Half-Life of the Doubly Magicr-Process NucleusN78i

Abstract

Nuclei with magic numbers serve as important benchmarks in nuclear theory. In addition, neutronrich nuclei play an important role in the astrophysical rapid neutron-capture process (r-process). 78 Ni is the only doubly-magic nucleus that is also an important waiting point in the r-process, and serves as a major bottleneck in the synthesis of heavier elements. The half-life of 78 Ni has been experimentally deduced for the first time at the Coupled Cyclotron Facility of the National Superconducting Cyclotron Laboratory at Michigan State University, and was found to be 110 +100 −60 ms. In the same experiment, a first half-life was deduced for 77 Ni of 128 +27 −33 ms, and more precise half-lives were deduced for 75 Ni and 76 Ni of 344 +20 −24 ms and 238 +15 −18 ms respectively.Doubly-magic nuclei with completely filled proton and neutron shells are of fundamental interest in nuclear physics. The simplified structure of these nuclei and their direct neighbors allows one to benchmark key ingredients in nuclear structure theories such as single-particle energies and effective interactions. Doubly-magic nuclei also serve as cores for shell model calculations, dramatically truncating the model space, thus rendering feasible shell model calculations in heavy nuclei. All this is of particular importance for nuclei far from stability, where doubly-magic nuclei serve as beachheads in the unknown territory of the chart of nuclides [1,2].When considering the classic nuclear shell gaps and excluding superheavy nuclei, there are only 10 doublymagic nuclei, and only four of these are far from stability: 48 Ni, 78 Ni, 100 Sn, and 132 Sn. Of these, 48 Ni and 78 Ni are the most exotic ones, and the last ones with experimentally unknown properties. 78 Ni therefore represents a unique stepping stone towards the physics of extremely neutron-rich nuclei. In a pioneering experiment, Engelmann et al. Very neutron-rich nuclei play an important role in the astrophysical rapid neutron-capture process (r-process) [5,6]. The r-process is responsible for the origin of about half of the heavy elements beyond iron in nature, yet its site and exact mechanism are still unknown. 78 Ni is the only doubly-magic nucleus that represents an important waiting point in the path of the r-process, where the reaction sequence halts to wait for the decay of the nucleus [7].One popular astrophysical site for the r-process is the neutrino driven wind off a hot, newborn neutron star in a core-collapse supernova explosion [8]. In this case the rprocess begins around mass number A = 90, with lighter nuclei being produced as less neutron-rich species in an α-rich freeze-out. For such a scenario 78 Ni would not be directly relevant. However, the α-rich freezeout fails to accurately reproduce the observed abundances for nuclei with A = 80−90 [9], and the associated r-process does not produce sufficient amounts of the heaviest r-process nuclei around A =195 [10].78 Ni is among the important r-process waiting points in models that try to address these issues. Examples ...

β-decay half-lives andβ-delayed neutron emission probabilities of nuclei in the regionA≲110, relevant for the r process

“…The Maximum Likelihood analysis (MLH) is well suited for determining decay half-lives in cases with low statistics [37,71,72,73,74,75,76]. The method, described in appendix A, defines decay sequences for up to three generations following an implantation.…”

The neutron long counter NERO for studies of neutron emission in the r-process

“…For the nuclear species analyzed in Refs. [5,6,35,36,37], β-neutron coincidences associated with descendant nuclei other than the β-decay daughter were negligible. In this case, using the Batemann equations [38], it is possible to write explicitly the value of N ββn as:…”

βdecay of nuclei around90Se: Search for signatures of aN=56subshell closure relevant to theQuinn1, Aprahamian2, Pereira3 et al. 2012Phys. Rev. CSelf Cite

“…The speed of the r-process reaction sequence and the resulting abundances of the elements are significantly influenced by the β-decay properties of the nuclei involved, which are in turn directly related to the evolution of nuclear structure and deformation. In a recent measurement of the half life of the doubly-magic nucleus 78 Ni [8,9], it was shown that the persistence of the closed neutron and proton shells had a significant impact on the r-process abundance patterns, and theoretical predictions for the half life were significantly longer than observed.…”