Four-fold data analysis of $^{252}$Cf fission products

Prompt gamma-ray 4-fold data were built to collect 2$\times $10$^{11}$ $\gamma -\gamma -\gamma -\gamma $ quadruple- and higher-fold $\gamma $ -coincidence events from the spontaneous fission of $^{252}$Cf with Gammasphere detector arrays. The nuclei $^{106}$Nb, $^{142}$La, Ba and Gd have been studied with these data. By using the new 4-fold data, we confirmed several weak tentative transitions in $^{106}$Nb, $^{142}$La, Ba, $^{148}$Ce which were observed previously from the $\gamma -\gamma $ -$\gamma $ triple cube. Some new transitions in $^{106}$Nb, $^{142}$La were identified by our new 4-fold data. Cascades in $^{145}$Ba are much clearer in four-fold data than the previous triple coincidence data. We will continue to study other nuclei by our 4-fold data with lower background than the previous triple cube. We thank M. Riley for urging us to build the 4-fold data set to get more accurate intensities.