The age of fallout ©): 22 crs. uch im tn ways, one from the sr&9/90 ratio, which varies as e functicn of time efter burst, and the ae other from percent contribtutisn of eet? be total ( activitye The theoretical sr&9/50 ratios usec te calculate burst cates were cbtained from the Hunter and Ballou yielc dats for these isctopes and their most recently reported half-life vaelus:.: The expected percent $r90 total 6 activity as a funetion <:f ti: i was obtained in the same waye The approximate burst times have besn calculated fer all the data, and the average burst months for each montn's samples, as obtained by both methods are listed in Table 3, Estimated burst dates of fallout in rain water. Burst month calculated from average Sr°9/30 ratios Burst month calculated from average % Sr70 March September 4j007 March 195, April Auguet toe, November 195, May Geptenver Llp. November 1951 June Ostober Too! February 1954 July February 125% October 195) been Sampling Month ee hemes eine an thd Table 4. ee te he in is high, indicating old debris, ‘e’> contribution to total 6 activity ra From this it can be seen that the AL tbe some time, the sr°9/o0 ratio is This is possibly due to enrichment of the Sr99 in fellout in a mixture ce large enough for a mush late; iurct i.ts te be realized. hat would occur of material from different test ser.-.. . Therefore, BO 4. the Ero?’ > vr > > the age of fallout anc can ke sens: 2) : is , o 4g ls more sensitive indicator of 1. vield a rore valid estimation 1 q x “SNS = “~ . AQE SS « * PTS ~ %2 TO g -~ Se ye . a * £ a * ” 4 = me - oh se tae *s “ as % LamRNA 8OO em Ty ~~eS ow me mq Se Sa SS 5, ~ - Thy - om* * 7S A