W. A. Peters

Reconstruction of nuclear charged fragment trajectories from a large-gap sweeper magnet

A new method to reconstruct charged fragment four-momentum vectors from measured trajectories behind an open, large gap, magnetic dispersion element (a sweeper magnet) has been developed. In addition to the position and angle behind the magnet it includes the position measurement in the dispersive direction at the target. The method improves the energy and angle resolution of the reconstruction significantly for experiments with fast rare isotopes, where the beam size at the target position is large.

Determining the 7 Li(n, γ) cross section via Coulomb dissociation of 8 Li

The applicability of Coulomb dissociation reactions to determine the cross section for the inverse neutron capture reaction was explored using the reaction 8Li(gamma,n)7Li. A 69.5 MeV/nucleon 8Li beam was incident on a Pb target, and the outgoing neutron and 7Li nucleus were measured in coincidence. The deduced (n,gamma) excitation function is consistent with data for the direct capture reaction 7Li(n,gamma)8Li and with low-energy effective field theory calculations.

Exploring Neutron-Rich Oxygen Isotopes with MoNA

The Modular Neutron Array (MoNA) was used in conjunction with a large‐gap dipole magnet (Sweeper) to measure neutron‐unbound states in oxygen isotopes close to the neutron dripline. While no excited states were observed in 24O, a resonance at 45(2) keV above the neutron separation energy was observed in 23O.

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 6Be and 3Li 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.

Efficiency of the Modular Neutron Array (MoNA)

The efficiency of the Modular Neutron Array (MoNA), located at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University, was measured and compared to simulations. The Coulomb dissociation of a 90 MeV/u beam of 11Be in a gold target was used to produce neutrons. The expected neutron production rate was calculated using the virtual photon method. The measured efficiency agrees with the efficiency calculated with GEANT simulations. The current configuration of MoNA has a 73% intrinsic detection efficiency for 90 MeV neutrons.

First Results from MoNA

We explore the limits of nuclear stability and the consequences on nuclear structure theory by measuring masses of neutron‐unbound nuclei and level energies above the neutron separation energy such as for the first excited state in 24O. Open problems in reaction theory are addressed by, among others, measurements of the Coulomb breakup of 8Li on a Pb and C target at 40 and 70 MeV/u.