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