Physical Insights

An independent scientist’s observations on society, technology, energy, science and the environment. “Modern science has been a voyage into the unknown, with a lesson in humility waiting at every stop. Many passengers would rather have stayed home.” – Carl Sagan

Posts Tagged ‘americium

Health physics implications of the ionisation smoke detector.

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It’s still occasionally heard from some sources, even after the technology has been in widespread use for many, many years, that the common household ionisation smoke detectors, which contain a very small radioactive source, present some kind of health hazard.
They don’t.

Such devices usually contain a sealed source, of 0.9 to 1.0 μCi of Americium-241. The source itself is a tiny little thing, about three millimeters in diameter – it’s a point source. *

241Am principally decays by emission of an alpha particle at 5.49 MeV, but this is not of any significance, since the α-particle cannot escape the device at all. However, 241Am also emits some low-energy gamma photons as it decays, principally a gamma ray of 59 keV, and it is this γ-radiation that can pass through the device and conceivably result in some dose to the householder, possibly.

The specific gamma-ray dose constant (“Γ factor”) for 241Am is 0.314 rem m2 Ci-1 h-1, or 3.14 x 10-9 Sv m2 μCi-1 h-1

(Here’s my source, a useful little reference table for this type of health physics information for several common, important, industrial, scientific and technical radionuclides.)

The dose rate, then, to the whole body from an external exposure to a point radioactive source which is emitting photon radiation is just the Γ-factor for the particular nuclide, multiplied by the activity of the source, multiplied by the familiar old inverse-squared distance term.

Suppose you’ve got such a detector on the ceiling, right next to your bed, which would place you about , say, 3 m away from the source. Suppose, additionally, that you spend your entire life in that bed. Of course, here I’m taking the most conservative scenario possible, to set an upper bound on the plausible dose.

3.14 x 10-9 Sv m2 μCi-1 h-1 x 1 μCi x 8765.8 hours x (3 m)-2 = 3.06 microsieverts per year.

Of course, the average worldwide dose from natural background radiation is almost 1000 times that – around 2.4 to 3 millisieverts per year, and in some places, far, far higher.

However, there is a more surprising and interesting context that we can put such a dose rate in. If you sleep in bed next to your partner every night, then the ionising radiation dose that you receive, due to the radioactivity of your partner’s body, from 40K and things like that, is about five microsieverts per year. [Source]

That’s assuming a realistic amount of sleep each day, of course – if you were actually in bed 24 hours per day, the dose from this source would be three times as much; 15 μSv per year.

Thus – the dose from sleeping next to your partner is 4.9 times what it is from the smoke detector. Surprising, isn’t it?

Obviously there is no reason to expect any significant health physics implications of any kind at such extremely low doses. Heck, such low doses are probably even too small to have potential significance with regards to radiation hormoesis. But we do know they’re proven to be extremely effective at protecting against the threat of fire destroying your home.

* Just as an aside, although technically illegal in the US and possibly in other localities as well, people (usually physicists and people that do know what they’re doing) do often remove these sources, and use them for educational demonstrations of radioactivity, of Geiger-Marsden style scattering, charged particle absorption, and to test and calibrate alpha and X-ray detectors – it’s quite tempting, since these devices are far, far less expensive than purchasing exactly the same sort of tiny sealed radioactive sources through “proper” channels, and no more dangerous.

Written by Luke Weston

October 23, 2008 at 10:33 am