Session 1WB: Workshop on Physics Opportunities with GRETINA I

Coulomb excitation and transfer reactions to study neutron-rich nuclei

At radioactive beam facilities, such as the Holified facility at ORNL and the future FRIB, neutron-rich fission fragments can be re-accelerated and used for in-beam nuclear structure studies. Two of the most promising reaction mechanisms for such experiments are Coulomb excitation and single-nucleon transfer reactions. When paired with a powerful gamma-ray detector such as GRETINA, these reactions can be used to perform detailed, systematic studies of nuclear collectivity and single-particle structures, and their dependence on neutron and proton numbers. Excited-state lifetimes, magnetic moments, spectroscopic factors, and single-particle energies are all accessible. Examples of some past measurements will be presented, together with an analysis of the reach of these techniques when used with GRETINA and GRETA. Some of the advantages of GRETINA for these experiments, such as energy resolution and high efficiency, will be discussed.   

Overview of RIBF and direct reactions with fast RI beams at RIBF

New-generation RI beam facilities such as RI Beam Factory (RIBF) will drastically enlarge the experimental opportunities to study the structure of unstable nuclei. In this field, direct-reaction measurements by the detection of the de-excitation $\gamma $ rays are the indispensable tools to probe the single particle and collective nature of the low-lying states. First, overview of RIBF and the BigRIPS Separator recently commissioned will be presented. Secondly, the experimental cases at RIBF with existing $\gamma $-ray detector arrays and advanced $\gamma $ -ray detector arrays will be discussed.   

Evolution of shell and nuclear structure in the neutron rich region

Evolution of shell structure in exotic nuclei has attracted significant interest both theoretically and experimentally. Recently, roles of the nuclear force in the evolution of shell structure have been clarified. Among them the tensor force has a unique feature that it reduces strongly the spin-orbit splitting at $j-j$ closed nuclei. This accounts for the disappearance of the $N=20$ magic number and appearance of a new $N=16$ magic number in exotic nuclei. Hence, it is of great interest whether an effective interaction based on the picture can describe a wide range of the nuclear chart in a natural way. In this talk, I will present how the shell evolution occurs far from stability and how its effect emerges in a nucleus as a many-body system based on the shell model. In particular, the structure in the $N=28$ region with a newly developed effective interaction is focused. With a natural choice of the tensor force and the central force, some exotic properties for $N=28$ nuclei are described such as a large deformation in $^{42}$Si and quenching of the proton's spin-orbit splitting at $^{48}$Ca probed by the distribution of spectroscopic factors. The present interaction predicts another interesting phenomenon beyond $N=28$: the shell gap does not change monotonically as a potential picture gives. The way of constructing the interaction is simple but highly predictive. By using that, the evolution of shell structure in heavier regions will also be discussed.   

Precision measurements of light neutron-rich nuclei

New {\it ab-initio} approaches starting from bare two-nucleon potentials and empirical three-nucleon potentials have been quite successful in reproducing many features of light nuclei. These sophisticated methods are constantly developing and precise measurements are necessary to challenge and refine the calculations. An improved Doppler Shift Attenuation method (DSAM) to measure lifetimes of bound states in light nuclei will be presented. Key aspects essential for achieving $<$5$\% $ accuracy with the DSAM include careful selection of the kinematic conditions for producing the states of interest at high velocities where the stopping powers are well categorized, control of feeding from higher levels, and advancements in $\gamma$-ray detection. The latter aspect will be discussed in terms of the advantages of using GRETINA to make measurements of high energy $\gamma$ rays from nuclei moving at high recoil velocities (v/c $\sim$ 10$\%$). The technique will be outlined for $^{10}$Be and the prospects/challenges of moving to more neutron-rich systems will be discussed.   

Nuclear astrophysics with tracking arrays

Efficient Gamma-ray tracking arrays enable a range of experiments that address open questions in nuclear astrophysics. I will give an overview of the types of measurements a next generation array would make possible, and the astrophysical problems these measurements can address.   

Spectroscopy of r-process nuclei using multi-nucleon transfer reaction

The $\beta $-decay properties of the neutron-rich isotopes with neutron number N = 126, as progenitors on the r-process path forming the third peak (A $\sim $ 195) in the r-abundance element distribution, are supposed to play a critical role for better understanding where the heavy elements such as gold and platinum were made. We will discuss our experimental proposal to study $\beta $-decay properties and nuclear structures around N = 126 nuclei. These isotopes could be obtained via the multi-nucleon transfer reactions induced by low-energy intense neutron-rich radioactive ion beams, such as $^{140}$Xe and/or $^{144}$Xe generated by the facility based on the ISOL and post-acceleration scheme. Since there has not been the facilities in the world yet, as the first step, we are going to produce $^{202}$Os (Z = 76, N = 126), which has not been produced in any other facilities, by using the multi-nucleon transfer reactions in $^{136}$Xe (stable beam) + $^{198}$Pt (target) collision. The Pt target ($\sim $5 $\mu $m) is used as the window of the gas catcher for collecting all reaction products by the collision, from which the $^{202}$Os will be extracted as singly-charged ions by laser resonance ionization and transferred to a detection chamber after being mass-separated. In this way, both the element (atomic number Z) and mass (A) separations could be possible, allowing study the products of rare reaction channels. The mass separated isotopes are implanted into a tape transport system, which allows us to measure their beta-decays by multi-layered plastic scintillation detectors and germanium detectors of GRETINA.