arXiv subjects

H. Oberhummer

Publications and source records attributed to H. Oberhummer.

At least 19 recordsLinked to original sources

arXiv2015

Theoretical neutron-capture cross sections for r-process nucleosynthesis in the $^{48}$Ca region

We calculate neutron capture cross sections for r-process nucleosynthesis in the $^{48}$Ca region, namely for the isotopes $^{40-44}$S, $^{46-50}$Ar, $^{56-66}$Ti, $^{62-68}$Cr, and $^{72-76}$Fe. While previously only cross sections resulting from the compound nucleus reaction mechanism (Hauser-Feshbach) have been considered, we recalculate not only that contribution to the cross section but also include direct capture on even-even nuclei. The level schemes, which are of utmost importance in the direct capture calculations, are taken from quasi-particle states obtained with a folded-Yukawa potential and Lipkin-Nogami pairing. Most recent deformation values derived from experimental data on $β$-decay half lives are used where available. Due to the consideration of direct capture, the capture rates are enhanced and the "turning points" in the r-process path are shifted to slightly higher mass numbers. We also discuss the sensitivity of the direct capture cross sections on the assumed deformation.

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arXiv2014

Measurement and analysis of the Am-243 neutron capture cross section at the n_TOF facility at CERN

Background:The design of new nuclear reactors and transmutation devices requires to reduce the present neutron cross section uncertainties of minor actinides. Purpose: Reduce the $^{243}$Am(n,$γ$) cross section uncertainty. Method: The $^{243}$Am(n,$γ$) cross section has been measured at the n_TOF facility at CERN with a BaF$_{2}$ Total Absorption Calorimeter, in the energy range between 0.7 eV and 2.5 keV. Results: The $^{243}$Am(n,$γ$) cross section has been successfully measured in the mentioned energy range. The resolved resonance region has been extended from 250 eV up to 400 eV. In the unresolved resonance region our results are compatible with one of the two incompatible capture data sets available below 2.5 keV. The data available in EXFOR and in the literature has been used to perform a simple analysis above 2.5 keV. Conclusions: The results of this measurement contribute to reduce the $^{243}$Am(n,$γ$) cross section uncertainty and suggest that this cross section is underestimated up to 25% in the neutron energy range between 50 eV and a few keV in the present evaluated data libraries.

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arXiv2003

Sensitivity of the C and O production on the 3-alpha rate

We investigate the dependence of the carbon and oxygen production in stars on the 3-alpha rate by varying the energy of the 02+-state of 12C and determine the resulting yields for a selection of low-mass, intermediate-mass, and massive stars. The yields are obtained using modern stellar evolution codes that follow the entire evolution of massive stars, including the supernova explosion, and consider in detail the 3rd dredge-up process during the thermally pulsating asymptotic giant branch of low-mass and intermediate-mass stars. Our results show that the C and O production in massive stars depends strongly on the initial mass, and that it is crucial to follow the entire evolution. A rather strong C production during the He-shell flashes compared to quiescent He burning leads to a lower sensitivity of the C and O production in low-mass and intermediate-mass stars on the 3-alpha-rate than predicted in our previous work. In particular, the C production of intermediate-mass stars seems to have a maximum close to the actual value of the 02+ energy level of 12C.

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arXiv2002

Temporal variation of coupling constants and nucleosynthesis

We investigate the triple-alpha process and the Oklo phenomenon to obtain constraints on possible cosmological time variations of fundamental constants. Specifically we study cosmological temporal constraints for the fine structure constant and nucleon and meson masses.

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arXiv2002

On the Solar Neutrino Problems, SNO experimental data and low-energy nuclear forces

The Solar Neutrino Problems (SNP's) are analysed within the Standard Solar Model (BP2000) supplemented by the reduction of the solar neutrino fluxes through the decrease of the solar core temperature. The former can be realized through the enhancement of the astrophysical factor for solar proton burning. The enhancement, the upper bound of which is restricted by the helioseismological data, goes dynamically due to low-energy nuclear forces described at the quantum field theoretic level. The agreement of the reduced solar neutrino fluxes with the experimental data is obtained within the scenario of vacuum two-flavour neutrino oscillations. We show that by fitting the mean value of the solar neutrino flux measured by HOMESTAKE Collaboration we predict the high energy solar neutrino flux measured by SNO Collaboration Phi(SNO(Boron))_th = 1.84 X 10^6 cm^(-2) s^(-1) in good agreement with the experimental value Phi(SNO(Boron))_exp = 1.75(0.14) X 10^6 cm^(-2) s^(-1) obtained via the measurement of the rate of reaction nu_e + D -> p + p + electron produced by boron solar neutrinos. The theoretical flux for low-energy neutrino flux measured by GALLIUM (GALLEX, GNO and SAGE) Collaborations S(Ga)_th = 65 SNU agrees with the experimental data averaged over experiments S(Ga)_exp = 75.6(4.8) SNU.

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arXiv2001

$^{92}$Mo($α,α$)$^{92}$Mo scattering,the $^{92}$Mo--$α$ optical potential, and the $^{96}$Ru($γ,α$)$^{92}$Mo reaction rate at astrophysically relevant energies

The elastic scattering cross section of$^{92}$Mo($α$,$α$)$^{92}$Mo has been measured at energies of $E_{\rm{c.m.}} \approx$ 13, 16, and 19 MeV in a wide angular range. The real and imaginary parts of the optical potential for the system $^{92}$Mo - $α$ have been derived at energies around and below the Coulomb barrier. The result fits into the systematic behavior of $α$-nucleus folding potentials. The astrophysically relevant $^{96}$Ru($γ$,$α$)$^{92}$Mo reaction rates at $T_9=2.0$ and $T_9=3.0$ could be determined to an accuracy of about 16 % and are compared to previously published theoretical rates.

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arXiv2001

On the D-wave state component of the deuteron in the Nambu-Jona-Lasinio model of light nuclei

The D-wave state component of the neutron-proton bound state in the deuteron is calculated in the Nambu-Jona-Lasinio model of light nuclei - the relativistically covariant quantum field theoretic approach to the description of low-energy nuclear forces. The theoretical value of the fraction of the D-wave state relative to the S-wave state is equal to eta_d = 0.0238. This agrees well with the phenomenological value eta_d = 0.0256(4) quoted by Kamionkowski and Bahcall (ApJ. 420, 884 (1994)).

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arXiv2000

Dynamics of low-energy nuclear forces and Solar Neutrino Problems in the Nambu-Jona-Lasinio model of light nuclei

The Solar Neutrino Problems (SNP's) are analysed in the Nambu-Jona-Lasinio model of light nuclei. In this model a possible clue to the solution of the SNP's is in the reduction of the solar neutrino fluxes relative to the predicted by the Standard Solar Model through the decrease of the solar core temperature. The former can be realized through the enhancement of the astrophysical factor for the solar proton burning. The enhancement the upper bound of which is restricted by the helioseismological data goes dynamically via the contribution of the nucleon tensor current coupled to the deuteron. The agreement of the reduced solar neutrino fluxes with the experimental data can be reached within a scenario of vacuum two-flavour neutrino oscillations without a fine tuning of the neutrino-flavour oscillation parameters. In the Nambu-Jona-Lasinio model of light nuclei an enhancement of the astrophysical factor for the solar proton burning entails a change of the cross sections for neutrino and anti-neutrino disintegration of the deuteron at low energies. This provides a theoretical foundation for a new check of a value of the astrophysical factor in terrestrial laboratories.

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arXiv2000

On the Delta-Delta component of the deuteron in the Nambu-Jona-Lasinio model of light nuclei

The probability P(Delta-Delta) to find the Delta-Delta component inside the deuteron, where Delta stands for the Delta(1232) resonance, is calculated in the Nambu-Jona-Lasinio model of light nuclei. We obtain P(Delta-Delta) = 0.3%. This prediction agrees good with the experimental estimate P(Delta-Delta) < 0.4% at 90% of CL (D. Allasia et al., Phys. Lett. B174 (1986) 450).

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arXiv2000

The Nambu-Jona-Lasinio model of light nuclei

The Nambu-Jona-Lasinio model of the deuteron suggested by Nambu and Jona-Lasinio (Phys. Rev. 124 (1961) 246) is formulated from the first principles of QCD. The deuteron appears as a neutron-proton collective excitation, i.e. a Cooper np-pair, induced by a phenomenological local four-nucleon interaction in the nuclear phase of QCD. The model describes the deuteron coupled to itself, nucleons and other particles through one-nucleon loop exchanges providing a minimal transfer of nucleon flavours from initial to final nuclear states and accounting for contributions of nucleon-loop anomalies which are completely determined by one-nucleon loop diagrams. The dominance of contributions of nucleon-loop anomalies to effective Lagrangians of low-energy nuclear interactions is justified in the large N_C expansion, where N_C is the number of quark colours.

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arXiv2000

Dynamics of low-energy nuclear forces for electromagnetic and weak reactions with the deuteron in the Nambu-Jona-Lasinio model of light nuclei

A dynamics of low-energy nuclear forces is investigated for low-energy electromagnetic and weak nuclear reactions with the deuteron in the Nambu-Jona-Lasinio model of light nuclei by example of the neutron-proton radiative capture (M1-capture) n + p -> D + photon, the photomagnetic disintegration of the deuteron photon + D -> n + p and weak reactions of astrophysical interest. These are the solar proton burning p + p -> D + positron + nu_e, the pep-process p + electron + p -> D + nu_e and the neutrino and anti-neutrino disintegration of the deuteron caused by charged nu_e + D -> electron + p + p, anti-nu_e + D -> positron + n + n and neutral nu_e(anti-nu_e) + D -> nu_e(ant-nu_e) + n + p weak currents.

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arXiv1999

On the Delta-Delta component of the deuteron in the relativistic field theory model of the deuteron

The Delta-Delta component of the deuteron, where Delta stands for the Delta(1232) resonance, is calculated in the relativistic field theory model of the deuteron. For the probability of the Delta-Delta component of the deuteron we give P(Delta-Delta) = 0.08 %. This prediction agrees good with the experimental estimate P(Delta-Delta) < 0.4 % at 90 % of CL (D. Allasia et al., Phys. Lett. B174 (1986) 450).

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arXiv1999

Neutron-proton radiative capture, photo-magnetic and anti-neutrino disintegration of the deuteron in the relativistic field theory model of the deuteron

The cross sections for the M1-capture n + p -> D + gamma, the photo-magnetic and anti-neutrino disintegration of the deuteron are evaluated in the relativistic field theory model of the deuteron (RFMD). The cross section for M1-capture is evaluated by taking into account the contributions of chiral one-meson loop corrections and the Delta(1232) resonance. The cross sections for the photo-magnetic and anti-neutrino disintegration of the deuteron are evaluated by accounting for final-state interaction of the nucleon pair in the spinsinglet S-wave state. The amplitudes of low-energy elastic np and nn scattering contributing to these processes are obtained in terms of the S-wave scattering lengths and the effective ranges. This relaxes substantially the statement by Bahcall and Kamionkowski (Nucl. Phys. A625 (1997) 893) that the RFMD is unable to describe a non-zero effective range for low-energy elastic nucleon-nucleon scattering. The cross sections for the anti-neutrino disintegration of the deuteron averaged over the anti-neutrino energy spectrum agree good with experimental data.

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arXiv1999

The relativistic field theory model of the deuteron from low-energy QCD

The relativistic field theory model of the deuteron (RFMD) is reformulated from the first principles of QCD. The deuteron appears as a neutron-proton collective excitation, i.e. a Cooper np-pair, induced by a phenomenological local four-nucleon interaction in the nuclear phase of QCD. The RFMD describes the deuteron coupled to hadrons through one-nucleon loop exchanges providing a minimal transfer of nucleon flavours from initial to final nuclear states and accounting for contributions of nucleon-loop anomalies which are completely determined by one-nucleon loop diagrams. The dominance of contributions of nucleon-loop anomalies to effective Lagrangians of low-energy nuclear interactions is justified in the large N expansion, where N is the number of quark colours.

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arXiv1999

Solar proton burning, neutrino disintegration of the deuteron and pep process in the relativistic field theory model of the deuteron

The astrophysical factor S_pp(0) for the solar proton burning, p + p -> D + positron + neutrino, is recalculated in the relativistic field theory model of the deuteron (RFMD). We obtain S_pp(0) = 4.08 x 10^{-25} MeV b which agrees good with the recommended value S_pp(0) = 4.00 x 10^{-25} MeV b. The amplitude of low-energy elastic proton-proton (pp) scattering in the singlet S-wave state with the Coulomb repulsion contributing to the amplitude of the solar proton burning is described in terms of the S-wave scattering length and the effective range. This takes away the problem pointed out by Bahcall and Kamionkowski (Nucl. Phys. A625 (1997) 893) that in the RFMD one cannot describe low-energy elastic pp scattering with the Coulomb repulsion in agreement with low-energy nuclear phenomenology. The cross section for the neutrino disintegration of the deuteron, neutrino + D -> electron + p + p, is calculated with respect to S_pp(0) for neutrino energies from threshold to 10 MeV. The results can be used for the analysis of the data which will be obtained in the experiments planned by SNO. The astrophysical factor S_pep(0) for the pep process, p + electron + p -> neutrino + D, is calculated relative to S_pp(0) in complete agreement with the result obtained by Bahcall and May (ApJ. 155 (1969) 501).

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arXiv1999

Reaction rates for Neutron Capture Reactions to C-, N- and O-isotopes to the neutron rich side of stability

The reaction rates of neutron capture reactions on light nuclei are important for reliably simulating nucleosynthesis in a variety of stellar scenarios. Neutron capture reaction rates on neutron-rich C-, N-, and O-isotopes are calculated in the framework of a hybrid compound and direct capture model. The results are tabulated and compared with the results of previous calculations as well as with experimental results.

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arXiv1999

Measurement of neutron capture on 50Ti at thermonuclear energies

At the Karlsruhe and Tuebingen 3.75 MV Van de Graaff accelerators the thermonuclear 50Ti(n,gamma)51Ti(5.8 min) cross section was measured by the fast cyclic activation technique via the 320.852 and 928.65 keV gamma-ray lines of the 51Ti-decay. Metallic Ti samples of natural isotopic composition and samples of TiO2 enriched in 50Ti by 67.53 % were irradiated between two gold foils which served as capture standards. The capture cross-section was measured at the neutron energies 25, 30, 52, and 145 keV, respectively. The direct capture cross section was determined to be 0.387 +/- 0.011 mbarn at 30 keV. We found evidence for a bound state s-wave resonance with an estimated radiative width of 0.34 eV which destructively interfers with direct capture. The strength of a suggested s-wave resonance at 146.8 keV was determined. The present data served to calculate, in addition to the directly measured Maxwellian averaged capture cross sections at 25 and 52 keV, an improved stellar 50Ti(n,gamma)51Ti rate in the thermonuclear energy region from 1 to 250 keV. The new stellar rate leads at low temperatures to much higher values than the previously recommended rate, e.g., at kT=8 keV the increase amounts to about 50 %. The new reaction rate therefore reduces the abundance of 50Ti due to s-processing in AGB stars.

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arXiv1999

Thermal neutron induced (n,p) and (n,alpha) reactions on 37Ar

The 37Ar(n_th,alpha)34S and 37Ar(n_th,p)37Cl reactions were studied at the high flux reactor of the ILL in Grenoble. For the 37Ar(n_th,alpha_0) and 37Ar(n_th,p) reaction cross sections, values of (1070+/-80)b and (37+/-4)b, respectively, were obtained. Both values are about a factor 2 smaller than results of older measurements. The observed suppression of the 37(n_th,alpha_1) transition could be verified from theoretical considerations. Finally, evidence was found for the two-step 37Ar(n_th,gamma-alpha) process.

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