Estimation of Exposure from 147Pm activated self-luminous Plate

The majority of radioactive self-luminous paint produced in Japan uses 147Pm. The characteristics of the radiation from a 147Pm-activated self-luminous plate which had a luminous area of 12×8 cm2 and radioactivity of 12.5 mCi were examined. Bremsstrahlung was produced from the luminous plate absorbing the 220 keV β-particles of 147Pm. The energy of the bremsstrahlung was analyzed with use of both NaI scintillation and Ge solid state detectors: the spectrum obtained showed a peak at around 50 keV and also a small peak due to γ-rays emitted from the 147Pm at 121 keV. Measurements of the bremsstrahlung made with Lauritsen electroscope, film badges, fluoroglass dosimeters and thermoluminescent dosimeters gave estimated exposure rates of at most only 0.5 and 0.3 mR/hr respectively at distances from the plate of 3 and 7 cm.