First Observation of 15Be

The neutron-unbound nucleus 15Be was observed for the first time. It was populated using neutron transfer from a deuterated polyethylene target with a 59 MeV/u 14Be beam. Neutrons were measured in coincidence with outgoing 14Be particles and the reconstructed decay energy spectrum exhibits a resonance at 1.8(1) MeV. This corresponds to 15Be being unbound by 0.45 MeV more then 16Be thus significantly hindering the sequential two-neutron decay of 16Be to 14Be through this state

Further Insights into the Reaction Be14(CH2,X)10He

A previously published measurement of the reaction of a 59 MeV/nucleon 14Be beam on a deuterated polyethylene target was further analyzed to search for 12He as well as initial state effects in the population of the 10He ground state. No evidence for either was found. A lower limit of about 1 MeV was determined for a possible resonance in 12He. In addition, the three-body decay energy spectrum of 10He could not be described by a reaction mechanism calculation based on the halo structure of the initial 14Be assuming a direct α-particle removal reaction

Structure and Decay Correlations of Two-Neutron Systems Beyond the Dripline

The two-neutron unbound systems of 16Be, 13Li, 10He, and 26O have been measured using the Modular Neutron Array (MoNA) and 4 Tm Sweeper magnet setup. The correlations of the 3-body decay for the 16Be and 13Li were extracted and demonstrated a strong correlated enhancement between the two neutrons. The measurement of the 10He ground state resonance from a 14Be(−2p2n) reaction provided insight into previous predictions that wavefunction of the entrance channel, projectile, can influence the observed decay energy spectrum for the unbound system. Lastly, the decay-in-target (DiT) technique was utilized to extract the lifetime of the 26O ground state. The measured lifetime of 4.5+1.1 −1.5 (stat.)±3(sys.) ps provides the first indication of two-neutron radioactivity

First Observation of Be-15

The neutron-unbound nucleus Be-15 was observed for the first time. It was populated using neutron transfer from a deuterated polyethylene target with a 59 MeV/u Be-14 beam. Neutrons were measured in coincidence with outgoing Be-14 particles and the reconstructed decay energy spectrum exhibits a resonance at 1.8(1) MeV. This corresponds to Be-15 being unbound by 0.45 MeV more then Be-16 thus significantly hindering the sequential two-neutron decay of Be-16 to Be-14 through this state

Unbound excited states​ of the N = 16 closed shell nucleus O​­24

Two low-lying neutron-unbound excited states of24O, populated by proton-knockout reactions on 26F, 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 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

High spin states of the normally deformed bands of y 83

Level lifetime and side-feeding time measurements were performed on the excited states of the normally deformed bands of Y83 using the Doppler-shift attenuation method (DSAM). The high spin states of Y83 were populated using the fusion-evaporation reaction Ni58(S32,α3p)Y83 at 135 MeV. Twenty-two level lifetime and side-feeding times were determined in most of the cases by comparing the line shapes gated with transitions above and below the state under study. Quadrupole moments determined from lifetime measurements are in the range 1.1-3.1 eb, and are similar to the ones found for some of the neighboring nuclei. The measured side-feeding times were compared with predictions made by simulations carried out with the Gammapace code. The results were in agreement with the experimental values by assuming reduced transition probabilities of the collective transitions in the continuum region, lying in the range 40-80 W.u. The discrete excited states were studied with paired cranked Nilsson-Strutinsky-Bogoliubov (CNSB) calculations carried out for the first time for an A≈80 nucleus. Unpaired cranked Nilsson-Strutinsky (CNS) calculations were used to specify configurations and study the band crossings. The measured |Qt| values show a general agreement with CNSB calculations. Cranked shell model analysis evinced that the smallest quadrupole moment appears at the sharpest band crossing of the bands studied and CNSB calculations show an increase of the deformation thereinafter

Competing Single-Particle and Collective Behavior in 71Se

The high-spin decay of 71Se was studied using the 54Fe(23Na,αpn) reaction at 80 MeV and the Florida State University Compton-suppressed Ge array consisting of three clover detectors and seven single-crystal detectors. Based on prompt γ-γ coincidences measured in the experiment, the known level scheme was enhanced and extended to higher spin with 19 new transitions. A band that was previously suggested to have positive parity was reassigned as the “missing” signature partner of an existing negative-parity band. Spins were assigned based on directional correlation of oriented nuclei ratios. Lifetimes of 17 excited states were measured using the Doppler-shift attenuation method. Experimental Qt values imply an intermediate degree of collective behavior for 71Se at high spin. Theoretical Qtvalues determined from cranked Woods-Saxon (CWS) calculations show better agreement with the experimental ones for the positive-parity states than the negative-parity states. Shape competition and γ softness characterize the low-spin states of the lowest positive- and negative-parity bands based on the CWS calculations. At high spin, triaxial shapes with γ\u3e0∘ are predicted

K-Hindered Decay of a Six-Quasiparticle Isomer in 176Hf

The structure and decay properties of high-K isomers in 176Hf have been studied using beam sweeping techniques and the Gammasphere multidetector array. A new ΔK=8 decay branch, from a Kπ=22−, six-quasiparticle, someric (t1/2=43μs) state at 4864 keV to the 20− state of a Kπ=14−band, has been identified. The reduced hindrance factor per degree of K forbiddenness for this decay is measured to be unusually low (fν=3.2), which suggests K mixing in the states involved. The deduced interaction matrix elements are discussed within the context of relevant K-mixing scenarios. The 3266-keV state, previously interpreted as a Kπ=16+ intrinsic state, is reassigned as the Jπ=16+ member of the band based on the Kπ=15+ state at 3080 keV. The systematics of fνvalues as a function of the degree of forbiddenness is discussed in light of this change

Unresolved Question of the He-10 Ground State Resonance

The ground state of He-10 was populated using a 2p2n-removal reaction from a 59 MeV/u Be-14 beam. The decay energy of the three-body system, He-8 + n + n, was measured and a resonance was observed at E = 1.60(25) MeV with a 1.8(4) MeV width. This result is in agreement with previous invariant mass spectroscopy measurements, using the Li-11(-p) reaction, but is inconsistent with recent transfer reaction results. The proposed explanation that the difference, about 500 keV, is due to the effect of the extended halo nature of Li-11 in the one-proton knockout reaction is no longer valid as the present work demonstrates that the discrepancy between the transfer reaction results persists despite using a very different reaction mechanism, Be-14(-2p2n). DOI: 10.1103/PhysRevLett.109.23250

Further insights into the reaction Be-14(CH2,X)He-10

A previously published measurement of the reaction of a 59 MeV/nucleon Be-14 beam on a deuterated polyethylene target was further analyzed to search for He-12 as well as initial state effects in the population of the He-10 ground state. No evidence for either was found. A lower limit of about 1 MeV was determined for a possible resonance in He-12. In addition, the three-body decay energy spectrum of He-10 could not be described by a reaction mechanism calculation based on the halo structure of the initial Be-14 assuming a direct alpha-particle removal reaction