In-beam γ spectroscopy using DIC with a radioactive Ne beam

A study of deep-inelastic and multi-nucleon transfer reactions to populate neutron-rich O, Ne and F nuclei is here presented. The reaction under analysis employed a beam of radioactive ²⁴Ne at 7.9 AMeV, provided by the SPIRAL facility at Ganil, impingin on a ²⁰⁸Pb target. The reaction products have been detected in the VAMOS spectrometer in coincidence with gamma rays measured by the EXOGAM array. Preliminary results here presented show a selectivity in the population of states of different nature.     

Two-neutron elastic transfer 4He(6He,4He)6He at E = 151 MeV

Using the framework of the coupled reaction channels (CRC) the elastic scattering and the elastic transfer in the system ⁶He + ⁴He measured at E = 151 MeV have been analysed. It is shown that the structure observed in the backward range of the angular distributions is influenced by the interference of the elastic 2n-transfer with a two-step process passing through the 2⁺ excitation in ⁶He. The two-neutron transfer mechanism is studied in the microscopic approach and it is found that for the ground-state transition the one step dominates by a factor 10 over the two-step mechanism at this energy. Received: 29 October 2001 / Accepted: 4 December 2001     

Spectroscopy of nuclei far from stability at GANIL: Recent experiments

The structure of neutron-rich light nuclei around N = 20, 28 has been investigated at GANIL by means of in-beam gamma spectroscopy using fragmentation reactions of ³⁶S and ⁴⁸Ca beams on a Be target. Gamma decay of relatively high-lying excited states have been measured for the first time in nuclei around ³²Mg and ⁴⁴S. Level schemes are proposed and discussed for a large number of these neutron-rich nuclei around N = 20 and N = 28. Received: 1 May 2001 / Accepted: 4 December 2001     

Coulomb breakup of 8B and the flux of 8B neutrinos from the Sun

A kinematically complete measurement was made of the Coulomb dissociation of ⁸B nuclei on a Pb target at 83 MeV/nucleon. The cross-section was measured at low relative energies in order to infer the astrophysical S-factor for the ⁷Be(p,γ)⁸B reaction. A first-order perturbation theory analysis of the reaction dynamics including E1, E2, and M1 transitions was employed to extract the E1 strength relevant to neutrino-producing reactions in the solar interior. By fitting the measured cross-section from E _rel = 130 keV to 400 keV, we find S ₁₇(0) = 17.8^+1.4 _-1.2 eV b. Semiclassical 1st-order perturbation theory and fully quantum-mechanical continuum-discretized coupled-channels analyses yield nearly identical results for the E1 strength relevant to solar-neutrino flux calculations, suggesting that theoretical reaction mechanism uncertainties need not limit the precision of Coulomb-breakup determinations of the ⁷Be(p,γ)⁸B S-factor. A recommended value of S ₁₇(0) based on a weighted average of this and other measurements is presented. This recommendation implies a revised value for the theoretical flux of ⁸B solar neutrinos, which is also given.-1 Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"Present address: Kernfysisch Versneller Instituut, Zernikelaan 25, 9747 AA Groningen, The Netherlands; e-mail: davids@kvi.nl     

Weak e+e− lines from internal pair conversion observed in collisions of 238U with heavy nuclei

We present the results of a Doppler-shift correction to the measured e⁺e⁻–sum-energy spectra obtained from e⁺e⁻–coincidence measurements in ²³⁸U +²⁰⁶Pb and ²³⁸U +¹⁸¹Ta collisions at beam energies close to the Coulomb barrier, using an improved experimental setup at the double-Orange spectrometer of GSI. Internal-Pair-Conversion (IPC) e⁺e⁻ pairs from discrete nuclear transitions of a moving emitter have been observed following Coulomb excitation of the 1.844 MeV (E1) transition in ²⁰⁶Pb and neutron transfer to the 1.770 MeV (M1) transition in ²⁰⁷Pb. In the collision system ²³⁸U +¹⁸¹Ta, IPC transitions were observed from the Ta-like as well as from the U-like nuclei. In all systems the Doppler-shift corrected e⁺e⁻–sum-energy spectra show weak lines at the energies expected from the corresponding γ–ray spectra with cross sections being consistent with the measured excitation cross sections of the γ lines and the theoretically predicted IPC coefficients. No other than IPC e⁺e⁻–sum-energy lines were found in the measured spectra. The transfer cross sections show a strong dependence on the distance of closest approach (R_min), thus signaling also a strong dependence on the bombarding energy close to the Coulomb barrier. Received: 22 July 1997 / Revised version: 15 October 1997     

Decay modes of 24Mg excited at 46.4 MeV

⁸Be_gs coincidences with ¹²C, ⁸Be_gs and α-particles produced in the ¹²C +¹²C interaction at 65 MeV have been measured in a wide in-plane angular range. The 3 −⁸Be_gs final state is found to be produced, even if poor statistics prevent any identification of the ¹⁶O states involved in the first stage of the process. The ⁸Be_gs−α and ⁸Be_gs−¹²C coincidence yields are found to be due to ¹²C and ¹⁶O excited states, decaying into the ⁸Be_gs+α and ¹²C +α systems, respectively. Received: 15 June 1998 / Revised version: 4 December 1998     

High-spin states in <Superscript>36</Superscript>Ar and their underlying shell model configurations

Eight high-spin states in ³⁶Ar below 10MeV excitation energy, among them a prospective J ^π = 8^- state at 9408keV and the J? 8 levels of the recently discovered superdeformed rotational band, have been observed by n-γ coincidence measurements with the ³³S(α, nγ) reaction at E _α = 14.4 and 13.4MeV. High-spin assignments of, respectively, J ^π = 6⁺ and 5^- were obtained for the E _p = 1209 and 1462keV (E _x = 9682 and 9927keV) resonances of the ³⁵Cl (p,γ) reaction by a measurement of γ-ray angular distributions. The spectrum of the high-spin and of the E _x? 7.4MeV levels is decomposed according to the underlying shell model configurations with n = 0, 1, 2, 4 particles excited from the N = 2 into the N = 3 major shell. The role of four-particle excitations, all connected with large prolate distortions, is elucidated for the entire A = 36-40 mass region. Received: 21 December 2001 / Accepted: 25 March 2002     

Positron spectra from internal pair conversion observed in 238U + 181Ta collisions

We present new results from measurements and simulations of positron spectra, originating from ²³⁸U + ¹⁸¹Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) processes in the measured e⁺-energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities of the observed γ-transitions ( E _γ∼ 1250-1600 keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines have been observed, is presented and the implications are discussed. Received: 27 April 2000 / Accepted: 14 September 2000     

Physics with heavy neutron-rich RIBs at the HRIBF

The Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory has recently produced the world's first post-accelerated beams of heavy neutron-rich nuclei. The first experiments with these beam are described, and the results discussed. B(E2;0⁺ → 2⁺) values for neutron-rich ^126,128Sn and ^132,134,136Te isotopes have been measured by Coulomb excitation in inverse kinematics. The results for ¹³²Te and ¹³⁴Te (N = 80, 82) show excellent agreement with systematics of lighter Te isotopes, but the B(E2) value for ¹³⁶Te (N = 84) is unexpectedly small. Single-neutron transfer reactions with a ¹³⁴Te beam on ^natBe and ¹³C targets at energies just above the Coulomb barrier have also been studied. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: radford@mail.phy.ornl.gov     

Depopulation of the Jπ = 9- isomer in 180Ta to the Jπ = 1+ ground state by Coulomb excitation

In-beam Coulomb excitation of the exotic odd-odd nucleus ¹⁸⁰Ta has been studied by using a ¹³⁶Xe beam and a setup consisting of five EUROBALL CLUSTER detectors and the Darmstadt-Heidelberg Crystal Ball array. Spectroscopic information on the extremely rare ¹⁸⁰Ta is obtained from the comparison between an enriched (3.6±0.3% ¹⁸⁰Ta) and a natural tantalum target. Possible evidence for a depopulation from the long-lived high-spin J ^π = 9^- isomer to the short-lived J ^π = 1⁺ ground state is searched for by different methods. The decay of low-K bandheads, which are nanosecond isomers, towards the ground-state band can be demonstrated in delayed spectroscopy. A γγ coincidence analysis provides indications of K = 5 in-band transitions. Finally, when the Crystal Ball is used as an energy and γ multiplicity filter, signals of decay into the K = 0 band are observed. Received: 23 January 2001 / Accepted: 8 February 2001     

Further measurement of the 7Be(p,γ)8B cross section at low energies with the coulomb dissociation of 8B

We have measured the dissociation of ⁸B in the Coulomb field of ²⁰⁸Pb at E_in=51.9 MeV/nucleon and extracted the cross section of the ⁷Be(p,γ)⁸B reaction at 0.4 ≤ E_rel≤ 3 MeV, which is of importance for the ⁸B solar-neutrino production rate. The extracted astrophysical S₁₇ factors are consistent with our earlier Coulomb dissociation measurement, and agree with the values deduced from the direct capture measurements by Filippone et al., Vaughn et al. and Hammache et al. The S factor at zero energy was extracted to be 18.9±1.8 eV-b with the help of theoretical energy-dependence. Received: 15 August 1998     

Dynamic polarization potential of 12C+12C system at molecular-resonance energies

The nuclear reaction dynamics leading to the formation of recently discovered resonance in the mutual-0₂ ⁺ channel of the ¹²C+¹²C inelastic scattering around E _c.m.≃ 32 MeV is studied in terms of the dynamic polarization potential (DPP) induced by the channel coupling among various excited states in ¹²C. The microscopic 3α cluster-model wave functions are used to generate the ¹²C−¹²C diagonal and coupling potentials in the double-folding model. It is found that DPP for the 0₂ ⁺+ 0₂ ⁺ channel is an unusually strong attractive potential which even exceeds the zeroth-order folding-model potential of this channel around the nuclear surface region and that the strong coupling between the 0₂ ⁺ and 2₂ ⁺ states is predominantly responsible for the unusual DPP in this channel. The effective potential, the sum of the original folding-model potential and the attractive DPP, is found to generates resonance states in the same energy region as that of the resonance states generated by the original folding-model potential but the former states are found to be higher-nodal states having four additional radial nodes. Similar but more moderate property of DPP is also found in the entrance (elastic) channel. These results suggest that the reaction dynamics of generating the resonance in the ¹²C(0₂ ⁺) +¹²C(0₂ ⁺) channel may rather differ from that of the simple crossing of the zeroth-order molecular band generated by the potentials in the entrance and exit channels suggested by the standard band-crossing model. Received: 17 December 1998 / Revised version: 1 March 1999     

Coulomb dissociation studies for reactions in steady and explosive hydrogen burning

Radiative capture processes in steady and explosive hydrogen burning sometimes involve unstable nuclei. The Coulomb dissociation method has been used to determine the cross-sections of such reactions indirectly with intermediate-energy RI beams. Recent studies on reactions in the pp chain, hot pp chain and CNO cycle are discussed. Received: 1 May 2001 / Accepted: 4 December 2001     

Shrinking  as a recipe for revealing classical-like properties of optical-potential cross-sections

A simple recipe for revealing classical-like properties of optical-potential cross-sections is proposed. The recipe is based on the fact that the classical properties are not expected to depend on the actual value of . This allows us to identify the classical-like characteristics of an optical-potential cross-section by simply repeating the calculation with different values of , and observing which properties of the cross-section are invariant. The method is applied to the cross-sections of a few optical potentials used to describe the recent data of light heavy-ion elastic scattering. An improved near-side/far-side decomposition is used to separate the near-side and far-side components of the optical-potential cross-sections. Received: 4 September 2001 / Accepted: 3 June 2002 / Published online: 26 November 2002 RID="a" ID="a"e-mail: anni@le.infn.it Communicated by V. Vento     

Coulomb excitation of 72 30Zn42

The reduced transition probability B(E2: 0₁ ⁺→ 2⁺) of ⁷²Zn has been measured for the first time by Coulomb excitation at intermediate energy. The result B(E2: 0₁ ⁺→ 2⁺) = 1740±210 e²fm⁴, corresponds to the deformation parameter β₂ of 0.23, in close agreement with expectations derived from the neighboring nucleus ⁷³Zn. A discussion of the evolution of the N = 40 sub-shell closure as a function of Z is presented. Received: 19 December 2001 / Accepted: 14 March 2002     

The 27Al (p,γ)28Si reaction: direct capture cross-section and resonance strengths at Ep = 0.2-1.12 MeV

The ²⁷Al(p,γ)²⁸Si reaction has been investigated using two different 4π NaI summing detectors. The strength of 26 resonances in the energy range from 200 to 1120 keV has been measured. In addition, yield measurements have been carried out at 9 beam energies in the 490 to 1150 keV range in order to determine the direct capture cross-section. The existing data on the non-resonant mechanism were analysed and combined with the results of the present work. The resulting resonance strengths as well as the deduced direct capture astrophysical S-factor are compared with previous work. Received: 4 October 2000 / Accepted: 7 December 2000     

1<Superscript>+</Superscript>(0<Superscript>+</Superscript>) state at 12.4 MeV in <Superscript>20</Superscript>Ne

Careful review of all the evidence makes it clear that at least three states are important at 12.4-MeV excitation in ²⁰Ne (four, if the broad (2⁺) at 12.5 MeV is included). The three states are 3 ^- , 0 ⁺ , and 1 ⁺ (0⁺). The latter, which is quite strong in ¹⁹F (³He, d) singles, is probably the state observed in coincidence with 6.13-MeV γ-rays in ¹⁹F(³He, dγ). Received: 19 August 2002 / Accepted: 28 October 2002 / Published online: 17 January 2003 RID="a" ID="a"e-mail: fortune@physics.upenn.edu Communicated by D. Guerreau