Glass darkening

["Scott O. Schwahn" <schwahn@CEBAF.GOV>]

Experimenters at our facility will be putting in a very expensive laser
system for determining beam energy via Compton scattering of the laser
beam.  One of the most expensive components of the laser, due mostly to its
size and perfection of machining, is the semi-reflective mirrors in the
lasing cavity.  It starts off with a "gain of 10,000" (their term - don't
ask me exactly what it means - apparently it is a measure of reflectance),
and is essentially useless below about 3,000 gain.  

I may be making a jump here, but I would guess that gain loss is directly
related to the darkening of the glass due to radiation.  We have observed
that here in controlled experiments in a cobalt irradiator, but mostly on
plastics or epoxies rather than glass.

So my question is - can anyone cite specific transmission loss data for
regular silicon dioxide glass as a function of dose absorbed?  I'm not
really looking for general terms such as "some darkening at 10^6 rad", but
studies of actual measured transmission loss vs. dose absorbed.  Even if
you could point to a reference for such information, that would be very
helpful.

Scott

Scott O. Schwahn, CHP
Thomas Jefferson National Accelerator Facility
Newport News, VA
schwahn@jlab.org