Re: Radioactive Boy Scouts and Smoke Detectors

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

The details given in the Reader's Digest article lead me to believe that
this is an urban legend. Clearly the author of the article understood
almost nothing about nuclear physics (except the scary stuff) and couldn't
sort out the facts from the rumors.

It would be almost impossible to accumulate enough radioactive material
from smoke detectors to create a "reactor" and it is entirely possible
(although I don't know for sure) that Am is not a fissionable material and
so couldn't be used to create a reactor at all. Since Am's decay mode
(alpha) is quite short ranged, it is highly unlikely that the radioactivity
could be detected "five houses away" unless there had been significant Am
released into the atmosphere, in which case it would be dispersed downwind.
Even if no Am had been released, a Geiger counter will still register
readings due to the inevitable background. Are we sure that, if these
readings were actually made, that after several stages of reporting the
results, that the backgrond became the supposedly released Am? Remember, if
the detection five houses away was due to primary alphas released at the
site of the student's work, the flux of alphas would have to be large
enough that everyone between the original house and the house where the
Geiger counter was located would have been either dead or dying.

Here is a scenario that could give one the impression that a "reactor" was
being formed. Am-243 decays by alpha emission to Np-239, which decays by
beta-minus to Pu-239, which can fission, but which requires several
kilograms to generate a "critical mass". (I realize that the smoke
detectors use Am-241, but I didn't have data on that radioactive chain
readily available.) Meanwhile, Am-243 has a half-life of 7370 years, while
Np-239 has a half-life of 2.35 days, so it decays rather quickly to Pu-239
which has a half-life of 24,100 years, and which is an alpha-emitter. So,
our intrepid inventor accumulates a few mg of Am-243 which is decaying
eventually to Pu-243, which has a longer half-life than the Am-243, and so
it is increasing in amount and thus in activity. Hence a Geiger counter
near the setup might actually detect increased activity with time, but note
that the student would have to have gathered thousands of old smoke
detectors in order to have collected as much as a gram of Am-243. This
would be nowhere near the amount necessary to create a reactor, either of
Am-243 or of Pu-239. (I am not saying that getting this much Am-243 in one
place wouldn't be dangerous. Breathing even a few micrograms could have
devastating effects on one's lungs, up to and including lung cancer--see
Ludwik's recent posting to get a feel for the numbers. But it's clear that
the author of this story whoever they may be, has focused on the most
sensational aspects of the story and neglected the real dangers.)

Of such stuff are urban legends made. One doesn't have to have read many
issues of Reader's Digest to know that they are not above a bit of
fear-mongering, and I doubt that they have a scientist on their editorial
staff to check out things like this. So the rumors get spread ever wider,
with no refutation, and pretty soon everyone thinks it happened. Viola!
Urban legend.

Hugh

> I just found the Reader's Digest version of the Radioactive Boy Scout on the
> Web at:
>
> www.readersdigest.com/rdmagazine/specfeat/
> archives/taleoftheradioactiveboyscout.htm.
>
> (It should all be on one line of course.)
>
> It is an interesting read. Many of the early steps taken seem plausible, but
> the article ends by implying that he had created a "reactor" that had an
> increasing level of radioactivity. The article claims that radiation was
> detectable five houses away with a Geiger counter. I don't doubt that one
> could
> make quite a mess with the materials said to have been collected, but I would
> love to read an experts analysis about what reactions and how many nuclear
> reactions were actually involved.
>
> That article also lead to a couple of Web pages of interest to the smoke
> detector discussion. A photo of the source in a smoke detector is given at:
>
> howstuffworks.com/inside-smoke.htm
>
> and a fact sheet on americium in smoke detectors is given at:
>
>
> www.uic.com.au/nip35.htm
>
> The fact sheet says that the americium is in the form of AmO_2 and that 1 gram
> of the oxide makes about 5000 detectors. The sheet says that since it is
> in the
> oxide form and therefore not soluable in the body, the small amount is not an
> ingestion hazard. It doesn't discuss possible inhalation hazards. The fact
> sheet is at the web site of an Australian company called the Uranium
> Information
> Centre, which appears to be supporting the Australian uranium industry, but
> which I know nothing else about.
>
> Tim Sullivan
> sullivan@kenyon.edu


********************************************************************************
Hugh Haskell

<mailto://haskell@odie.ncssm.edu>
<mailto://hhaskell@mindspring.com>

The box said "Requires Windows 95 or better." So I bought a Macintosh.
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