Radiative decay of neutron-unbound intruder states inO19

Abstract

The 9 Be( 14 C, αγ) reaction at E Lab =30 and 35 MeV was used to study excited states of 19 O. The Florida State University (FSU) γ detector array was used to detect γ radiation in coincidence with charged particles detected and identified with a silicon ∆E-E particle telescope. Gamma decays have been observed for the first time from six states ranging from 368 to 2147 keV above the neutron separation energy (S n =3962 keV) in 19 O. The γ decaying states are interspersed among states previously observed to decay by neutron emission. The ability of electromagnetic decay to compete successfully with neutron decay is explained in terms of neutron angular momentum barriers and small spectroscopic factors implying higher spin and complex structure for these intruder states. These results illustrate the need for complementary experimental approaches to best illuminate the complete nuclear structure. 1The decay mode(s) of a nuclear quantum state is (are) one of its most important properties after energy. Usually nuclear decay follows the hierarchy of the fundamental forces of nature.Decay by emission of one or more particles mediated by the strong nuclear interaction is normally the dominant mode for those states unbound to particle emission. Bound excited states usually decay by emission of electromagnetic radiation to the ground state. The ground state, in turn, decays much more slowly by β decay mediated by the weak nuclear interaction until the lowest energy neutron-to-proton ratio has been reached. Of course, there are exceptions: lower energy charged particle decay from unbound states is inhibited by the Coulomb barrier, and large spin change or low emission energy can inhibit γ decay so much that β decay occurs first. Even decay by neutrons which face no Coulomb barrier can be inhibited by large spin change, as is well known in high-spin spectroscopy of medium and heavy nuclei. However, based on the simple picture of barrier penetrability, neutron decay is usually assumed to dominate over radiative decay when angular momentum barriers are not very high. But Fermi's Golden Rule [1] states that the decay rate is the product of the coupling strength, the density of final states, and the matrix element between the wavefunction of the parent state and the daughter state (usually called spectroscopic factor (S)). What is often overlooked is that this latter factor due to the nuclear structure may be more instrumental than the angular momentum barrier in limiting neutron decay rates below those of electromagnetic decay. Examples are states with complex structure often involving intruder configurations.The role of factors beyond barrier penetrability is highlighted by observations reported in this paper of the decays of unbound states in 19 O populated in the 9 Be( 14 C,αγ) reaction which favors higher-spin states, which are typically more complex and may involve intruder configurations. The importance of 19 O with a closed major proton shell and an exactly halffilled neutron d 5/2 orbital has been recognized wi...

Strong Neutron-γCompetition above the Neutron Threshold in the Decay ofCo70

“…This extremely small spectroscopic overlap can explain the reduction in neutron emission and the strong γ emission above the neutron threshold. Similar conclusions were drawn in the recent work of Dungan et al [59], in a different case of γ-neutron competition in 19 O, from states populated in a transfer reaction. We therefore conclude that in the decay of neutron-rich nuclei, the neutron emission can be hindered due to the small spectroscopic overlap of the involved states, with a major impact on the calculated neutron emission probability.…”

Cross-shell excitations inSi31

“…values in the range of 10 −2 to 10 −4 or less. This is consistent with previous observations [35,36]. As a test of these estimates, the neutron decay spectroscopic factor for the 6661 keV state was calculated in the shell model to be 0.0012, a factor of 2 above the upper limit in A bigger challenge is understanding the structure of the negative-parity states which must involve an odd number of particles outside the sd shell.…”