Search

INPC 2013 – International Nuclear Physics Conference; 4 pags.; 3 figs.; Open Access funded by Creative Commons Atribution Licence 2.0, The astrophysical S-factor for the 3He(α,γ)7Be reaction plays an important role in the Solar Standard Model and in the Big Bang Nucleosynthesis scenario. The advances from two recent experiments performed using complementary techniques at center of mass (C.M.) energies between 1 and 3 MeV are discussed., The work was partly financed by the Spanish Research Funding Agency CICYT under Project FPA2012-32443 and was supported by the UK STFC. Further M.C.G. acknowledges his CSICJAEpredoc grant partly funded by the European Social Funds.

International audience

ITO International Research Center, on the campus of the University of Tokyo, June 1 to 6, 2014; http://ribf.riken.jp/ARIS2014/index.html, Due to its relevance to the Standard Solar Model and to the Big Bang Nucleosynthesis calculations, the cross section of the 3He(α, γ)7Be reaction have been widely studied both from the theoretical and the experimental fronts. We report here on cross section measurements in the energy region Ec.m.=1- 3 MeV using the direct recoil counting method in an attempt to solve the discrepancies among the previous data sets and calculations in this energy region and thus to constrain the extrapolations of the S34(E) curve to astrophysical energies.

Preliminary studies in preparation for an absolute cross-section measurement of the radiative capture reaction 3He(α,γ)7Be with the DRAGON recoil separator have demonstrated beam suppression >1014 at the 90% confidence level. A measurement of this cross section by observation of 7Be recoils at the focal plane of the separator should be virtually background free. & 2012 Elsevier B.V.

An experimental campaign to perform the first accurate measurement of the 17O(a,g)21Ne reaction rate was carried out during May and November 2009 at the DRAGON recoil separator in the TRIUMF laboratory, Vancouver, Canada. The goals were to differentiate between two previous conflicting theoretical predictions, and to establish the (a,g)/(a,n) reaction rate ratio.Two distinct energy regions were scanned: a higher region, Ecm 1.5 MeV, where the recoil cone was sufficiently confined and the cross section sufficiently large so that a statistically significant yield could be achieved, and a lower region, Ecm 0.7 MeV, chosen to approach the astrophysicalenergies and reduce errors from extrapolation. The experiment, performed in inverse kinematics, used the highest intensity beam ever deliveredto DRAGON, 11012 pps of 17O incident on a 4He gas target. An array of 30 Bismuth Germanate (BGO) detectors [1] was used in conjunction with two microchannel plates (MCP) [2] for a local time-of-flight measurement, and ion chamber for coincident event detection.

Active targets are perfect tools for the study of reactions induced by radioactive ion beams of very low intensity. Thus, they make possible the study of direct and compound reactions at the same time. In this talk, we will present the results obtained with the MAYA active target for low energy reactions induced by the halo nucleus 11Li. A method used to measure the mass of short-lived nuclei, applied to the 11Li case will be discussed. The experimantal angular distribution of the two halo-neutron transfer reaction 1H(11Li,9Li)t at 4.3A MeV, using the MAYA detector as a thin target, will be presented and compared to coupled reaction channel calculations including 3-body models in order to extract informations on the structure of the 11Li halo. The extraction of the 11Li+p optical potential using the data on the elastic scattering, will also be discussed. Finally, the excitation function of the resonant elastic scattering reaction, determined using the MAYA detector as a thick target, will be presented. Extraction of information on the structure of possible 12Li isobaric analog states observed in 12Be using several R-matrix calculations will be shown.

The first measurement of the momentum distribution for one-neutron removal from 24 O at 920 A MeV performed at GSI, Darmstadt is reported. The observed distribution has a width (FWHM) of 99 4 MeV =c in the projectile rest frame and a one-neutron removal cross section of 63 7mb . The results are well explained with a nearly pure 2 s 1 = 2 neutron spectroscopic factor of 1 : 74 0 : 19 within the eikonal model. This large s -wave probability shows a spherical shell closure thereby confirming earlier suggestions that 24 O is a new doubly magic nucleus. ispartof: Physical Review Letters vol:102 issue:15 ispartof: location:United States status: published

Active target MAYA, expérience TRIUMF; The mass of 11Li has been determined from Q-value measurements of the 1H(11Li, 9Li)3H reaction. The experiment was performed at TRIUMF laboratory with the GANIL active target MAYA. Energy-energy and angle-angle kinematics reconstruction give a Q value of 8.119(22) MeV for the reaction. The derived 11Li two-neutron separation energy is S2n = 363(22) keV.

The direct and statistical neutron decay of the isoscalar giant dipole resonance was studied in Zr-90, Sn-116, and Pb-208 using the (alpha,alpha(')n) reaction at a bombarding energy of 200 MeV. The spectra of fast decay neutrons populating valence hole-states of the Z,N-1 nuclei were analyzed, and estimates for the branching ratios were determined. The results were compared to recent continuum random-phase approximation calculations.

Excitation functions for total reaction cross sections, sigma(R), were measured for the light, mainly proton-rich nuclei Li-6, Be-7, B-10, C-9,C-10,C-11, N-12, O-13,O-15, and Ne-17 incident on a Si telescope at energies between 15 and 53 MeV/nucleon. The telescope served as target, energy degrader and detector. Proton-removal cross sections, sigma(2p) for Ne-17 and sigma(p) for most of the other projectiles, were also measured. The strong absorption model reproduces the A-dependence of sigma(R), but not the detailed structure. Glauber multiple scattering theory and the Jeukenne, Lejeune, and Mahaux (JLM) folding model provided improved descriptions of the measured sigma(R) values. Rms radii, extracted from the measured sigma(R) using the optical limit of Glauber theory, are in good agreement with those obtained from high energy data. One-proton removal reactions are described using an extended Glauber model, incorporating second order noneikonal corrections, realistic single particle densities, and spectroscopic factors from shell model calculations.

The Doppler-shift attenuation method was applied to measure the lifetime of the 4.03 MeV state in 19Ne. Utilizing a 3He-implanted Au foil as a target, the state was populated using the 20Ne(3He,alpha)19Ne reaction in inverse kinematics at a 20Ne beam energy of 34 MeV. De-excitation gamma rays were detected in coincidence with alpha particles. At the 1 sigma level, the lifetime was determined to be 11 +4, -3 fs and at the 95.45% confidence level the lifetime is 11 +8, -7 fs., Comment: 6 pages, submitted to Phys. Rev. C

NESTER PTH; We have used the 43 MeV/nucleon primary tritium beam of the AGOR facility with an intensity of 4×107 pps and the BBS experimental setup to study the (t,3He) reaction between 0° and 5° lab angles on 12C, 48Ca, and 58Ni targets. The standard ray-tracing procedure has allowed us to obtain excitation-energy spectra up to 30 MeV in six angular bins for each residual nucleus, with an average energy resolution of 350 keV. The reaction mechanism has been described in distorted-waves Born approximation (DWBA) using the DWBA98 code. In this approximation, the form factor is treated as a folding of an effective projectile-nucleon interaction with a transition density. The effective projectile-nucleon interaction has been adjusted to reproduce the 0° cross section of the 1+ ground state of 12B populated in the 12C(t,3He) reaction. We have employed random-phase approximation (RPA) wave functions of excited states to construct the form factor instead of the normal modes wave functions used earlier. This new DWBA+RPA analysis is used to compare calculated and experimental cross sections directly and to discuss the giant resonance excitations in 48K and 58Co nuclei.

A feasibility study was made of an important aspect of the Coulomb-dissociation method, which has been proposed for the determination of the rate of the astrophysically important C-12(alpha, gamma)160 reaction. A crucial aspect is the disentanglement of nuclear and Coulomb interactions on one hand and the separation of dipole and quadrupole contributions on the other. As a first step the resonant breakup via two well-known 2(+) states of 160 was measured. The differential cross section of Pb-208(O-16, O-16*)Pb-208 and the angular correlations of the fragments C-12 and alpha in the center of mass were measured and compared to theoretical predictions calculated in DWBA and the coupled-channel method. The best agreement was found for the state at 11.52 MeV associated to a one-step excitation from the ground state, while the 9.84 MeV requires coupling to the first-excited 2(+) state and is not well described. (c) 2005 Elsevier B.V. All rights reserved.

We have used the (d,He-2) reaction to obtain the GT(+) strength distribution for Sn-116 ->In-116 transitions. Here, In-116 is the intermediate nucleus in the second-order perturbative description of Cd-116 double beta (beta beta) decay. In this paper we will review what is known about the nuclear matrix elements for the Cd-116 2 nu beta beta decay, discuss the single-state dominance hypothesis and combine our new data with other existing data from charge-exchange reactions to obtain a 2 nu beta beta-decay half-life. The deduced value T-1/2=(4 +/- 1)x10(19)y compares well with the one known from direct counting experiments.

Cross sections for the charge-exchange reaction (d,He-2) on V-50, the only odd-odd stable nucleus in the pf shell, have been measured at an incident energy of 171 MeV. The two correlated protons of the unbound He-2 system were both momentum analyzed and detected by the same spectrometer and detector. Measurements have been carried out at forward center-of-mass angles up to 6degrees. The GT(+) strength distribution has been extracted up to about 12 MeV of excitation energy. Because charge-exchange reactions from V-50 provide a unique opportunity to probe Gamow-Teller response from an odd-odd nucleus of the pf shell, the present study serves as important test case for model calculations and astrophysical processes that proceed through GT-type transitions. Consequently, the experimental result is compared with a large-scale shell-model calculation, and the implications for electron-capture processes in presupernova environments are discussed.

The question, whether the high-quality nucleon-nucleon potentials can successfully describe the three-nucleon system, and to what extent three-nucleon forces (3NFs) play a role, has become very important in nuclear few-body physics. One kinematic region where effects because of 3NFs show up is in the minimum of the differential cross section of elastic nucleon-deuteron scattering. Another observable, which could give an indication about the contribution of the spin to 3NFs, is the vector analyzing power. To investigate the importance of 3NFs systematically over a broad range of intermediate energies, both observables of elastic proton-deuteron scattering have been measured at proton bombarding energies of 108, 120, 135, 150, 170, and 190 MeV, covering an angular range in the center-of-mass system between 30(degrees) and 170(degrees). The results show unambiguously the shortcomings of calculations employing only two-body forces and the necessity of the inclusion of 3NFs. They also show the limitations of the results of the present day models for few-nucleon systems at backward angles, especially at higher beam energies. New calculations based on chiral perturbation theory are also presented and compared with the data at the lowest energy.

The two-neutrino double-beta (2nubetabeta) decay represents a test case for our knowledge about the nuclear structure of the involved isobars. The decay of Ca-48 is an especially interesting case because it holds an anomaly of a uniquely high Q(betabeta) value and a comparatively long half-life, which points to a peculiar nuclear structure situation. The nuclear matrix element relevant for betabeta decay can be calculated, if the complete set of Gamow-Teller (GT) matrix elements for the two virtual transitions in the perturbative description are known. Using the high-resolution (d, He-2) probe, we have measured the GT(+) strength distribution in Sc-48, which is the intermediate nucleus in the Ca-48 betabeta decay. By combining our measured GT distribution with data from a Ca-48(p, n) experiment and taking into account relative phases between individual matrix elements, which can be gained from theoretical models, one can compute the double-GT matrix element and deduce the half-life of the Ca-48 2nubetabeta decay.

NESTER PTH

A 130 MeV primary tritium beam, newly developed at the superconducting cyclotron AGOR, has been used for the investigation of 1 hw and 2 hw isovector giant resonances in Co-58 via the r(+) channel. The angular distributions of double-differential cross sections have been extracted between E-x and 30 MeV from theta(lab) = 0degrees to 6degrees. A DWBA analysis have been extracted between E. with normal-mode (NM) wave functions and a multipole-decomposition analysis (MDA) indicate strength for the monopole resonances (dominated by the spin-flip transition) fragmented between 10 and 27 MeV excitation energy, in rather good agreement with preliminary RPA calculations.

Populating states in Mg-22 via the (p,t) reaction in inverse kinematics with a 55-MeV /nucleon Mg-24 beam, we have measured the proton-decay branching ratios of the levels at 5.96 MeV and 6.05 MeV and obtained an experimental upper limit on the branching ratio of the 5.71-MeV state. On the basis of the present and previous measurements, we assign spins and parities to the 5.96-MeV and 6.05-MeV states. We combine our branching ratios with independent measurements of the lifetimes of these states or their Ne-22 analogs to compute the resonance strengths and thereby the astrophysical rate of the Na-21(p,gamma)Mg-22 reaction. We perform hydrodynamic calculations of nova outbursts with this new rate and analyze its impact on Na-22 yields.

We present S factor data obtained from the Coulomb dissociation of 83 MeV/nucleon B-8, and analyze Be-7 longitudinal momentum distributions measured at 44 and 81 MeV/nucleon using a potential model, first-order perturbation theory, and dynamical solution of the time-dependent Schrodinger equation. Comparing our results with independent continuum-discretized coupled channels calculations, we study the reaction model and beam energy dependence of the E2 contribution to the dissociation cross section. By fitting radiative capture and Coulomb breakup data taken below relative energies of 400 keV with potential models constrained by Li-7+n and Be-7+p elastic scattering data, we examine the mutual consistency of recent S-17 measurements and obtain a recommended value for S-17(0) of 18.6+/-0.4 (experimental) +/-1.1 (extrapolation) eV b (1sigma). This result is in good agreement with recent experimental determinations of the asymptotic normalization coefficient of the valence proton wave function in B-8.

The V-51(d,He-2)Ti-51 charge-exchange reaction has been investigated at an incident energy of 171 MeV and scattering angles near 0degrees. The two protons in the S-1(0)(pp) state (denoted as He-2) were both momentum analyzed and detected by the same spectrometer and detector. Spectra with a resolution of about 125 keV (full width at half maximum) have been obtained allowing identification of many levels in Ti-51 with high precision. The GT(+) strength distribution for transitions to levels up to about 6 MeV of excitation energy has been extracted. The results are compared with a large-scale shell-model calculation in the full pf shell.

A recoil coincidence technique has been applied to measure the alpha-decay branching ratios of near-threshold states in Ne-19. Populating these states using the (p,t) reaction in inverse kinematics, we detected the recoils and their decay products with 100% geometric efficiency using a magnetic spectrometer. Combining our branching ratio measurements with independent determinations of the radiative widths of these states, we calculate the astrophysical rate of O-15(alpha,gamma)Ne-19. Using this reaction rate, we perform hydrodynamic calculations of nova outbursts and conclude that no significant breakout from the hot CNO cycles into the rp process occurs in novae via O-15(alpha,gamma)Ne-19.

The 15O(alpha,gamma)19Ne reaction is one of two known routes for breakout from the hot CNO cycles into the rp process. Its astrophysical rate depends on the decay properties of excited states in 19Ne lying just above the 15O + alpha threshold. We have measured the alpha-decay branching ratios for these states using the p(21Ne,t)19Ne reaction at 43 MeV/u. Combining our branching ratio measurements with previous determinations of the radiative widths of these states, we calculate the astrophysical rate of 15O(alpha,gamma)19Ne. Using this reaction rate, we perform hydrodynamic calculations of nova outbursts and conclude that no significant breakout from the hot CNO cycles into the rp process occurs in classical novae via 15O(alpha,gamma)19Ne., Comment: Contribution to proceedings of the 2003 Hirschegg Workshop on Nuclear Structure and Dynamics at the Limits

Populating states in 22Mg via the (p,t) reaction in inverse kinematics with a 55 MeV/nucleon 24Mg beam, we have measured the proton-decay branching ratios of the levels at 5.96 MeV and 6.05 MeV and obtained an experimental upper limit on the branching ratio of the 5.71 MeV state. On the basis of the present and previous measurements, we assign spins and parities to the 5.96 MeV and 6.05 MeV states. We combine our branching ratios with independent measurements of the lifetimes of these states or their 22Ne analogs to compute the resonance strengths and thereby the astrophysical rate of the 21Na(p,g)22Mg reaction. We perform hydrodynamic calculations of nova outbursts with this new rate and analyze its impact on 22Na yields., Comment: 8 pages, 6 figures, 1 table, submitted to PRC