The 95zr(n, gamma)96zr cross section from the surrogate ratio method and its effect on the s-process nucleosynthesis

The 95Zr(n,gamma)96Zr reaction cross section is crucial in the modelling of s-process nucleosynthesis in asymptotic giant branch stars because it controls the operation of the branching point at the unstable 95Zr and the subsequent production of 96Zr. We have carried out the measurement of the 94Zr(18O,16O) and 90Zr(18O,16O) reactions and obtained the gamma-decay probability ratio of 96Zr* and 92Zr* to determine the 95Zr(n,gamma)96Zr reaction cross sections with the surrogate ratio method. Our deduced maxwellian-averaged cross section of 66+-16 mb at 30 keV is close to the value recommended by Bao et al. (2000), but 30% and more than a factor of two larger than the values proposed by Toukan & Kappeler (1990) and Lugaro et al. (2014), respectively, and routinely used in s-process models. We tested the new rate in stellar models with masses between 2 and 6 Msun and metallicities 0.014 and 0.03. The largest changes - up 80% variations in 96Zr - are seen in models of mass 3-4 Msun, where the 22Ne neutron source is mildly activated. The new rate can still provide a match to data from meteoritic stardust silicon carbide grains, provided the maximum mass of the parent stars is below 4 Msun, for a metallicity of 0.03.Comment: 10 pages, 6 figures, accepted for publication in Ap

Experimental Study of Photonuclear Reactions of ^4He below Pion Threshold(I. Nuclear Physics)

An experimental method for the precise measurement of the photonuclear reactions of ^4He below the pion threshold has been tested. We used a tagged photon beam and a time projection chamber containing helium gas, which served as an active target. It was proved that the chamber could successfully detect the tracks of the charged particles from the photonuclear reactions in a high radiation level due to the irradiation of a high-intensity photon beam. It was found that the background was mainly due to the noise of the chamber, and could be suppressed by taking coincidence of the signals from the chamber and the tagging counter

Role of Multichance Fission in the Description of Fission-Fragment Mass Distributions at High Energies

Fission-fragment mass distributions were measured for U237-240, Np239-242, and Pu241-244 populated in the excitation-energy range from 10 to 60 MeV by multinucleon transfer channels in the reaction O18+U238 at the Japan Atomic Energy Agency tandem facility. Among them, the data for U240 and Np240,241,242 were observed for the first time. It was found that the mass distributions for all the studied nuclides maintain a double-humped shape up to the highest measured energy in contrast to expectations of predominantly symmetric fission due to the washing out of nuclear shell effects. From a comparison with the dynamical calculation based on the fluctuation-dissipation model, this behavior of the mass distributions was unambiguously attributed to the effect of multichance fission

Measurement of fission-fragment mass distributions in the multinucleon transfer channels of the 18O+237Np^18O+^237Np reaction

International audienceFission-fragment mass distributions for 23 nuclei (U234−237, Np236−239, Pu238−241, Am240−243, Cm242−245, and Bk244−246) were measured using the multinucleon transfer approach in the reaction of O18+Np237, and their excitation-energy dependence was obtained up to a maximum of 70 MeV. Among them, the low-energy fission of Np236, Pu238, and Cm245 is reported for the first time. The experimental data for all the studied nuclei were compared to the Langevin calculations. The calculation which takes into account the effects of multichance fission well reproduced the peak-to-valley ratio and mass-asymmetric peak positions of the distributions. The angular momentum given to the fissioning nucleus is also discussed

Observation of the competing fission modes in 178 Pt

Fragment mass distributions from fission of the excited compound nucleus 178 Pt have been deduced from the measured fragment velocities. The 178 Pt nucleus was created at the JAEA tandem facility in a complete fusion reaction 36 Ar + 142 Nd, at beam energies of 155, 170 and 180 MeV. The data are indicative of a mixture of the mass-asymmetric and mass-symmetric fission modes associated with higher and lower total kinetic energies of the fragments, respectively. The measured fragment yields are dominated by asymmetric mass splits, with the symmetric mode contributing at the level of ≈1/3. This constitutes the first observation of a multimodal fission in the sub-lead region. Most probable experimental fragment-mass split of the asymmetric mode, A L /A H ≈79/99, is well reproduced by nuclear density functional theory using the UNEDF1-HFB and D1S potentials. The symmetric mode is associated by theory with very elongated fission fragments, which is consistent with the observed total kinetic energy/fragment mass correlation

Fission fragments mass distributions of nuclei populated by the multinucleon transfer channels of the 18O + 232Th reaction

It is shown that the multinucleon transfer reactions is a powerful tool to study fission of exotic neutron-rich actinide nuclei, which cannot be accessed by particle-capture or heavy-ion fusion reactions. In this work, multinucleon transfer channels of the 18O +  232Th reaction are used to study fission of fourteen nuclei 231,232,233,234Th, 232,233,234,235,236Pa, and 234,235,236,237,238U. Identification of fissioning nuclei and of their excitation energy is performed on an event-by-event basis, through the measurement of outgoing ejectile particle in coincidence with fission fragments. Fission fragment mass distributions are measured for each transfer channel, in selected bins of excitation energy. In particular, the mass distributions of 231,234Th and 234,235,236Pa are measured for the first time. Predominantly asymmetric fission is observed at low excitation energies for all studied cases, with a gradual increase of the symmetric mode towards higher excitation energy. The experimental distributions are found to be in general agreement with predictions of the fluctuation–dissipation model

Search for a decay of Te-104 with a novel recoil-decay scintillation detector

A search for superallowed α decay of N=Z nuclei Te104 and Xe108 was carried out using a novel recoil-decay scintillator detector at the tandem accelerator facility at the Japan Atomic Energy Agency (JAEA). Inorganic crystal scintillation material YAP:Ce (yttrium aluminum perovskite) coupled to a position-sensitive photomultiplier tube (PSPMT) was implemented for the first time in a radioactive decay experiment. Residues from the fusion-evaporation reaction Ni58+Fe54→Xe∗112 were separated by the JAEA Recoil Mass Separator (RMS) and implanted into the YAP:Ce crystal. α decays of neutron-deficient tellurium isotopes were identified and proton emission of I109 was observed. The α-decay chain Xe109→Te105→Sn101 was recorded with a time interval of 960 ns between two α pulses. Position localization in the crystal for decays and ions in the energy range from hundreds of keV to 60 MeV was achieved with an accuracy of 0.67 mm, proving that this detector is capable of making temporal and spatial correlations for fast decay events. No conclusive evidence was found for the decay chain Xe108→Te104→Sn100 within 3 days of experiment. However, two events were observed with properties consistent with the reported observation at the Fragment Mass Analyzer (FMA), but with a separation between signals of less than 4 ns. The cross section limit of 130 pb was obtained for production of two events of Xe108, about an order of magnitude below the expectation based on earlier cross section measurements and the hivap fusion-evaporation code