As far as I know, Geiger counters are not sensitive to alpha particles
(helium nuclei), since they don't make it into the tube. They are sensitive
to beta particles (electrons or positrons) and gamma rays (high-energy
photons). Putting a thin piece of metal in between the radioactive element
and the tube cuts the detected radioactivity about in half. Since the
electrons shouldn't make it through the metal, this suggests that about half
of the radiation is gamma rays. A thin lead sheet has about the same effect
as the thin piece of metal, and a thick lead brick stops all the detected
radiation.
I would guess that the electrons would have no effect on the film, but the
gamma rays could fog film, similar to x-rays. I would also guess that the
rate of gamma rays coming out of the lens is many orders of magnitude less
than a typical airport x-ray machine, so it would take much much longer for
the lens to fog the film than the airport x-ray machine.
Incidentally, if alpha particles can hit you, they cause more damage than
beta particles or gamma rays, since they're big and heavy (well, compared to
electrons and photons they are). So, eating alpha emitters is definitely a
Bad Thing, since the emitter would be close to tissue and the alphas could
then hit it before being stopped. This is why plutonium is so dangerous.
Mark Marr-Lyon.
You wrote:
> Just been thinking about this. They're radioactive due to Rare Earth
> Elements in the glass, am I right? These elements emit alpha particles,
> which do not travel very far or fast. A few centimetres of air is plenty
> enough to stop an alpha particle, so if the shutter's closed the film is
> absolutely safe, and even if you were to leave it on B for a month, the air
> between the lens and the film would absorb pretty much all the radiation.
> The alpha particles are also not going to have any effect on the
> photographer - unless you eat the lens, the particles can't get anywhere
> where they can do any damage.
>
> Of course, if you find that you're sprouting an extra thumb a few years
> down the line, I'll stand entirely corrected.
>
> Roger
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