The capture reaction4He(12C, γ)16O (E
cm= 134–338 MeV) as well as the elastic scattering process4He(12C,12C)4He (E
cm=144–338 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 692 MeV state is dominated by the direct capture mechanism and plays a small role at stellar energies

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

The scattering of alpha particles from 12C and the 12C(α, γ)16O stellar reaction rate

The partial cross sections of heavy residual nuclei produced in the heavy ion fusion of12C+20Ne have been measured atE
c.m.=6–15 MeV viaγ-ray spectroscopy with a Ge(Li) detector. Windowless and recirculating gas target systems have been used. The dominant residual nuclei are24Mg,27Al,28Si,30Si,30P and31P, which arise from two- and three-body breakups in the exit channels. The observed excitation functions are smooth in their energy dependence and give no indications for the existence of pronounced resonance structures, in contrast to theoretical predictions. The Coulomb excitation of20Ne served as an intrinsic calibration standard in the determination of absolute partial and total fusion cross sections. The same experimental set-up was also used in the reaction studies of16O+16O atE
c.m.=7–14 MeV, going through the same compound nucleus32S at similar excitation energies. The observed energy dependence in the excitation functions is in good agreement with previous work. The total fusion cross section agrees fairly well with two sets of values reported previously, but deviates significantly from other reported absolute cross section values. The relative evaporation distributions of the residual nuclei are similar for both heavy ion reactions. However, the ratio of their total fusion cross sections deviates from model predictions and suggests that compound nucleus formation does depend on the microscopic structure of the colliding nuclei in the entrance channel. From the observed energy dependence of the above ratio, particularly at subcoulomb energies, geometrical effects in the entrance channel (due to deformed and spherical nuclei) appear to be weak. The astrophysical aspects of the data in the context of late stellar nucleosynthesis are discussed.

A. Richter

Papers

Operation results of the Dynamitron Tandem at Bochum

Wobble target holder for thin targets in intense light-and heavy-ion beams