  We propose a simple quartet condensation model (QCM) which describes with
very high accuracy the isovector pairing correlations in self-conjugate nuclei.
The quartets have an alpha-like structure and are formed by collective
isovector pairs. The accuracy of the QCM is tested for N=Z nuclei for which
exact shell model diagonalizations can be performed. The calculations are done
with two isovector pairing forces, one extracted from standard shell model
interactions and the other of seniority type, acting, respectively, upon
spherical and axially-deformed single-particle states. It is shown that for all
calculated nuclei the QCM gives very accurate values for the pairing
correlations energies, with errors which do not exceed 1%. These results show
clearly that the correlations induced by the isovector pairing in
self-conjugate nuclei are of quartet type and also indicate that QCM is the
proper tool to calculate the isovector proton-neutron correlations in mean
field pairing models.
