In order to elucidate the background level of artificial radionuclides (108mAg, 110mAg, 54Mn and60Co) 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 for108mAg 110Ag and60Co (-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 fallout137Cs 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.
The possibility of a position-sensitive proportional counter based on the tilted-wire position reading method has been examined for thermal neutron detection. The aluminum angle plate with five lines of natural boron painted in 1 mm width and 10 mm spacing is placed inside of the counter. On the same plane including the anode wire, a pair of position reading wires is extended in both sides of the anode wire. The position reading wires are in parallel each other, but tilted against the anode wire. The induced charges on the position reading wires due to the electron avalanche around the anode wire depend on the distance between the avalanche and position reading wire, so that position information can be deduced with the charge division technique. It is demonstrated that the five boron lines can be clearly observed in the two-dimensional spectrum of position vs. energy, and this kind of simple sensing-wire structure is feasible for neutron position detection.
The effect of gas barrier silica coatings on the tensile strength and elongation at break for gamma-irradiated 1 mm thick polypropylene was studied. The irradiation was carried out in air at a constant dose rate of 125 Gy·h-1with a60Co source and the silica coatings were formed by magnetron sputtering for a thickness up to 120 nm. The elongation at break for the sample without silica coatings was reduced at an irradiation time of 350 hours, whereas the reduction occurred at significantly longer times for the samples with silica coatings. This result shows that the gas barrier silica coating can improve the radiation resistance of a polymer as a result of suppressed radiation-induced oxidation.