For the design and construction of a viewing window, one has a
choice of roughly a dozen different optical glass types.
There are approximately 250 different optical glass types on the
market, including cerium stabilized optical glasses, but only a
small portion have been selected as shielding glasses from the
standpoint of efficiency, production size limitations, melting
techniques, and economics.
The radiation absorption coefficient of a glass type, i.e. "μ", defining its
shielding capability, is generally related with its specific
gravity. Glass is, like any other shielding material, subject to
the exponential law of attenuation. The absorption coefficient μ increases as the specific gravity of the glass increases, thus
leading to a higher efficiency with regard to shielding. The
introduction of lead oxide into the glass batch is a common means to
achieve these higher densities.
It is incorrect to believe that only high density glasses are
useful as shielding glasses. While each type has its own shielding
efficiency, optical properties, and unique response to irradiation,
all types are subject to the so called "browning effect"
when irradiated. This is a discoloration process within the
molecular structure of the glass.
In the irradiation process, electrons that are displaced by
high energy
photons are set free and settle in "vacant" spots
within the glass molecules. This causes a shift in the absorption
band, meaning a loss of light transmittance. Such a loss can lead,
in the extreme case, to a 0% transmittance. In order to overcome
this browning process, a glass type which is less prone to that
effect must be placed before the high-density glass.
Such glass types are lower-density, typically between 2.53 and 3.7g/cm3,
and are usually doped with cerium-oxide. This additive is an
effective browning inhibitor and accounts for 0.5 to 2.5% of the
glass composition (by weight).
It prevents, or at least slows down the ionization process which
causes the browning effect. The absorption band of the given glass
type thus remains at the intended range under irradiation. Cerium
oxide changes the natural color of the base glass to a
certain extent, adding a more or less dominant yellow tint. Lead
free glasses of 2.53-2.7 g/cm3 density show a less noticeable tint,
but as the specific gravity is increased due to introduction of lead
oxide, the more yellowish a cerium-oxide doped glass will appear.
Glass producers manufacture these so-called non-browning glasses in
the range of 2.53 to 5.6 g/cm3. The above mentioned non-browning,
non-leaded glasses are in most cases applied as hot side cover plate
glasses and first hot side shielding slabs. They offer moderate
shielding, but are capable to reduce doses, rates and lifetime doses
to a level compatible with other high-density glass types, gaskets,
oil or other materials of construction.
Today's glass manufacturers produce types having density ranges
between 2.53 to 6.2 g/cm3. The latter density represents a limit,
beyond which, the glass looses transparency rapidly, and thus light
transmittance.
Cerium oxide has another positive feature: after a browning
incident, it helps the glass to recover to a light transmittance
near or identical to an unirradiated condition. The rate of this
recuperation period varies with the given glass type, fluctuating
between 5-10 minutes and several months. Therefore, a properly
designed window having non-browning glass components will always
offer a light transmittance value which is close to the unirradiated
condition. Actual transmission losses over a design lifetime of
typically 20 years are not much more than 10-15% from original,
provided of course, that the window will not be over exposed beyond
design limits and provided that a routine maintenance program is
enforced.
The table in the section,Radiation Shielding Glass, summarizes
the most common available shielding glasses produced by Schott for
our windows, and lists their recommended application limits.
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Data and information contained in the tutorial was written by Dale A. Tobias, Premier Technology, Inc. and Hienz E. Hoffman and William G. Wash, Schott Glass Technologies, Inc. and may be reproduced only with written consent.
For information, contact Lyle Freeman
Vice President of Business Development
(208) 782-9129 lfreeman@ptius.net
