Low level 137Cs measurements in deep seawater samples

The ammonium phosphomolybdate (AMP) procedure for low level Cs-137 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 1 samples and the radiochemical Cs-137 yield was 96-100%. The absolute efficiency of the HPGe coaxial well detector was 16% for Cs-137 at 662 keV for 4 g AMP/Cs compound. Using the improved procedure,Cs-137 activities in 20-100 l samples from sea surface to near bottom in the western North Pacific were measured. The Cs-137 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 Cs-137 as a transient tracer for oceanographic purposes, as high resolution water profiles are obtained due to the smaller volume of samples needed. (C) 2000 Elsevier Science Ltd. All rights reserved.