Toshitaka Kajino

New s-process path and its implication to 187Re-187Os nucleo-cosmochronometer

We study a new s-process path through an isomer of 186Re to improve a 187Re-187Os nucleo-cosmochronometer. The nucleus 187Re is produced by this new path of 185Re(n, γ)186Rem(n, γ)187Re. We measure a ratio of neutron-capture cross sections for the 185Re(n, γ)186Rem and 185Re(n, γ)186Reg reactions at thermal neutron energy because the ratio with the experimental uncertainty has not been reported. Using an activation method with reactor neutrons, we obtain the ratio of Rth = 0.54% ± 0.11%. From this ratio we estimate the ratio of Maxwellian-averaged cross sections in a typical s-process environment at kT = 30 keV with the help of the temperature dependence given in a statistical-model calculation because the energy dependence of the isomer/ground ratio is smaller than the absolute neutron-capture cross section. The ratio at kT = 30 keV is estimated to be Rst = 1.3% ± 0.8%. We calculate the s-process contribution from the new path in a steady-flow model. The additional abundance of 187Re through this path is estimated to be Ns = 0.56% ± 0.35% relative to the abundance of 186Os. This additional increase of 187Re does not make any remarkable change in the 187Re-187Os chronometer for an age estimate of a primitive meteorite, which has recently been found to be affected strongly by a single supernova r-process episode.

Principle of Universality of γ-Process Nucleosynthesis in Core-Collapse Supernova Explosions

Using core-collapse supernova explosion models, we investigate the principle of the universality of the γ-process, according to which the s/p abundance ratios produced by individual nucleosynthesis episode are almost constant over a wide range of atomic number. The universality originates from the shift of the γ-process layers, the independence of the s/p abundance ratios of the nuclear reactions, and the weak s-process in presupernovae. Our calculations further suggest an extended universality in which the s/p ratios in the γ-process layers are not only constant but also centered around a specific value of 3. To verify this, we propose an astronomical observation of indium isotopic abundance fractions.

The new record holder for the most iron-poor star: HE 1327 2326, a dwarf or subgiant with (Fe/H) = 5.4

We describe the discovery of HE 1327−2326, a dwarf or subgiant with (Fe/H) = −5.4. The star was found in a sample of bright metal-poor stars selected from the Hamburg/ESO survey. Its abundance pattern is characterized by very high C and N abundances. The detection of Sr which is overabundant by a factor of 10 as compared to iron and the Sun, suggests that neutron-capture elements had already been produced in the very early Galaxy. A puzzling Li depletion is observed in this unevolved star which contradicts the value of the primordial Li derived from WMAP and other Li studies. Possible scenarios for the origin of the abundance pattern (Pop. II or Pop. III) are presented as well as an outlook on future observations.

Isomer residual ratio of odd-odd isotope {sup 180}Ta in supernova nucleosynthsis

The nucleosynthesis of 180Ta has remained an unsolved problem and as its origin many nucleosynthesis mechanisms have been proposed. This isotope has the unique feature that the naturally occurring abundance of 180Ta is actually a meta‐stable isomer (half‐life of ≥1015 yr), while the ground state is a 1+ unstable state which β‐decays with a half‐life of only 8.15 hr. We have made a new time‐dependent calculation of 180Ta meta‐stable isomer residual ratio after supernova neutrino‐induced reactions. This residual isomer ratio is crucial for understanding the production and survival of this naturally occurring rare isotope. We have constructed a new model under temperature evolution after type II supernova explosion. We include the explicit linking between the isomer and all known excited states and found that the residual ratio is insensitive to astrophysical parameters such as neutrino energy spectrum, explosion energy, decay time constant. We find that the explicit time evolution of the synthesis of 180Ta ...

Thermal neutron capture cross-section to [sup 113]Cd isomer for the study of s-process origin of [sup 115]Sn

The astrophysical origin of a p‐nucleus 115Sn has remained still an open question. The nucleus 115Sn may be produced by a weak branch of the s‐process through a β‐unstable isomer in 113Cd. However, a neutron capture cross‐section to this isomer has not been measured with high accuracy at any energy. A neutron capture cross‐section for the 112Cd(n,γ)113Cdm reaction has been measured with neutrons provided from a nuclear reactor. The nucleus 115Sn may be produced by a nucleosynthesis flow through 113Cdm in the s‐process. We have obtained the thermal neutron capture cross‐section of 0.028±0.009 [b] and the resonance integral of 1.1±0.3 [b] using a cadmium difference method. The cross‐section ratio of the isomer to the ground state has been calculated as a function of the incident neutron energy, E, by using a statistical model. The calculated ratios are almost constant over a wide range of E<100 keV. We have evaluated the s‐process contribution to the solar abundance of 115Sn using the classical steady‐flow ...

Study on the α+[sup 8]He resonant scattering

The α+8He low‐energy reactions are studied by the application of the generalized two‐center cluster model with the α+α+4N structure. The calculation predicts prominent resonant peaks in the two‐neutron transfer reactions, α+8Heg.s.→6Heg.s.+6Heg.s., with Jπ = 0+. Experimental conditions to identify these resonances are also discussed.

Direct Measurement of [sup 21]Na+α Stellar Reaction

The measurement of the resonant alpha scattering and the {sup 21}Na({alpha}, p) reaction were performed for the first time in inverse kinematics with the thick target method using a {sup 21}Na radioisotope (RI) beam. This paper reports the current result of alpha scattering measurement and its astrophysics implication.

The [sup 12]C(α, γ)[sup 16]O E2 cross section at stellar energies

The E2 component of the 12 C (α, γ) 16 O cross section is investigated by a microscopic cluster model, and by R‐matrix fits. The first approach provides S E2 (300 keV )≈50 keV ‐ b for ground‐state transitions. In the R‐matrix theory, we show that the background term plays a crucial role, and cannot be determined without ambiguity. Only an upper limit on the extrapolated S factor can be obtained [S E2 (300 keV ) keV ‐ b ]. To constrain the R‐matrix analysis, we use the GCM Asymptotic Normalization Constant (ANC) of the 2 1 + level. This procedure strongly reduces the uncertainties on the R‐matrix fit, and we end up with a recommended value of S E2 (300 keV ) =42±2 keV ‐ b . As ANC values derived from indirect methods are not consistent with the 12 C (α, γ) 16 O cascade transitions to the 2 1 + state, we suggest a remeasurement of this cross section.The E2 component of the {sup 12}C({alpha}, {gamma}){sup 16}O cross section is investigated by a microscopic cluster model, and by R-matrix fits. The first approach provides S{sub E2}(300 keV){approx_equal}50 keV-b for ground-state transitions. In the R-matrix theory, we show that the background term plays a crucial role, and cannot be determined without ambiguity. Only an upper limit on the extrapolated S factor can be obtained [S{sub E2}(300 keV)<190 keV-b]. To constrain the R-matrix analysis, we use the GCM Asymptotic Normalization Constant (ANC) of the 2{sub 1}{sup +} level. This procedure strongly reduces the uncertainties on the R-matrix fit, and we end up with a recommended value of S{sub E2}(300 keV) =42{+-}2 keV-b. As ANC values derived from indirect methods are not consistent with the {sup 12}C({alpha}, {gamma}){sup 16}O cascade transitions to the 2{sub 1}{sup +} state, we suggest a remeasurement of this cross section.

The Trojan Horse Method as a tool to investigate low-energy resonances: the [sup 18]O(p, α)[sup 15]N and [sup 17]O(p, α)[sup 14]N cases

The 18O(p, α)15N and 17O(p, α)14N reactions are of primary importance in several as‐trophysical scenarios, including nucleosynthesis inside Asymptotic Giant Branch stars and oxygen and nitrogen isotopic ratios in meteorite grains. They are also key reactions to understand exotic systems such as R‐Coronae Borealis stars and novae. Thus, the measurement of their cross sections in the low energy region can be crucial to reduce the nuclear uncertainty on theoretical predictions, because the resonance parameters are poorly determined. The Trojan Horse Method, in its newly developed form particularly suited to investigate low‐energy resonances, has been applied to the 2H(18O, α15N)n and 2H(17O, α14N)n reactions to deduce the 18O(p, α)15N and 17O(p, α)14N cross sections at low energies. Resonances in the 18O(p, α)15N and 17O(p, α)14N excitation functions have been studied and the resonance parameters deduced.

Study of [sup 12]C(γ, 2α)[sup 4]He with NewSUBARU laser Compton scattered gamma-ray beam

The 16O(γ, 2α)4He reaction cross section was measured in the γ‐ray energy range from 16 MeV up to 39 MeV using an active target method and a quasi‐monochromatic γ‐ray beam provided at the Laboratory of Advanced Science and Technology for Industry (LASTI) of the University of Hyogo. The cross section is found to be rather small in the energy region corresponding to the 2+ and 4+ excited states of the intermediate 8Be nucleus, while it becomes large above the energy corresponding to the 8Be 1− state, being in contrast to the latest result reported by Afanas’ev and Khodyachikh. The present result suggests the cross sections are dominated by the contributions of the 1− states in 12C which are excited with the E1 transition.