Low level 137Cs measurements in deep seawater samples

The ammonium phosphomolybdate (AMP) procedure for low level 137Cs measurements in deep seawater samples was examined, as a marked decrease in the recovery of AMP has been observed over several years. An improved AMP procedure is proposed: adjust the pH to 1.6–2.0, next add 0.26 g CsCl to form an insoluble compound, then add 4 g AMP to every 20–100 l seawater sample. Stir at a rate of 25 l air/min for 1 h, then recover the AMP/Cs compound after 6–24 h. The weight yield of AMP/Cs compound was about 98% of AMP for 20 l samples and the radiochemical 137Cs yield was 96–100%. The absolute efficiency of the HPGe coaxial well detector was 16% for 137Cs at 662 keV for 4 g AMP/Cs compound. Using the improved procedure, 137Cs activities in 20–100 l samples from sea surface to near bottom in the western North Pacific were measured. The 137Cs concentrations in deep water samples were lower than 0.1 Bq m−3 in 1997 and 1998. This procedure allows a wide range of applications of 137Cs as a transient tracer for oceanographic purposes, as high resolution water profiles are obtained due to the smaller volume of samples needed.