Mike Williams wrote:
> Wasn't it Jack Twilley who wrote:
> 
>>Hash: SHA1
>>
>>I have been wondering how useful POV-Ray would be in tracing gamma
>>rays instead of visible light.  This could be useful for modeling
>>gamma ray shielding and gamma ray buildup.  Reflection, refraction,
>>and dispersion are very similar between the two types of EM radiation,
>>as far as I can see.  Obviously, I'd have to create my own material
>>definitions and I'd only be able to test one energy level at a time,
>>but #ifdef's would help with that.  The biggest obstruction I can see
>>is how to associate exposure levels with brightnesses, but I imagine
>>that could be done by creating very simple scenes matching calibrated
>>environments with accurate tested values.
>>
>>Any thoughts?
> 
> 
> I was under the impression that there were differences in reflection and
> refraction. I guess the underlying principles are the same, but there
> aren't any available materials that make useful gamma ray lenses or
> conventional mirrors. I thought that gamma ray telescopes used a
> different reflection principle to produce grazing mirrors, and that
> principle is probably not modelled in POV-Ray (unless it works by
> fresnel reflection). 
> 
> I would have thought that for most gamma ray applications you could
> ignore reflection, refraction and dispersion and model all your
> materials as partially absorbing media. If you set everything except the
> photographic plate to be no_image, then the shadows that POV casts on
> that plate should correspond to the photographic image that would be
> recorded.
> 

The previous post is correct about using grazing mirrors for gamma rays. 
  The physical model of how gamma rays interact with matter s very 
different than for visible light.  Gamma rays are pentrating radiation 
(for non-grazing angles) and how they react with matter depends on their 
energies and the composition of the material.  At low energies, gamma 
rays can excite one of the inner electrons of the an atom and then when 
that electron is filled the atom releases an x-ray.  At moderate 
energies (100s of KeV to a few MeV) they are most likely to experience 
Compton scattering where the gamma ray transfers some of its energy to 
an electron. That is a probabilistic process (both in where in the 
material it occurs and the scattering angle) so where POVRay doing 
photon mapping can send out one photon and see what happens to it, for 
gamma rays you would probably have to send out thousands to get a 
representative sample of the possibilities.  In addition after Compton 
scattering, that electron may have enough energy to emit additional 
radiation (brehmstrahlung) as it slows down in the material.  At higher 
energies the gamma ray can also experience pair production where it 
spawns an electron-positron pair where the positron will likely 
annihilate with an electron emitting more gamma rays (if the electron 
has enough energy it will emit radiation too).

So while you could take the POVRay framework and re-write the photon 
mapping code to be a gamma mapping program, it would be an enormous 
amount of work.  If you're looking for a master's thesis topic, you're 
in luck (it may be more work than I think and you can get a Ph.D. out of 
it.)

If you want to look into gamma interactions with matter try "Radiation 
Detection and Measurement (Third Edition)" by Glenn F. Knoll.  It covers 
all types of radiation and detectors, but spends a lot of time on gamma 
rays and has good references if you need more detail.  I'd suggest 
checking it out of a library since it costs $120.

Good Luck,
Richard Kline

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