Chong Qi

Universal decay law in charged-particle emission and exotic cluster radioactivity.

A linear universal decay formula is presented starting from the microscopic mechanism of the charged-particle emission. It relates the half-lives of monopole radioactive decays with the Q values of the outgoing particles as well as the masses and charges of the nuclei involved in the decay. This relation is found to be a generalization of the Geiger-Nuttall law in alpha radioactivity and explains well all known cluster decays. Predictions on the most likely emissions of various clusters are presented.

Evidence for a spin-aligned neutron-proton paired phase from the level structure of 92Pd

Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work that introduced a strong spin–orbit interaction to the nuclear shell model potential. However, knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron–proton pairing, in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus 92Pd. Gamma rays emitted following the 58Ni(36Ar,2n)92Pd fusion–evaporation reaction were identified using a combination of state-of-the-art high-resolution γ-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron–proton coupling scheme, different from the previous prediction. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron–proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.

SPIN-ALIGNED NEUTRON-PROTON PAIR MODE IN ATOMIC NUCLEI

Shell model calculations reveal that the low-lying spectrum of the N = Z nucleus 92 Pd is generated from a correlated isoscalar spin-aligned neutron-proton pair mode, exhibiting a new form of collectivity different from vibrational and rotational excitations. Already the ground state structure of 92 Pd is mostly built from isoscalar pairs each carrying angular momentum J = 9. This structure is different from all other even-even nuclei studied so far. The energy spectrum generated by the correlated neutron-proton pairs has two distinctive features: i) it is almost equidistant for low-lying energies and ii) the transition probability I ! I − 2 is approximately constant and independent of I. This exotic coupling scheme is predicted to correspond to the yrast structures of the heaviest nuclei approaching the doubly-magic 100 Sn.

On the validity of the Geiger–Nuttall alpha-decay law and its microscopic basis

a b s t r a c t The Geiger-Nuttall (GN) law relates the partial α-decay half-life with the energy of the escaping α particle and contains for every isotopic chain two experimentally determined coefficients. The expression is supported by several phenomenological approaches, however its coefficients lack a fully microscopic basis. In this paper we will show that: (1) the empirical coefficients that appear in the GN law have a deep physical meaning, and (2) the GN law is successful within the restricted experimental data sets available so far, but is not valid in general. We will show that, when the dependence of logarithm values of the α formation probability on the neutron number is not linear or constant, the GN law is broken. For the α decay of neutron-deficient nucleus

Abrupt changes in alpha-decay systematics as a manifestation of collective nuclear modes

An abrupt change in alpha-decay systematics around the N = 126 neutron shell closure is discussed. It is explained as a sudden hindrance of the clustering of the nucleons that eventually form the a ...

Alpha decay as a probe for the structure of neutron-deficient nuclei

The advent of radioactive ion beam facilities and new detector technologies have opened up new possibilities to investigate the radioactive decays of highly unstable nuclei, in particular the proton emission, α decay and heavy cluster decays from neutron-deficient (or proton-rich) nuclei around the proton drip line. It turns out that these decay measurements can serve as a unique probe for studying the structure of the nuclei involved. On the theoretical side, the development in nuclear many-body theories and supercomputing facilities have also made it possible to simulate the nuclear clusterization and decays from a microscopic and consistent perspective. In this article we would like to review the current status of these structure and decay studies in heavy nuclei, regarding both experimental and theoretical opportunities. We then discuss in detail the recent progress in our understanding of the nuclear α formation probabilities in heavy nuclei and their indication on the underlying nuclear structure.

N = Z nuclei: a laboratory for neutron–proton collective mode

The neutron-neutron and proton-proton pairing correlations have long been recognized to be the dominant many-body correlation beyond the nuclear mean field since the introduction of pairing mechani ...

Mirror energy difference and the structure of loosely bound proton-rich nuclei around A=20

The properties of loosely bound proton-rich nuclei around A = 20 are investigated within the framework of the nuclear shell model. In these nuclei, the strength of the effective interactions involving the loosely bound proton s(1/2) orbit is significantly reduced in comparison with that of those in their mirror nuclei. We evaluate the reduction of the effective interaction by calculating the monopole-based-universal interaction (V-MU) in the Woods-Saxon basis. The shell-model Hamiltonian in the sd shell, such as USD, can thus be modified to reproduce the binding energies and energy levels of the weakly bound proton-rich nuclei around A = 20. The effect of the reduction of the effective interaction on the structure and decay properties of these nuclei is also discussed.

Effects of formation properties in one-proton radioactivity

Academy of Romanian Scientists, 54 Splaiul Independentei, RO-050085 Bucharest, Romania(Dated: January 19, 2012)It is shown that the proton formation probability, extracted from experimental data correspondingto one-proton radioactivity, is divided into two regions when plotted as a function of an universalparameter. This parameter is derived from a microscopic description of the decay process. Inthis way we explain the systematics of proton emission half-lives. At the same time the formationprobability is shown to be a useful quantity to determine the deformation property of the mothernucleus.

Multistep shell model description of spin-aligned neutron-proton pair coupling

The multistep shell model was extended recently to incorporate both neutron and proton degrees of freedom and applied to study the structure of