Elsevier

Nuclear Physics A

Volume 944, December 2015, Pages 376-387
Nuclear Physics A

Nuclear spectroscopy in nuclei with Z  110

https://doi.org/10.1016/j.nuclphysa.2015.09.002Get rights and content

Abstract

The nuclear structure of species at the extreme of highest atomic numbers Z and nuclear masses A promises to reveal intriguing new features of this exotic hadronic matter. Their stability itself they owe to quantum-mechanic effects only. They form metastable states which, governed by the subtle interplay of α decay and spontaneous fission versus quantum-mechanic stabilization via shell effects, are in some cases more robust against disintegration than their ground states. Following the isotopic and isotonic trends of single particle levels, as well as collective features like deformation, may reveal the path towards the gap in the level densities, expected for the next closed proton and neutron shells at the so-called “island of stability” of spherical superheavy nuclei. Their atomic configuration offers via X-ray spectroscopy a tool to identify the atomic number of heavy species, where other more traditional methods like evaporation residue (ER)–α correlation are not applicable.

Introduction

In the section of this issue of Nucl. Phys. A on “Nuclear structure of elements with 100Z109 from α spectroscopy” the major features valid also for the spectroscopy of heavy nuclear species (in the range Z109) and the methods for their study have already been introduced [1]. With increasing atomic number the production probability decreases and the access to specific parameters becomes more and more difficult. Methods demanding a minimum production rate like in-beam γ and conversion electron (CE) spectroscopy will not be applicable below a certain production cross section. In Table 1 a few examples for typical reactions are listed with their production cross sections, respective rates and the possible types of investigation. For the lowest rates with single or few event statistics only synthesis experiments are feasible, yielding information on the basic decay features like decay mode (α decay, spontaneous fission (sf) or β decay), decay energy and decay time. For moderate event numbers, of the order of a few tenths, advanced detection techniques of decay spectroscopy can be applied, which are adopted to special requirements. They allow for detailed decay spectroscopy studies typically after separation of the species of interest from the projectile beam by ion optical arrangements like e.g. velocity filters, gas-filled separators or magnetic mass spectrometers (see also [1]). Features like the nuclear structure close to the ground state of daughter nuclei populated by α decay, isomeric states surviving separation or atomic decays can be studied with comprehensive particle and photon detection set-ups coupled to a separator (see Fig. 1).

For the heaviest species the tracking of single particle levels and deformation towards the next closed shell gaps for protons and neutrons are of paramount interest. Metastable states are formed for which decay is hindered by large differences in total angular momentum of initial and final state, caused by special orientation configurations of deformed nuclei, so-called K-isomeric states. Their investigation in nuclei with increasing Z and A can yield information on the development of deformation towards the predicted region of spherical shell stabilized nuclei as well as on the dependence of the competition between α decay and sf on the quantum character of those states. This will be discussed here as well as the possible employment of X-rays for the Z identification of nuclei which otherwise cannot be characterized in terms of their atomic number by the usually applied evaporation residue (ER)–α correlations, due to the missing link to known nuclides.

After a brief discussion of the particularities of low yield spectroscopy of extremely heavy species in the following section, I will discuss in the next section two examples of advanced decay spectroscopy features: the discovery and study of K-isomeric states in the heaviest species for which such states have been observed, 270Ds and 266Hs, and the spectroscopy of members of a decay chain of one of the nuclides produced in 48Ca-induced reactions on actinide targets, 288115. I will conclude this short discourse on the intriguing facets of studying the heaviest nuclear species available in the laboratory with an outlook towards the possible achievements, envisaged with the advanced facilities presently in the planning or under construction in the last section.

Section snippets

Spectroscopic tools for extreme low cross section processes

Processes of extremely low production rates like the investigation of the species discussed here with Z100, call for special adopted methods which have been developed throughout the last decades.

As mentioned above the detailed understanding of nuclear structure and its development in the vicinity of closed shells, in regions of deformation and towards higher Z is a necessary ingredient for a successful progress towards a possible exploration of the predicted spherical shell stabilized

Example 1: K-isomers in SHN and the decay chain of 270Ds

Among the most interesting features to be studied for SHN is the observation of K-isomeric states (see e.g. [8], [9]). These states are formed by the excitation of particle–hole configurations, so-called 2-quasiparticle states consisting of a proton or a neutron elevated to a higher single-particle level and the corresponding hole generated in its original quantum state. The projection Ω of the total angular momentum, consisting of the sum of the quasiparticle's orbital angular momentum and the

Example 2: Spectroscopy along decay chains of element 115

An intriguing facet of odd–A or odd–odd heavy and superheavy nuclei is that their α decay preferably populates excited states in the daughter nucleus, because the unpaired nucleons tend to remain in their (deformed) single-particle orbitals during the α decay process [40], [41]. The decay of those excited states can be used as an efficient tool to study the nuclear and, as we will see, to some extend the atomic structure of those decay daughter nuclei. Results from this type of investigation

Future prospects

Future activities for the investigation of SHN will rely on the presently ongoing developments in heavy ion acceleration, separation and detection technologies. Projects like the envisioned cw-linac at GSI or accelerator projects presently under construction like the SHE factory at FLNR/JINR in Dubna, Russia, and LINAG of the SPIRAL2 project at GANIL in Caen, France, open new perspectives in terms of intensities, and will allow the extension of the nuclides in reach for spectroscopic methods

Acknowledgement

I would like to thank Dirk Rudolph, Department of Physics of Lund University, for his constructive discussions, advice and input, in particular, concerning his recent experimental investigations of decay chains of Z=115 isotopes.

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    On leave from GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany.

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