Session DB: Mini-Symposium on Nuclear Structure of Neutron-rich Exotic Nuclei I

Physics of neutron-rich exotic nuclei

Gaining a better understanding of heavy neutron-rich nuclei is central to the resolution of a number of open questions in nuclear structure physics, nuclear astrophysics and various nuclear physics applications. Access to these nuclei has been greatly improved recently with new medium and large scale facilities and knowledge of this region of the chart of nuclei is expanding rapidly. The big questions involving neutron-rich nuclei will be briefly presented in this talk, together with the measurements that are needed to address them and our current ability to fill these needs. The current capabilities will be concisely illustrated by recent spectacular experimental results in the field. The expected next steps will be highlighted by concentrating on the limits of what is possible now and the limitations that remain and what is needed to address them.   

Determination of the density distribution of halo nuclei by proton elastic scattering

A current issue in nuclear physics is how large the core excitation is in halo nuclei. It was recently shown in $^{31}$Ne that $^{30}$Ne as a core nucleus is highly excited. The angular distribution of proton elastic scattering is considered to be useful to investigate the core excitation, because it is reported that the position of the first diffraction minimum is sensitive to the density distribution of core nucleus. In this presentation, we concentrate our analysis on $^{6}$He as a typical case of neutron halo. We use the $n + n + ^{4}$He model to obtain the density distribution of $^{6}$He and analyze elastic cross sections for $^{6}$He scattering from a proton target at about 0.7 GeV/nucleon by using the single folding model. The calculated results are successful in reproducing the experimental data with no adjustable parameter. We also investigate the effects of core and valence neutron distributions on the angular distribution of elastic cross section.   

Study of the T$=$5/2 states in $^{9}$Li (analogs of the lowest states in $^{9}$He) as a test of nuclear structure theory for drip line nuclei

About 20 years ago, a group of Hahn-Meitner Institute made precision measurements of a multi nucleon transfer reaction to populate the lowest states in $^{9}$He. They found [1,2] a state of $^{9}$He(1/2$^{-})$ at 1.27 $\pm$ 0.10 MeV above the $^{8}$He $+$ n threshold with $\Gamma =$ 0.10 $\pm$ 0.06 MeV. Since then, many groups tried to obtain detailed information on $^{9}$He mainly using rare isotope beams. However, the energy resolution and counting statistics was never sufficient to test the data [1,2] (see a review in [3]). Additionally an MSU group [4] found a virtual s-wave state within 0.2 MeV of the $^{8}$He$+$n threshold which they claimed to be the ground state of $^{9}$He. The theoretical calculations demonstrate rare unanimity. A variety of approaches including the recent [5]\textit{ ab initio} calculations predict a broad state, approximately ten times broader than given in Refs. [1,2]. So it can be that our understanding of nuclear structure at the border of nuclear stability is seriously deficient. To date, it looks like all straightforward ways to obtain spectroscopic information on $^{9}$He were tested. So, we populated T$=$5/2 states in $^{9}$Li (analogs of $^{9}$He) in $^{8}$He$+$p resonance elastic scattering using the TTIK method [5,6]. The measurements were performed using 4 MeV/A $^{8}$He beam provided by TRIUMF facilities. The scattering chamber was filled with CH$_{4}$ gas. The proton recoils were detected by an array of position sensitive proportional counters and silicon detectors. The experimental equipment was tested using 3.5 and 7 MeV/A $^{12}$C beams of Cyclotron Institute at TAMU.   

Proton radii of neutron-rich B isotopes and neutron surface thickness in $^{17}$B

As the neutron to proton asymmetry increases nuclei develop exotic structures such as neutron skin and halo. It is important to investigate how this asymmetry affects the proton distribution. The matter and proton radii have started unfolding a complete picture of the halo. For two-neutron halos the correlation between the halo neutrons and their distance from the core can be derived to define the average halo geometry. The proton radii are crucial information to extract the neutron skin thickness to constrain the equation of state of asymmetric nuclear matter. Very limited information is available on the proton radii of very neutron-rich nuclei. In this presentation, we will describe the new technique of extracting proton radii from charge changing cross sections using relativistic beams at GSI, Germany. The presentation will show first measurements of proton radii of the neutron-rich boron isotopes. The implications of the results in understanding the neutron surface thickness in the Borromean $^{17}$B and its possible halo structure will be discussed.   

Reaction cross sections of $^{14}$B and $^{8}$He on proton targets for the separation of proton and neutron density distributions

In order to discuss the exotic surface structures of neutron- / proton-rich nuclei such as halo and skin in detail, it is important to clarify the distributions of neutron and proton respectively. For this purpose, we utilize the isospin (p-p or p-n) asymmetry of nucleon-nucleon total cross sections in the intermediate energy region. We employed a Proton target as the isospin asymmetric target, which is the most asymmetric one. Also, we employed Be, C, and Al targets which are isospin symmetric to be contrast with Proton target. In the present work, we have measured reaction cross sections for $^{14}$B and $^{8}$He on Proton, Be, C, and Al targets at intermediate energies. The experiments were carried out at the HIMAC heavy ion synchrotron facility, Japan. We will report results of the analyses.   

Current status of the no-core Monte Carlo shell model

One of the major challenges in nuclear physics is to describe nuclear structure and reactions from first principles. Such $ab$ $initio$ calculations have recently become feasible for nuclear many-body systems beyond $A = 4$ due to the development of quantum many-body methods along with the rapid evolution of computational technologies. The No-Core Shell Model (NCSM) is one of the relevant $ab$ $initio$ methods and is now available for the study of nuclear structure and reactions in the $p$-shell nuclei. As the NCSM treats all the nucleons on an equal footing, computational demands for the calculations explode exponentially as the number of nucleons increases. In order to access heavier nuclei, many efforts have been devoted to the NCSM calculations. Among them, the no-core Monte Carlo shell model (MCSM) is one of the promising candidates to go beyond the Full Configuration Interaction method. Here, we report recent developments of the MCSM and its application to the no-core calculations. No-Core Full Configuration results are also presented as full $ab$ $initio$ solutions extrapolated to the infinite basis limit. We compare the NCFC results with the MCSM results extrapolated to the infinite basis space.   

Structure of Be isotopes based on Monte Carlo shell model

We study the properties of low-lying states of Be isotopes based on Monte Carlo shell model (MCSM). We calculate the 2$^+$ energies and the $B(E2; 2_1^+ \rightarrow 0_1^+)$. These values are compared with the experiments and other models. The calculated $B(E2)$ values are comparable with the experiments and other models especially for $^{8,10}Be$. To see the origin of these values, we analyse the intrinsic density. We find two equally weighted distributions of the proton intrinsic density in the ground states of Be isotopes (from $A=$ 8 to 12). These distributions correspond to the two $\alpha$ particles. The gradual annihilations of the clear $2\alpha$ shape with the neutron number are found. After that, We focus on the valence neutrons which are distributed around them. The distributions are understood by the molecular orbits such as $\sigma$ and $\pi$ orbits. The role of the valence neutrons for the shapes and $E2$-transition properties is analyzed. We discuss whether these behavior of the structure is consistent with the prediction by other models. We also discuss the results in terms of the breaking of $N=8$ magicity in $^{12}$Be.   

Invariant mass spectroscopy of $^{17}$C via one-neutron knockout reaction from $^{18}$C

The nuclei away from the $\beta$-stability line are expected to have exotic nuclear structures. For example, the ground states of neutron-rich carbon isotopes, $^{15}$C, $^{17}$C, and $^{19}$C, have been predicted to be 5/2$^{+}$ states in the naive shell model. However, they were identified as 1/2$^{+}$, 3/2$^{+}$, and 1/2$^{+}$, respectively, due to the halo structure and/or nuclear deformation. To understand the properties of the valence orbit relative to the inner orbit in those neutron-rich carbon isotopes, the study of the negative parity states is decisive. The present study focuses on the low-lying negative parity states in $^{17}$C above the neutron decay threshold. The experiment was performed for the C($^{18}$C,$^{17}$C$^*$) one-neutron knockout reaction channel at 250 MeV/nucleon using the SAMURAI spectrometer at RIKEN-RIBF, during the first physics runs of the apparatus. The nucleon knockout reaction utilizing the secondary beams in inverse kinematics has become recognized as a sensitive tool for spectroscopy of the nuclei far from the $\beta$-stability line. In the presentation, details of the measurement and analysis will be reported together with new results on the low-lying negative parity states in $^{17}$C.   

Magnetic Transition Rate Measurement in $^{19}$C

The magnetic transition rate of the 3/2$^{+}$$\rightarrow$1/2$^{+}_{g.s.}$ transition in the one-neutron halo nucleus $^{19}$C has been determined via lifetime measurement. This represents the first measurement of the magnetic response of a halo nucleus. The lifetime was determined using two independent Doppler-shift techniques at the National Superconducting Cyclotron Laboratory along with the state-of-the-art gamma tracking array GRETINA. The observed transition strength $B$(M1; 3/2$^{+}$$\rightarrow$1/2$^{+}$) is one of the most hindered among known M1 transitions in the mass region. In this talk, the lifetime results are presented, and the strong hindrance is discussed in terms of the halo configurations in $^{19}$C.   

Reaction cross section of $^{22}$C

Reaction cross section of $^{22}$C on a carbon target at an energy of 240~MeV/nucleon have been measured by using the transmission method. The most neutron-rich carbon isotopes $^{22}$C is a candidate of a two-neutron halo nucleus. Tanaka {\it et al.} [1] measured the reaction cross section of $^{22}$C on a hydrogen target at 40~MeV/nucleon. It is showed $^{22}$C to have a large matter radius of $5.9 \pm 0.9$~fm, which is much larger than the ones of carbon isotopes with $N \leq 14$, suggesting $^{22}$C is the halo nucleus. This reported radius has a large uncertainty due to a lack of statistics. To deduce a more accurate matter radius of $^{22}$C, the measurement of reaction cross section with higher statistics at a higher beam energy are required. The experiment was performed by using the SAMURAI spectrometer at RIBF. The $^{22}$C beam at 240~MeV/nucleon was impinged on a carbon target, and the reaction product was identified by using SAMURAI spectrometer. In the present talk, the extracted reaction cross section and derived matter density distribution of $^{22}$C will be presented.   

Spectroscopy of single-particle states in oxygen unstable isotopes via (pol p,2p) reaction

The spin-orbit coupling has an important part in building the nuclear structure. It is so strong in nuclei, unlike atomic system, that it can shuffle the level sequences with different principal quantum numbers, or in different major shells. Our goal is to elucidate how the spin-orbit splitting, which is a direct measure of the strength of the spin-orbit coupling and is defined as a single-particle energy difference of spin-doublet, changes as a function of nucleon number in oxygen isotopes. To this end, the ($\vec{p}$,2$p$) reaction is an effective spectroscopic tool because of its property of selectively populating single-hole states. We performed $^{14,22-24}$O($\vec{p}$,2$p$) reaction experiment with a polarized proton target at RIKEN RIBF to measure single-particle spectra and to determine the spin-orbit splitting in $^{14,22-24}$O. The beam consisting of unstable isotopes of oxygen at $\sim$250 A MeV bombarded the solid polarized proton target. Two scattered protons were detected by using a pair of detector units consisting of a plastic scintillator and a multi-wire drift chamber. Excitation energy for the residual nitrogen was obtained from the energies and the scattering angles of the protons. The details of the experiment and its result will be reported.