Particle number conserving BCS approach in the relativistic mean field model and its application to 32-74Ca

fixed particle number BCS (FBCS) approach is formulated in the relativistic mean field (RMF) model. It is shown that the RMF+FBCS model obtained can describe the weak pairing limit. We calculate the ground-state properties of the calcium isotopes( 32-74)Ca and compare the results with those obtained from the usual RMF+BCS model. Although the results are quite similar to each other, we observe the interesting phenomenon that for Ca-54, the FBCS approach can enhance the occupation probability of the 2p(1/2) single particle level and slightly increases its radius, compared with the RMF+BCS model. This leads to the unusual scenario that although Ca-54 is more bound with a spherical configuration, the corresponding size is not the most compact. We anticipate that such a phenomenon might happen for other neutron-rich nuclei and should be checked by further more systematic studies.