F. de Oliveira

First observation of 54Zn and its decay by two-proton emission

The nucleus 54Zn has been observed for the first time in an experiment at the SISSI/LISE3 facility of GANIL in the quasifragmentation of a 58Ni beam at 74.5 MeV/nucleon in a (nat)Ni target. The fragments were analyzed by means of the ALPHA-LISE3 separator and implanted in a silicon-strip detector where correlations in space and time between implantation and subsequent decay events allowed us to generate almost background free decay spectra for about 25 different nuclei at the same time. Eight 54Zn implantation events were observed. From the correlated decay events, the half-life of 54Zn is determined to be 3.2(+1.8)(-0.8) ms. Seven of the eight implantations are followed by two-proton emission with a decay energy of 1.48(2) MeV. The decay energy and the partial half-life are compared to model predictions and allow for a test of these two-proton decay models.

Astrophysical rate of the 11C+p reaction from the Coulomb break-up of a 12N radioactive beam

Abstract The Coulomb break-up technique has been used to determine the radiative width of one excited level in 12 N and the direct capture contribution to 11 C(p,γ) 12 N. The 12 N radioactive beam was produced through transfer and fragmentation reactions of a primary 95.5 MeV/u 14 N beam at GANIL. From the extracted cross section we deduce the rate of the 11 C+p reaction. Consequences for the hot pp chain are analysed.

Shape coexistence and the N = 28 shell closure far from stability

A mass measurement experiment by a time of flight method with the SPEG spectrometer at GANIL has been performed to investigate the N=20 and N=28 shell closures far from stability. The masses of 31 neutron-rich nuclei in the range A=29–47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S and P isotopes are seen to exhibit a change in shell structure around N=28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. The evidence of an isomeric state in the 43S and its interpretation by a shell model calculation confirm the analysis of the masses and constitutes the first evidence of the predicted shape coexistence around N=28.

TONNERRE: an array for delayed-neutron decay spectroscopy

Abstract The design and construction of an array for delayed-neutron decay spectroscopy is presented. The array is composed of 32 plastic scintillator (BC400) modules each of which provide for time-of-flight (energy) and position measurements. The intrinsic detection efficiency is 30% at E n =2 MeV and the resolution δ E n / E n ∼10% (FWHM). The array can cover a total solid angle of up to 50% of 4π. Details of studies performed to optimize the performances of the individual modules are reported, as well as source testing and in-beam commissioning.

β-decay of 35Ca

Abstract The β-decay of the T z =− 5 2 nucleus 35 Ca was studied at the LISE3 spectrometer at GANIL. The 35 Ca decay scheme was deduced from its β-delayed proton emission into the ground and excited states of 34 Ar and from its β-delayed two-proton emission. The 35 Ca half-life was determined to be 25.7±0.2 ms. The measured transition strength function B(GT) is compared to results obtained from large-scale sd-shell model calculations.

Prolate-Spherical Shape Coexistence at N=28 in 44 S

The structure of 44S has been studied by using delayed γ and electron spectroscopy. The decay rates of the 02+ isomeric state to the 2(1)+ and 0(1)+ states, measured for the first time, lead to a reduced transition probability B(E2: 2(1)+→0(2)+)=8.4(26) e(2) fm4 and a monopole strength ρ2(E0: 0(2)+→0(1)+)=8.7(7)×10(-3). Comparisons to shell model calculations point towards prolate-spherical shape coexistence, and a two-level mixing model is used to extract a weak mixing between the two configurations.

Beta-decay studies at the N=28 shell closure

Abstract First results on a β-γ decay spectroscopy experiment of nuclei in the region of N=28 are reported. New β half-lives for nuclei from Mg to Ar have been extracted. For the heavier ones, the new periods of 48Ar and 47Cl are of importance in the understanding of solar abundance ratio 48 Ca 46 Ca . The measured half-lives of Si isotopes have been extended from N=25 to N=28 and are discussed in the light of possible deformation occuring in this region. Finally, preliminary spectroscopy of 45Ar is also reported.

Comparison of low-energy resonances in15N(α,γ)19F and15O(α,γ)19Ne and related uncertainties

A disagreement between two determinations of Gamma_alpha of the astro- physically relevant level at E_x=4.378 MeV in 19F has been stated in two recent papers by Wilmes et al. and de Oliveira et al. In this work the uncertainties of both papers are discussed in detail, and we adopt the value Gamma_alpha=(1.5^{+1.5}_{-0.8})10^-9eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei Gamma_gamma(19F) approx Gamma_gamma(19Ne), theta^2_alpha(19F) approx theta^2_alpha(19Ne) are discussed, together with the resulting uncertainties on the resonance strengths in 19Ne and on the 15O(alpha,gamma)19Ne rate.A disagreement between two determinations of {Gamma}{sub {alpha}} of the astrophysically relevant level at E{sub x}=4.378 MeV in {sup 19}F has been stated in two recent papers by Wilmes {ital et al.} and de Oliveira {ital et al.} In this work the uncertainties of both papers are discussed in detail, and we adopt the value {Gamma}{sub {alpha}} = (1.5{sub {minus}0.8}{sup +1.5}){times}10{sup {minus}9} eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei {Gamma}{sub {gamma}}({sup 19}F){approx}{Gamma}{sub {gamma}}({sup 19}Ne), {theta}{sub {alpha}}{sup 2}({sup 19}F){approx}{theta}{sub {alpha}}{sup 2}({sup 19}Ne) are discussed, together with the resulting uncertainties on the resonance strengths in {sup 19}Ne and on the {sup 15}O({alpha},{gamma}){sup 19}Ne rate. {copyright} {ital 1997} {ital The American Physical Society}

Proton reaction cross-section measurements on stable and neutron-rich nuclei as a probe of the nucleon–nucleus interaction

PCT No. PCT/JP95/02403 Sec. 371 Date Jul. 25, 1996 Sec. 102(e) Date Jul. 25, 1996 PCT Filed Nov. 27, 1995 PCT Pub. No. WO96/16993 PCT Pub. Date Jun. 6, 1996The present invention relates to a process for controlling molecular weight of a styrenic polymer by adding an organometallic compound such as triethylaluminum when a crystalline styrenic polymer having a highly syndiotactic configuration is produced by using a catalyst for producing a styrenic polymer having the syndiotactic configuration. A styrenic polymer having a highly syndiotactic configuration can efficiently be obtained by homopolymerizing or copolymerizing a styrenic compound in accordance with the process of the present invention. Therefore, the process of the present invention is expected to be advantageously used for producing the styrenic polymer.

Determination of α-widths in 19F relevant to fluorine nucleosynthesis

Abstract Nucleosynthesis of fluorine in the context of helium burning occurs through the 15 N(α,γ) 19 F reaction. At temperatures where fluorine formation takes place in most astrophysical models, the narrow resonance associated with the 4.378 MeV level of 19 F is expected to dominate the reaction rate, but its strength is not known. We used a 15 N confined gas target to study this level by means of the transfer reaction 15 N 7 Li,t) 19 F at 28 MeV. Reaction products products were analysed with a split pole magnetic spectrometer and the angular distributions for the first 16 levels of 19 F were extracted. These distributions are fairly well reproduced by FR-DWBA calculations in the framework of an α-cluster transfer model with a compound-nucleus contribution obtained by Hauser-Feshbach calculations. α-spectroscopic factors were deduced and, for unbound levels, the α-widths were determined and compared with the existing direct measurements. The α-width of the level of astrophysical interest ( E x = 4.378 MeV) was found to be Γ α = 1.5 × 10 −9 eV, a value 60 times smaller than the commonly used one. The astrophysical consequences for 19 F production in AGB stars are discussed.