  We present deep, high-resolution (R~40000) UVES at VLT spectrophotometric
data of the planetary nebula NGC 3918. This is one of the deepest spectra ever
taken of a planetary nebula. We have identified and measured more than 700
emission lines and, in particular, we have detected very faint lines of several
neutron-capture elements (s-process elements: Kr, Xe and Rb) that enable us to
compute their chemical abundances with unprecedented accuracy, thus
constraining the efficiency of the s-process and convective dredge-up.
