Radioanalytical studies of fallout 63Ni

Fallout of Nickel-63 (T1/2 = 100 a) produced in small amounts at nuclear weapon tests following the neutron activation of weapon construction material was investigated by studying carpets of lichen collected during 1961 to 1988 at the Lake Rogen district in central Sweden (62.3 degrees N, 12.4 degrees E). The maximal level of 63Ni in the lichen carpet, which occurred in 1964, was about 0.6 Bq kg-1, dry weight, and decreased to 0.1 Bq kg-1 in 1988. The deposition pattern for 63Ni was similar to other fallout radionuclides such as 137Cs, 90Sr and 239 + 240Pu. The concentrations of stable Ni were relatively constant at 0.5 to 1.0 microgram g-1 throughout the years resulting in, for example, a specific activity (63Ni/stable Ni) of 0.5 Bq mg-1 in 1964 and 0.1 Bq mg-1 in 1988. The total area content of 63Ni was estimated to be 1.0 Bq m-2 and the activity ratio 63Ni/60Co was estimated to be 0.03 in 1966. The Chernobyl accident in April 1986 did not significantly increase the levels of 63Ni. For the measurement of these extremely low-levels of 63Ni, 200 g of dry material (about 1 kg fresh) were ashed and leached with aqua regia after hydroxides had been precipitated with ammonia, leaving Ni in the aqueous phase. Nickel was extracted as a dimethylglyoxime complex by chloroform and back-extracted with HCl. Finally, Ni was electroplated onto copper discs from an ammonium sulfate medium at high pH. The radiochemical yield was determined by atomic absorption spectrometry of stable Ni before and after electrodeposition.(ABSTRACT TRUNCATED AT 250 WORDS)