RE: blue flash and criticality

[Thomas Morgan <tmorgan@isotopeproducts.com>]

Here is the basics of Cerenkov light (from R.D. Evans The Atomic Nucleus, 
1982):

Cerenkov radiation is produced by charged particles.  The threshold for 
emission of Cerenkov radiation is the phase velocity of light in the medium 
through which the charged particle travels (c/n, where n = refractive 
index).  As I indicated in a previous posting, this is 240 keV in water and 
21 MeV in air, for electrons.  At these kinetic energies, the particles are 
relativistic (beta = V/c ~ 1.0).  A charged particle, such as an electron, 
may clearly achieve a kinetic energy in excess of these limits, either 
naturally or with human assistance.

The electromagnetic field of a charged particle will distort the electronic 
structure of an atom.  As a result of the atom relaxing back to its 
unstimulated state, light is emitted.  At velocities below the Cerenokov 
threshold (beta*n <1), complete distructive interference occurs and no 
light is visible beyond the immediate vacinity of the affected atom.

Above the Cerenkov threshold (beta*n >1), constructive interference occurs. 
 This results in light being propogated in the forward direction (along the 
axis of travel of the electron). The angle of emission is dependent upon 
particle velocity and refractive index.

The quantum yield is only dependent on the charge and velocity of the 
particle and the refractive index of the medium.  It is not dependent on 
the particle's mass.  The intensity is inversely proportional to the 
wavelenth, so short wavelenght light is preferred.  This accounts for the 
bluish-white appearance.

Thomas L. Morgan, Ph.D.
Director, Health Physics
Radiation Safety Officer
Isotope Products Laboratories
1800 N. Keystone St.
Burbank, CA 91504
818-558-1375 (Voice)
818-843-6168 (Fax)
tmorgan@isotopeproducts.com

-----Original Message-----
From:	Dave Brown [SMTP:david.brown@nist.gov]
Sent:	Friday, November 10, 2000 7:48 PM
To:	Multiple recipients of list
Subject:	RE: blue flash and criticality


>My question now is, IF, you had a camera, and the source of criticality
>was shielded from the camera's angle, in other words that the source was
>going away from the camera instead of at it, and you you were able to
>establish the original spectrum from long distance face on photography,
>would it be red shifted going away?

....from my limited understanding of Cherenkov radiation I believe the
answer to your question to be..No.  Cherenkov light is not the result of a
doppler shift in the emitted light of the particle as your question
infers.  It is a Secondary result of the phenomenon associated with the
speed of the particles in the given medium, with a characteristic
wavelength.  I like the analogy of the sonic boom.... though I'm not sure I 
could explain the phenomenology associated with it.

Regards,
Dave Brown
david.brown@nist.gov

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