Standardization of 129I using the movable 4πβ(LS)-X(NaI(Tl)) system

Abstract

The 129I standardization, using the movable 4πβ(LS)−X(NaI(Tl)) coincidence system, was performed for two 129I radioactive sources – one was dissolved in 0.1M NaOH solution and the other in 0.1M HNO3 solution. The system incorporates three movable PM tubes for a β–counter placed on a plane and a X-ray detector that can be moved up to the bottom of the vial.The β-efficiency depending on the amount of radioactive solution was investigated with 14 liquid scintillation samples prepared by gravimetrically dispensing 4.4–145 mg of 129I radioactive solution. The β-efficiencies above 90% were observed at less than 56 mg, but it was at most 70% at 145 mg. This occurred regardless of the activity of the sample or the type of chemical solution used to dissolve 129I source. The activity concentration of each 129I source was determined by efficiency-extrapolation method for samples with an activity range of 0.28–4.5 kBq. The β-efficiency points were derived over 10 intervals by moving 3-PM tubes in fine steps of about 1 mm from the sample. The highest value for β-efficiency was 95%. The combined uncertainty were 0.25% and 0.26%, respectively. The stated precision obtained using the system is better than that previously reported in the literature obtained by the triple to double coincidence ratio (TDCR) or the CIEMAT/NIST efficiency tracing method.