to account for the evident odd-even staggering effect observed in many experimental cross sections. Comparisons with extensive cross sections measured in various fragmentation reactions reveal that FRACS is in much better agreement with experimental data and can reproduce measured cross sections in most cases within a factor of 1.84, which is a much smaller rms deviation as compared to that of the recent parametrization EPAX3.

Improved empirical parameterization for projectile fragmentation cross sections

A search for new isotopes in the neutron-rich rare-earth region has been carried out using a 345 MeV/nucleon 238U beam at the RIKEN Nishina Center RI Beam Factory. Fragments produced were analyzed and identified using the BigRIPS in-flight separator. We observed a total of 29 new neutron-rich isotopes: 153Ba, 154,155,156La, 156,157,158Ce, 156,157,158,159,160,161Pr, 162,163Nd, 164,165Pm, 166,167Sm, 169Eu, 171Gd, 173,174Tb, 175,176Dy, 177,178Ho, and 179,180Er.

Identification of new neutron-rich isotopes in the rare-earth region produced by 345MeV/nucleon 238U

Production cross sections for neutron-rich nuclei from the fragmentation of a ${}^{82}$Se beam at 139 MeV/u were measured. The longitudinal momentum distributions of 126 neutron-rich isotopes of elements $11\ensuremath{\le}Z\ensuremath{\le}32$ were scanned using an experimental approach of varying the target thickness. Production cross sections with beryllium and tungsten targets were determined for a large number of nuclei including several isotopes first observed in this work. These are the most neutron-rich nuclides of the elements $22\ensuremath{\le}Z\ensuremath{\le}25$ (${}^{64}$Ti, ${}^{67}$V, ${}^{69}$Cr, and ${}^{72}$Mn). One event was registered consistent with ${}^{70}$Cr and another one with ${}^{75}$Fe. The production cross sections are correlated with ${Q}_{g}$ systematics to reveal trends in the data. The results presented here confirm our previous results from a similar measurement using a ${}^{76}$Ge beam and can be explained with a shell model that predicts a subshell closure at $N=34$ around $Z=20$. This is demonstrated by systematic trends and calculations with the abrasion-ablation model that are sensitive to separation energies.

https://physics.aps.org/featured-article-pdf/10.1103/PhysRevC.87.054612

C. Sumithrarachichi

Papers

Production cross sections from 82 Se fragmentation as indications of shell effects in neutron-rich isotopes close to the drip-line