Identification of 90 Sr and 40 K based on cherenkov radiation at lower background suppressed cosmic rays

A new detector, real-time 90 Sr counter, was developed with sensitivity to 90 Sr and less sensitivity to 137 Cs, 40 K and Cosmic rays, based on Cherenkov radiation. The detector is a threshold type Cherenkov counter using silica aerogel with a refractive index less than 1.042. Since of the threshold energy of 1.31 MeV, the beta ray from 90 Y can be identified. This detector would be applied for recovery Fukushima. By the Fukushima Nuclear Accident in 2011, particularly the fisheries were severely damaged due to radioactive contamination in Fukushima, Japan. Recent study is focused to radiation with longer half-life, 90 Sr and 137 Cs, in the contaminated water. In the study, the veto counter suppressed background event from cosmic rays was upgraded to performance with the detection efficiency of 99.9% or more. As the results, an absolute efficiency of 90 Sr, 137 Cs, and 40 K as the performance of detector was evaluated to 2.01 × 10 −3 , 1.87 × 10 −6 , and 1.75 × 10 −5 Bq −1 s −1 using the sources, respectively. The efficiency uniformity depending on the source position was measured as peak at the center of the detector and half of the maximum value at the edge, the mean efficiency is estimated as (1.68 ± 0.24) × 10 −3 Bq −1 s −1 . The relation between the radioactivity and count rate was observed as response linearity, which is fitted a good linear function. Therefore, the count rate was corresponded to radioactivity concentration of contamination in a sample. Detection limit was estimated to be 1.93 (seawater) and 57.3 Bq kg −1 (seafood).