Sunflower - In-beam gamma-ray spectroscopy at the RIBF

MINOS (acronym for MagIc Numbers Off Stability) is a device dedicated to perform in-flight gamma spectroscopy of very exotic nuclei in knockout reactions. It consists of a thick liquid hydrogen target (15-20 cm) surrounded by a TPC acting as a tracking detector. The vertex position is reconstructed from the direction of the emitted protons and the beam. In this way one can profit of the increase of luminosity (up to one order of magnitude) due to the thick target without losing resolution in the Doppler correction, as would occur if the vertex position in the target was not measured. The MINOS device has been assembled and tested at CEA Saclay. An in-beam test is scheduled in October at the HIMAC facility in Chiba, Japan. It will be ready since the beginning of 2014 to perform experiments at the RIBF facility coupled with the DALI2 gamma array and the SAMURAI or ZeroDegree spectrometer. It is expected to allow the spectroscopy of key nuclei for the question of the evolution of magic numbers (e.g. 78Ni and 40Mg) that nowadays would be hard (if not impossible) to reach without such a thick target. The device will be described and the status of the project will be presented.

In 2012, Coulomb excitation and secondary fragmentation experiments using radioactive ion beams at relativistic energies have been performed for the first time with the new PreSPEC-AGATA setup. PreSPEC-AGATA is a unique combination of the FRagment Separator(FRS), used for providing and selecting specific radioactive ion beams, the Lund-York-Cologne CAlorimeter (LYCCA), which discriminates heavy ions produced in nuclear reactions taking place in a secondary target, the HECTOR+ (LaBr3/BaF2) array and the Advanced Gamma Tracking Array(AGATA), for the precise measurement of gamma-ray energies. The first campaign was dedicated to the questions of how the collectivity is build-up from single particle excitations and how it evolves away from magic nuclei. Excitation probabilities of the first excited states in nuclei south-west of 208Pb were measured, including heavy Pb, Hg and Pt isotopes. The level scheme of 52Fe,nucleus with only two valence proton and neutron holes in the doubly magic 56Ni, shows rotational behavior for the low spin states. The nucleus was not only populated in its ground state but also in the 12+ isomeric excited state, which was Coulomb excited using a gold target. For 64Fe the Pygmy dipole resonance was studied, which probes the properties of neutron skin. Finally, neutron rich Zr isotopes were excited to determine their shape evolution. An overview of the 2012 physics campaign and the planed PreSPEC-AGATA experiments for 2014 will be presented.

The study of neutron-rich Ni isotopes, towards 78Ni, is one of the main topics in nuclear physics with exotic beams. In fact, the persistency of the N=50 shell gap far from stability is still an open question with many profound implications on the study of the nuclear structure. The availability of radioactive beams has provided the opportunity to study such neutron-rich Ni isotopes with the in-flight separation technique. Results have been obtained for 70Ni with coulex at GANIL (LISE facility) [1], and for 72Ni with the plunger technique at MSU, although the experiment is still under analysis. For 74Ni a p,p’ measurement was performed at MSU [2], providing information on its nuclear deformation. In this talk we will report the preliminary results on the coulex of 73,74,75Ni, performed at Riken. The isotopes of interest were obtained from the fission of a primary 238U beam, at an energy of 345 MeV/u and an intensity of about 8-9 pnA. The separated secondary beam impinged on a 208Pb target at the centre of the DALI array, made of 186 scintillators for gamma detection. The first excited states in 73,74,75Ni were observed, allowing one to estimate the E2 strength from the ground state. The experiment was run in parallel with another experiment of the EURICA stopped beam campaign, aimed at studying neutron-rich nuclei below Z=28 via decay spectroscopy. [1] O. Perru et al., Phys. Rev. C96, 232501 (2006) [2] N. Aoi et Al., Phys. Lett. B692, 302 (2010)

The structure and nature of the pygmy dipole resonance (PDR) state below and above the neutron threshold in exotic nuclei is still far away from beeing understood. Recent experimental observations give only few information on this subject. New and combined experiments using different methods are needed. The presentation we will give information on how relativistic coulomb excitation combined with gamma ray detectors could help to measure the PDR.

This letter of intent is aiming to investigate the collective nature of low lying Octupole states, in the region of the N=Z 112Ba, by measuring the octupole deformation and the neutron and proton contribution to the octupole excitation by means of the isovector (p,p’) and isoscalar (d,d’) reactions, in inverse-kinematics, of secondary BigRIPS beams in the target position of the DALI2 - BigRIPS/ZeroDegree setup. The goal is to investigate the nature of the enhanced octupoles located in the vicinity of the N=Z line close to 112Ba and in particular of the 114Xe octupole state, the lightest one with sufficient production yield

The present proposal aims to exploit the unique capabilities of BigRIPS in delivering Pb beams from 238U fragments at RIKEN. Coulomb excitation studies of fast radioactive beams will be extended to very neutron-deficient Pb nuclei and to excited states inaccessible with other methods. The measurements of B(E2) values to the 2+ states in the even-mass 184-188Pb nuclei in Coulomb excitation experiments are discussed. The proposed studies will be carried out with 345 AMev primary 238U beam. The Pb fragments will be separated with BigRIPS and the Coulomb excitation gamma-ray yield will be recorded with DALI2 at the F7 focal plane of Big RIPS. The proposed measurements will be unique to RIKEN and complementary to the lifetime measurements at JYFL and low-energy Coulomb excitation measurements at REX-ISOLDE providing stringent experimental constraints to the related theory development.

We will propose an experiment to investigate proton shell evolution towards 78Ni by means of in-beam gamma-ray spectroscopy with MINOS at RIBF. The goal of the experiment is to characterize a proton f7/2 hole states in the Cu isotopes populated by one-proton knockout reaction: (p,2p). This will allow us to understand a migration of shell structure induced by the tensor part of the nucleon-nucleon interaction. In the workshop, a physics motivation and feasibility for MINOS@RIBF campaign will be discussed.

Nuclei with two-proton and two-neutron holes and particles will be studied in order to get a better understanding of the proton-neutron interactions. The nuclei and properties aimed at include: (i) around 56Ni: 56Zn (2+ energy), 58,60Zn (B(E2) transition strength), and (ii) 82Zn (2+ energy and possibly B(E2), 80Zn (B(E2)) and possibly 74Fe. The nuclei around 56Ni and 78Ni will be populated using the primary beams 78Kr and 238U, respectively.