Abundances of s-process elements in planetary nebulae: Br, Kr & Xe
  We identify emission lines of post-iron peak elements in very high signal-to-noise spectra of a sample of planetary nebulae. Analysis of lines from ions of Kr and Xe reveals enhancements in most of the PNe, in agreement with the theories of s-process in AGB star. Surprisingly, we did not detect lines from Br even though s-process calculations indicate that it should be produced with Kr at detectable levels. 
Y. Zhang, R. Williams, E. Pellegrini, K. Cavagnolo, J. A. Baldwin, B.   Sharpee, M. Phillips, X.-W. Liu