Decay Pattern of Pygmy States Observed in Neutron-Rich 26 Ne

TL;DR

This study measured low-lying E1 strength in neutron-rich Ne26 via Coulomb excitation, revealing a decay pattern inconsistent with some theoretical models, and providing new insights into pygmy states in exotic nuclei.

Contribution

First experimental measurement of decay pattern of low-lying pygmy states in a neutron-rich nucleus, challenging existing mean-field theories.

Findings

Observed E1 strength between 6 and 10 MeV.

Deduced B(E1)=0.49+-0.16 e^2fm^2, about 4.9% of TRK sum rule.

Decay pattern inconsistent with several theoretical models.

Abstract

Coulomb excitation of the exotic neutron-rich nucleus Ne26 on a Pb208 target was measured at 58 MeV/u in order to search for low-lying E1 strength above the neutron emission threshold. This radioactive beam experiment was carried out at the RIKEN Accelerator Research Facility. Using the invariant mass method in the Ne25+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV excitation energy. By performing a multipole decomposition of the differential cross section, a reduced dipole transition probability of B(E1)=0.49+-0.16e2fm2 is deduced, corresponding to 4.9+-1.6% of the Thomas-Reiche-Kuhn sum rule. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured. The extracted decay pattern is not consistent with several mean-field theory descriptions of the pygmy states.