Application of Low Background Gamma-ray Spectrometry to Seaweed Samples

—Improvement of Detection Limit for Low Level Artificial Radionuclides—

Abstract

In order to elucidate the background level of artificial radionuclides (^108mAg, ^110mAg, ⁵⁴Mn and ⁶⁰Co) in seaweed (Sargasso) sample, we improved the detection limit by the following methods; method-1 (use of GeLEPS in Ogoya underground laboratory), method-2 (⁴⁰K removal from sample + Ge-LEPS in the underground lab.) and method-3 (⁴⁰K removal + well type Ge in the underground lab.) . In method-3, extremely low detection limit was obtained for ^108mAg ¹¹⁰Ag and ⁶⁰Co (-8 mBq/kg-fresh) .
As a practical application, we applied low background gamma-ray spectrometry (method-2) to 72 Sargasso samples collected in the vicinity of Shika Nuclear Power Station from December 1998 to June 2002. The activity level of the fallout ¹³⁷Cs in Sargasso samples was found to be below 0.2 Bq/kg-fresh (mostly<0.1 Bq/kg-fresh) and the activities of low level artificial radionuclides were below the detection limit (^108mAg, ^110mAg, <15 mBq/kg-fresh; ⁵⁴Mn, <20 mBq/kg-fresh; ⁶⁰Co, <25 mBq/kg-fresh) . Contaminations derived from Shika Nuclear Power Station and/or Russian dumping were not clearly observed in Sargasso around Shika Nuclear Power Station.