RE: blue flash and criticality

["Sugarman, Steve" <SugarmaS@orau.gov>]

This is as good a basic explanation as I have found for Cherenkov radiation.
It came from the Univ. of Missouri-Rolla (http://www.umr.edu) website.
There is also a pretty good picture of their reactor, complete with the
"blue glow."  

"...While no particle can exceed the speed of light in a vacuum (3.0E+8
m/sec), it is possible for a particle to travel faster than light in certain
mediums, such as water. The speed of light in a particular medium is related
to the speed of light in a vacuum and by the index of refraction. Water has
an index of refraction of approx. 1.3; thus the speed of light in water is
2.3E+8 m/s. Beta particles with kinetic energies in excess of 0.26 MeV
travel at speeds in excess of 2.3E+8 m/s.

When the charged beta particle moves through water it tends to "POLARIZE"
(or orient) the water molecules in a direction adjacent to its path thus
"DISTORTING" the local electric charge distribution. After the beta particle
has passed, the molecules realign themselves in their original, random
charge distribution. A pulse of electromagnetic radiation in the form of
BLUE LIGHT is emitted as a result of this "REORIENTATION". When the speed of
the beta particle is less than the speed of the light, the pulses tend to
"CANCEL" themselves by destructive interference; however, when the speed of
the beta particle is greater than the speed of light then the light pulses
are "AMPLIFIED" through constructive interference. The phenomenon is
analogous to the acoustic "SONIC BOOM" observed when an object exceeds the
speed of sound in air.

The intensity of the blue glow is directly proportional to the number of
fissions occurring and the reactor power level. This property is utilized in
Cherenkov detectors that measure the magnitude of Cherenkov radiation
produced in a detector made of Lucite."

Interestingly, the source  that was involved in the Goiania, Brazil Accident
in 1987 (approx. 1375 Ci Cs-137) appeared to glow after the capsule was
breached, but the color was greener than it was blue.  The general consensus
is that this was Cherenkov radiation due to at some time after the capsule
was breached, enough ambient moisture was absorbed by the cesium chloride
salt source to allow this phenomenon to be seen, but the moisture levels
affected the color of the "glow".

Steve Sugarman
Health Physicist, REAC/TS
sugarmas@orau.gov

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