At 12:03 PM +0000 8/7/03, olympus-digest wrote:
>Date: Thu, 7 Aug 2003 08:58:03 +0100
>From: Kennedy McEwen <rkm@xxxxxxxxxxxxxxxxxxxx>
>Subject: Re: [OM] Radioactive glass
>
>In article , Joe Gwinn <joegwinn@xxxxxxxxxxx> writes
> >At 12:40 AM +0000 8/7/03, olympus-digest wrote:
> >>Date: Wed, 6 Aug 2003 18:47:23 +0100
> >>From: Kennedy McEwen <rkm@xxxxxxxxxxxxxxxxxxxx>
> >>Subject: Re: [OM] Radioactive glass
> >>
> >>In article , Joe Gwinn <joegwinn@xxxxxxxxxxx> writes
> >> >
> >> >Whoa! It's not nearly that dangerous, as it's very difficult to get
> >> >the thorium out of the glass, even if the glass is reduced to powder.
> >> >
> >>Its not an issue of getting the radioactive elements out of the glass.
> >>It is what happens to the alpha emission once you have ingested any of
> >>the fine glass particles or dust. Alpha radiation, not the decaying
> >>element, is then absorbed by LIVE tissue and that is the danger and the
> >>case of carcinogenic mutation.
> >
> >Given the relative density of skin and glass, how much of the alpha
> >radiation will in fact escape the glass, even if it's been crushed? In
> >practice, the danger comes from chemically disolved material, not
> >powder, as the range of the alpha radiation in glass and in water is
> >very short.
> >
>That is the point - once ingested it has no skin to penetrate. This is
>the major danger with all alpha emitters. Outside of the body they are
>relatively harmless because the radiation is effectively stopped by the
>layer of dead skin cells - that doesn't exist to provide any protection
>once the material is ingested. The radiation is absorbed directly by
>living cells in your lungs, liver, blood, intestine or other internal
>organs. Absorption means they are stopped by the cells themselves, and
>that means cell damage with the usual probability of that damage being
>mutation.
The point being made is that only a very thin layer of glass can be
contributing, and the radiation from that layer cannot go very far in flesh,
which is 90% water. Attenuation of ionizing radiation varies as the square of
the atomic number, so glass will have far higher attenuation than water, which
in turn will have far higher attenuation than air.
If one flys coast-to-coast, one gets about 100 millirems of radiation, about
the same as a medical X-ray. Uranium and Thorium are widely distributed,
appearing in stone and brick and dirt. This is the source of the radon gas one
hears about.
Factoid: A coal-fired power plant cannot be licenced under Nuclear Regulatory
Commission regulation, as it emits too much radioactive material. Nuclear
powerplants pass, though. The reason is that there is a lot of radioactive
stuff (Uranium, Thorium, Radium, Radon, etc) trapped in the coal, and this is
released when the coal is burned.
> >Do we know the actual amount of thorium in the glass, as a mass percentage?
> >
>Depending on the glass used, the radioactive material can be up to 10%
>of the mass.
OK. I assume that it's thorium oxide that's 10% by weight.
> >Americium is *much* hotter than an equal mass of thorium.
> >
>That is not an issue since it only determines how fast the material
>decays. Eventually it all decays and, providing that a significant
>proportion of that occurs within your lifetime (and in the case of the
>radioactive materials used in glass this is true) then all of that
>emission will be absorbed if the material is retained in the body.
The half-life of americium 241 is 475 years, while the half-life of thorium 232
(the isotope that occurs naturally) is 1.39*10^10 years. If you assume equal
energy, which is clse enough, the ratio of activities is 29 million to one.
> >I wonder what happened to the folk that ground the glass into lenses
> >for a living. Given that they did this all day every day, they would
> >be the first to go, not us duffers. I don't recall ever reading
> >stories about them dying like rabbits from this or anything else
> >associated with lens manufacture.
> >
>Certainly here in our optics facility, where we used thorium containing
>coatings for many years, the protection measures taken for the workforce
>were extreme. The cost of that and the cleanup operations when optics
>were damaged in the factory or in the field, was one of the main drivers
>for an alternative coating material. We never used radioactive glass,
>however I would expect that similar precautions were also required at
>manufacture and just as costly if not more so. Coating tends to be a
>fairly sterile environment in the first place, leading to relatively
>simple safety precautions. Glass polishing is generally messy with a
>lot of waste slurry produced. Of course they were probably made in 3rd
>world countries where the dangers were probably ignored and life
>expectancy was short in any case. That doesn't mean you should make
>yours equally short by being irresponsible with the product.
When those lenses were made, I don't think people worried about such things
nearly as much as they do today.
People that use a substance all day every day have to be more careful than the
duffers, because the key is total dose. The duffers just don't have the
opportunity to accumulate the dose that the workers do. This may be the key.
The classic example was the people that applied the radium paint to the faces
of clocks, watches, and instruments. They would form the point on the brush
with their lips, and so ingested a little bit of the paint every time. They
all died of it, slowly, mostly of cancer. Their customers didn't have the
slightest problem.
Joe Gwinn
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