Studi Eksperimental Unjuk Kerja Termal Vertical Straight Wickless-Heat Pipe sebagai Sistem Pendingin Pasif pada Kolam Penyimpan Sumber Radioaktif Co-60 Iradiator Gamma = EXPERIMENTAL STUDY ON THERMAL PERFORMANCE OF VERTICAL STRAIGHT WICKLESS-HEAT PIPE AS PASSIVE COOLING SYSTEM IN RADIOACTIVE CO-60 SOURCE STORAGE POOL OF GAMMA IRRADIATOR