Neutrinos and explosive nucleosynthesis

Core-collapse supernovae produce a hot protoneutron star that cools emitting huge amounts of neutrinos of all flavors. The interaction of these neutrinos with the outer layers of the protoneutron star produces an outflow of matter whose composition is determined by the luminosities and energies of the emitted neutrinos and antineutrinos. The presence of light nuclei like deuterons and tritons can have a big impact in the average energies of the emitted antineutrinos and consequently in the neutron-richness of the ejected matter. Recent hydrodynamical models show that the ejected matter is in fact proton-rich and constitutes the site of the νp-process where antineutrino absorption reactions catalyze the nucleosynthesis of nuclei with A>64.     

Destruction of26Al in explosive nucleosynthesis

The²⁶Al(n, p)²⁶Mg reaction has been studied using neutron spectra which closely resembled Maxwell-Boltzmann (M.-B.) distributions with thermal energies ofkT=40 ×10^?6, 31 and 71 keV and also forE _n =270–350 keV. These energies correspond to stellar temperaturesT ₉=4.6×10^?7, 0.36, 0.82 and 3.1–4.1, whereT ₉ is in units of 10⁹ K. The partial cross sections for thep ₀-(p ₁-)transition are found to equal 26±10(1,850 ±150), 13±6(124±17), 16±13(84±14) and 21±8(72±15) mb for the above neutron spectra, respectively. The astrophysical reaction rate is determined for the combinedp ₀- andp ₁-transitions to beN _A 〈σν〉=(0.324±0.026, 20.5±2.7, 22.6±4.3 and 38.7±11.1) ×10⁶ cm³ mole^?1 s^?1. The results are compared with previous investigations and with statistical model calculations.     

Magic numbers of ultraheavy nuclei

For nuclei where the number of protons lies in the range 76 ≤ Z ≤ 400, proton and neutron shell corrections are calculated along the beta-stability line described by Green’s formula. The magic numbers of protons and neutrons are determined for ultraheavy nuclei. Alpha-decay half-lives and fission barriers are estimated for ultraheavy doubly magic nuclei.     

Proton-rich unstable nuclei in explosive hydrogen burning

Onset of the rapid-proton process is discussed based on the recent experimental data of proton-rich unstable nuclei. A new experimental result on the Ne-E problem is also discussed. The low-energy radioactive nuclear beam project at INS is briefly reported that is under construction for nuclear astrophysics.     

Magic gaps and intruder levels in triaxially superdeformed nuclei

Nuclear shell model calculations based on a modified harmonic-oscillator potential result in amazingly stable triaxial nuclear shapes. Major gaps in the single-particle energy spectra at proton number 71 and neutron number 94 combine constructively at low and intermediate rotational frequencies. At high frequencies, gaps at proton number 72 and neutron number 97 combine in an equally favourable way. The sizes of the gaps may be as large as 35% of the values for the gaps at the classical magic numbers 50 and 82 at spherical shape. The dependence on the positions of the intruder levels in forming the gaps is discussed. Experimentally observed rotational bands in lutetium (Z = 71) and hafnium (Z = 72) appear in isotopes and frequency ranges, which are consistent with the gaps in the theoretical single-particle energy spectra.     

R-process nucleosynthesis and nuclei far from the region of β-stability

The gross theory of β-decay is applied to make improved estimates of the absolute magnitude of β-decay half-lives and related variables in the r-process calculation. The time necessary for synthesis seems to be considerably longer than assumed in some of the recent studies of the dynamic r-process. This means the necessity of suitable re-scaling of the expansion rates in the dynamics. Although the application of the improvements to the dynamics is most interesting, the present paper has limited itself to the classical quasi-static model of Seeger, Fowler and Clayton in performing the abundance calculation. The calculation method of the final abundance curve can allow several nuclear processes to interrupt β-decay cascades before the β-stability region is reached. The sudden freezing of neutron flux and temperature is assumed to precede these processes. In particular, the effect of β-delayed neutron emission after freezing is extensively studied. The β-delayed fission is also discussed. The calculated β-strength functions are compared with the recent experimental data obtained at the ISOLDE and OSIRIS facilities.     

Toward precision mass measurements of neutron-rich nuclei relevant to r-process nucleosynthesis

The open question of where, when, and how the heavy elements beyond iron enrich our Universe has triggered a new era in nuclear physics studies. Of all the relevant nuclear physics inputs, the mass of very neutron-rich nuclides is a key quantity for revealing the origin of heavy elements beyond iron. Although the precise determination of this property is a great challenge, enormous progress has been made in recent decades, and it has contributed significantly to both nuclear structure and astrophysical nucleosynthesis studies. In this review, we first survey our present knowledge of the nuclear mass surface, emphasizing the importance of nuclear mass precision in r-process calculations. We then discuss recent progress in various methods of nuclear mass measurement with a few selected examples. For each method, we focus on recent breakthroughs and discuss possible ways of improving the weighing of r-process nuclides.     

Simulating transuranium isotope yields upon explosive nucleosynthesis with allowance for elements of process dynamics

A two-group model of transuranium isotope production under the conditions of explosive nucleosynthesis is studied. The slow neutron effect on the yields of transuranium isotopes is considered. The static model of multiple neutron capture is supplemented with features of dynamics with allowance for the adiabatic expansion of matter at t ?? 10^?6 s. The results from calculations in the two-group model for mass numbers A = 246?C257 are compared to those of the Par experiment.     

Neutrino-induced nucleosynthesis of A>64 nuclei: the nu p process

We present a new nucleosynthesis process that we denote as the nu p process, which occurs in supernovae (and possibly gamma-ray bursts) when strong neutrino fluxes create proton-rich ejecta. In this process, antineutrino absorptions in the proton-rich environment produce neutrons that are immediately captured by neutron-deficient nuclei. This allows for the nucleosynthesis of nuclei with mass numbers A>64, , making this process a possible candidate to explain the origin of the solar abundances of (92,94)Mo and (96,98)Ru. This process also offers a natural explanation for the large abundance of Sr seen in a hyper-metal-poor star.     

Evidence for nucleosynthesis in the supernova gamma process: universal scaling for p nuclei

Analyzing the solar system abundance, we find two universal scaling laws concerning the p and s nuclei. They indicate that the gamma process in supernova (SN) explosions is the most probable origin of the p nuclei that has been discussed with many possible nuclear reactions and sites in about 50 years. In addition, the scalings lead to new concepts: a universality of the gamma process and a new nuclear cosmochronometer. We carry out gamma-process nucleosynthesis calculations for typical core-collapse SN explosion models, and the results satisfy the observed scalings.     

Reaction matrix for nonlocal potentials. I. Magic nuclei

The binding energies of⁴He,¹⁶O, and⁴⁰Ca are computed applying the Brueckner theory of the reaction matrix for the Tabakin and the Mongan nonlocal separable potentials. Binding energies (10 MeV/nucleon) obtained are substantially greater than similar values for the local Hamada-Johnston potential (approx. 4÷5 MeV/nucleon). Detailed comparison of reference and exact reaction matrices for both types of potentials (local and nonlocal) in the coupled³ S-³ D channel confirms the existence of a strong correlation between the magnitude of the matrix elements of the reaction matrix and the intensity of a tensor force. A similar correlation also exists in the¹ S ₀ channel between the magnitude of matrix elements of the reaction matrix and the singlet scattering length. A different off-shell behaviour of individual potentials plays also a certain role.     

Magic numbers in exotic nuclei and spin-isospin properties of the NN interaction

The magic numbers in exotic nuclei are discussed, and their novel origin is shown to be the spin-isospin dependent part of the nucleon-nucleon interaction in nuclei. The importance and robustness of this mechanism is shown in terms of meson exchange, G-matrix, and QCD theories. In neutron-rich exotic nuclei, magic numbers such as N = 8, 20, etc. can disappear, while N = 6, 16, etc. arise, affecting the structure of the lightest exotic nuclei to nucleosynthesis of heavy elements.     

Explosive processes in nucleosynthesis

There are many explosive processes in nucleosynthesis: big bang nucleosynthesis, the rp-process, the γ-process, the ν-process, and the r-process. However, I will discuss just the rp-process and the r-process in detail, primarily because both seem to have been very active research areas of late, and because they have great potential for studies with radioactive nuclear beams. I will also discuss briefly the γ-process because of its inevitability in conjunction with the rp-process. Received: 1 May 2001 / Accepted: 4 December 2001     

Beta decay rates for nuclei with 115<A<140 for r-process nucleosynthesis

For r-process nucleosynthesis the β-decay rates for a number of neutron-rich intermediate heavy nuclei are calculated. The model for the β-strength function is able to reproduce the observed half-lives quite well.     

Gamow-Teller strength in the exotic odd-odd nuclei 138La and 180Ta and its relevance for neutrino nucleosynthesis

The Gamow-Teller strength distributions below the particle threshold in 138La and 180Ta, deduced from high-resolution measurements of the (3He,t) reaction at 0 degrees, allow us to evaluate the role of charged-current reactions for the production of these extremely rare nuclides in neutrino-nucleosynthesis models. The analysis suggests that essentially all 138La in the Universe can be made that way. Neutrino nucleosynthesis also contributes significantly to the abundance of 180Ta but the magnitude depends on the unknown branching ratio for population of the long-lived isomer.     

Big bang nucleosynthesis

A brief review of standard big bang nucleosynthesis theory and the related observations of the light element isotopes is presented. Implications of BBN on chemical evolution and constraints on particle properties will also be discussed.