Search Results (20 total)

The ¹³C(α,n)¹⁶O reaction is considered the main neutron source for the s process in low mass asymptotic giant branch (AGB) stars. In the Gamow peak, the cross section sensitively depends on the 1/2⁺ subthreshold state of ¹⁷O (E_x=6.356 MeV). In this work, we determined the astrophysical S factor through an evaluation of the α spectroscopic factor and the corresponding asymptotic normalization factor (ANC) of the 6.356 MeV state using the transfer reaction ¹³C(⁷Li,t)¹⁷O at two different incident energies. Our result confirms that the contribution of the 1/2⁺ state is dominant at astrophysical energies. Our reaction rate at T=0.09 GK is slightly lower than the value adopted in the Nuclear Astrophysics Compilation of REaction rates (NACRE), but two times larger than the one obtained in a recent ANC measurement.

γ-ray production cross sections for proton and α-particle interactions with ¹²C, ¹⁶O, ²⁴Mg, and Fe have been measured in the energy range 5–25 MeV with proton beams and 5–40 MeV with α-particle beams. Isotopically pure foils of ²⁴Mg and foils of natural isotopical composition of C, MgO, and Fe have been used. γ-ray angular distributions were obtained with five high-purity Ge detectors with bismuth germanate Compton shields placed at angles of 45^° to 157.5^°. Cross sections for more than 50 different γ-ray transitions were extracted, and for many of them no data have been published before. Comparison of present data with data available in the literature shows mostly good to excellent agreement. In addition to the production cross sections, high-statistics, low-background line shapes of the 4.438 MeV ¹²C γ ray from inelastic scattering off ¹²C and spallation of ¹⁶O were obtained. Comparison with nuclear reaction calculations shows that these data place interesting constraints on nuclear reaction models.

The ¹⁷O(p,α)¹⁴N and ¹⁷O(p,γ)¹⁸F reactions are of major importance to hydrogen-burning nucleosynthesis in a number of different stellar sites. In particular, ¹⁷O and ¹⁸F nucleosynthesis in classical novae is strongly dependent on the thermonuclear rates of these two reactions. The previously estimated rate for ¹⁷O(p,α)¹⁴N carries very large uncertainties in the temperature range of classical novae (T=0.01–0.4 GK), whereas a recent measurement has reduced the uncertainty of the ¹⁷O(p,γ)¹⁸F rate. We report on the observation of a previously undiscovered resonance at E_c.m.=183.3 keV in the ¹⁷O(p,α)¹⁴N reaction, with a measured resonance strength ωγ_pα=(1.6±0.2)×10^-3 eV. We studied in the same experiment the ¹⁷O(p,γ)¹⁸F reaction by an activation method, and the resonance strength was found to amount to ωγ_pγ=(2.2±0.4)×10^-6 eV. The excitation energy of the corresponding level in ¹⁸F was determined to be 5789.8±0.3 keV in a Doppler shift attenuation method measurement, which yielded a value of τ<2.6 fs for the level lifetime. The ¹⁷O(p,α)¹⁴N and ¹⁷O(p,γ)¹⁸F reaction rates were calculated using the measured resonance properties and reconsidering some previous analyses of the contributions of other levels or processes. The ¹⁷O(p,α)¹⁴N rate is now well established below T=1.5 GK, with uncertainties reduced by orders of magnitude in the temperature range T=0.1–0.4 GK. The uncertainty in the ¹⁷O(p,γ)¹⁸F rate is somewhat larger because of remaining obscurities in the knowledge of the direct capture process. These new resonance properties have important consequences for ¹⁷O nucleosynthesis and γ-ray emission of classical novae.

A new experiment to determine the thermonuclear cross section of the ¹²C(α,γ)¹⁶O reaction has been performed in regular kinematics using an intense α-particle beam of up to 340 μA from the Stuttgart DYNAMITRON. For the first time, a 4π-germanium-detector setup has been used to measure the angular distribution of the γ rays at all angles simultaneously. It consisted of an array of nine EUROGAM high-purity Ge detectors in close geometry, actively shielded individually with bismuth germanate crystals. The ¹²C targets were isotopically enriched by magnetic separation during implantation. The depth profiles of the implanted carbon in the ¹²C targets were determined by Rutherford backscattering for purposes of cross-section normalization and absolute determination of the E1 and E2  S factors. Angular distributions of the γ decay to the ¹⁶O ground state were measured in the energy range E_c.m.=1.30–2.78 MeV and in the angular range (lab.) 30°–130°. From these distributions, astrophysical E1 and E2  S-factor functions vs energy were calculated, both of which are indispensable to the modeling of this reaction and the extrapolation toward lower energies. The separation of the E1 and E2 capture channels was done both by taking the phase value ϕ₁₂ as a free parameter and by fixing it using the results of elastic α-particle scattering on ¹²C in the same energy range.

We report on the observation of a previously unknown resonance at E_R^lab=194.1±0.6  keV in the ¹⁷O(p,α)¹⁴N reaction, with a measured resonance strength ωγ_pα=1.6±0.2  meV. We studied in the same experiment the ¹⁷O(p,γ)¹⁸F reaction by an activation method and the resonance-strength ratio was found to be ωγ_pα/ωγ_pγ=470±50. The corresponding excitation energy in the ¹⁸F compound nucleus was determined to be 5789.8±0.3  keV by γ-ray measurements using the ¹⁴N(α,γ)¹⁸F reaction. These new resonance properties have important consequences for ¹⁷O nucleosynthesis and γ-ray astronomy of classical novae.

γ-ray production cross sections have been measured for γ-ray lines copiously emitted in the ³He bombardment of ¹⁶O nuclei: the 937-, 1042-, and 1081-keV lines of ¹⁸F and the 1887-keV line of ¹⁸Ne. Four Ge detectors with BGO shielding for Compton suppression were used to measure the angular distributions of the γ rays. The excitation functions have been obtained for ³He bombarding energies from 3.7 to 36 MeV. Total cross sections are tabulated for calculations relevant to γ-ray astronomy. The importance of these lines as diagnosis for the presence and properties of accelerated ³He in solar flares is discussed in light of the measured cross sections.

The ¹⁸F(p,α)¹⁵O reaction is recognized to be one of the most important reactions for nova γ-ray astronomy, as it governs the early E<~511 keV γ emission. However, in the nova temperature regime, its rate remains largely uncertain due to unknown low-energy resonance strengths. We report here the measurement of the ²H(¹⁸F,p)¹⁹F(α)¹⁵N one-nucleon transfer reaction, induced by a 14-MeV ¹⁸F radioactive beam impinging on a CD₂ target; outgoing protons and ¹⁵N (or α particles) were detected in coincidence in two silicon strip detectors. A distorted-wave Born approximation analysis of the data resulted in new limits to the contribution of low-energy resonances to the rate of the ¹⁸F(p,α)¹⁵O reaction.

The neutron-rich ^66,68Ni have been produced at GANIL via interactions of a 65.9A MeV ⁷⁰Zn beam with a ⁵⁸Ni target. Their reduced transition probability B(E2;0₁⁺→2⁺) has been measured for the first time by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. The B(E2) value for ⁶⁸Ni₄₀ is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the B(E2) values and indicates the erosion of the N  =  40 harmonic-oscillator subshell by neutron-pair scattering.

We calculated in detail the angular distribution of γ rays and the resulting shape of the γ-ray line produced by the nuclear deexcitation of the 4.439 MeV state of ¹²C following proton and α-particle interactions with ¹²C and ¹⁶O in the energy range from threshold to 100 MeV per nucleon, making use of available experimental data. In the proton energy range from 8.6 to 20 MeV, the extensive data set of a recent accelerator experiment on γ-ray line shapes and angular distributions was used to deduce parametrizations for the γ-ray emission of the 2⁺, 4.439 MeV state of ¹²C following inelastic proton scattering off ¹²C and proton induced spallation of ¹⁶O. At higher proton energies and for α-particle induced reactions, optical model calculations were the main source to obtain the needed reaction parameters for the calculation of γ-ray line shapes and angular distributions. Line shapes are predicted for various interaction scenarios of accelerated protons and α particles in solar flares.

We have measured the cross section of the ⁷Be(p,γ)⁸B reaction for E_c.m.  =  185.8, 134.7, and 111.7 keV using a radioactive ⁷Be target (132 mCi). Single and coincidence spectra of β⁺ and α particles from ⁸B and ⁸Be^* decay, respectively, were measured using a large acceptance spectrometer. The zero energy S factor inferred from these data is 18.5±2.4 eV b and a weighted mean value of 18.8±1.7 eV b (theoretical uncertainty included) is deduced when combining this value with our previous results at higher energies.

We present direct evidence for the process of internal conversion between bound atomic states (BIC) when the binding energy of the converted electron becomes larger than the nuclear transition energy. This process has been proposed as an explanation of the measured, unexpectedly short lifetime of the first excited state of ¹²⁵Te with charge state larger than 44⁺. We have detected the K_α x rays emitted in flight which follow the filling of the K-shell vacancy created by the bound internal conversion process, together with γ rays from Te ions in charge states ranging between 44⁺ and 48⁺. For Te⁴⁵⁺ and Te⁴⁶⁺, the comparison of the x-ray to γ-ray ratios with the theoretical calculations of the internal conversion coefficients including decay to bound atomic states, assuming Te ions in their ground electronic state, show poor agreement. The agreement becomes good if account is taken of BIC decay of excited initial states with different occupancies of the 2p_1/2 and 2p_3/2 subshells. In this situation, the half-life becomes sensitive to the precise initial state and simple specification of the charge state alone is no longer appropriate.

Excitation functions for the production of the 2.74, 6.13, 6.92, and 7.12 MeV γ rays by inelastic proton scattering on ¹⁶O have been measured in steps of 200–500 keV for proton energies E_p=8.4–20 MeV. For the 4.44 MeV γ rays of ¹²C produced by inelastic proton scattering on ¹²C data have been obtained for proton energies E_p=8.4–14 MeV and at E_p=17.25, 18.25, and 19.75 MeV. Eight high efficiency Ge detectors with anti-Compton shielding for the γ-ray detection were used to obtain the laboratory γ-angular distributions. The results of Legendre polynom fits to these distributions are discussed and presented in suitable form for use in γ-ray astronomy.

Cross sections for the ⁷Be(p,γ)⁸B reaction have been measured for E_c.m.  =  0.35–1.4 MeV using radioactive ⁷Be targets. Two independent measurements carried out with different beam conditions, different targets, and detectors are in excellent agreement. A statistical comparison of these measurements with previous results leads to a restricted set of consistent data. The deduced zero-energy S factor S(0) is found to be 15%–20% smaller than the previously recommended value. This implies a ⁸B solar neutrino flux lower than previously predicted in various standard solar models.

A disagreement between two determinations of Γ_α of the astrophysically relevant level at E_x=4.378 MeV in ¹⁹F has been stated in two recent papers by Wilmes et al. and de Oliveira et al. In this work the uncertainties of both papers are discussed in detail, and we adopt the value Γ_α = (1.5_-0.8^+1.5)×10^-9  eV for the 4.378 MeV state. In addition, the validity and the uncertainties of the usual approximations for mirror nuclei Γ_γ(¹⁹F)≈Γ_γ(¹⁹Ne), θ_α²(¹⁹F)≈θ_α²(¹⁹Ne) are discussed, together with the resulting uncertainties on the resonance strengths in ¹⁹Ne and on the ¹⁵O(α,γ)¹⁹Ne rate.

We have studied the ionic charge state dependence of the nuclear lifetime of the 35.5-keV first excited state of ¹²⁵Te. We found for the 47⁺ and 48⁺ ions, 400 and 670 % variations with respect to the neutral-atom half-life (1.49 ns). These unusually large effects are due to the energetic blocking of the K-shell internal conversion as the charge state increases past 47⁺. For the 46⁺, we suggest a new internal conversion mode without any electron emission into the energy continuum.

We have studied the atomic charge state dependence of the nuclear lifetime of the 35.5-keV first excited state of ¹²⁵Te. For the 47⁺ and 48⁺ ions, 300% and 640% increases, respectively, of the half-life were found with respect to the neutral-atom value (1.49 ns). These unusually large effects are due to the energetic blocking of K-shell internal conversion as the charge state increases past 47⁺.

We present some preliminary calculations on cross sections for the breakup of ¹⁶O around 100 MeV/nucleon with emphasis on the effect of nuclear breakup on the angular distributions. Underlying the results of these calculations, the possibilities and problems of extracting the astrophysical S factor for the ¹²C(α,γ)¹⁶O reaction at very low energies are discussed. Some considerations on the experimental conditions for a ¹⁶O breakup experiment aiming at this astrophysical information are given.

Clear experimental evidence of plasma stripping enhancement for 1.5 MeV/u chlorine ions is contrasted with its cold gas homologue at the same density (1×10¹⁹ e^-/cm^-2). The plasma is created by an electrical discharge in hydrogen and diagnosed with optical spectroscopic methods and beam probe analysis. The velocity dependence for the charge distributions is modeled both in cold gas and in plasma experiments. The calculations are found to be in good agreement with the hypothesis of a strongly reduced capture rate in the plasma case. Those results document quantitatively the basic mechanisms underlying the enhanced ion projectile charge in a plasma target. They also add credence to currently advocated scenarios for heavy-ion-driven fusion.

Coulomb dissociation of light nuclear projectiles in the electric field of heavy target nuclei has been experimentally investigated as an alternative access to radiative capture cross sections at low relative energies of the fragments, which are of astrophysical interest. As a pilot experiment the breakup of 156 MeV ⁶Li projectiles at ²⁰⁸Pb with small emission angles of the α particle and deuteron fragments has been studied. Both fragments were coincidentally detected in the focal plane of a magnetic spectrograph at several reaction angles well below the grazing angle and with relative angles between the fragments of 0°–2°. The experimental cross sections have been analyzed on the basis of the Coulomb breakup theory. The results for the resonant breakup give evidence for the strong dominance of the Coulomb dissociation mechanism and the absence of nuclear distortions, while the cross section for the nonresonant breakup follows theoretical predictions of the astrophysical S factor and extrapolations of corresponding radiative capture reaction cross section to very low c.m. energies of the α particle and deuteron. Various implications of the approach are discussed.