Model of a cobalt-60 source
1985
1985
1981
1981
1980
1981
1981
1981
1981
1980
1981
1981
1982
1988-1998
circa 1990
1981-1982
1981
1981
1993
1993
1978-1979
1980
1978-1979
1981
1981
1981
1981
1981
1981
1981
1978-1979
1978-1979
1981
1981