Strong Evidence for

The boundaries of the chart of nuclides contain exotic isotopes that possess extreme proton-to-neutron asymmetries. Here we report on strong evidence of ^{9}N, one of the most exotic proton-rich isotopes where more than one half of its constitute nucleons are unbound. With seven protons and two neutrons, this extremely proton-rich system would represent the first-known example of a ground-state five-proton emitter. The invariant-mass spectrum of its decay products can be fit with two peaks whose energies are consistent with the theoretical predictions of an open-quantum-system approach; however, we cannot rule out the possibility that only a single resonancelike peak is present in the spectrum.

Observation of the Exotic Isotope

A ^{13}F resonance was observed following a charge-exchange reaction between a fast ^{13}O beam and a ^{9}Be target. The resonance was found in the invariant-mass distribution of 3p+^{10}C events and probably corresponds to a 5/2^{+} excited state. The ground state was also expected to be populated, but was not resolved from the background. The observed level decays via initial proton emissions to both the ground and first 2^{+} state of ^{12}O, which subsequently undergo 2p decay. In addition, there may also be a significant proton decay branch to the second 2^{+} level in ^{12}O. The wave function associated with the observed level may be collectivized due to coupling to the continuum as is it located just above the threshold for proton decay to the 2_{2}^{+} state of ^{12}O.

First Observation of the Four-Proton Unbound Nucleus

^{18}Mg was observed, for the first time, by the invariant-mass reconstruction of ^{14}O+4p events. The ground-state decay energy and width are E_{T}=4.865(34) MeV and Γ=115(100) keV, respectively. The observed momentum correlations between the five particles are consistent with two sequential steps of prompt 2p decay passing through the ground state of ^{16}Ne. The invariant-mass spectrum also provides evidence for an excited state at an excitation energy of 1.84(14) MeV, which is likely the first excited 2^{+} state. As this energy exceeds that for the 2^{+} state in ^{20}Mg, this observation provides an argument for the demise of the N=8 shell closure in nuclei far from stability. However, in open systems this classical argument for shell strength is compromised by Thomas-Ehrman shifts.

Probing the Symmetry Energy with the Spectral Pion Ratio.

Many neutron star properties, such as the proton fraction, reflect the symmetry energy contributions to the equation of state that dominate when neutron and proton densities differ strongly. To constrain these contributions at suprasaturation densities, we measure the spectra of charged pions produced by colliding rare isotope tin (Sn) beams with isotopically enriched Sn targets. Using ratios of the charged pion spectra measured at high transverse momenta, we deduce the slope of the symmetry energy to be 42

First Observation of Unbound

The structure of the extremely proton-rich nucleus _{8}^{11}O_{3}, the mirror of the two-neutron halo nucleus _{3}^{11}Li_{8}, has been studied experimentally for the first time. Following two-neutron knockout reactions with a ^{13}O beam, the ^{11}O decay products were detected after two-proton emission and used to construct an invariant-mass spectrum. A broad peak of width ∼3.4 MeV was observed. Within the Gamow coupled-channel approach, it was concluded that this peak is a multiplet with contributions from the four lowest ^{11}O resonant states: J^{π}=3/2_{1}^{-}, 3/2_{2}^{-}, 5/2_{1}^{+}, and 5/2_{2}^{+}. The widths and configurations of these states show strong, nonmonotonic dependencies on the depth of the p-^{9}C potential. This unusual behavior is due to the presence of a broad threshold resonant state in ^{10}N, which is an analog of the virtual state in ^{10}Li in the presence of the Coulomb potential. After optimizing the model to the data, only a moderate isospin asymmetry between ground states of ^{11}O and ^{11}Li was found.

Observation of long-range three-body coulomb effects in the decay of (16)Ne.

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.

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.

Correlations in intermediate energy two-proton removal reactions.

We report final-state-exclusive measurements of the light charged fragments in coincidence with (26)Ne residual nuclei following the direct two-proton removal from a neutron-rich (28)Mg secondary beam. A Dalitz-plot analysis and comparisons with simulations show that a majority of the triple-coincidence events with two protons display phase-space correlations consistent with the (two-body) kinematics of a spatially correlated pair-removal mechanism. The fraction of such correlated events, 56(12)%, is consistent with the fraction of the calculated cross section, 64%, arising from spin S=0 two-proton configurations in the entrance-channel (shell-model) (28)Mg ground state wave function. This result promises access to an additional and more specific probe of the spin and spatial correlations of valence nucleon pairs in exotic nuclei produced as fast secondary beams.

Beam particle identification and tagging of incompletely stripped heavy beams with HEIST.

A challenge preventing successful inverse kinematics measurements with heavy nuclei that are not fully stripped is identifying and tagging the beam particles. For this purpose, the HEavy ISotope Tagger (HEIST) has been developed. HEIST utilizes two micro-channel plate timing detectors to measure the time-of-flight, a multi-sampling ion chamber to measure energy loss, and a high-purity germanium detector to identify isomer decays and calibrate the isotope identification system. HEIST has successfully identified 198Pb and other nearby nuclei at energies of about 75 MeV/A. In the experiment discussed, a typical cut containing 89% of all 198Pb80+ in the beam had a purity of 86%. We examine the issues of charge state contamination. The observed charge state populations of these ions are presented and, using an adjusted beam energy, are well described by the charge state model GLOBAL.

Ground-state proton decay of 69Br and implications for the 68Se astrophysical rapid proton-capture process waiting point.

We report on the first direct measurement of the proton separation energy for the proton-unbound nucleus (69)Br. Bypassing the (68)Se waiting point in the rp process is directly related to the 2p-capture rate through (69)Br, which depends exponentially on the proton separation energy. We find a proton separation energy for (69)Br of Sp((69)Br )= -785(-40)(+34) keV; this is less bound compared to previous predictions which have relied on uncertain theoretical calculations. The influence of the extracted proton separation energy on the rp process occurring in type I x-ray bursts is examined within the context of a one-zone burst model.

The SπRIT time projection chamber.

The Superconducting Analyzer for MUlti-particles from RAdioIsotope (SAMURAI) Pion-Reconstruction and Ion-Tracker Time Projection Chamber (SπRIT TPC) was designed to enable measurements of heavy ion collisions with the SAMURAI spectrometer at the RIKEN radioactive isotope beam factory and provides constraints on the equation of state of neutron-rich nuclear matter. The SπRIT TPC has a 50.5 cm drift length and an 86.4 × 134.4 cm2 pad plane with 12 096 pads that are equipped with the generic electronics for TPCs. The SπRIT TPC allows for an excellent reconstruction of particles and provides isotopic resolution for pions and other light charged particles across a wide range of energy losses and momenta. The details of the SπRIT TPC are presented, along with discussion of the TPC performance based on cosmic rays and charged particles emitted in heavy ion collisions.

Neutron-proton asymmetry dependence of spectroscopic factors in ar isotopes.

Spectroscopic factors have been extracted for proton-rich 34Ar and neutron-rich 46Ar using the (p, d) neutron transfer reaction. The experimental results show little reduction of the ground state neutron spectroscopic factor of the proton-rich nucleus 34Ar compared to that of 46Ar. The results suggest that correlations, which generally reduce such spectroscopic factors, do not depend strongly on the neutron-proton asymmetry of the nucleus in this isotopic region as was reported in knockout reactions. The present results are consistent with results from systematic studies of transfer reactions but inconsistent with the trends observed in knockout reaction measurements.

Constraints on the density dependence of the symmetry energy.

Collisions involving 112Sn and 124Sn nuclei have been simulated with the improved quantum molecular dynamics transport model. The results of the calculations reproduce isospin diffusion data from two different observables and the ratios of neutron and proton spectra. By comparing these data to calculations performed over a range of symmetry energies at saturation density and different representations of the density dependence of the symmetry energy, constraints on the density dependence of the symmetry energy at subnormal density are obtained. The results from the present work are compared to constraints put forward in other recent analyses.

Survey of excited state neutron spectroscopic factors for Z=8-28 nuclei.

We have extracted 565 neutron spectroscopic factors of sd and fp shell nuclei by systematically analyzing more than 2000 measured (d, p) angular distributions. We are able to compare 125 of the extracted spectroscopic factors to values predicted by large-basis shell-model calculations and evaluate the accuracies of spectroscopic factors predicted by different shell-model interactions in these regions. We find that the spectroscopic factors predicted for most excited states of sd-shell nuclei using the latest USDA or USDB interactions agree with the experimental values. For fp shell nuclei, the inability of the current models to account for the core excitation and fragmentation of the states leads to considerable discrepancies. In particular, the agreement between data and shell-model predictions for Ni isotopes is not better than a factor of 2 using either the GXPF1A or the XT interaction.

Mechanisms in knockout reactions.

We report the first detailed study of the relative importance of the stripping and diffraction mechanisms involved in nucleon knockout reactions, by the use of a coincidence measurement of the residue and fast proton following one-proton knockout reactions. The measurements used the S800 spectrograph in combination with the HiRA detector array at the NSCL. Results for the reactions 9Be(9C,8B+X)Y and 9Be(8B,7Be+X)Y are presented and compared with theoretical predictions for the two reaction mechanisms calculated using the eikonal model. The data show a clear distinction between the stripping and diffraction mechanisms and the measured relative proportions are very well reproduced by the reaction theory. This agreement adds support to the results of knockout reaction analyses and their applications to the spectroscopy of rare isotopes.

Isotopic dependence of the nuclear caloric curve.

The A/Z dependence of projectile fragmentation at relativistic energies has been studied with the ALADIN forward spectrometer at SIS. A stable beam of (124)Sn and radioactive beams of (124)La and (107)Sn at 600 MeV per nucleon have been used in order to explore a wide range of isotopic compositions. Chemical freeze-out temperatures are found to be nearly invariant with respect to the A/Z of the produced spectator sources, consistent with predictions for expanded systems. Small Coulomb effects (DeltaT approximately 0.6 MeV) appear for residue production near the onset of multifragmentation.

Tidal effects and the proximity decay of nuclei.

We examine the decay of the 3.03 MeV state of (8)Be evaporated from an excited projectilelike fragment following a peripheral heavy-ion collision. The relative energy of the daughter alpha particles exhibits a dependence on the decay angle of the (8)Be(*), indicative of a tidal effect. A comparison of the measured tidal effect with a model suggests a measurable nuclear proximity interaction.

Neutron and proton transverse emission ratio measurements and the density dependence of the asymmetry term of the nuclear equation of state.

Recent measurements of preequilibrium neutron and proton transverse emission from (112,124)Sn+(112,124)Sn reactions at 50 MeV/A have been completed at the National Superconducting Cyclotron Laboratory. Free nucleon transverse emission ratios are compared to those of A=3 mirror nuclei. Comparisons are made to Boltzmann-Uehling-Uhlenbeck (BUU) transport calculations and conclusions concerning the density dependence of the asymmetry term of the nuclear equation of state at subnuclear densities are made. Comparison to BUU model predictions indicate a density dependence of the asymmetry energy that is closer to a form in which the asymmetry energy increases as the square root of the density for the density region studied. A coalescent-invariant analysis is introduced as a means of reducing suggested difficulties with cluster emission in total nucleon emission.

Survey of ground state neutron spectroscopic factors from Li to Cr isotopes.

The ground state neutron spectroscopic factors for 80 nuclei ranging in Z from 3 to 24 have been extracted by analyzing the past measurements of the angular distributions from (d, p) and (p, d) reactions. We demonstrate an approach that provides systematic and consistent values with minimum assumptions. For the 61 nuclei that have been described by large-basis shell-model calculations, most experimental spectroscopic factors are reproduced to within 20%.

Isotopic scaling in nuclear reactions.

A three parameter scaling relationship between isotopic distributions for elements with Z< or =8 has been observed. This allows a simple description of the dependence of such distributions on the overall isospin of the system. This scaling law (termed isoscaling) applies for a variety of reaction mechanisms that are dominated by phase space, including evaporation, multifragmentation, and deeply inelastic scattering. The origins of this scaling behavior for the various reaction mechanisms are explained. For multifragmentation processes, the systematics is influenced by the density dependence of the asymmetry term of the equation of state.