Discovery of 40Mg and 42Al suggests neutron drip-line slant towards heavier isotopes

Abstract

A fundamental question in nuclear physics is what combinations of neutrons and protons can make up a nucleus. Many hundreds of exotic neutron-rich isotopes have never been observed; the limit of how many neutrons a given number of protons can bind is unknown for all but the lightest elements, owing to the delicate interplay between single particle and collective quantum effects in the nucleus. This limit, known as the neutron drip line, provides a benchmark for models of the atomic nucleus. Here we report a significant advance in the determination of this limit: the discovery of two new neutron-rich isotopes--40Mg and 42Al--that are predicted to be drip-line nuclei. In the past, several attempts to observe 40Mg were unsuccessful; moreover, the observation of 42Al provides an experimental indication that the neutron drip line may be located further towards heavier isotopes in this mass region than is currently believed. In stable nuclei, attractive pairing forces enhance the stability of isotopes with even numbers of protons and neutrons. In contrast, the present work shows that nuclei at the drip line gain stability from an unpaired proton, which narrows the shell gaps and provides the opportunity to bind many more neutrons.

Shell and shape evolution atN=28: TheMg40ground state

“…Exotic combinations of protons (Z) and neutrons (N ) can significantly affect the underlying shell structure, and for weakly bound nuclei at or near the dripline, the proximity to continuum states may further alter nuclear properties. Benchmarking and constraining theory at the very limits of existence is critical, and one of the most exotic neutron-rich nuclei currently accessible to experiment is 40 Mg.First observed following fragmentation of a 48 Ca primary beam at the National Superconducting Cyclotron Laboratory in 2007 (with three events) [1], 40 12 Mg 28 lies at an intersection for nucleon magic numbers and the neutron dripline. It is expected to exhibit [2] the collective and deformed properties characteristic of the N = 28 isotones below 48 Ca, which is a region of rapidly changing nuclear shapes.…”

“…First observed following fragmentation of a 48 Ca primary beam at the National Superconducting Cyclotron Laboratory in 2007 (with three events) [1], 40 12 Mg 28 lies at an intersection for nucleon magic numbers and the neutron dripline. It is expected to exhibit [2] the collective and deformed properties characteristic of the N = 28 isotones below 48 Ca, which is a region of rapidly changing nuclear shapes.…”

Analysis of the unbound spectrum of 12Li

“…The core in these calculations was assumed to be 4 He and the Shell Model single-particle states were the shells 0p only. As pointed out in that reference, the study of many particles moving in states lying in the complex energy plane can be a challenging task.…”

“…These conditions are fulfilled by the nucleus 11 Li and also heavier Li isotopes. There are a number of experiments which have been performed in these very unstable isotopes in order to get information about the structure of halos [4]. In particular we will concentrate our attention to Refs.…”

New dimensions of the periodic system: superheavy, superneutronic, superstrange, antimatter nuclei

“…Two first methods are extensively used today for the production of new isotopes in the light and medium mass region including those which are close to the drip lines. For example, in the fragmentation of the 48 Ca beam with energy of about 140 MeV per nucleon the neutron rich nuclides 44 Si, 42 Al and 40 Mg have been observed recently [5] with extremely low cross section of their production at the level of 1 pb. In this region of the nuclear map the neutron drip line may stretch up to very exotic nuclei like 40 O, 110 Ni, 152 Zr or 288 Pb [6] (see Fig.…”