FALI.-OUl- MEASURKMENTS IN AUSTRALIA 



could not l)e satisfactorily correlated with the gummcd-film results. The 

 maximum \c\c\ of fission-product contamination detected by the air 

 samplers was 0-01 per cent of that allowable under the recommendations 

 of the International Commission on Radiolot^ical Protection. 



Of the rainwater samjiles collected Ijy the 12 stations using this monitoring 

 technique, none exhibited an activity greater than 1 (i.[jiC/m^, corresponding 

 to the background in the detector. 



Of the other measurements of gross fission-product activity carried out 

 during the series, the most interesting results were obtained from a com- 

 parison of gummed films exposed side l^y side^. It is obvious that the 

 sampling processes are strongly dependent on the particulate nature of the 

 fall-out material ; large activities, especially close to the trial site and shortly 

 after firing, occasionally resulted from a single large particle and so did not 

 truly represent the radiation levels in the locality. 



RADIO-IODINE MEASUREMENTS 



It is well known that animal thyroid tissue has an affinity for iodine, so that 

 radioiodine, ingested by a grazing animal from fodder contaminated with 

 fission-products, is quickly concentrated in the thyroid gland. It may be 

 estimated in vivo or, more precisely, after thyroidectomy. 



During 1956, an Australia-wide survey was made of radioiodine in the 

 thyroid glands of grazing sheep and cattle. It was apparent that measure- 

 ments close to the weapon test site would adequately monitor the maximum 

 levels of radioiodine contamination likely to result in unrestricted areas 

 from the testing of a low yield nuclear weapon. During Operation 'Antler' 

 thyroids were collected from only those sheep grazing within 150 miles to 

 200 miles of Maralinga, in areas close to the trajectories of the radio-active 

 clouds. 



For about 10 days after each explosion and at intervals of about 3 days, 

 three sheep were sacrificed in each of the chosen localities and the thyroid 

 glands were forwarded to Maralinga for analysis. Eight out of the 68 glands 

 examined contained activities less than the minimum detectable in the equip- 

 ment. The remaining tissue contained low activities of ^^^I but in a few cases 

 ^^^I and isomeric ^^^Xe were also identified and estimated from their y-ray 

 spectra. 



The highest individual level of ^^^I detected was 42 m[j,C/g of thyroid, 

 with a mean value of 24 m[jLC/g of thyroid for the group of three glands. 

 This should be compared with calculated and measured radioiodine 

 burdens, which can give rise to biological damage. 



Maximum permissible levels of ^^^I for short-term exposure of animals 

 have been given by Scott Russell, Martin and Wortley'^ of the British 

 Agricultural Research Council. They calculate that no detcctal)le bio- 

 logical damage would be caused to sheep from the daily ingestion of 26,000 

 m[iC of ^^^I from pasture contaminated with fission-products. They assume 

 that grazing on this pasture is limited to the period from the first to the 

 fourteenth day after fission and the activity is referred to the first day. This 

 ingestion would give a mean thyroid burden of about 3000 m[j,C ^''^I/g 

 thyroid for the period, and a total dose to the thyroid of 1000 rad. This 

 group has also shown that the contribution to the dose to the thyroid gland 



142 



