Model of a cobalt-60 source
1985
1985
1993
circa 1990
1988-1998
1993
1982
1981
1981
1981
1987
1981
1987
1980
1981
1981
1981-1982
1981
1980
1980
1981
1972-1982
1972-1982
1972-1982
1972-1982
1972-1982
1987
1987
unknown
unknown
unknown
unknown
unknown
unknown
unknown
1981
1981
1981
1972-1982
1972-1982
1972-1982
1987
1987
unknown
unknown
unknown
unknown
1981
1981
1981
1981