Study of the 20.3 MeV state in 4He by the cluster model

The first excited state of 4He at about 20.3 MeV has been observed in many nuclear reactions during the past several years. However, the various measurements of its excitation energy and width are not consistent. A recent investigation by Gross et al yields a value of 20.28+-0.05 MeV for the excitation energy and 0.41+-0.05 MeV for the width of this state. On the basis of an analysis of their data, these authors observe that the state is consistent with a O+ assignment and corresponds to a pure 1S-wave resonance in the p-3H system. In the present study, therefore, a variational procedure has been used to calculate the energy of this state described as a triton cluster plus a proton. Properly antisymmetrized wave function and a nucleon-nucleon potential yielding good fit to the low energy scattering data have been used. The calculation yields a value of 20.54 MeV for the excitation energy of this state relative to the ground state which is in quite good agreement with the experimental value obtained by Gross et al. (author)