Capture reactions at astrophysically relevant energies: extended gas target experiments and GEANT simulations

Abstract

Several resonances of the capture reaction ${}^{20}\text{Ne}\text{(}\text{α}\text{,}\text{γ}\text{)}{}^{24}\text{Mg}$ were measured using an extended windowless gas target system. Detailed GEANT simulations were performed to derive the strength and the total width of the resonances from the measured yield curve. The crucial experimental parameters, which are mainly the density profile in the gas target and the efficiency of the γ-ray detector, were analyzed by a comparison between the measured data and the corresponding simulation calculations. The excellent agreement between the experimental data and the simulations gives detailed insight into these parameters.

Introduction

Capture reactions play a leading role in nucleosynthesis of elements. The experimental determination of resonance strengths and astrophysical S-factors and thereby the exact knowledge of reaction rates is necessary for the modelling of stellar nucleosynthesis. Since the cross sections to be determined are very small at astrophysically relevant energies, a high-current particle accelerator combined with a gas target is needed for the experiments because many of the target substances are gaseous. Additionally, a windowless gas target system is required because of the high currents and the low projectile energies. Both components are available at the Institut für Strahlenphysik at the University of Stuttgart [1], [2].

The use of an extended gas target in combination with a γ detector arranged in close geometry to the target chamber demands some considerations in the analysis of the measured yield curves. Up to now, in most cases, resonance strengths were gained from the analysis of experimental data in the yield maximum [3]. That means that only γ rays emitted from the vicinity of the center of the chamber come into consideration. With this restriction the efficiency of the γ detector in most cases can be determined straightforward using pointlike calibration sources.

The use of large volume HPGe detectors leads to a large efficiency for the detection of γ rays. BGO detectors arranged around the HPGe detector and used as an active shield reduce the background drastically. This detector combination used for the measurement of capture reactions results in a very large dynamical range of the yield curves. The values found in the maxima of the yield resonances and those found in the minima between them differ up to six orders of magnitude [5]. This result opens the possibility to gain more and better information by an analysis of the full yield curve instead only an analysis of the peak maximum. In many cases, not only the strength, but also the total width of a resonance can be determined.

In order to simulate the shape of peaks in the yield curves data for the gas pressure along the beam line in the interior and in either side of the extended gas target have to be known. Entering the resulting values for the mean energies and the energy straggling of the incoming particles in dependence of the position of γ ray emission, and the value of the resonance width Γ, the yield curve of an isolated single resonance can be computed over many orders of magnitude if the efficiency of the γ detector for the emitted γ rays is well known. The Monte Carlo program GEANT [6] developed for applications in high energy physics facilitates the calculation of the detector efficiency using all geometrical data of the experimental set up and taking into consideration the energy and the angular distribution of the γ ray emission. Comparing the simulated and the experimental data over a wide dynamical range yields values of the branching ratios, of the resonance strengths, and of the widths of the resonances.

In Section 2 of this paper a short description of the experimental set up is given and, as an example, a yield curve of the reaction ${}^{20}\text{Ne}\text{(}\text{α}\text{,}\text{γ}\text{)}{}^{24}\text{Mg}$ is discussed which has been measured in this arrangement. In Section 3 some results of simulations calculated with the program GEANT and the experimental verification of the γ detector efficiency are displayed. In Section 4 the simulation of yield curves is described together with the representation of the different components whose knowledge is necessary for these calculations. Finally, some results are shown and a conclusion is given.

Note that E_α is the α energy in the laboratory system in this paper.