From MediaWikiInternational Medcom Radalert Geiger-Müller counter, located at 48°46'54" North x 123°29'31" West, between Vancouver and Victoria, British Columbia, Canada.
These records are in reverse chronological order, with the most recent events first.
As of 6 May 2011, we've not seen any abnormal readings. The three-day average count remains below 10.
I have removed readings from the graph prior to 16 March, 7:44PM, when I moved the location of the counter. Prior readings had apparently been influenced by some books that the counter was inadvertently pointed at! (You can see prior versions of the graph by clicking it.)
What does this mean?
Any sustained rise of a count or more above average, especially if only from the outdoor samples, might indicate atmospheric contamination -- or it might be a solar flare. But it would take a huge increase to be dangerous. Note that the background count of Denver is four times ours.
We have defined a radiation fallout plan to follow, should readings increase.
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- The RadAlert is sitting on a north-facing windowsill. It should be detecting gamma background radiation in this indoor location.
- paper sample
- The RadAlert is pointing directly at a plain piece of white cardboard (the target), indoors, as a control for later outdoor sampling. This should show any alpha or beta radiation -- hopefully, none! -- coming from the blank, fallout-free paper.
- The RadAlert is pointing at the cardboard, as above, but the cardboard has been outdoors for some period of time, collecting dust from the atmosphere, which would include alpha and beta emitters not able to be sampled indoors.
The counter has an LCD event display. I let it run continuously, then at random intervals, I note the next full minute, count down to that, then log the reading with that timestamp. This web page calculates the difference in time and displays the counts-per-minute, which is an average over that time period.
For background readings, the counter is on the windowsill of a north-facing window, placed so that sunlight cannot strike the face of the Geiger-Müller tube. This should only detect gamma rays, primarily from the sun, but also cosmic rays from outer space.
For environmental contamination readings, a target (an 8.5"x11" piece of white cardboard) is periodically placed outside, directly beneath the open sky. It is placed approximately 1.5 metres above the ground and about three metres from the nearest horizontal obstruction. This is left at this location for the duration of the current indoor background reading.
Being careful not to dislodge any dust on the target, it is retrieved and brought indoors and positioned on a flat surface approximately two metres away from the background reading location. The counter is supported about three centimetres above the target, so that the Geiger-Müller tube alpha window is directly exposed to the target, enabling it to record any alpha or beta events emanating from any radioactive particles that may have fallen on the target.
The records are thus alternating background and environmental records -- the background reading being taken while the environment is sampled.
In the event that a target count is significantly higher than the background rate, I'll attempt to discriminate the radiation type with blockage:
- If an additional, clean sheet of cardboard reduces the count to normal, then it is probably alpha radiation, which could come from radon or plutonium.
- If the sheet of cardboard does not change the count, but an inch of closed cell foam reduces the count to normal, then it is probably beta radiation, which could come from iodine-131, caesium-137, or strontium-90.
- If neither of these blockages changes the count, which remains high, then it is probably gamma radiation, which is unlikely to come from Fukushima fallout. I'll have to do further experiments to ensure it is a valid count.
I am planning to automate the event logging via a Vernier LabPro data logger, but the necessary "BTD" cable is on order.
UPDATE: I've been puzzled that the outdoor target readings were consistently lower than the background readings. I'm switching to doing the two in exactly the same location, with the same orientation of the counter; the only thing different will be the insertion of a sheet of exposed white card stock.
UPDATE: Counts have all been under 10 since re-orienting my indoor location. The books that the counter had been pointed at must be mildly radioactive!
UPDATE: As of 5 April 2011, I am using a rag instead of a paper target. It seems most of the risk is during rain, and a rag is a better way to collect a sample in the rain.
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