Tohru Motobayashi

Identification of 45 New Neutron-Rich Isotopes Produced by In-Flight Fission of a 238U Beam at 345 MeV/nucleon

A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out at the RI Beam Factory at the RIKEN Nishina Center. Fission fragments were analyzed and identi...

MEASUREMENT OF THE COULOMB DISSOCIATION OF 8B AT 254 MEV/NUCLEON AND THE 8B SOLAR NEUTRINO FLUX

We have measured the Coulomb dissociation of 8B into 7Be and proton at 254 MeV/nucleon using a large-acceptance focusing spectrometer. The astrophysical S17 factor for the 7Be(p,gamma)8B reaction at E{c.m.} = 0.25-2.78 MeV is deduced yielding S17(0)=20.6 \pm 1.2 (exp.) \pm 1.0 (theo.) eV-b. This result agrees with the presently adopted zero-energy S17 factor obtained in direct-reaction measurements and with the results of other Coulomb-dissociation studies performed at 46.5 and 51.2 MeV/nucleon.

Evidence for a new nuclear /`magic number/' from the level structure of 54Ca

Atomic nuclei are finite quantum systems composed of two distinct types of fermion—protons and neutrons. In a manner similar to that of electrons orbiting in an atom, protons and neutrons in a nucleus form shell structures. In the case of stable, naturally occurring nuclei, large energy gaps exist between shells that fill completely when the proton or neutron number is equal to 2, 8, 20, 28, 50, 82 or 126 (ref. 1). Away from stability, however, these so-called ‘magic numbers’ are known to evolve in systems with a large imbalance of protons and neutrons. Although some of the standard shell closures can disappear, new ones are known to appear. Studies aiming to identify and understand such behaviour are of major importance in the field of experimental and theoretical nuclear physics. Here we report a spectroscopic study of the neutron-rich nucleus 54Ca (a bound system composed of 20 protons and 34 neutrons) using proton knockout reactions involving fast radioactive projectiles. The results highlight the doubly magic nature of 54Ca and provide direct experimental evidence for the onset of a sizable subshell closure at neutron number 34 in isotopes far from stability.

Identification of New Isotopes 125 Pd and 126 Pd produced by In-flight Fission of 345 MeV/nucleon 238 U: First Results from the RIKEN RI Beam Factory

A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out in the commissioning experiment of the next-generation in-flight radioactive isotope beam separator BigRIPS at the RI Beam Factory at the RIKEN Nishina Center. Two neutron-rich palladium isotopes 125 Pd and 126 Pd were observed for the first time, which demonstrates the great potential of the RIKEN RI beam factory.

Isomeric 0+ state in 12Be

An isomeric 0+ state at E x = 2.24 MeV in 12Be was found in the projectile fragmentation of 18O at 100 A MeV on a Be target. The excitation energy and the spin assignment were deduced by the measurements of the energies and the angular correlation function of the coincident two γ-rays corresponding to the 0+ → 2+ → 0+ decay, respectively.

Determination of the astrophysical S factor of the 8B(p,γ)9C capture reaction from the d(8B,9C)n reaction

The asymptotic normalization coefficients for the virtual decay 9 C → 8 B + p have been determined by measuring the crosssection of the 8 B(d, n) 9 C reaction in inverse kinematics at 14.4 MeV/u, using the RIPS facility. The deduced astrophysical S factor S18 of the 8 B(p ,γ ) 9 C capture reaction in the center of mass energy range 1–100 keV is S18 = 45 ± 13 eV b.  2001

Extension of the N = 40 Island of Inversion towards N =50: Spectroscopy of 66Cr,70,72Fe

We report on the measurement of the first 2(+) and 4(+) states of (66)Cr and (70,72)Fe via in-beam γ-ray spectroscopy. The nuclei of interest were produced by (p,2p) reactions at incident energies of 260  MeV/nucleon. The experiment was performed at the Radioactive Isotope Beam Factory, RIKEN, using the DALI 2γ-ray detector array and the novel MINOS device, a thick liquid hydrogen target combined with a vertex tracker. A low-energy plateau of 2(1)(+) and 4(1)(+) energies as a function of the neutron number was observed for N≥38 and N≥40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modified Lenzi-Nowacki-Poves-Sieja (LNPS) interaction in the pfg(9/2)d(5/2) valence space reproduce the observations. Interpretation within the shell model shows an extension of the island of inversion at N=40 for more neutron-rich isotopes towards N=50.

Superconducting Dipole Magnet for SAMURAI Spectrometer

A superconducting dipole magnet for a large-acceptance spectrometer named SAMURAI has been constructed and installed at the RIKEN RI Beam Factory. The important features of the SAMURAI superconducting dipole magnet are a large pole gap, a wide horizontal opening, and a large momentum bite. The magnet is an H-type dipole, having circular superconducting coils and cylindrical pole pieces with a diameter of 2 m and a pole gap of 880 mm. The coils are orderly wound by the wet winding method developed by Toshiba using a Nb/Ti superconducting wire. The upper and lower coils are installed in two separate cryostats and cooled by the liquid helium bath cooling method. Each cryostat has six cryocoolers: one for a coil vessel at 4 K, four for thermal shields, and one for high- TC superconducting power leads. The size of the iron yoke is 6.7 m wide, 3.5 m deep, 4.64 m tall, and the total weight of the magnet is about 650 tons. The maximum magnetic field is 3.08 T at 563 A (1.922 MA turns/coil), which gives a bending power (field integral) of 7.05 Tm. The maximum stored energy amounts to 27.4 MJ and the inductance varies from 396 H to 150 H as the magnetic field increases. The fringe fields are smaller than 5 mT at 0.5 m from the magnet. The construction of the SAMURAI magnet started in 2008 and was completed in June 2011. The commissioning of the SAMURAI spectrometer was successfully performed using RI beams in March 2012.

Indirect measurements of the solar-neutrino production reaction 7Be(p,γ)8B

Abstract Low-energy cross sections of the 7Be(p ,γ )8B reaction is of crucial importance, because they determine the high-energy solar neutrino flux. Coulomb dissociation with intermediate-energy 8B beams and low-energy proton-transfer reactions with 7Be beams have been investigated to determine indirectly the astrophysical S 17 factors of the 7Be(p ,γ )8B reaction. The results of these studies are generally in good agreement with the ones obtained by direct capture measurements using intense proton beams and radioactive 7Be targets.

Study of the 7Be(p, γ)8B Reaction with the Coulomb Dissociation Method

The Coulomb-dissociation cross section for the process B-8 --> Be-7 + p in the field of Pb-208 was measured at E(B-8) = 46.5 A MeV. The Be-7(p,gamma)B-8 cross section relevant to the high-energy B-8 solar-neutrino production was extracted at E(C.M.) = 0.6 - 1.7 MeV. Two theoretical models on the structure of 8B were used to evaluate possible corrections due to M1 and E2 admixtures to the dominant E1 cross sections. Their overall effect is shown to be small regardless of the choice of the models. The present result is consistent with the results of the direct measurements of Filippone et al. and Vaughn et al. within errors. This result suggests further studies with the Coulomb-dissociation method to extract astrophysical S-17 factor at similar to 20 keV relevant to the high-energy solar-neutrino flux.