Coulomb dissociation experiments

The 7Be(p,g)B8 reaction

Fig. 1: Setup for coulomb dissociation with the magnet "Kaos".

The low-energy cross section of the radiative proton-capture reaction 7Be(p,γ)8B is directly linked to the flux of high-energy solar neutrinos and their flavor oscillations, which are currently the subject of intense studies at terrestrial real-time solar-neutrino detectors, e.g. at SNO in Canada [1] or Super-Kamiokande in Japan [2].

Fig. 2: Proton in-plane transverse momenta.

Recently, a direct-proton-capture measurement on 7Be performed at Seattle [3] found a low-energy cross section for this reaction, which differed remarkably from previous results. We have measured the same quantity by a completely independent method, namely Coulomb dissociation (CD) of a 254 A MeV radioactive 8B beam.

For this purpose, the coincident outgoing fragments after the CD reaction, protons and 7Be, were identified and their momenta measured at the KaoS spectrometer at GSI [4]. The lowest data point could be measured at a proton center-of-mass energy of 160±18 keV. Of particular interest are the p-7Be angular correlations in the moving frame because they are sensitive to the electromagnetic multipolarities that contribute to the CD process. We could show that, contrary to theoretical expectations, these angular correlations can be explained with good accuracy by pure E1 multipolarity [4]. Fig. 2 depicts the proton in-plane transverse momenta, which do not show, for large Rutherford scattering angles θ8, the forward-backward asymmetries typical for E1-E2 mixtures (dashed lines). Instead, the data follow closely the full lines drawn for E1-multipolaity only. This allows to convert easily the measured differential CD cross sections into the (also mostly E1) radiative-proton-capture cross sections measured directly. As usual in nuclear astrophysics, the astrophysical S-factor is plotted instead of the capture cross section (Fig. 3). As can be seen from Fig. 3, our results are in good agreement with those of Ref. [3] and also with those of a similar experiment, Ref. [5]. The energy-dependence was also found to agree well with that of a refined cluster-model calculation [6]. That means that this cluster model provides a reliable basis for the necessary extrapolation of the cross sections towards the solar-energy range of about 20 keV.

Fig. 3: S-factor of this experiment compared to other measurements.

More details about this experiment can be found in the published paper [4] in Physical Review C.

References