Nuclear uncertainties in the NeNa-MgAl cycles and production of 22Na and 26Al during nova outbursts

Classical novae eject significant amounts of nuclear processed material into the interstellar medium. Among the isotopes synthesized during such explosions, two radioactive nuclei deserve a particular attention: 22Na and 26Al. In this paper, we investigate the nuclear paths leading to 22Na and 26Al production during nova outbursts by means of an implicit, hydrodynamic code that follows the course of the thermonuclear runaway from the onset of accretion up to the ejection stage. New evolutionary sequences of ONe novae have been computed, using updated nuclear reaction rates relevant to 22Na and 26Al production. Special attention is focused on the role played by nuclear uncertainties within the NeNa and MgAl cycles in the synthesis of such radioactive species. From the series of hydrodynamic models, which assume upper, recommended or lower estimates of the reaction rates, we derive limits on the production of both 22Na and 26Al. We outline a list of nuclear reactions which deserve new experimental investigations in order to reduce the wide dispersion introduced by nuclear uncertainties in the 22Na and 26Al yields.Comment: 46 pages, 4 figures. Accepted for publication in The Astrophysical Journa

Thermonuclear Reaction Rate of 23Mg(p,gamma)24$Al

Updated stellar rates for the reaction 23Mg(p,gamma)24Al are calculated by using all available experimental information on 24Al excitation energies. Proton and gamma-ray partial widths for astrophysically important resonances are derived from shell model calculations. Correspondences of experimentally observed 24Al levels with shell model states are based on application of the isobaric multiplet mass equation. Our new rates suggest that the 23Mg(p,gamma)24Al reaction influences the nucleosynthesis in the mass A>20 region during thermonuclear runaways on massive white dwarfs.Comment: 13 pages (uses Revtex) including 3 postscript figures (uses epsfig.sty), accepted for publication in Phys. Rev.

Measurement of direct neutron capture by neutron-rich sulfur isotopes

Thermal neutron capture cross sections for $^{34}$S(n,$\gamma$)$^{35}$S and $^{36}$S(n,$\gamma$)$^{37}$S have been measured and spectroscopic factors of the final states have been extracted. The calculated direct-capture cross sections reproduce the experimental data.Comment: 4 pages (uses espcrc1.sty), 1 postscript figure (uses psfig), accepted for publication in Nucl. Phys. A (Suppl.), uuencoded tex-files and postscript-files available at ftp://is1.kph.tuwien.ac.at/pub/ohu/Stherm.u

Direct neutron capture of 48Ca at kT = 52 keV

The neutron capture cross section of 48Ca was measured relative to the known gold cross section at kT = 52 keV using the fast cyclic activation technique. The experiment was performed at the Van-de-Graaff accelerator, Universitaet Tuebingen. The new experimental result is in good agreement with a calculation using the direct capture model. The 1/v behaviour of the capture cross section at thermonuclear energies is confirmed, and the adopted reaction rate which is based on several previous experimental investigations remains unchanged.Comment: 9 pages (uses Revtex), 2 postscript figures, accepted for publication as Brief Report in Phys. Rev.

Up-Down Quark Mass Difference Effect in Nuclear Many-Body Systems

A charge-symmetry-breaking nucleon-nucleon force due to the up-down quark mass difference is evaluated in the quark cluster model. It is applied to the shell-model calculation for the isovector mass shifts of isospin multiplets and the isospin-mixing matrix elements in 1s0d-shell nuclei. We find that the contribution of the quark mass difference effect is large and agrees with experiment. This contribution may explain the Okamoto-Nolen-Schiffer anomaly, alternatively to the meson-mixing contribution, which is recently predicted to be reduced by the large off-shell correction

Thermonuclear reaction rate of 23 Mg ( p , γ ) 24 Al

Updated stellar rates for the reaction 23Mg(p,g )24Al are calculated by using all available experimental information on 24Al excitation energies. Proton and g -ray partial widths for astrophysically important resonances are derived from shell-model calculations. Correspondences of experimentally observed 24Al levels with shell-model states are based on application of the isobaric multiplet mass equation. Our new rates suggest that the 23Mg(p,g )24Al reaction influences the nucleosynthesis in the mass A.20 region during thermonuclear runaways on massive white dwarfs

Deep learning-based parameter mapping for joint relaxation and diffusion tensor MR Fingerprinting

Magnetic Resonance Fingerprinting (MRF) enables the simultaneous quantification of multiple properties of biological tissues. It relies on a pseudo-random acquisition and the matching of acquired signal evolutions to a precomputed dictionary. However, the dictionary is not scalable to higher-parametric spaces, limiting MRF to the simultaneous mapping of only a small number of parameters (proton density, T1 and T2 in general). Inspired by diffusion-weighted SSFP imaging, we present a proof-of-concept of a novel MRF sequence with embedded diffusion-encoding gradients along all three axes to efficiently encode orientational diffusion and T1 and T2 relaxation. We take advantage of a convolutional neural network (CNN) to reconstruct multiple quantitative maps from this single, highly undersampled acquisition. We bypass expensive dictionary matching by learning the implicit physical relationships between the spatiotemporal MRF data and the T1, T2 and diffusion tensor parameters. The predicted parameter maps and the derived scalar diffusion metrics agree well with state-of-the-art reference protocols. Orientational diffusion information is captured as seen from the estimated primary diffusion directions. In addition to this, the joint acquisition and reconstruction framework proves capable of preserving tissue abnormalities in multiple sclerosis lesions

Shell model calculation of the beta- and beta+ partial halflifes of 54Mn and other unique second forbidden beta decays

The nucleus 54Mn has been observed in cosmic rays. In astrophysical environments it is fully stripped of its atomic electrons and its decay is dominated by the beta- branch to the 54Fe ground state. Application of 54Mn based chronometer to study the confinement of the iron group cosmic rays requires knowledge of the corresponding halflife, but its measurement is impossible at the present time. However, the branching ratio for the related beta+ decay of 54Mn was determined recently. We use the shell model with only a minimal truncation and calculate both beta+ and beta- decay rates of 54Mn. Good agreement for the beta+ branch suggests that the calculated partial halflife of the beta- decay, (4.94 \pm 0.06) x 10^5 years, should be reliable. However, this halflife is noticeably shorter than the range 1-2 x 10^6 y indicated by the fit based on the 54Mn abundance in cosmic rays. We also evaluate other known unique second forbidden beta decays from the nuclear p and sd shells (10Be, 22Na, and two decay branches of 26Al) and show that the shell model can describe them with reasonable accuracy as well.Comment: 4 pages, RevTeX, 2 figure

New stellar reaction rates for 25Mg(p,γ) 26Al and 25Al(p,γ) 26Si

Existing experimental proton stripping reaction data on 25Mg leading to threshold states in 26Al are reinvestigated and reanalyzed in a consistent and improved manner. We use unbound state form factors in the DWBA analysis of the measured deuteron angular distributions to deduce absolute rather than relative proton partial widths. For higher-lying resonances these values are compared to widths obtained from (p,γ) work. It is also shown that several of the unique Jπ values assigned previously to 26Al states are erroneous. This paper reports on a reanalysis of spins, parities, and isospins for 26Al states located at Ex<8.00 MeV. We deduce new stellar rates for the reaction 25Mg(p,γ)26Al and compare our results with previous values. Furthermore, shell-model calculations for the mass A = 26 system are performed. Theoretical excitation energies, Jπ values, γ-ray transition strengths, spectroscopic factors, and proton partial widths are compared to experimental data and new shell-model assignments of experimental states in 26Al are derived. We estimate Coulomb displacement energies of excited 26Mg and 26Si mirror states and present new analog assignments for T = 1 triplet states in A = 26 nuclei. Based on shell-model results and analog state information we present updated stellar rates for the 25Al(p,γ)26Si reaction

Positron-neutrino correlation in the 0^+ \to 0^+ decay of ^{32}Ar

The positron-neutrino correlation in the $0^+ \to 0^+ \beta$ decay of $^{32}$Ar was measured at ISOLDE by analyzing the effect of lepton recoil on the shape of the narrow proton group following the superallowed decay. Our result is consistent with the Standard Model prediction. For vanishing Fierz interference we find $a=0.9989 \pm 0.0052 \pm 0.0036$, which yields improved constraints on scalar weak interactions