Re: cherenkov radiation

["Dukelow, James S Jr" <jim.dukelow@pnl.gov>]

Jaro Franta wrote:
-----Original Message-----

	<snip>

I refer specifically to the "blue glow" phenomenon.
Kevin Roark of LANL says much the same thing in his article posted at URL
http://www.lanl.gov/worldview/news/releases/archive/00-099.html

<BEGIN QUOTE>
Criticality accidents report issued
LOS ALAMOS, N.M., July 19, 2000- Since 1945 there have been 60 criticality
accidents world-wide 
<snip>
In most criticality accidents this chain reaction is very short lived,
causing a neutron population spike and resultant radiation, heat and, in
many cases, an ethereal "blue flash," a phenomenon of the air surrounding a
neutron burst becoming ionized and giving off a flash of blue light.
<END QUOTE>

But as I understand it, this is NOT CORRECT.
This "flash of blue light" would occur if the criticality had been inside a
tank of water, like a pool-type research reactor, due to Cerenkov radiation.
But Daghlian's and Slotin's plutonium sphere was in a dry air environment
with no way to produce Cerenkov radiation in the lab. 
But there's more to it than that. 
Recall that in the case of the Tokaimura criticality accident the excursion
occurred in an aqueous solution of enriched uranium. A blue flash of
Cerenkov radiation would therefore have been produced. But the liquid was in
a closed tank, so the light would not have shined outside. 
Nevertheless the victims of the accident, just like Daghlian, Slotin and
their colleagues, saw a blue flash. 
But this was not in the laboratory. It was INSIDE THEIR EYEBALLS, where the
gamma & neutron particles caused Cerenkov radiation to form "in-situ." (
BTW, the end of the "blue flash" also signalled the end of the fission
pulse, due to thermal expansion of the metal -- no point leaping at the
device to try to shut it down, since its all over by then anyway ! )

	<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.

Best regards.

Jim Dukelow
Pacific Northwest National Laboratory
Richland, WA
jim.dukelow@pnl.gov

These comments are mine and have not been reviewed and/or approved by my
management or by the U.S. Department of Energy.

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