B. Čujec

The 12C + 12C sub-coulomb fusion cross section☆

Abstract The 12 C + 12 C fusion cross section has been studied over the energy range 2.46 ≦ E c.m. ≦ 5.88 MeV. The yields of the γ-rays emitted from the first excited states of 23 Na and 20 Ne, following 24 Mg compound nucleus decay via proton- and α-emissions, were measured using a Ge(Li) detector. The fusion cross section was obtained by normalizing these yields to previously reported 12 C( 12 C, p) 23 Na and 12 C( 12 C, α) 20 Ne cross sections. The data indicate that the cross section below 3.5 MeV is dominated by two or more resonances, and that the average trend in this energy region does not show the absorption-under-the barrier features of the optical model. For astrophysical extrapolations to lower energies, the new results are consistent with the extrapolation proposed by Fowler, Caughlan and Zimmerman.

Boron+boron sub-Coulomb energy total reaction cross sections☆

Abstract Total reaction cross sections have been measured for the following reactions and energy intervals: 11 B+ 11 B, E c.m. = 1.56–3.65 MeV; 10 B+ 11 B, E c.m. = 1.61–3.94 MeV; 10 B+ 10 B, E c.m. = 1.84–3.66 MeV. Absolute cross sections were extracted from the prompt γ-rays emitted by the various residual nuclei and measured by large NaI detectors. The absolute accuracy of the method is thoroughly discussed and tested via a measurement of the 12 C+ 12 C reaction. For all three boron cases measured neither intermediate nor giant type resonances were observed. The cross sections are well described by optical model calculations based on lowenergy 11 B+ 11 B elastic scattering parameter sets.

Fusion cross sections for 12C + 12C, 12C + 13C and 13C + 13C at low energies

Abstract Fusion cross sections for 12 C + 12 C and 12 C + 13 C have been measured by the total-γ-ray- yield method over the c.m. energy intervals 4.2–7.0 MeV and 3.1–6.7 MeV, respectively. Comparing the present 12 C + 12 C fusion cross sections with those obtained previously by various methods, the discrepancies between data are clarified. The fusion cross sections for 12 C + 13 C agree with those of Dayras et al . and confirm the unusual energy dependence below the interaction barrier. The fusion cross sections for the 12 C + 13 C and 13 C + 13 C systems are found to be enhanced with respect to the 12 C + 12 C system more than the increase in the nuclear radius suggests. The enhancement, which is larger for 12 C + 13 C than for 13 C + 13 C, is attributed to the attractive action of the valence neutron of the 13 C nucleus. A new resonance has been observed in the 12 C + 12 C system at E c.m. ≈ 5.22 MeV.

Mesures de corrélations angulaires et de vies moyennes dans 57Co

Abstract The reaction 54Fe(α, p)57Co at 9.86, 10.02 and 10.40 MeV incident energies was used to measure the proton-gamma angular correlations in the Litherland-Ferguson geometry and to measure the mean lifetimes by the Doppler-shift attenuation method. The mixing ratios have been obtained for levels at 1.897, 2.134, 2.311 and 2.879 MeV. The mean lifetimes have been determined for 14 levels up to 2.9 MeV. The spins have been unambiguously assigned to all levels up to 2.311 MeV and to the 2.879 ( 3 2 − ) and 2.981 ( 1 2 + ) MeV levels. The partial radiative widths have been determined for all levels, except 1.505 MeV, up to 2.311 MeV.

Fusion and neutron transfer cross sections for 9Be + 12C at sub-Coulomb energies

Abstract The energy dependence of the fusion cross section for 9 Be + 12 C has been measured over the range E c.m. = 2.8–6.3 MeV by detecting γ-rays from the various possible residual nuclei with two large NaI(Tl) detectors placed close to the target. The single-neutron transfer reactions 9 Be( 12 C, 13 C) 8 Be leading to the 3.088 MeV ( 1 2 + ) and 3.854 MeV ( 5 2 + ) states of 13 C were measured over the range E c.m. = 1.0–5.8 MeV and 2.1–5.8 MeV, respectively by detecting the characteristic γ-rays with a Ge(Li) detector. The energy dependence of the 9 Be( 12 C, 13 C 1 ) 8 Be transfer cross section was found, in the sub-Coulomb energy region, to be in good agreement with the simple calculation proposed by Switkowski et al . The fusion cross sections for 9 Be + 12 C are compared with the optical model calculations, using various parameter sets, and with experimental results for other light-nuclei systems. The fusion cross sections for 9 Be + 12 C, 12 C + 13 C and 9 Be + 16 O are found to be enhanced with respect to other systems at incident energies below the Coulomb barrier. It is suggested that a loosely bound neutron fosters, via polarisation, the fusion of the two nuclei at energies just below the Coulomb barrier.

Level structure of 55Co, 51V and 49V from γ-decay

Abstract Gamma-rays following 54 Fe( 3 He, d) 55 Co, 50 Ti( 3 He, d) 51 V, 48 Ti(α,p) 51 V and 46 Ti(α,p) 49 V reactions have been measured. Branching ratios have been deduced for levels in 55 Co up to 4.185 MeV, in 51 V up to 2.699 MeV and in 49 V up to 2.409 MeV. On the basis of the decay schemes and previously established l -values, the following J π assignments have been made: 55 Co 2.162 MeV, 3 2 − ; 2.559 MeV, 3 2 − ; 2.938 MeV, 1 2 − ; 51 V 2.410 MeV, 3 2 − ; 49 V 1.141 MeV, 5 2 − ; 2.234 MeV, 3 2 or case5 2 . The 1 2 + states in 51 V (at 2.547 MeV) and in 49 V (at 1.646 MeV) were observed to decay predominantly to the lowest 3 2 − state. This indicates some admixture to the main component of the 1 2 + state, which gives rise to the E1 transition.

Étude de 53Mn suivant la réaction (α, pγ)

Abstract The 53Mn levels are investigated up to 2.4 MeV excitation following the 50Cr(α, pγ)53Mn reaction. The (αp, γ) angular correlations are measured in the Litherland-Ferguson geometry II. The measured excitation function between 8 and 11 MeV permitted to choose for each level a suitable incident energy. For the levels at 1.438, 1.617, 2.271 and 2.404 MeV the following Jπ values are proposed: 11 2 − , 9 2 − , 5 2 and 3 2 − . Gamma-ray branching and mixing ratios were also obtained. In 53Mn, the M1 transitions are inhibited so that they are in the shell-model equivalent nucleus 51V. Also the level ordering is the same for both nuclei. The results are compared with previous experiments and theoretical predictions.

Fusion cross sections for 16O + 13C at low energies

Abstract The energy dependence of the fusion cross section has been measured over the range E c.m. = 3.05–6.88 MeV by detecting the γ-rays from residual nuclei in a 4π geometry. Analyzing the 1.37 MeV photopeak, originating from 24 Mg 1.37 MeV → g.s. transition , the cross sections for 24 Mg+2n channel were also deduced. The measured fusion cross sections have been compared with those for 12 C + 12 C and 12 C + 13 C systems and found to be significantly different. For 13 C+ 13 C the fusion cross sections agree with the standard optical-model prediction down to the lowest measured energies, while for 12 C + 12 C and 12 C + 13 C they are, at the lowest energies, too low. It is suggested that the unpaired valence nucleons facilitate fusion at energies well below the Coulomb barrier.

Energy levels of 57Co

Abstract The reaction 54Fe(α, p)57Co is used to study the energy levels of 57Co up to 4 MeV excitation with 25 keV resolution. Several new levels are found, and it is proposed that the levels at 1.22 MeV and 1.68 MeV have spin and parity 9 2 − and 11 2 − , respectively.

Cross section measurements for the 9Be+9Be System at subbarrier energies

Abstract The total reaction cross section and the characteristic y-ray cross sections have been measured for the 9 Be+ 9 Be reaction in the energy range E cm = 1.4–3.4 MeV, detecting the prompt γ-rays emitted by the various residual nuclei with two Nal detectors in nearly 4π geometry and with a germanium detector, respectively. The differential elastic cross sections for the same system have also been measured at e c.m. = 2.2, 2.7 and 3.2 MeV. The cross sections calculated with the “standard” and the proximity optical model potentials, which describe well the total reaction cross sections of the light nuclei, agree with the 9 Be + 9 Be elastic-scattering data, but underpredict the total reaction cross section by a factor of 2 to 3. The characteristic γ-ray measurements show that all two-particle emission channels, nα 13 C, nn 16 O, np 16 N and αα 10 Be are enhanced by about that factor, while the single-particle emission channel, p 17 N, is not enhanced.