It's not the MFP of the x-rays that makes tesla coils okay in air, it's
the MFP of the electrons. In order to get x-rays the electrons need to
reach sufficient kinetic energy before they collide with a target so
they can knock out inner electrons in the target, which will
subsequently cause target-characteristic (line) x-rays... or... the
electrons need to reach sufficient kinetic energy before they interact
with a target so they produce can produce the continuous
"bremsstrahlung" x-rays.
In air, the MFP of the electrons is so small, or said differently, the
interactions resulting in energy loss are so frequent, that the
electrons never attain enough kinetic energy to allow the creation of
x-rays.
Even if a tesla coil or van de Graaff generator is at tens or hundreds
of kilovolts, no electrons are going to reach energies anywhere close to
kilovolts if they are always colliding with air. Thus, no x-rays are
generated.
On the other hand, of there is a vacuum in the high-field region such
that some electrons have enough MFP that they can achieve tens of keV of
kinetic energy before having an interaction with matter, then we can
get x-rays.
Once we have x-rays, the air does not attenuate them very much.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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