Heiko Scheit

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.

Well Developed Deformation in 42Si

Excited states in (38,40,42) Si nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of ^{40}S and (44)S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742(8) keV in (42) Si confirms the 2(+) state reported in an earlier study. Among the γ lines observed in coincidence with the 2^{+} → 0+ transition, the most probable candidate for the transition from the yrast 4(+) state was identified, leading to a 4(1)+) energy of 2173(14) keV. The energy ratio of 2.93(5) between the 2(1)+ and 4(1)(+) states indicates well-developed deformation in (42) Si at N = 28 and Z = 14. Also for 38,40)Si energy ratios with values of 2.09(5) and 2.56(5) were obtained. Together with the ratio for (42)Si, the results show a rapid deformation development of Si isotopes from N = 24 to N = 28.

Exploring the "Island of Inversion" by in-beam γ-ray spectroscopy of the neutron-rich sodium isotopes 31,32,33 Na

The structure of the neutron-rich sodium isotopes {sup 31,32,33}Na was investigated by means of in-beam gamma-ray spectroscopy following one-neutron knockout and inelastic scattering of radioactive beams provided by the RIKEN Radioactive Ion Beam Factory. The secondary beams were selected and separated by the fragment separator BigRIPS and incident at approx =240 MeV/nucleon on a natural carbon (secondary) target, which was surrounded by the DALI2 array to detect coincident de-excitation gamma rays. Scattered particles were identified by the spectrometer ZeroDegree. In {sup 31}Na, a new decay gamma ray was observed in coincidence with the known (5/2{sub 1}{sup +})->3/2{sub g.s.}{sup (+)} transition, while for {sup 32,33}Na excited states are reported for the first time. From a comparison to state-of-the-art shell-model calculations it is concluded that the newly observed excited state in {sup 31}Na belongs to a rotational band formed by a 2p2h intruder configuration within the Island of Inversion.

Collectivity evolution in the neutron-rich Pd isotopes toward the N=82 shell closure

The neutron-rich, even-even 122,124,126Pd isotopes has been studied via in-beam gamma-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Excited states at 499(9), 590(11), and 686(17) keV were found in the three isotopes, which we assign to the respective 2+ -> 0+ decays. In addition, a candidate for the 4+ state at 1164(20) keV was observed in 122Pd. The resulting Ex(2+) systematics are essentially similar to those of the Xe (Z=54) isotopic chain and theoretical prediction by IBM-2, suggesting no serious shell quenching in the Pd isotopes in the vicinity of N=82.

Bremsstrahlung inαDecay Reexamined

A high-statistics measurement of bremsstrahlung emitted in the alpha decay of (210)Po has been performed, which allows us to follow the photon spectra up to energies of approximately 500 keV. The measured differential emission probability is in good agreement with our theoretical results obtained within the quasiclassical approximation as well as with the exact quantum mechanical calculation. It is shown that, due to the small effective electric dipole charge of the radiating system, a significant interference between the electric dipole and quadrupole contributions occurs, which is altering substantially the angular correlation between the alpha particle and the emitted photon.

Inelastic Scattering of32Mg at 190 MeV/Nucleon from a Thick Proton Target

The first excited 2+ state (885 keV) of 32Mg and its corresponding deformation parameter β2 are studied with proton inelastic scattering at 190 MeV/nucleon using in-beam γ technology with a thick target. Angular distributions of the inelastic scattering cross sections are analyzed with coupled-channel calculations. The deformation parameter β2 = 0.41(3) obtained in this work agrees with the earlier experimental results obtained at lower beam energies. Our work demonstrates that the structure of unstable nuclei can be studied with proton inelastic scattering at high incident energies in the vicinity of 200 MeV/nucleon using thick targets.

On the analysis of intermediate–energy Coulomb excitation experiments

In a recent publication [C.A. Bertulani, G. Cardella, M. De Napoli, G. Raciti, E. Rapisarda, Phys. Lett. B 650 (2007) 233] the validity of analysis methods used for intermediate-energy Coulomb excitation experiments was called into question. Applying a refined theory large corrections of results in the literature seemed needed. We show that this is not the case and that the large deviations observed in above mentioned reference are due to the use of the wrong experimental parameters in that publication. We furthermore show that an approximate expression derived in above mentioned reference is in fact equivalent to the theory of Winther and Alder, an analysis method often used in the literature.

Exploring the Southern Boundaries of the “Island of Inversion” at the RIBF

The nuclear structure of neutron rich Ne isotopes has been investigated at the recently commissioned Radioactive Ion Beam Factory (RIBF) at the RIKEN Nishina Center by means of inelastic scattering and knockout reactions. A cocktail beam of unstable nuclei around 32Ne was provided by the BigRIPS fragment separator and incident on a natural carbon target at energies [approximate]220 A MeV. Reaction products were identified with the Zero Degree Spectrometer and de-excitation gamma-rays detected by the DALI2 array. The reported results demonstrate the potential for in-beam gamma-ray experiments at the RIBF.

Population Of Neutron Unbound States Via Two-Proton Knockout Reactions

The two-proton knockout reaction 9Be(26Ne,O2p) was used to explore excited unbound states of 23O and 24O. In 23O a state at an excitation energy of 2.79(13) MeV was observed. There was no conclusive evidence for the population of excited states in 24O.

Coulomb excitation of 68/28Ni40 at "safe" energies

The B(E2;0+2+) value in 68Ni has been measured using Coulomb excitation at safe energies. The 68Ni radioactive beam was postaccelerated at the CERN on-line isotope mass separator (ISOLDE) facility to 2.9 MeV/u and directed to a 108Pd target. The emitted rays were detected by the MINIBALL detector array. Not only directly registered but also indirectly deduced information on the nucleus emitting the ray was used to perform the Doppler correction, leading to a larger center-of-mass angular range to infer the excitation cross section. The obtained value of 2.8×102e2 fm4 is in good agreement with the value measured at intermediate energy Coulomb excitation, confirming the low 0+2+ transition probability.