Development of an underground radon detector using an optical fiber

We developed and tested a new underground radon detector using an optical fiber. Previous underground radon detector used a photo-multiplier tube (PMT) of small diameter behind the chamber thus the diameter of the underground radon detector was determined by the size of the PMT. The larger diameter of the detector needed considerable labor for drilling a hole in under ground soil. The new underground radon detector consists of a small chamber, an optical fiber, and a PMT. The small chamber is a scintillation detector using a ZnS(Ag) film. The optical fiber transfers the scintillation light produced in the chamber to the PMT that positioned out of the soil. In these configurations, the size of the detector could not be determined by the size of the PMT. The diameter of the optical fiber used was 5mm and the outside diameter of the detector that buried into the soil could be reduced to be 12mm. Although the light loss due to the optical fiber was about 90%, the signal level of scintillation was much larger than the noise level of the PMT and electronics. We measured performance of the underground radon detector. The energy response had a clear distribution due to alpha particles by radon and its decay products. Both the rise time and fall time of the detector was approximately 2 hours. Sensitivity was approximately 0.01 counts/h/Bq/m³, one third of the previous underground radon detector. These results indicate the developed radon detector can be used for continuous measurements of radon concentration in underground soil with easy handling.