R. Shane

SπRIT: A time-projection chamber for symmetry-energy studies

A time-projection chamber (TPC) called the SAMURAI Pion-Reconstruction and Ion-Tracker (S pi RIT) has recently been constructed at Michigan State University as part of an international effort to constrain the symmetry-energy term in the nuclear Equation of State (EoS). The S pi RIT TPC will be used in conjunction with the SAMURAI spectrometer at the Radioactive Isotope Beam Factory (RIBF) at RIKEN to measure yield ratios for pions and other light isospin multiplets produced in central collisions of neutron-rich heavy ions, such as Sn-132+Sn-124. The S pi RIT TPC can function both as a TPC detector and as an active target. It has a vertical drift length of 50 cm, parallel to the magnetic field. Gas multiplication is achieved through the use of a multi-wire anode plane. Image charges, produced in the 12096 pads, are read out with the recently developed Generic Electronics for TPCs

Democratic Decay of 6Be Exposed by Correlations

The interaction of an E/A=70-MeV (7)Be beam with a Be target was used to populate levels in (6)Be following neutron knockout reactions. The three-body decay of the ground and first excited states into the α+p+p exit channel were detected in the High Resolution Array. Precise three-body correlations extracted from the experimental data allowed us to obtain insight into the mechanism of the three-body democratic decay. The correlation data are in good agreement with a three-cluster-model calculation and thus validate this theoretical approach over a broad energy range.

Democratic Decay ofBe6Exposed by Correlations

The interaction of an E/A=70-MeV (7)Be beam with a Be target was used to populate levels in (6)Be following neutron knockout reactions. The three-body decay of the ground and first excited states into the α+p+p exit channel were detected in the High Resolution Array. Precise three-body correlations extracted from the experimental data allowed us to obtain insight into the mechanism of the three-body democratic decay. The correlation data are in good agreement with a three-cluster-model calculation and thus validate this theoretical approach over a broad energy range.

Observation of Long-Range Three-Body Coulomb Effects in the Decay of Ne 16

The interaction of an E/A=57.6-MeV ^{17}Ne beam with a Be target is used to populate levels in ^{16}Ne following neutron knockout reactions. The decay of ^{16}Ne states into the three-body ^{14}O+p+p continuum is observed in the High Resolution Array (HiRA). For the first time for a 2p emitter, correlations between the momenta of the three decay products are measured with sufficient resolution and statistics to allow for an unambiguous demonstration of their dependence on the long-range nature of the Coulomb interaction. Contrary to previous measurements, our measured limit Γ<80  keV for the intrinsic decay width of the ground state is not in contradiction to the small values (of the order of keV) predicted theoretically.

Three-body decay of Be-6

Three-body correlations for the ground-state decay of the lightest two-proton emitter Be-6 are studied both theoretically and experimentally. Theoretical studies are performed in a three-body hyperspherical-harmonics cluster model. In the experimental studies, the ground state of Be-6 was formed following the alpha decay of a C-10 beam inelastically excited through interactions with Be and C targets. Excellent agreement between theory and experiment is obtained demonstrating the existence of complicated correlation patterns that can elucidate the structure of Be-6 and, possibly, of the A = 6 isobars.

Pion production in rare-isotope collisions

Pion energy spectra are presented for central collisions of neutron-rich 132Sn+124Sn and neutron-deficient 108Sn+112Sn systems using simulations with Boltzmann-Uehling-Uhlenbeck transport model. These calculations, which incorporate isospin-dependent mean field potentials for relevant baryons and mesons, display a sensitivity to the pion spectra that could allow significant constraints on the density dependence of the symmetry energy and its mean field potential at supra-saturation densities. The predicted sensitivity increases with the isospin asymmetry of the total system and decreases with incident energy.

N,N,N',N'-Tetra-methyl-ethylene-diammonium tetra-chloridocobaltate(II).

The asymmetric unit of the title compound, [(CH3)2NH(CH2)2NH(CH3)2][CoCl4], contains a tetrachloridocobaltate(II) dianion and two halves of two centrosymmetric, crystallographically-independent, dications. One independent dication is disordered between two conformations in a 0.784 (13):0.216 (13) ratio. In the crystal, intermolecular N—H⋯Cl hydrogen bonds link cations and anions into chains propagated in [01]. These hydrogen bonds contribute to the distorted tetrahedral geometry at the CoII atom.

Two-proton decay of the 6Be ground state and the double isobaric analog of 11Li

Two-proton decay is discussed in a number of light isobaric multiplets. For the lightest two-proton emitter, 6Be, the momentum correlations between the three decay products were measured and found to be consistent with quantum-mechanical three-cluster-model calculations. Two-proton decay was also found for two members of the A=8 and A=11 quintets. Finally, a third member of the A=11 sextet, the double isobaric analog of the halo nucleus 11Li in 11B was observed by its two-proton decay.

Exploring Correlations in Exotic Nuclei

The difference in neutron total cross section between 40 Ca and 48 Ca is sensitive to the n/p asymmetry dependence of the surface imaginary potential in a dispersive optical model analysis, from which spectroscopic information can be extracted. The measured cross sections imply that the strength of neutron correlations changes little as neutron number increases in neutron-rich isotopes. In contrast, for the light-nucleus-induced knockout of the valence neutron in 36 Ca, the small experimental knockout cross section (as compared to an eikonal reaction theory) implies a very small spectroscopic factor (and a strong trend in correlations with asymmetry). If this type of knockout analysis is correct, it implies that standard shellmodel calculations miss spectroscopic strength as the Fermi energy nears the continuum. If this is not the case, it may be that there is a problem with the application of the eikonal theory to very deeply bound nucleons at these intermediate energies. The term correlations refers to interactions between nucleons that go beyond the mean field considered in an independent-particle-model (IPM) approach. The effect of these correlations is that in real nuclei, the spectroscopic strength of a singleparticle orbital is fragmented over energy. The spectroscopic factor (SF) quantifies the strength found at a discrete energy, and the sum of spectroscopic strength over all energies below the Fermi energy EF gives the occupation of the orbital. Correlations reduce the occupation of a single-particle state relative to its IPM value, since some of the fragmented spectroscopic strength is shifted above EF . The nuclear shell model (SM) can account for some of these correlations by including mixing between single-particle states (within a finite basis set). But due to the finite model space, the presence of the hard-core of the N-N interaction, and the tensor interaction (which further reduce the occupancies by shifting strength to high-momentum states), standard SM calculations still overestimate the occupancies of bound states. The reduction in occupancy due to correlations is not a direct experimental observable, but it will affect particle removal cross sections. By comparing experimental cross sections to calculated single-particle cross sections, one can learn about the strength of correlations. Electron-induced proton knockout (e,e’p) experiments have indicated a roughly constant 30% reduction in spectroscopic strength for valence protons in beta-stable nuclei. For unstable nuclei, light-nucleus-induced knockout experiments in inverse kinematics suggest that the affect of correlations is weaker for weakly-bound nucleons and much stronger for very deeply-bound nucleons (e.g.

2p-2p decay of {sup 8}C and isospin-allowed 2p decay of the isobaric-analog state in {sup 8}B

8 C is found to decay to four protons and an α particle in two 2p emission steps. The correlations between the protons in the first step ( 8 C to 6 Be) exhibit a significant enhancement in the region of the decay phase space where the two protons have small relative energy, a region sometimes called the diproton region. The decay of the isobaric analog of 8 C in 8 B is also found to decay by 2p emission. This is the first case of isospin-allowed 2p decay between isobaric analog states.