TRAPPING AND MEASURING RADIOIODINE (I-129) IN CARTRIDGE FILTERS

To understand the distribution of I-129 trapped in AgS (silver-impregn ated silica gel) adsorbent and to determine a calibration curve for me asurement of this I-129, cartridges containing 10 in-line (arranged in series) filter elements, each packed with a 10-mm thickness of adsorb ent, were fed from 20 Bq (3.3 mu g) to 4.6 MBq (768 mg) of I-129 at 15 0 degrees C at a gas velocity of 22 cm/s. The I-129 in each filter ele ment was measured after the adsorbent had been mixed homogeneously unt il the counting rates at the front and back surfaces of the filter ele ment became equal. The amount of input I-129 was allocated to each fil ter in proportion to the counting rates thus obtained. The first 7 of the 10 filter elements completely confined 4.6 MBq of I-129. Therefore , the width of the adsorption zone was 7 cm. Although each filter elem ent in the cartridge could theoretically adsorb 2.92 MBq (485 mg) of I -129, the first element captured only 1.42 MBq, which was 49% of its s aturation value. Its DF had dropped to a level of 2.21 for its adsorpt ion of only 17.4% of the saturated amount. As I-129 continued to be de posited, the DF dropped to 1.45. The high gas velocity causes this phe nomenon. Plots of counting rates vs. adsorption quantity provide a cal ibration curve covering a range of 20 Bq to 2 MBq of I-129 with deviat ion of +/- 13%. This curve is expressed by In y = -0.57 + 0.96 In x, w here y is the counting rate (counts/300 s) and x the quantity of I-129 in Bq.