I’ve been collecting a number of (relatively) low-level radioactive samples for use with my cloud chamber project.
Now that I’ve been playing with the stuff for a while – I figure it’s time to get a better understanding of radiation measurements so I can be sure I’m not putting myself in danger.
Disclaimer: I DON’T REALLY KNOW ANYTHING ABOUT THIS STUFF – DO YOUR OWN RESEARCH IF THIS MIGHT IMPACT YOUR HEALTH. Further – if you think I got something wrong – please leave me a comment.
A rad is a unit of absorbed radiation dose causing 0.01 joule of energy to be absorbed per kilogram of matter. It’s also a pretty large amount of radiation – so millirads is more commonly used.
A rem is a rad measurement that’s been multiplied by a weighting factor depending on how biologically damaging the type of radiation it is.
For example – beta and gamma particles have weighting values of 1. Alpha particles are rated at 20.
So alphas are really dangerous? Yes and no. You don’t want to get them inside your body – but they also can’t penetrate your layer of dead outer skin.
So – assuming we’re just worried about beta and gamma radiation – 1 rad equals 1 rem.
You can use this tool to calculate your own exposure. About 300 millirem / year is a common level.
Nuclear workers are allowed up to 5 rem (5000 millirem) per year. Maybe that’s safe – but it sounds scary to me.
I figure I don’t want my playing with radioactivity to bump my exposure rate any more than 10% above my annual background exposure. That’s 30 millirem / year.
About 2ft from my radioactive collection – I get a reading on my geiger of about .07 mrem / hr (I’m having to guestimate this – at such low counts the needle moves erratically). At 8760 hrs / year – that would come out to 613 mrem annually (but presumably 300 mrem of this is background I’m already getting).
I can chalk up about 90% of the radiation to a piece of red fiesta ware – that stuff is pretty hot!
So – from two feet away – my radioactive collection is exposing me to about as much background radiation as everything else in the world does. If I spent 10% of my time at this location – I’d hit my self-imposed 30 millirem limit (it’s probably more like 1%).
However – at 6 feet away – I only get the dozen-or-so clicks each minute that I normally see as background radiation. I think it’s safe to say that just having the collection in my house (and not next to my bed or computer) isn’t getting close to 30 millirem / year.
So what about handling the materials when I take them in and out of the cloud chamber?
My “hottest” sample (a piece of Autunite – third sample in the video below) rates at 20 millirad / hour if I hold my geiger directly against it. That’s 20 millirem / hour based on my prior assumptions.
[youtube=http://www.youtube.com/watch?v=WzygctySeYk]
So – that means to get 30 millirem – I’d have to hold the piece for 90 minutes straight (which I would not do…). Further – “limits” typically tend to be about 10x higher for extremities – but I’ll stick with my ultra-conservative 30 millirem to play it safe.
Note – rem and rad are considered obsolete units – in favor of the Sievert – but my Geiger counter reads in millirads – so I’m going sticking with old-school.