Masahisa Ohta

NACRE II: an update of the NACRE compilation of charged-particle-induced thermonuclear reaction rates for nuclei with mass number A < 16

Abstract An update of the NACRE compilation [3] is presented. This new compilation, referred to as NACRE II, reports thermonuclear reaction rates for 34 charged-particle induced, two-body exoergic reactions on nuclides with mass number A 16 , of which fifteen are particle-transfer reactions and the rest radiative capture reactions. When compared with NACRE, NACRE II features in particular (1) the addition to the experimental data collected in NACRE of those reported later, preferentially in the major journals of the field by early 2013, and (2) the adoption of potential models as the primary tool for extrapolation to very low energies of astrophysical S-factors, with a systematic evaluation of uncertainties. As in NACRE, the rates are presented in tabular form for temperatures in the 10 6 ≲ T ⩽ 10 10 K range. Along with the ‘adopted’ rates, their low and high limits are provided. The new rates are available in electronic form as part of the Brussels Library (BRUSLIB) of nuclear data. The NACRE II rates also supersede the previous NACRE rates in the Nuclear Network Generator (NETGEN) for astrophysics. [ http://www.astro.ulb.ac.be/databases.html ]

Photodisintegration cross section measurements on {sup 186}W, {sup 187}Re, and {sup 188}Os: Implications for the Re-Os cosmochronology

Cross sections of the {sup 186}W, {sup 187}Re, {sup 188}Os({gamma},n) reactions were measured using quasimonochromatic photon beams from laser Compton scattering with average energies from 7.3 to 10.9 MeV. The results are compared with the predictions of Hauser-Feshbach statistical calculations using four different sets of input parameters. In addition, the inverse neutron capture cross sections were evaluated by constraining the model parameters, especially the E1 strength function, on the basis of the experimental data. The present experiment helps to further constrain the correction factor F{sub {sigma}} for the neutron capture on the 9.75 keV state in {sup 187}Os. Implications of F{sub {sigma}} for the Re-Os cosmochronology are discussed with a focus on the uncertainty in the estimate of the age of the galaxy.

Radiative Capture Cross Section for 16O(n, γ)17O and 16O(p, γ)17F below Astrophysical Energies

In this paper we aim to construct a reliable theoretical framework to analyze nuclear reaction cross sections concerning the nucleosyntheses of light nuclei. To explain the reaction cross sections of O(n, γ)O and O(p, γ)F, we propose a theoretical procedure combining an O+N model and the complex scaling method with the Lippmann-Schwinger solution for the complex-scaled Green’s operator. We demonstrate that this procedure is a powerful tool in the analysis of the reactions observed in the lower-energy region including astrophysical energies.

Fusion Cross Section of Massive Nuclei by Fluctuation-Dissipation Dynamics

Fluctuation-dissipation dynamics is applied to the nearly mass-symmetric fusion reactions forming nuclei of Z> 80. We take into account the angular momentum of the system. The fusion cross section is calculated, and the hindrance of fusion is discussed. We estimate the amount of extra energy above the Coulomb barrier needed for fusion. In a fusion reaction with nearly symmetric projectile-target combination forming a compound nucleus with Z> 80 (or fissility larger than 0.65), additional kinetic energy above the incident Coulomb barrier is needed to realize the formation of a spherical compound nucleus. This supplementary energy was defined as the extraextra push energy EXX by Swiatecki 1) and systematic studies of this phenomenon, the so-called fusion hindrance, have been reported. 2) - 4) The origin of the necessity of the extra-extra push energy lies in the characteristics of the potential energy landscape in nuclear deformation parameter space. 5),6) As the system becomes heavier than Z around 80, the contact configuration comes to be located outside the ridge curve of the potential energy surface. Note that the fission saddle point is located on the ridge curve; it divides the deformation space into the mono-nucleus (fusion) region and di-nucleus (fission) region. This implies that the colliding nuclei must overcome the ridge to attain compact shapes, such as a sphere. Fusion hindrance has been investigated in the framework of the classical trajectory method 7) which describes fusion-fission dynamics in terms of the time evolution of the nuclear shape initially located at the contact configuration in the deformation space with a certain collective kinetic energy. To obtain information on EXX, the classical trajectory in a multi-dimensional deformation space has been calculated by Blocki et al. 7) taking into account the dissipative force stemming from the coupling between the collective and the single particle degrees of freedom. In their framework, the critical energy of the fusion reaction was obtained. There is no fusion event below the critical energy, while the trajectory always reaches the spherical region above the energy. The fluctuation of the trajectory which appears as the counterpart of the dissipative force was taken into account by Aguiar et al. 8) By the inclusion of fluctuations, it becomes possible to calculate the fusion probability; the trajectory becomes that of a Brownian particle and can thus be calculated using the Langevin equation. The aim of the present paper is to confirm the validity of the application of fluctuation-dissipation dynamics to the study of fusion of massive nuclei in the mass

Origin of the drastic decrease of fusion probability in superheavy mass region

Abstract The fusion–fission process in the superheavy mass region is studied systematically by solving the time evolution of nuclear shape in three-dimensional deformation space using the Langevin equation. By analyzing the trajectory in the deformation space, we identify the critical area when the trajectorys destination is determined to be the fusion or the quasi-fission process. It is also clarified that the potential landscape around the critical area is crucial for estimating the fusion probability, and its dependence on the atomic number is presented.

Pre-scission neutron multiplicity associated with the dynamical process in the superheavy-mass region

The fusion?fission process accompanied by neutron emission is studied in the superheavy-mass region on the basis of the fluctuation?dissipation model combined with a statistical model. The calculation of the trajectory or the shape evolution in the deformation space of the nucleus with neutron emission is performed. Each process (quasi-fission, fusion?fission and deep quasi-fission processes) has a characteristic travelling time from the point of contact of colliding nuclei to the scission point. These dynamical aspects of the whole process are discussed in terms of the pre-scission neutron multiplicity, which depends on the time spent on each process. We have presented the details of the characteristics of our model calculation in the reactions 48Ca + 208Pb and 48Ca + 244Pu, and shown how the structure of the distribution of the pre-scission neutron multiplicity depends on the incident energy.

Tracking dissipation in capture reactions

Nuclear dissipation in capture reactions is investigated using backtracing, a new analysis protocol. Combining analysis procedure with dynamical models, the difficult and long-standing problem of competition and mixing between quasifission and fusion-fission is solved for the first time. The nature of the relevant dissipation is determined as one-body dissipation. At low excitation energy where shell effects are strongly effective, the shape of the mass distribution could be a powerful check of the nature and the magnitude of the dissipation.

Analysis of fusion-fission process with neutron evaporation in superheavy mass region

The fusion-fission process for the synthesis of superheavy elements is discussed on the basis of the fluctuation-dissipation dynamics. We analyze the experimental data using a three-dimensional Langevin calculation. We take the neutron emission into account in the Langevin calculation and compare the results with experimental data. Also we discuss the evaporation residue cross section for superheavy elements.

Fission Width of Compound Nuclei Calculated Using the Mean First Passage Time Method

In order to estimate the fission width for the case in which the Bohr-Wheeler model is inapplicable due to a fission barrier Bf 0, we introduce the mean first passage time method and discuss the usefulness of this method. By using this method, the fission decay time can be obtained without solving the Smoluchowski equation, which involves a time consuming calculation. This method is useful when we need to calculate the fission width with a vanishing fission barrier, as when we estimate the fusion evaporation residue cross section for superheavy nuclei with Z > 114.

EFFECT OF 16O CORE EXCITATION FOR PARTICLE CAPTURE REACTIONS AT ASTROPHYSICAL ENERGY

In order to investigate the particle capture reactions at the astrophysical energy, we consider that the structure of the negative-parity states near the threshold energy becomes important. To study the structure, it is necessary to extend the model space take into account the effect of the core excitation. In this paper, we discuss the effect of the core excitation for the reaction cross sections at the astrophysical energy.