Although prompt (i.e. not beta-delayed) proton emission was observed from an isomer in cobalt-53 as early as 1969, no other proton-emitting states were found until 1981, when the proton radioactive ground states of lutetium-151 and thulium-147 were observed at experiments at the GSI in West Germany.
Meskipun emisi proton yang cepat (yaitu bukan beta tertunda) teramati dari sebuah isomer kobalt-53 pada awal 1969, belum ada keadaan emisi proton yang ditemukan hingga tahun 1981, ketika keadaan dasar proton radioaktif lutesium-151 dan tulium-147 teramati pada percobaan di GSI, Jerman Barat.