I’ve been watching these graphs for a few days:
This is the time series taken closest to our place. The live graph (at the link above allows you to read the values off at certain times. There are three points we’re interested in 1) on the far left you can just see the background level, we can read this off as approximately 40nGy/h; 2) in the middle of the graph there is a peak, this is roughly 150; 3) the latest reading which is around 130.
What we’d like to know is it this radiation is due to Iodine-131, with a half-life of 8 days, what would be expect the level to be? There are a bunch of handy half-life calculations, but the one for which we can easily plug in all the numbers is:
N(t) = N0(1/2)t/t1/2
N(t) – level at time t (what we want to know in nGy/h), N(0) – level at time zero (where we’re starting from, 150nGy/h), t – the elapsed time (41hrs), t1/2 the half-life of the isotope (192hrs). As we’re only interested in additional radiation levels we can subtract away the background level of 40 and end up with:
~94 = 110(1/2)41/192
Which matches up rather nicely with what we see on the graph, a reading of 130 (minus the ~40, to account for the background level).
Wasn’t that fun!
This makes me feel a little better about my theory that the higher radiation levels that we are seeing now are due to the Iodine-131 released into the atmosphere last week, and subsequently washed back out again when the rains started back on Monday.
[Disclaimer: i’m not a nuclear scientist / radiation specialist. Consequently, you should not take anything away from these calculations other than the sad fact that doing them cheered me up immensely.]