A. Mengoni

The s-process branching at W-185

The neutron capture cross section of the unstable nucleus W-185 has been derived from experimental photoactivation data of the inverse reaction W-186(gamma, n) W-185. The new result of sigma = 687 +/- 110 mbarn confirms the theoretically predicted neutron capture cross section of W-185 of sigma approximate to 700 mbarn at kT = 30 keV. A neutron density in the classical s-process of n(n) = (3.8(-0.9)(+1.1)) x 10(8) cm(-3) is derived from the new data for the W-185 branching. In a stellar s-process model, one finds a significant overproduction of the residual s-only nucleus Os-186.

Determination of the astrophysical S factor of the 8B(p,γ)9C capture reaction from the d(8B,9C)n reaction

The asymptotic normalization coefficients for the virtual decay 9 C → 8 B + p have been determined by measuring the crosssection of the 8 B(d, n) 9 C reaction in inverse kinematics at 14.4 MeV/u, using the RIPS facility. The deduced astrophysical S factor S18 of the 8 B(p ,γ ) 9 C capture reaction in the center of mass energy range 1–100 keV is S18 = 45 ± 13 eV b.  2001

Coulomb excitation of 11Be

Abstract The Coulomb excitation of a halo nucleus 11 Be to its first bound excited state (E1: 1 2 + → 1 2 − ) has been studied using a 64 MeV/u 11 Be radioactive beam incident on a 208 Pb target. The electric dipole strength deduced by a first-order perturbation analysis is in good agreement with the result of previous life-time measurements of the 11 Be 1 2 − state.

Neutron capture cross section of unstable 63Ni: implications for stellar nucleosynthesis.

The 63Ni(n,γ) cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from   kT=5-100  keV with uncertainties around 20%. Stellar model calculations for a 25M⊙ star show that the new data have a significant effect on the s-process production of 63Cu, 64Ni, and 64Zn in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova.

On the figure of merit in neutron time-of-flight measurements

Abstract We address the task of evaluating the basic performances of neutron time-of-flight spectrometers, by means of an appropriate figure of merit. Calculations of neutron flux, energy resolution, and figure of merit are presented with a critical discussion on their definitions. The results of Monte Carlo simulations for two presently operating facilities, CERN n_TOF and GELINA, are commented and compared.

The C 14 ( n , γ ) cross section between 10 keV and 1 MeV

The neutron capture cross section of 14C is of relevance for several nucleosynthesis scenarios such as inhomogeneous Big Bang models, neutron induced CNO cycles, and neutrino driven wind models for the r process. The 14C(n,gamma) reaction is also important for the validation of the Coulomb dissociation method, where the (n,gamma) cross section can be indirectly obtained via the time-reversed process. So far, the example of 14C is the only case with neutrons where both, direct measurement and indirect Coulomb dissociation, have been applied. Unfortunately, the interpretation is obscured by discrepancies between several experiments and theory. Therefore, we report on new direct measurements of the 14C(n,gamma) reaction with neutron energies ranging from 20 to 800 keV.

Coulomb dissociation of halo nuclei

Abstract Coulomb dissociation of one neutron halo nuclei 11Be and 15,19C has been studied at energies around 70A MeV by kinematically complete measurements of breakup particles at RIKEN. The experiment of 11Be breakup on a Pb target aims at studying the characteristic features of low-lying E1 strenght. In particular the effects of higher order excitation and nuclear contribution have been closely studied by using the impact parameter analysis. We have found that these contributions are very small, and the selection of the large impact parameter is useful to extract the spectroscopic information of the halo around state. We have then successfully applied this analysis method to 15C and 19C, where we have extracted the spectroscopic factor for the s-wave halo configuration. We also show preliminary results on the breakup of 11Be on a carbon target in order to study the nuclear breakup mechanism and low-lying discrete states of the halo nucleus.

Measurement of the stellar cross sections for the reactions 9Be(n,?)10Be and 13C(n,?)14C via AMS

We started a series of experiments, aiming at obtaining neutron-capture cross sections from measurements of the production of both long-lived nuclides 10Be and 14C, in the energy region of interest for astrophysical applications. Neutron-rich scenarios lead to the production of the long-lived radionuclides 10Be (t1/2 = 1.51 Myr) and 14C (t1/2 = 5730 yr) via neutron-capture reactions on 9Be and 13C, respectively. Data for both neutron-capture reactions are sparse and disagree in the astrophysical relevant energy region between 10 and 250 keV. Since (n,γ) cross sections in this mass and energy range are expected to be very small (some tens of µbarn), the combination of the activation technique and accelerator mass spectrometry (AMS) offers the powerful experimental approach which is necessary for such investigations. Beryllium oxide and enriched 13C graphite samples have been irradiated in a quasi-stellar neutron spectrum of kT = 25 keV at Forschungszentrum Karlsruhe. The subsequent AMS measurements were performed at the VERA (Vienna Environmental Research Accelerator) laboratory. The first AMS results confirm the expected low cross sections of about 10 µbarn for both reactions.

Nuclear Physics of the s Process

Starting from a sketch of the s-process concept formulated 50 years ago, the nuclear physics data for s-process calculations are briefly reviewed with emphasis on the status of neutron capture cross sections and beta decay rates. Accurate and comprehensive experimental data are mandatory as direct input for s-process calculations as well as for improving the complementary information from nuclear theory. The current challenges of the field are discussed in the light of new or optimized methods and state-of-the- art facilities, indicating the potential for accurate measurements and the possibility to study cross sections of radioactive isotopes. These opportunities will be considerably enriched by the enormous improvements provided by new facilities.

AMS Applications in Nuclear Astrophysics: New Results for 13C(n,γ)14C and 14N(n,p)14C

The technique of accelerator mass spectrometry (AMS) offers a complementary tool for studying long-lived radionuclides in nuclear astrophysics: (1) as a tool for investigating nucleosynthesis in the laboratory; and (2) via a direct search of live long-lived radionuclides in terrestrial archives as signatures of recent nearby supernova-events. A key ingredient to our understanding of nucleosynthesis is accurate cross-section data. AMS was applied for measurements of the neutron-induced cross sections 13C(n,γ) and 14N(n,p), both leading to the long-lived radionuclide 14C. Solid samples were irradiated at Karlsruhe Institute of Technology with neutrons closely resembling a Maxwell–Boltzmann distribution for kT = 25 keV, and with neutrons of energies between 123 and 178 keV. After neutron activation the amount of 14C nuclides in the samples was measured by AMS at the VERA (Vienna Environmental Research Accelerator) facility. Both reactions, 13C(n,γ)14C and 14N(n,p)14C, act as neutron poisons in s-process nucleosynthesis. However, previous experimental data are discordant. The new data for both reactions tend to be slightly lower than previous measurements for the 25 keV Maxwell–Boltzmann energy distribution. For the higher neutron energies no previous data did exist for 13C(n,γ), but model calculations indicated a strong resonance structure between 100 and 300 keV which is confirmed by our results. Very limited information is available for 14N(n,p) at these energies. Our new data at 123 and 178 keV suggest lower cross sections than expected from previous experiments and data evaluations.