A compact and high efficiency GAGG well counter for radiocesium concentration measurements

After the Fukushima nuclear disaster, social concern about radiocesium (¹³⁷Cs and ¹³⁴Cs) contamination in food increased. However, highly efficient instruments that can measure low level radioactivity are quite expensive and heavy. A compact, lightweight, and reliable radiation detector that can inexpensively monitor low level radiocesium is highly desired. We developed a compact and highly efficient radiocesium detector to detect ∼32 keV X-rays from radiocesium instead of high energy gamma photons. A 1-mm thick GAGG scintillator was selected to effectively detect ∼32 keV X-rays from ¹³⁷Cs to reduce the influence of ambient radiation. Four sets of 25 mm×25 mm×1 mm GAGG plates, each of which was optically coupled to a triangular-shaped light guide, were optically coupled to a photomultiplier tube (PMT) to form a square-shaped well counter. Another GAGG plate was directly optically coupled to the PMT to form its bottom detector. The energy resolution of the GAGG well counter was 22.3% FWHM for 122 keV gamma rays and 32% FWHM for ∼32 keV X-rays. The counting efficiency for the X-rays from radiocesium (mixture of ¹³⁷Cs and ¹³⁴Cs) was 4.5%. In measurements of the low level radiocesium mixture, a photo-peak of ∼32 keV X-rays can clearly be distinguished from the background. The minimum detectable activity (MDA) was estimated to be ∼100 Bq/kg for 1000 s measurement. The results show that our developed GAGG well counter is promising for the detection of radiocesium in food.