A. Gillibert

SPEG: AN ENERGY LOSS SPECTROMETER FOR GANIL

Abstract Since July 1985, an energy loss spectrometer (SPEG) is under operation at the National Heavy Ion Laboratory (GANIL), at Caen (France). It has been designed to allow the study of quantum states populated in reactions induced by nuclei accelerated at energies up to 100 A MeV. The spectrometer has been designed by P. Birien. The optical properties and the main magnetic features have been calculated by Birien and Valero. A detailed reported of their study is given in ref. [1]. In the first part of the present paper, after recalling the specifications of the spectrometer, we shall give an overall description of the main characteristics, together with indications about the various shimming procedures which have been used to achieve the desired resolution (sections 1–4). In the second part, we shall describe various accessories and the different kinds of detectors which are used during experiments, with several illustrations of experimental results (sections 5 and 6).

New mass measurements of neutron-rich nuclei near N= 20

Abstract The masses of 39 neutron-rich nuclei in the mass range 17–37 have been measured using a direct time-of-flight technique following the fragmentation of a 48 Ca beam at 55 MeV/nucleon. The masses of 29,30 Ne, 34,35 Mg and 36,37 Al are reported for the first time. The very neutron-rich nuclei, 31–33 Na, are found to be 2–4 MeV less bound than previously believed. A comparison is made with recently available large scale shell model calculations encompassing the deformed A ∼32 nuclei. Conclusions are drawn regarding the extent of the region of deformation, which is found to include 30 Ne.

No enhancement of fusion probability by the neutron halo of 6He

Quantum tunnelling through a potential barrier (such as occurs in nuclear fusion) is very sensitive to the detailed structure of the system and its intrinsic degrees of freedom. A strong increase of the fusion probability has been observed for heavy deformed nuclei. In light exotic nuclei such as 6He, 11Li and 11Be (termed ‘halo’ nuclei), the neutron matter extends much further than the usual nuclear interaction scale. However, understanding the effect of the neutron halo on fusion has been controversial—it could induce a large enhancement of fusion, but alternatively the weak binding energy of the nuclei could inhibit the process. Other reaction channels known as direct processes (usually negligible for ordinary nuclei) are also important: for example, a fragment of the halo nucleus could transfer to the target nucleus through a diminished potential barrier. Here we study the reactions of the halo nucleus 6He with a 238U target, at energies near the fusion barrier. Most of these reactions lead to fission of the system, which we use as an experimental signature to identify the contribution of the fusion and transfer channels to the total cross-section. At energies below the fusion barrier, we find no evidence for a substantial enhancement of fusion. Rather, the (large) fission yield is due to a two-neutron transfer reaction, with other direct processes possibly also involved.

Proton elastic scattering on light neutron-rich nuclei

Abstract Proton-nucleus elastic scattering angular distributions have been measured for 6 He, 7 Li, 10 Be, 11 Be secondary beams. These data, together with other elastic scattering data for proton on unstable nuclei, have been analyzed using phenomenological and microscopic optical model approaches. To reproduce the experimental angular distributions for unstable nuclei, it was necessary to renormalize either the real or imaginary part of the optical potential. The influence of the break-up channels on the results is discussed.

Direct mass measurements of 19 B, 22 C, 29 F, 31 Ne, 34 Na and other light exotic nuclei

We report on direct time-of-flight based mass measurements of 16 light neutron-rich nuclei. These include the first determination of the masses of the Borromean drip-line nuclei (19)B, (22)C, and (29)F as well as that of (34)Na. In addition, the most precise determinations to date for (23)N and (31)Ne are reported. Coupled with recent interaction cross-section measurements, the present results support the occurrence of a two-neutron halo in (22)C, with a dominant ν2s(1/2)(2) configuration, and a single-neutron halo in (31)Ne with the valence neutron occupying predominantly the 2p(3/2) orbital. Despite a very low two-neutron separation energy the development of a halo in (19)B is hindered by the 1d(5/2)(2) character of the valence neutrons.

Direct Mass Measurements ofB19,C22,F29,Ne31,Na34and Other Light Exotic Nuclei

We report on direct time-of-flight based mass measurements of 16 light neutron-rich nuclei. These include the first determination of the masses of the Borromean drip-line nuclei (19)B, (22)C, and (29)F as well as that of (34)Na. In addition, the most precise determinations to date for (23)N and (31)Ne are reported. Coupled with recent interaction cross-section measurements, the present results support the occurrence of a two-neutron halo in (22)C, with a dominant ν2s(1/2)(2) configuration, and a single-neutron halo in (31)Ne with the valence neutron occupying predominantly the 2p(3/2) orbital. Despite a very low two-neutron separation energy the development of a halo in (19)B is hindered by the 1d(5/2)(2) character of the valence neutrons.

The elastic scattering of 6Li+28Si at near-barrier energies

The 6 Li + 28 Si elastic scattering was studied at near-barrier energies with the aim to probe the threshold anomaly. Angular distributions were measured over a wide angular range (θlab = 25 ◦ to 150 ◦ ) at 4 energies, namely, 7.5, 9, 11 and 13 MeV. The present data together with previous ones at higher energies, as well as elastic scattering data of 6 Li on various targets at near barrier energies, were analyzed by using optical potentials obtained in a double-folding framework. It was found that the strength of the real part of the potential remains almost independent of energy down to, and possible even below, barrier, while the strength of the imaginary part presents an increase at near barrier energies. The results are discussed.  2003 Elsevier Science B.V. All rights reserved.

Coupling effects in the elastic scattering of the exotic nucleus 6He on protons

Abstract Cross sections for the elastic scattering of 6He radioactive nuclear beam on proton targets have been measured at 38.3 MeV/nucleon. With a view to test the ability of general optical potentials to reproduce the data for scattering of unstable nuclei, the present results, as well as other existing data for 6,8He, have been analyzed within the framework of the microscopic Jeukenne–Lejeune–Mahaux nucleon–nucleus potential. The angular distributions were found to be best reproduced by reducing the real part of the optical potential. This renormalization can be seen as a consequence of the complex polarization potential produced by the coupling to the continuum due to the weakly bound nature of the unstable nuclei. This effect can be simulated in a phenomenological analysis by a surface potential.

EXCITATION OF GIANT-RESONANCES IN PB-208, SN-120, ZR-90 AND NI-60 BY 84 MEV/NUCLEON O-17 IONS

Abstract Elastic and inelastic scattering of 1435 MeV 17O ions on 208Pb, 120Sn, 90Zr and 60Ni have been measured. Hindrance in the excitation of the first 3− states is observed. Parameters of the isoscalar giant monopole and quadrupole resonances are obtained. The quadrupole resonance exhausts ∼ 55% of the energy-weighted sum rule while the observed monopole resonance corresponds to more than 100% of the sum rule. Strength is also observed at high excitation energy in all targets.

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.