General ConsiderationsCaution: 254 Es (t 1/2 = 275.7 days, 70.9 TBq g −1 ) and 249 Cf (t 1/2 = 351 years, 150 GBq g −1 ) are highly radioactive, and decay to α-, β-and γ-emitting isotopes. These isotopes and their decay daughters present substantial health risks, and were manipulated only in facilities specifically designed for the safe handling of long-lived radioactive materials. All measurements were taken in controlled facilities and/or using multiple containment procedures.