The 18O(p, α)15N reaction at stellar energies

The ¹⁸O(p, α)¹⁵N reaction has been investigated in the energy range E_p = 72–935 keV. The three known resonances above E_p = 620 keV have been confirmed and four new resonances have been found below E_p = 340 keV. All observed resonances correspond to known compound states in ¹⁹F. Information on resonance energies, total widths and ωγ values is reported. The low-energy resonances are superimposed on a non-resonant reaction yield, which varies smoothly with beam energy and which exhibits pronounced α-particle angular distributions asymmetric around 90°. The explanation of these data requires either interferring amplitudes of broad resonances with differing parities or a direct (p, α) reaction mechanism. The investigated energy range corresponds to the important temperature range of T = (0.05–2.5) × 10⁹ K. The energy averaged astrophysical reaction rates are compared with predictions.     

The 13N(p, γ)14O reaction at stellar energies

We present a microscopically-founded potential model for the ${}^{13}\text{N(p}\text{, γ)}{}^{14}\text{O}$ reaction at energies of astrophysical interest. The model is shown to reproduce the ${}^{13}\text{C(p}\text{, γ)}{}^{14}\text{O}$cross section very well. The ${}^{13}\text{N(p}\text{, γ)}{}^{14}\text{O}$ reaction is dominated by the first excited J = 1^? resonance in ¹⁴O. For this resonance we find a proton width of Γ_c.m. = 40.1 keV and a photon width of Γ_γ = 1.50 eV.     

The15N(p,α 0)12C reaction at stellar energies

The¹⁵N(ρ, α₀)¹²C reaction has been investigated in the energy range ofE _p (lab)=78-810keV. The measurement of the excitation functions and α-particle angular distributions involved solid targets as well as a quasi-point supersonic jet gas target. The determination of absolute cross sections has been carried out with the gas target. The observed energy dependence of the total cross sections can be described in terms of two-level Breit-Wigner shapes including the resonances atE _p (J ^π)=335(1^?) and 1,028(1^?)keV. The data lead to a zero-energy intercept of the astrophysicalS(E) factor ofS(0)= 65±4MeV-b. The angular distributions are asymmetric around 90° and require an additional amplitude in the reaction mechanism, which interferes predominantly with the 335 keV resonance. The origin of this background amplitude is discussed.     

The3He(d,p) α reaction at astrophysical energies

We have performed a microscopic study of the³He(d,p) α reaction at astrophysical energies within the framework of the Resonating Group Method adopting three different effective nucleon-nucleon interactions. The calculations suggest that the low-energy³He(d,p) α cross section is in good approximation given by the contribution arising from the 3/2⁺ resonant state atE _R =245 keV. In fact, the low-energy data can be better (and more physically) described by a single Breit-Wigner resonance parametrization than by a polynomial fit. Our fit to the³He(d,p) α cross section results in a noticeable reduction in the uncertainties of the resonance parameters of the 3/2⁺ resonant state as well as in a significantly improved extrapolation of the data to astrophysically important energies. On the basis of this extrapolation we were able to quantitatively deduce the enhancement of the low-energy cross section due to electron screening effects from the data of Krauss et al. and Engstler et al. These experimental enhancement factors were compared with various theoretical models which are all based on the Born-Oppenheimer approximation. As the models underestimate the observed enhancement we suggest that the theoretical study of electron screening effects requires a dynamical treatment.     

The Fe(α, γ)Ni reaction at excitation energies in the region of the giant dipole resonance

Excitation functions for the ⁵⁶Fe(α, γ₀)⁶⁰Ni and ⁵⁶Fe(α, γ₁)⁶⁰Ni reactions have been measured at 90° to the beam direction over the bombarding energy range 8.0–17.6 MeV. Gamma-ray angular distributions were measured at ten bombarding energies. Excitation functions for the ⁵⁹Co(p, γ₀)⁶⁰Ni and ⁵⁹Co(p, γ₁)⁶⁰Ni reactions were measured over the range E_x = 16.58–16.92 MeV and compared with the (α, γ) data. The angular distribution data indicate that the (α, γ₀) and (α,γ₁) reactions proceed through 1⁻, and 1⁻ and 3⁻ states, respectively. The (α, γ) excitation functions are discussed with respect to isospin splitting of the ⁶⁰Ni giant dipole resonance. The fine structure observed in the excitation functions is shown to be most probably due to Ericson fluctuations. The gross (α, γ) cross sections are shown to be in reasonable agreement with the results of calculations made using the theory of Hauser and Feshbach assuming excitation of the giant dipole resonance.     

The4He(12C, γ)16O reaction at stellar energies

The capture reaction⁴He(¹²C, γ)¹⁶O (E _c.m.= 1.34–3.38 MeV) as well as the elastic scattering process⁴He(¹²C,¹²C)⁴He (E _c.m.=1.44–3.38 MeV) have been investigated with the use of an intense¹²C beam and a windowless and⁴He recirculating gas target system. The measurements involved two large NaI(T1) crystals in close geometry to an extended gas target, whereby angle-integrated γ-ray yields were obtained. A large area plastic detector was used for the suppression of time-independent background. A search for cascade γ-ray transitions was carried out by coincidence techniques. The measurement of absolute cross sections is also reported. Theoretical fits of the excitation function for the groundstate γ-ray transition requireE1 as well asE2 capture amplitudes, which are of equal importance at stellar energies. This result increases significantly the stellar burning rate of⁴He(¹²C, γ)¹⁶O and leads to¹⁶O as the dominant product at the end of helium burning in massive stars. The observed capture yield to the 6.92 MeV state is dominated by the direct capture mechanism and plays a small role at stellar energies.     

The 7Li (n,γ)8Li radiative capture at astrophysical energies

The possibility to construct intercluster interaction potentials in continuous and discrete spectra is shown in one‐channel cluster model based on the classification of orbital states according to Young schemes. These potentials usually contain Pauli forbidden states, and correctly describe elastic scattering phase shifts taking into account resonance behavior and main characteristics of the bound states of nuclei in the considering cluster channel. The versions of intercluster interaction potentials describing the resonance nature of some phase shifts of the n⁷Li elastic scattering at low energies and the P₂ ground state of ⁸Li in the n⁷Li cluster channel have been constructed for the demonstration of this approach. The possibility of describing the total cross sections of ⁷Li (n,γ)⁸Li within the energies from 5 meV (5 · 10^‐3 eV) to 1 MeV, including resonance at 0.25 MeV, has been demonstrated for the potentials obtained in the potential cluster model with forbidden states.     

The Bi(d,γ)Po reaction

The yield of the ²⁰⁹Bi(d, γ)^211g.s.Po and ^211mPo (T_1/2 = 25.2s) reaction was measured for deuteron energies E_d = 8–11.5 MeV. The reaction was identified by the -activities of the Po isotope. At E_d = 10.43 MeV, the (d, γ) cross section for the population of the ground state of ²¹¹Po is σ_g.s. = 16 ± 3 μb, the ratio relative to the cross section for the metastable state is σ_g.s./σ_m = 25.4 ± 0.9. These values and the yield curves were compared with calculations using a simple model for the population of the two states. In the excitation region E* = 15–19 MeV, the branching ratio of γ- to particle emission is nearly constant and has a value of about 0.4 × 10⁻⁴.     

The16O(α, γ)20Ne direct capture reaction at low energies

The¹⁶O(α, γ)²⁰Ne direct capture cross section has been calculated in a microscopically founded cluster model which reproduces simultaneously both the correct binding energies and the deformations of the²⁰Ne bound states. Theα+¹⁶O scattering states are derived from a microscopically derived local potential. The astrophysicalS-factor is found to increase linearly with energy in the energy rangeE _cm≈0.4–2 MeV and might therefore be determinable experimentally.     

The (γ,p) reaction in Au

Proton energy spectra of the ¹⁹⁷Au(e,p) reaction were measured in the region between 17 and 30 MeV at three angles: 40°, 90° and 140°. Two prominent bumps were observed in the (γ,p) spectra converted using virtual photon theory. The higher-energy bump shifts with photon energies and the lower-energy one stays at 10.5 MeV. The higher-energy bump is much larger at 40° than at 140°; on the contrary the angular dependence of the lower-energy bump is small. Neither bump can be described by a statistical calculation. A calculation of a microscopic shell model shows that the lower-energy bump is attributed to the decay of proton-particle–neutron-hole pairs in the T_> states, leaving a neutron hole around the Fermi surface. The higher-energy bump can be ascribed to the direct–semidirect mechanism. This paper gives the solution to a part of the long-standing question about the origin of photo-proton emission in heavy nuclei.     

Studies on the Ne(p, d)Ne and Ne(d, p)Ne reactions

Angular distribution measurements have been performed on the ²¹Ne(p, d)²⁰Ne and ²¹Ne(d, p)²²Ne reactions at E_p = 20 MeV and E_d = 10.2 MeV, respectively. In the ²¹Ne(p, d) ²⁰Ne reaction, the prolific formation of the J^π = 2⁺, 1.63 MeV state was characterized by l_n = 2 pickup, and the distribution associated with the 4⁴, 4.25 MeV state was suggestive of a weak l_n = 2 pickup. All of the observed l_n = 1 pickup strength is associated with formation of the 2⁻, 4.97 MeV ²⁰Ne level. The ²¹Ne(d, p)²²Ne results indicate that l_n = 2 transfer is involved in the formation of the 1.28, 3.36, 5.52, 5.63 and 6.65 MeV ²²Ne states. The angular distribution observed for the 2⁺, 4.46 MeV state and also the unresolved 5.33, 5.36 MeV composite of states required both l_n = 0 and l_n = 2 components in the associated distorted-wave Born approximation fits. The spectroscopic factors extracted from the present results are compared with those predicted by the Nilsson model without mixing: Applications of the angular momentum projection rule to the ²¹Ne(d, p)²²Ne reaction are considered.     

The N(p,γ)O low-energy S-factor

The ¹⁴N(p,γ)¹⁵O low-energy S-factor is analyzed using the R-matrix model. We find that the g.s. contribution is less than previously reported. The S-factor is mainly given by the 6.79 MeV state contribution which is determined by its asymptotic normalization constant (ANC). Consequently, the S-factor at zero energy is lower by a factor of 1.7 compared to the values given in recent compilations. This result may affect the nucleosynthesis and time scale evolution in massive stars. New measurements of the ¹⁴N(p,γ)¹⁵O cross section over a wide energy range, and especially at low energies, are highly desirable. Significant improvement could be also obtained from the ANC measurement of the 6.79 MeV state.     

Inclusive (γ, N), (γ, NN) and (γ, Nπ) reactions in nuclei at intermediate energies

Differential cross sections of nucleons excited in photonuclear reactions in medium and heavy nuclei are studied by considering all relevant reaction mechanisms leading to the excitation of protons or neutrons. We take advantage of previous microscopic studies for the absorption and scattering of photons and photoproduced pions, and implement a simulation code in order to take into account the propagation of the nucleons as well as their collisions with other nucleons in the nuclear medium, which generate secondary excited nucleons. Comparison with experimental data is done. Cross sections for nucleon emission in coincidence with one pion are also calculated, and some coincidence observables are discussed.     

The (γ, p n) and (γ, 3p3n) reactions in40Ca at intermediate energies

The yields of the reactions⁴⁰Ca(γ, p n)^38g K and⁴⁰Ca(γ, 3p 3n)^34m Cl have been measured by the activation method in the energy range 80–800 MeV. From the measured yields the cross sections are deduced. The experimental cross sections are compared to calculations with a cascade-evaporation model for photo-induced reactions.     

3He break-up and quasi-free7Li(p, α)4He reaction at low energies

Using the reactions³⁵Cl(¹³⁰Ba;4,5n) and³⁶Ar(¹³³Cs;6,7n), the half-lives of^160,161Ta were measured to be 1.2(3) and 2.7(2) s, respectively, and theα-lines observed at 4.88(1) and 4.63(1) MeV were assigned to¹⁶²Ta and¹⁶³Ta, respectively. Theα-decay branch of¹⁶²Ta was determined to be 0.065(14)%.     

Energy levels of K from Ar(p, p)Ar and Ar(p, p′γ)Ar

The elastic and inelastic scattering of protons from ³⁶Ar has been investigated from 2.1 to 5.3 MeV using a gas target. Experimental energy resolution varied from 11 keV to 5 keV over this range. The results are interpreted in terms of 56 states in ³⁷K between the energies of 4.0 and 7.0 MeV excitation with the aid of single-level dispersion theory. The width and spin and parity or proton l-value are determined for the 38 states seen in the elastic scattering. Upper limits are placed on the total widths of levels seen only in the inelastic scattering.     

The astrophysical S-factor of the reaction 7Be (p, γ)8B in the direct capture model

The astrophysical S-factor for the reaction ⁷Be(p, γ)⁸B up to an energy of 2 MeV (c.m.) and the capture cross section of ⁷Li(n,γ)⁸Li up to 1 MeV (c.m.) are calculated using the Direct Capture model (DC). Both calculations are in good agreement with experimental data.     

Two-step mechanism in the 22Ne(p,t) reaction

The forward-angle differential cross sections have been measured for the ²²Ne(p,t) reaction leading to the 4.97 MeV, 2^? state in ²⁰Ne. Comparison of the data with the results of calculations shows that the two-step mechanism responsible for exciting this state is (p-p′-t) rather than (p-d-t).