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9th September 2013 - I recently collected 82 grams of moss that had been growing in roof gutter here, and tested it. It was very wet here for the first half of 2013. August has had no rain, so the moss sample was dry. Sample collected here on the east coast of Australia. http://sccc.org.au/monitoring/Australian-Map.jpg
Moss growing in roof gutter
Moss in marinelli beaker for testing
Test chart of moss sample using experimental Theremino MCA version 5. (Mouse click on image to see larger version)
If my assessment is correct, this Southern Hemisphere detection is not a good sign. This is a lot of Iodine I-129. The other possibility is it is Lead Pb-210, or a mixture of both. The main peak is much closer to 40 keV than to 47 keV for lead Pb-210. I have tested this sample with different software, and calibrated a number of times, it very much looks like I-129.
This means it is bio-accumulating. So even though our background levels have not risen a lot, bio-accumulations could present a more serious issue. The moss also had some other material mixed in with it, that it was growing on, leaves and other organic matter. This test result also suggests that there would be more 1-129 in fallout in the Northern Hemisphere.
I have discussed this test result with some of my contacts here.
Why so much I-129, and not Cesium etc.?
The conclusion is because of its volatility. It can also easily volatilize in an environment, as the temperature increases.
This would allow it to easily spread long distances, like Iodine I-131.
Japanese study of I-129 pre-Fukushima Nuclear disaster. http://www.ncbi.nlm.nih.gov/pubmed/23829385
Another pre-Fukushima Argentina study, indicates that Norther Hemisphere I-129, can reach the Southern Hemisphere. This study shows I-129 has migrated from nuclear fuel reprocessing plants at Cape de La Hague (France) and Sellafield (UK). http://www.sciencedirect.com/science/article/pii/S0048969712006353
For the technically minded - Testing procedures, and Becquerel activity calculations.
Scintillator Model:
Alpha Spectra NaI 818/2B, resolution 7%
Voltage setting; 800 Volts
Efficiency 7% at 662 keV
I used radioactive Americium (Am-241), from a smoke detector in the test chamber, to calibrate the low energy peak position, and also the scintillator efficiency at around 40 keV.
Here is the calibration chart for reference. (Mouse click on image to see larger version)
Estimate of scintillator efficiency at 40 keV
Smoke detector Am-241 activity = 37,000 cps, from my research approximately 1% of this activity is Gamma.
1% is Gamma = 3700 cps (36% of that 1% is at 60 keV)
37 x 36 = 1332 cps = 100% gamma at 60 kev
1295 / 100 = 13.32 = 1%
693 cps was the actual cps in chamber for this peak region
693 / 13.32 = 52% efficiency at 60 keV
Estimated efficiency at 40 keV is approximately 52%.
Estimated Activity of Iodine Sample
I used Japanese BecqMoni MCA software to calculate the activity.
Using 52% efficiency at 40 keV.
net count – background = 80039
80039 / 52 = 1,539 (1%)
1,539 x 100 = 153,900 (100%)
Time of test = 79700 seconds
153900 / 79700 = 1.931 cps
82 grams in weight 1000 / 82 = 12.195
1.931 X 12.195 = 23.5 Bq/kg
This may not be an absolute quantification of the scintillator tube's efficiency at 40 keV, but we are dealing with moss in a large marinelli beaker here. The efficiency of activity is going to be less than the 52% for the Am-241. So the 23.5 Bq/kg activity estimation is probably conservative.
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