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If this is still topical I would like to note
the following:
- If the area of one spectacle lens is 10 cm2,
then the volume ThF4 at 1000 a0ngstro"m (?) thickness is 1E-4
cm3. With a density of
6.1g/cm3 this volume contains 6.1E-4 g ThF4 or 4.6 E-4 g Th 232.
- The
specific activity of monoisotopic Th (Th232) is 4.05 E+3 Bq/g (depending on
the half life chosen) which would then imply a Th 232 activity of about 1.9
Bq per lens. Th 232 is an alpha emitter
and if the ThF4 is on the outside of the lens the alphas emitted in the
direction of the eye will be stopped in the glass. If the coating is on the
inside some alphas will be stopped in the ThF4 itself. Those that are
emitted from the surface will be emitted in a 2pi angle, while the eye
subtends only a fraction of this angle.
- It must be
noted however that ThF4 has been prepared from Th which was present in a
mineral, most probably in equilibrium with its decay chain. The chemical
processes involved in the preparation of ThF4 would, certainly (?) ensure
that the Th is separated from all its decay products, except Th 228 which
has practically the same chemistry as Th 232. Th 228 present in purified Th
will decay, but due to the decay of Th 232, the whole decay series of Th 232
will slowly grow towards equilibrium again. This aspect has been discussed
before - also in the literature (Health Phys. Vol 48 (1) p124, 1985). The
total activity in the ThF4 at any time is thus dependent on the time elapsed
since the Th was purified.
- When one
considers the possibility that some Rn 220 can escape from the ThF4 coating,
then a calculation of the total activity and the dose to the eye from betas
and gammas as a function of time, can become quite
interesting.
Dolf Brits
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