C. S. Lee

Candidate Resonant Tetraneutron State Populated by the He 4 (He 8, Be 8) Reaction

A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ^{4}He(^{8}He,^{8}Be) at 186  MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst)  MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the (^{8}He,^{8}Be) reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

Spin-Isospin Response of the Neutron-Rich Nucleus 8He via the \((p,n)\) Reaction in Inverse Kinematics

A systematic study of the nucleon density distributions of finite nuclei is important for understanding the isospin dependence of the nuclear many-body system. For example, the neutron skin thicknesses are known to be strongly correlated with the density-dependent term of the symmetry energy, and the isospin-dependent term of the nuclear equation of state. In previous works, we have succeeded in extracting the neutron density distributions of stable nuclei such as Sn and Pb isotopes via proton elastic scattering. At the same time we have launched a new project aimed at measuring the Elastic Scattering of Protons with RI beams (ESPRI) with the purpose of the extraction of proton and neutron density distributions of unstable nuclei. We have newly developed and tested unique devices for the ESPRI measurements at NIRSHIMAC in Chiba and GSI in Germany. Finally, we have successfully performed the ESPRI experiment at RIBF in April, 2013. Unlike the case of stable nuclei, however, we have no information on the nuclear charge densities of unstable nuclei. Thus, we have proposed a new method to extract proton and neutron densities via two-energy proton elastic scattering. This method is based on the large difference between the energy dependences of the p-p and p-n interactions. Recently, we performed an experiment to demonstrate this new method using real data of Zr isotopes. In this report, we show the preliminary results of Zr only. For other isotopes (Zr), the analysis is still ongoing. The experiment was performed at RCNP, Osaka University. Polarized proton elastic scattering from Zr and Ni at 200 and 300 MeV was measured by using the Grand Raiden magnetic spectrometer. Figure 1 shows the angular distributions of cross sections (dσ/dΩ) and the analyzing powers (Ay) of Zr(p⃗,p) at 200 and 300 MeV and of Ni(p⃗,p) at 200 MeV. The red lines denote the result of relativistic impulse approximation (RIA) with relativistic-Hartree (RH) densities. The Ni data was used to determine the effective interaction at 200 MeV (solid lines). Using the effective interaction, the proton and neutron densities of Zr were simultaneously searched and the results are denoted by the solid lines in Fig. 1 and 2. The

Complete Set of Deuteron Analyzing Powers for dp Elastic Scattering at Intermediate Energies and Three Nucleon Forces

Few-nucleon scattering at intermediate energies (E/A ∼ 200 MeV) is one attractive approach to investigate the dynamical aspects of 3NFs, such as momentum and/or spin dependences. Direct comparison between the data and the rigorous numerical calculations based on bare nuclear potentials provides information of 3NFs. So far large 3NF effects are theoretically predicted and experimentally confirmed in the cross section minimum for dp scattering at ∼ 100 MeV/nucleon. With the aim of clarifying roles of the 3NFs in nuclei the experimental programs with polarized deuterons beams at intermediate energies are in progress at RIKEN RI Beam Factory (RIBF) [1]. We have measured all deuteron analyzing powers (iT11, T20, T21, and T22) for deuteron–proton (dp) elastic scattering at 70–300 MeV/nucleon, typically in step of 50 MeV/nucleon. The vector and tensor polarized deuteron beams were accelerated by three cyclotrons, AVF, RRC and SRC. The measurement of deuteron analyzing powers for elastic dp scattering was carried out using the polarimeter BigDpol installed at the extraction beam line of the SRC. The deuteron beams bombarded a polyethylene (CH2) target in the scattering chamber. Scattered deuterons and recoil protons were detected by plastic scintillators in kinematical coincidence conditions. The obtained high precision data are compared with the results of three-nucleon Faddeev calculations based on the nucleon-nucleon (NN) potentials; i.e. CD Bonn, Argonne V18, Nijmegen I, and II, alone or combined with the Tucson-Melbourne’99 and the Urbana IX 3NFs. Large discrepancies between the pure NN theory and the data, which are not resolved even by adding the current 3NFs, are found at the c.m. backward angles for almost all the deuteron analyzing powers with increasing an incident energy. In the conference the comparison between the data and the calculations based on the χ EFT potentials [2] will also be presented.

Measurement of the 8 He( p,n ) 8 Li Reaction at Intermediate Energy in Inverse Kinematics

We performed the first measurement of the He(p, n)Li reaction at a beam energy of 190A MeV in inverse kinematics at the RIKEN RI Beam Factory (RIBF). It is well known that (p, n) measurements at intermediate energies are powerful tools to study spin-isospin responses of nuclei. A facility for (p, n) measurements in inverse kinematics was constructed at RIBF to extend the field of spin-isospin physics to unstable nuclei. The main component of this facility is the neutron detector WINDS, which was recently developed for detection of recoil neutrons with low kinetic energies. A fundamental study on the structure of He is its β-decay, for which the Q-value is 10.7 MeV. In contrast to β-decay, charge-exchange reactions, such as (p, n) reactions, do not suffer from the Q-value limitation and can be used to study transitions to highly-excited states. Figure 1 shows a preliminary spectrum of the He(p, n)Li reaction by tagging the Li isotopes. Two loci are observed in this spectrum. The locus at low excitation energy is most likely the Ex = 0.98 MeV state with B(GT) = 0.24, which has been observed in previous β-decay studies. A large transition strength is found at around Ex ∼ 8 MeV, which is a candidate for the Gamow-Teller state. In the present work, the angular distribution of the differential cross sections will be analyzed to clarify the nature of this state.

Complete set of deuteron analyzing powers for dp elastic scattering at intermediate energies

With the aim of clarifying roles of the 3NFs in nuclei experimental programs with polarized deuterons beams at intermediate energies are in progress at RIKEN RI Beam Factory. As the first step, we have measured a complete set of deuteron analyzing powers (iT11, T20, T21, T22) in deuteron-proton elastic scattering at 250 and 294 MeV/nucleon. The obtained data are compared with the Faddeev calculations based on the modern nucleon-nucleon forces together with the Tucson-Melbourne’99, and UrbanaIX three nucleon forces.