Bill Hunter writes:
>Radiation damage, chemically induced color
>change from either the rare earth ingredient or a contaminant or both
>remain equally suspect?
Under what circumstances is a silicate compound with a rare earth element
likely to be chemically unstable resulting in a gradual colour change
throughout the substrate? What external factors (heat, light etc) would be
likely to contribute to the rate of this colour change? If this is a
chemical reaction unrelated to the isotopic decay (since even "radiation
damage" is still obviously a chemical change) it should be relatively simple
to test your hypothesis, albeit destructively for the lens involved. Take
one mildly yellowed lens, cut in two (being careful not to inhale any dust),
refrigerate one half, heat the other, compare after x weeks/months.
>The absence of any focal defects supports
>chemically induced color change.
Other than your anecdotal evidence, what is the basis for your postulate
that focal defects as opposed to uniform change are characteristic of
"radiation damage"? Surely with a glass of relatively homogenous formulation
(including homogenous distribution of the unstable isotope) one would expect
uniform change? Are you suggesting autocatalytic free radical reactions?
Based on the nature of the emitted radiation, I would condider focal defects
(other than pinpoint) to be unlikely, as it would imply poor mixing of the
glass with high concentration gradients between areas containing unstable
isotope and those not. Given the origin of the "rare" in rare earth elements
(ie difficulty of purifying them), this would also imply a concentration
gradient between areas containing rare earth element and those not. I would
be surprised to hear that this did not similarly affect the optical
characteristics of the glass. If one of the radioactive lens owners had a
counter with a small head, this would also help answer the homogeneity
question - how evenly distributed is emission from the element surface?
>Short of more definitive chemical analysis, your photos support a
non-destructive trial of 'sunlight' or other UV treatment.
This may answer the question as to whether the coloured compound in the
glass can be broken down by UV, but doesn't address how the coloured
compound came to form. As a specious example, UV will bleach out a stain
from my shirt whether it was caused by wine or tomato sauce. (Unless the
suggestion is that the daughter product of decay is the source of the colour
change, which may or may not be the case. If however this _is_ the case,
it's not per se a case of "radiation damage".)
>If removed, a simple but definitive
assessment can be made from the 'feel' of the lens surface. Is the
surface smooth just like the other lens elements or rough like fine
sandpaper. The latter would be consistent with radiation induced
change in material structure; a smooth surface STRONGLY weighs towards
chemically induced color change.
Would you seriously expect to see a macroscopic change in the material
structure of a silicate from thorium decay?? Ablation of the surface by
those high energy particles <g>?
Dylan <not wanting to be left behind in the "windbag" stakes> Sutton
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