RE: cherenkov radiation

["Franta, Jaroslav" <frantaj@aecl.ca>]

Dukelow, James wrote

	<snip>

It is my understanding, reinforced by a brief Internet search, that Jaro's
understanding of Cherenkov radiation is not correct.

Cherenkov radiation occurs when charged particles move through some medium
faster than the speed of light in that medium.  If the medium is
transparent,
then we can see the Cherenkov radiation. There is nothing special or
privileged
about water, although that is the context that most of us are familiar with.
Cherenkov "glow" can occur in air, even dry air, where it is the basis for
an
"instrument", the STACEE (Solar Tower Atmospheric Cherenkov Effect
Experiment),
that uses a heliostat array and a photomultiplier to detect the atmospheric
trace of particle "showers" caused by extremely high-energy gamma rays
arriving
from active galactic nuclei.  There is another instrument based on detecting
Cherenkov radiation in an aerogel material.

The Tokaimura blue flash was probably in the air near the vessel, but was
certainly not caused by gamma rays and neutrons in the eyeballs, since
neither
of those are charged particles.  Secondary charged particles might have
resulted
in some Cherenkov in the eyeball, but it wouldn't be perceived as a blue
flash
localized near the criticality.
<><><><><><><><><><><><><>

A colleague suggested the following reasoning :

<BEGIN QUOTE>
A blue glow (Cerenkov radiation) occurs only if ionizing radiation travels
through a material faster than the local speed of light.   The speed of
light in air is 99.97% of the speed of light in a vacuum (3.00x108 m/s).
The difference in the speed of light between air and a vacuum is far too
small for visible Cerenkov radiation to occur.   The speed of light in
water, however, is 75% of the speed of light in a vacuum (that's why light
refracts when passing through water, but that's another discussion).   Thus
visible Cerenkov radiation occurs in water surrounding a strong radiation
source, such as an operating reactor.   There was no blue flash filling
Slotin's laboratory, but rather the Cerenkov radiation flash occurred in the
fluid (mainly water) within the eyeballs of those present.   The
experimenters did not detect "the beginning of a chain reaction" by a blue
glow as a regular experimental procedure - if they did so, it was too late
and they had induced a criticality accident! 
<END QUOTE>

...which I thought was an excellent explanation ( probably would have been
nice to say something about that high "c" in air having something to do with
its low density - much nearer that of vacuum than liquid water....)

Any further comments ?

Thanks

Jaro
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