[LargeFormat] radioactive lenses

[Michael Briggs]

On 25-Nov-01 Clive Warren wrote:

> Perhaps you could explain briefly the nature of gamma radiation - all 
> I know is that this is quite penetrative short wavelength radiation. 
>  From what you say, this is emitted from the lenses but what about 
> frequency?? If there are an exponential number of daughters and when 
> the daughters decay gamma and xray radiation is emitted then surely 
> this is not too healthy?  What sort of shielding is required to stop 
> any gamma and xrays reaching me here at my seat - eyes large Aero 
> Ektar with increasing respect ;-)

Gamma-rays are like X-rays, only more so.

Seriously, gamma-rays are photons with higher energies than X-rays.  Visible
light photons have energies of a few electron volts, X-rays energies from a few
thousand electron volts to a few ten thousand electron volts.  Gamma-rays are
photons with energies above about a hundred thousand electron volts.   Typical
gamma-rays from radioactive isotopes are a few hundred thousand electron volts
to one or two million electron volts.

Photon energy is one way to define the difference between X-rays and gamma-rays.
The other is origin.  Visible light photons can originate from transitions of
the outer electrons of atoms, while X-rays can originate from the transitions of
inner atomic electrons.  Gamma-rays can originate from transitions of the
particles inside a nucleus.

Thorium and its daughters emit all of these.  Most of the photons are of the
lower-energy X-ray range, but there are gamma-rays all the way up to 2.6
million electron volts.

My calculations of the amount of radiation exposure possible from Aero-Ektars
are still VERY PRELIMINARY.   Assuming that my calculations are correct, a few
hours exposure would give a person a radiation dose much smaller than a
trans-Atlantic plane flight.    Most people completely ignore the increased
radiation dose from flying when the decide whether they want to travel. 
Similarly, the radiation dose from using an Aero-Ektar is probably from minor
compared to what one gains from  using it.    Prudence suggests minimizing ones
exposure when one is not using an Aero-Ektar--the simplest way is to store the
lenses far away from people.

Any shielding will help.  The more material (mass) the better.   Shielding alone
shouldn't be relied upon to stop all of the radiation unless you really know
what you are doing.   Stopping million electron volt photons takes centimeters
of lead.  The alpha and beta particles, and X-rays can be eliminated with
smaller amounts of shielding.  There are a lot more of these than high energy
gamma-rays, so shielding will help. Again, the simplest and most reliable
approach for most is probably to use distance.

Perhaps more confidence inspiring than my preliminary calculation: another
indication of the lack of serious danger from Aero-Ektars is that their
possession is legal for ordinary people (i.e., people without a radiation
license) in the U.S.    Title 10, Chapter I, Part 40, Section 13 of the Code of
Federal Regulations says in part:

"Any person is exempt from the regulations in this part and from  the
requirements for a license set forth in section 62 of the Act to the extent
that such person receives, possesses, uses, transfers or delivers source
material in" ......

"(7) Thorium contained in finished optical lenses, provided that each 
lens does not contain more than 30 percent by weight of thorium; and 
that the exemption contained in this subparagraph shall not be deemed to 
authorize either:
    (i) The shaping, grinding or polishing of such lens or manufacturing 
processes other than the assembly of such lens into optical systems and 
devices without any alteration of the lens; or
    (ii) The receipt, possession, use, transfer, or of thorium contained 
in contact lenses, or in spectacles, or in eyepieces in binoculars or 
other optical instruments."

Two elements of the f2.5 Aero-Ektars are made of glass with approximately 10%
Thorium Oxide by weight, so this rule clearly allows possession of Aero-Ektars.
Just don't try to grind a contact lens from glass obtained from an Aero-Ektar!
:)

I learned of this regulation at the AtomicRocks web site, 
http://www.atomicrocks.com/html/rules.html.   Another source:
http://lula.law.cornell.edu/cfr/cfr.php?title=10&type=part&value=40


--Michael