One critical issue relating to the proper glass selection for shielding purposes is the
electrostatic discharge phenomenon. This phenomenon may occur in either a dry or oil filled
window.
Ionizing radiation from high-energy gamma photons impinging on shielding glass causes
an internal electrostatic charge to accumulate toward the hot side within the shielding
glass. This phenomenon is caused by the high-energy photons displacing outer shell electrons
toward the center of a shielding glass, creating positive and negative ionized locations; this
is very similar in concept to charging a huge capacitor. The same electrons that settle in the
vacant spots within the glass molecule and cause the browning effect remain within certain glass
types and build up a high voltage potential static charge. After subjection of the glass to
high gamma radiation dose rates for a significant length of time, a dielectric discharge can
be initiated (triggered) by a mechanical impact stress and may result in permanent
deterioration/damage of the glass slab, with the discharge energy forming the well known
"Lichtenberg" branch pattern. A spontaneous discharge could also occur without a trigger
mechanism if the glass slab has reached charge potential saturation limits. Conductive
characteristics of a given glass type influence this discharge phenomenon considerably.
Utilizing specific glass properties and glass characteristics data for design
information, a window design engineer has eight (8) or more different glass densities,
degrees of stabilization and charging tendencies to select from to provide a window design
that combines adequate radiation attenuation with the least potential for dielectric discharge
or darkening (browning), while not compromising exceptional light transmission and viewing
capabilities. In addition to these fixed design properties, variables such as glass thickness
and relative position in a window structure are also important window design considerations.
Borosilicate crown (unleaded), cerium stabilized glass plates (less than or equal to 1.50 inches thick)
are essentially totally discharge resistant and are therefore primarily used as contamination
barriers (alpha shields and hot side cover plates) and as physical shields to protect the
leaded glass from impact stresses. These plate glasses also provide some radiation attenuation,
but are usually not used as significant radiation shields. They may be utilized as a thick slab
for very high radiation dose rates.
Discharge resistance depends to a high degree on the chemical and mineral composition of
the glass, and the impingement dose rate (R/hr). Borosilicate glasses (RS 253 G18, RS 253
G25) utilized as alpha shields and hot side cover plates, are extremely resistant to charge
buildup and are rated at a dose rate limit of 4 x 104 R/hr for G18 and 1 x 106 R/hr for G25.
Both have a discharge resistance threshold of 1 x 1010 R for saturation.
The 3.23 density shield slabs provide the primary radiation attenuation mechanism in a
shielding window and are located near the hot side of the window structure, immediately
behind the hot side cover plate. At this location, the radiation dose is only slightly
attenuated. Therefore, the medium leaded shield glass of 3.23-gm/cc density (RS 323 G15) with
cerium stabilization, has the greatest propensity for dielectric discharge; this is due to three
(3) primary factors:
- The chemical and mineral composition of the glass,
- Position in the window structure,
- Slab thickness.
A slab thickness of less than three (3) inches in this density will rarely discharge
under any circumstance, but due to the low structural strength of this leaded glass, it
may fracture during polishing and handling if it is less than four (4) inches thick.
Therefore, if multiple slabs of 3.23-density glass are used as shielding, the thinnest
slab is greater than four (4) inches thick and is placed toward the hot side of the
window to minimize a dielectric discharge possibility. The RS 323 G15 glass, with cerium
stabilization for non-browning, has a radiation dose limit of 1 x 104 R/hr (10,000 R/hr)
and a discharge resistance threshold of 5 x 106 R for saturation.
The heavily leaded (flint) silicate glass, RS 520, is generally not cerium stabilized
for applications because the 3.23 density glass design thickness has reduced the
impingement dose rate at this position location to less than 5 R/hr. Schott's RS 520
glass has an application dose rate limit of 1 x 103 R/hr (1,000 R/hr) and a discharge
resistance threshold of 2 x 106 R for saturation. If the dose rate is higher than 5 R/hr,
the cerium stabilized RS 520 G 05 glass should be used.
These leaded shield glasses, like a charged capacitor, will slowly bleed-off the
internal electrical charge if the impingement dose rate is decreased or eliminated. The
potential buildup of internal electrostatic charge in a window may be minimized by
physically shielding the window when not in use, or repositioning a high gamma source
away from a window or in a floor pit.
Electrostatic discharge has to be taken seriously, not only from the viewpoint of
monetary losses due to the destruction of the glass slab, but also from the viewpoint of
personnel protection. The glass manufacturers realize this problem and have developed
specific glass types which are highly static discharge resistant. These glass types also
feature good browning resistance and are thus very suitable hot side cover glasses. Many
of them employ a higher alkali content, especially NaO2, in order to increase electric
conductivity. The application of such glass types is highly recommended when dose rates
of more than 4 x 104R/h @ 1.0-2.0 MeV are encountered. The glass type designation, their
appropriate CeO2 content, and their density vary from manufacturer to manufacturer.
Discharge resistance is always thoroughly evaluated by Premier engineers in the
conceptual design of facility windows, and the glass densities, percent of cerium
stabilization, charging tendencies, plate or slab thickness and window structure
positions are optimized to provide an extremely discharge resistant window design with
excellent viewing properties.
Back to Topic List
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
