Speaker
Description
The radionuclide iodine-125 (I-125) is an important agent in radiation therapy of various cancers, especially for brachytherapy. Reliable and safe application of I-125 activity require primary standardization as the foundation of traceability. The I-125 decay scheme, proceeding primarily through emission of two photons in coincidence, lends itself to primary standardization through coincidence methods. Often, the sum-peak method is used as it can be accomplished using a single detector, or two detectors operated as one without the need of complicated coincidence logic. However, to date at NIST, this method was limited to evaporated point sources both to mitigate photon attenuation and geometric variability effects.
The need for evaporated sources presents a challenge due to the volatility of iodine. Adapting this method to aqueous samples would improve the safety and accessibility of this method but requires consideration of geometric effects.
In this work we present sum-peak counting for an aqueous solution in a flame-sealed ampoule, counted in a commercial NaI(Tl) well counter along with thorough Monte Carlo modelling. Five ampoules were prepared, with activities spanning a factor of two, all gravimetrically linked to a NIST SRM, which had been standardized by the evaporated point-source method. The ampoules were measured in two different ampoule holders (PTFE and nylon) on two occasions. The highest total count rate was 22000 s-1 on the first occasion and 500 s-1 on the second occasion. Activities were calculated using the formulism of J.S. Eldridge and P. Crowther (Nucleonics 22 56, 1964), with updated nuclear decay data. Extrapolations of calculated activity at zero count rate were adopted as nominal values that differed between PTFE and nylon by 0.80 % and 0.08 % and differed from the SRM certificate by (0.24 ± 0.81) % and (0.69 ± 0.20) %, for the high and low activity measurements respectively, thus agreeing with the SRM within its stated uncertainty due to the extrapolation (1.1 %).
In addition, significant Monte Carlo simulation studies were carried out to analyze the effect of sample holder material (air, PTFE, nylon, aluminum), sample holder geometry (cylinder or cup), solution volume (0.01 mL to 5.0 mL), and pileup (0 or 5 %) on the resulting extrapolated activity. Further, the underlying physics was studied by simulating I-125 decay with varied decay parameters to understand the relative importance of multiple assumptions of the method. It was found that an important assumption involves variation of efficiency over the source volume, similar to the requirement for beta-gamma coincidence (C. Bobin, Metrologia 44 S27, 2007). From this understanding, the "ideal" geometry is described as well as a practical geometry such that a standard 5 mL ampoule source can be measured accurately without applying corrections. Alternatively, other geometries could be used with corrections tabulated from the Monte Carlo model results.
