Nuclear reaction cross sections are usually very small in typical astrophysical environments. It has been one of the major challenges of experimental nuclear astrophysics to assess the magnitude of these cross sections in the laboratory. For a successful experiment high luminosity beams are needed. Increasing the target width, one also increases the reaction yields. But, this is of limited use due to multiple scattering in the target. Storage rings are appropriate tools to overcome these difficulties. Storage rings can work with large luminosities, and the beams cross the interaction region many times per second (typically one million/s), compensating low density internal gas targets, or low reaction rates in beam-beam collisions. Storage rings are also ideal tools for precise measurements of masses and beta-decay lifetimes of nuclei of relevance for astrophysics.
CENTRAL PROBLEMS IN NUCLEAR ASTROPHYSICS
1.1. Screening
Nuclear astrophysics requires the knowledge of the reaction rate Rij between the nuclei i and j. It is given by Rij = nlnj < cry >/(1 + ~ij), where cr is the cross section, v is the relative velocity between the reaction partners, n~ is the nmnber density of the nuclide i, and <> stands for energy average. Extrapolation procedures are often needed to obtain cross sections in the energy region of astrophysical relevance. While non-resonant cross sections can be rather well extrapolated to the low-energy region, the presence of continuum, or subthreshold resonances, complicates these extrapolations. Another problem is that charged-particle induced reactions are electron-screened in stellar environments [1]. Applying the Debye-Hiickel approach, one finds that the plasma enhances reaction rates, e.g., 3He(3He, 2p)4He and 7Be(p, 7)sB, by as much as 20%. Laboratory nuclear reactions are also modified by screening effects due to electrons which are inevitably present in the target [2 I. As can be seen in figure 1 for the reaction aHe(d, p)4He (probably the best studied example of laboratory electron screening effects, both experimentally and theoretically -Data are from ref. [3]. Calculations are from ref. [4]), the effects of laboratory electron screening are far from being understood and require further investigation.
While corrections due to plasma screening effects have to rely on theoretical models, laboratory screening can be avoided experimentally using crossed beams in storage rings.