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UV and ionizing radiation



Ray Stanford asks,
"Maybe I'm missing something, but I've never heard of UV being described
as "ionizing radiation".  I though that description was reserved for certain
higher energy radiation (e.g., gamma radiation)."

Ionizing radiation is often taken to start at 13.6 electron volts,
or wavelength of 912 Angstroms (the ionization potential of
hydrogen, which is abundant not only cosmically but biologically).
Normally this isn't a biological hazard because it's not very penetrating
until you get to much higher energies, but one could imaging it
being a factor if there were vastly increased deep-UV radiation
and the ozone layer had been trashed.

Bill Keel
Astronomy, University of Alabama