Unbound excited states of theN=16closed shell nucleusO24

Abstract

Two low-lying neutron-unbound excited states of 24 O, populated by proton-knockout reactions on 26 F, have been measured using the MoNA and LISA arrays in combination with the Sweeper Magnet at the Coupled Cyclotron Facility at the NSCL using invariant mass spectroscopy. The current measurement confirms for the first time the separate identity of two states with decay energies 0.51(5) MeV and 1.20(7) MeV, and provides support for theoretical model calculations, which predict a 2 + first excited state and a 1 + higher energy state. The measured excitation energies for these states, 4.70(15) MeV for the 2 + level and 5.39(16) MeV for the (1 + ) level, are consistent with previous lower-resolution measurements, and are compared with five recent model predictions.

Two-neutron sequential decay ofO24

“…The first tentative evidence that one of these resonances decays by sequential emission of two neutrons was deduced from a measurement of two discrete neutron energies in coincidence similar to a γ-ray cascade [23]. In this Rapid Communication we present the first observation of a twoneutron sequential decay, exposed by energy and angular correlations, in 24 O. The experiment was performed at the National Superconducting Cyclotron Laboratory (NSCL), where a 140 MeV/nucleon 48 Ca beam impinged upon a 9 Be target with a thickness of 1363 mg/cm 2 to produce an 24 O beam at 83.4 MeV/nucleon with a purity of 30% at the end of the A1900 fragment separator.…”

“…So far, there has been no correlation measurement of a state decaying by sequential emission of two neutrons, although evidence for sequential decay has been reported for the high-energy continuum of 14 Be [12]. The structure of 24 O, the heaviest bound isotope for which the neutron drip-line is established [17], has been well studied and there is substantial evidence for the appearance of a new magic number N = 16 [18][19][20][21][22]. Two unbound resonances have been observed in 24 O above the one-neutron separation energy [19,23,24], and there is also evidence for resonances above the two-neutron separation energy [19,23] around 7.5 MeV.…”

“…The structure of 24 O, the heaviest bound isotope for which the neutron drip-line is established [17], has been well studied and there is substantial evidence for the appearance of a new magic number N = 16 [18][19][20][21][22]. Two unbound resonances have been observed in 24 O above the one-neutron separation energy [19,23,24], and there is also evidence for resonances above the two-neutron separation energy [19,23] around 7.5 MeV. The first tentative evidence that one of these resonances decays by sequential emission of two neutrons was deduced from a measurement of two discrete neutron energies in coincidence similar to a γ-ray cascade [23].…”

“…Together MoNA, LISA, and the charged particle detectors provide a complete kinematic measurement of the neutrons and charged particles, from which the decay of 24 O can be reconstructed.…”

“…A Monte Carlo simulation was used to model the decay of 24 O and included the beam characteristics, the reaction mechanism, and subsequent decay. The efficiency, acceptance, and resolution of the charged particle detectors following the dipole sweeper magnet and MoNA-LISA were fully incorporated into the simulation mak- ing the result directly comparable with experiment.…”