Identification of ⁹⁰Sr/⁴⁰K Based on Cherenkov Detector for Recovery from the Fukushima Nuclear Accident

Abstract

Although five years have passed since the Fukushima nuclear accident of 2011, the local fisheries have yet to recover from its effects. One reason for this situation is the difficulty of measuring the radioactivity owing to 90 Sr in seafood. After the accident, the radioactivity due to Cs isotopes in samples was measured with precision, which facilitated the enforcement of the maximum concentration of Cs radioisotopes in food at 100 Bq/kg, as defined by the Ministry of Health, Labour and Welfare in Japan. However, 90 Sr is more dangerous than Cs isotopes because it has an effective halflife of 18 years and accumulates in the bone. The radioactivity owing to 90 Sr in a sample is difficult to measure because the beta rays from 137 Cs or 40 K also contribute to the signal. When measured based on the endpoint pulse height as determined by a conventional survey meter, the beta ray signal from 90 Y (daughter of 90 Sr) cannot be differentiated from the beta rays from other sources. To overcome this difficulty, in this study, we develop a Cherenkov detector based on a silica aerogel with a refractive index of 1.034 that can identify beta rays from 90 Y within a background of beta rays from 137 Cs and 40 K. This instrument involves a detector that is sensitive to beta rays from 90 Sr but less sensitive to radiation from other sources. This detector comprises a trigger counter that uses scintillating fibers, an aerogel Cherenkov counter with wavelength-shifting fibers, and a veto counter to suppress cosmic rays. We characterize the detector using a 90