Clustering in $^{18}$O - absolute determination of branching ratios via high-resolution particle spectroscopy

Stuart Pirrie,Carl Wheldon,D Mengoni,H.-F Wirth,Stephanie Bailey,Daniele Dell'Aquila,Thomas Faestermann,R Hertenberger,Jack Bishop,Tzany Kokalova,Robin Smith,Ari Turner,Domenico Torresi,Neil Curtis

doi:10.21468/scipostphysproc.3.009

Abstract

The determination of absolute branching ratios for high-energy states in light nuclei is an important and useful tool for probing the underlying nuclear structure of individual resonances: for example, in establishing the tendency of an excited state towards \alphaα-cluster structure. Difficulty arises in measuring these branching ratios due to similarities in available decay channels, such as (\mathbf{^{18}}18O,\mathbf{n}𝐧) and (\mathbf{^{18}}18O,\mathbf{2n}2𝐧), as well as differences in geometric efficiencies due to population of bound excited levels in daughter nuclei. Methods are presented using Monte Carlo techniques to overcome these issues.

Highlights

The arrangement of nucleons in the nuclear system, especially in cases when these arrangements present physics outside of the largely successful nuclear shell model, is of great interest and importance in the elucidation of the nuclear force
Nuclear molecules are cluster structures in which increased stability is achieved between two clusters due to binding provided by valence nucleons, typically neutrons due to the lack of Coulomb repulsion
The likelihood of clustering in 18O is in part due to both the properties of the 12C and 14C nuclei: as 12C presents with a doubly-closed p3/2 sub-shell, it has an increased stability accentuated by the energy of its first excited state, 4.4 MeV, which indicates the potential of 12C as a good cluster; the 14C nucleus is similar in this regard, with complete neutron and partial proton p-shell closure, and has a first excited state at above 6 MeV

Summary

Introduction

The arrangement of nucleons in the nuclear system, especially in cases when these arrangements present physics outside of the largely successful nuclear shell model, is of great interest and importance in the elucidation of the nuclear force. Experimental work performed by von Oerzten et al [2] found 30 new resonances in 18O through use of the 12C(7Li,p)18O∗ reaction, measuring the energy of the recoil proton with the Q3D magnetic spectrograph at the Maier-Leibnitz Laboratory in Munich From these and previously measured states, rotational fitting was performed from which several rotational bands were tentatively assigned. In order to determine the validity of these rotational bands, the tendency towards αclustering must be established by comparison of the reduced partial α-width, γ2α, to the Wigner limit. This limit is a theoretical reduced partial α-width representing a case in which, for a particular excited state of a nucleus, an α-particle is permanently preformed inside of the nucleus [9]. For states in a cluster band, a ratio of larger than 0.1 would typically be expected, as well as a consistent value of this ratio across the composite states

Experimental method

Reconstruction of decay products

Kinematic cones

Excited levels in daughter nuclei

Determination through hit position on detectors

Conclusion