Confirmation of a new resonance in Si 26 and contribution of classical novae to the galactic abundance of Al 26

The 25Al(p,¿) reaction has long been highlighted as a possible means to bypass the production of 26Al cosmic ¿ rays in classical nova explosions. However, uncertainties in the properties of key resonant states in 26Si have hindered our ability to accurately model the influence of this reaction in such environments. We report on a detailed ¿-ray spectroscopy study of 26Si and present evidence for the existence of a new, likely l=1, resonance in the 25Al + p system at Er=153.9(15) keV. This state is now expected to provide the dominant contribution to the 25Al(p,¿) stellar reaction rate over the temperature range, T˜0.1-0.2 GK. Despite a significant increase in the rate at low temperatures, we find that the final ejected abundance of 26Al from classical novae remains largely unaffected even if the reaction rate is artificially increased by a factor of 10. Based on new, galactic chemical evolution calculations, we estimate that the maximum contribution of novae to the observed galactic abundance of 26Al is ˜0.2M¿. Finally, we briefly highlight the important role that super-asymptotic giant branch stars may play in the production of 26Al.Postprint (published version

Eksperimentalni dokazi (pokazatelji) molekulske strukture lakih jezgri

Experimental studies of the cluster structures in 9,10Be and 13,14C have been performed. Evidence for the α+5He decay of 9Be excited states, α+6He decays of 10Be∗ , α+9Be decay of 13C ∗ and for the decays of 14C ∗ into α+10Be were found. These decay processes indicate the cluster structures of the excited states. The results are compared with recent suggestions for the presence of molecule-like structures based upon α-particles and valence neutrons in Be and C isotopes.Eksperimentalno smo proučavali nakupinske strukture u 9,10Be i 13,14C. Našli smo α+5He raspad uzbuđenog stanja 9Be, α+6He raspad 10Be∗ , α+9Be raspad 13C ∗ te raspad 14C ∗ u α+10Be. Ovi procesi ukazuju na nakupinske strukture uzbuđenih stanja tih jezgri. Ishode mjerenja uspoređujemo s novim objašnjenjima o prisutnosti molekulama-sličnih struktura zasnovanim na α-česticama i valentnim neutronima u izotopima Be i C

Anomalies in the Charge Yields of Fission Fragments from the U(n,f)238 Reaction

Fast-neutron-induced fission of 238U at an energy just above the fission threshold is studied with a novel technique which involves the coupling of a high-efficiency γ-ray spectrometer (MINIBALL) to an inverse-kinematics neutron source (LICORNE) to extract charge yields of fission fragments via γ−γ coincidence spectroscopy. Experimental data and fission models are compared and found to be in reasonable agreement for many nuclei; however, significant discrepancies of up to 600% are observed, particularly for isotopes of Sn and Mo. This indicates that these models significantly overestimate the standard 1 fission mode and suggests that spherical shell effects in the nascent fission fragments are less important for low-energy fast-neutron-induced fission than for thermal neutron-induced fission. This has consequences for understanding and modeling the fission process, for experimental nuclear structure studies of the most neutron-rich nuclei, for future energy applications (e.g., Generation IV reactors which use fast-neutron spectra), and for the reactor antineutrino anomaly

Coulomb dissociation of N 20,21

Neutron-rich light nuclei and their reactions play an important role in the creation of chemical elements. Here, data from a Coulomb dissociation experiment on N20,21 are reported. Relativistic N20,21 ions impinged on a lead target and the Coulomb dissociation cross section was determined in a kinematically complete experiment. Using the detailed balance theorem, the N19(n,γ)N20 and N20(n,γ)N21 excitation functions and thermonuclear reaction rates have been determined. The N19(n,γ)N20 rate is up to a factor of 5 higher at

Direct measurement of astrophysically important resonances in 38K( p,γ )39Ca

Background: Classical novae are cataclysmic nuclear explosions occurring when a white dwarf in a binary system accretes hydrogen-rich material from its companion star. Novae are partially responsible for the galactic synthesis of a variety of nuclides up to the calcium ( A ∼ 40 ) region of the nuclear chart. Although the structure and dynamics of novae are thought to be relatively well understood, the predicted abundances of elements near the nucleosynthesis endpoint, in particular Ar and Ca, appear to sometimes be in disagreement with astronomical observations of the spectra of nova ejecta. Purpose: One possible source of the discrepancies between model predictions and astronomical observations is nuclear reaction data. Most reaction rates near the nova endpoint are estimated only from statistical model calculations, which carry large uncertainties. For certain key reactions, these rate uncertainties translate into large uncertainties in nucleosynthesis predictions. In particular, the 38 K ( p , γ ) 39 Ca reaction has been identified as having a significant influence on Ar, K, and Ca production. In order to constrain the rate of this reaction, we have performed a direct measurement of the strengths of three candidate ℓ = 0 resonances within the Gamow window for nova burning, at 386 ± 10 keV, 515 ± 10 keV, and 689 ± 10 keV. Method: The experiment was performed in inverse kinematics using a beam of unstable 38 K impinged on a windowless hydrogen gas target. The 39 Ca recoils and prompt γ rays from 38 K ( p , γ ) 39 Ca reactions were detected in coincidence using a recoil mass separator and a bismuth-germanate scintillator array, respectively. Results: For the 689 keV resonance, we observed a clear recoil- γ coincidence signal and extracted resonance strength and energy values of 120 + 50 − 30 ( stat . ) + 20 − 60 ( sys . ) meV and 679 + 2 − 1 ( stat . ) ± 1 ( sys . ) keV , respectively. We also performed a singles analysis of the recoil data alone, extracting a resonance strength of 120 ± 20 ( stat . ) ± 15 ( sys . ) meV, consistent with the coincidence result. For the 386 keV and 515 keV resonances, we extract 90 % confidence level upper limits of 2.54 meV and 18.4 meV, respectively. Conclusions: We have established a new recommended 38 K ( p , γ ) 39 Ca rate based on experimental information, which reduces overall uncertainties near the peak temperatures of nova burning by a factor of ∼ 250 . Using the rate obtained in this work in model calculations of the hottest oxygen-neon novae reduces overall uncertainties on Ar, K, and Ca synthesis to factors of 15 or less in all cases.</p