Do alpha particles cluster inside heavy nuclei?

Abstract

Excited states in212 Po were populated by transfer using the 208 Pbð 18 O; 14 CÞ reaction, and their deexcitation rays were studied with the Euroball array. Several levels were found to decay by a unique E1 transition (E < 1 MeV) populating the yrast state with the same spin value. Their lifetimes were measured by the Doppler-shift attenuation method. The values, found in the range 0.1-1.4 ps, lead to very enhanced transitions, BðE1Þ ¼ 2 Â 10 À2 -1 Â 10 À3 W:u: These results are discussed in terms of an -cluster structure which gives rise to states with non-natural-parity values, provided that the composite system cannot rotate collectively, as expected in the '' þ 208 Pb'' case. Such states due to the oscillatory motion of the -core distance are observed for the first time. DOI: 10.1103/PhysRevLett.104.042701 PACS numbers: 25.70.Hi, 21.60.Gx, 23.20.Àg, 27.80.+w In the early days of the development of nuclear theory, the particle was considered as the basic building block of any nucleus, providing a simple explanation for the emission of s by heavy nuclei or the fact that all of the light nuclei with A ¼ 4n have higher binding energies per particle than any of their neighbors. But many arguments [1] were rapidly developed against this picture, which was almost completely abandoned to the benefit of singleparticle description of nuclei based on the hypothesis of a common mean field for all nucleons.A strong revival of the -cluster model occurred in the 1960s when both experimental and theoretical studies revealed that the concept of clustering is essential for the understanding of the structure of light nuclei. The states based on particles (and other bound substructures) are not so much found in the ground states but rather observed as excited states close to the decay thresholds into clusters, as suggested by Ikeda [2]. In particular, the Hoyle state, i.e., the 0 þ 2 state at 7.65 MeV in 12 C which has recently been interpreted as an -particle condensate [3], and other similar states in heavier n nuclei, have attracted much renewed attention (see, e.g., [4]). Also, so-called nuclear molecules, such as the -core system rotating about its center of mass, have lately been studied intensely and with great success (for a review, see [5]).The persistence of clustering in heavy nuclei is much less documented. This is even the case of 212 Po, a typical nucleus with two protons and two neutrons outside the doubly magic core, 208 Pb. Up to now, there was no clearcut evidence of its cluster structure. Shell model (SM) configurations involving a few orbits account reasonably well for the excitation energies of its low-lying yrast states [6]. Nevertheless, the width of its ground state is predicted more than 1 order of magnitude smaller than the experimental value. This has called for a hybrid model comprising both shell and cluster configurations [7]. Then the -decay energy and the half-life are well reproduced and a large amount of clustering is found (30%). Such a result suggests that the low-lying yrast spectrum of ...

Pairing Effect on the Binding Energy Curve of N = Z Atomic Nuclei

“…When the masses of the nuclear fragments are measured, an abundance of alpha particles or their multiples is consistently found. Alpha clustering is not only now well established in light nuclei [9,10] but also in all nuclei, as shown in this paper. MacGregor [7] "pointed out that there is a huge penalty paid by, particularly, the large nuclei for having a core region with equal numbers of protons and neutrons because that implies an abundance of proton charge in the nuclear interior".…”