|
No. 13
Scattering Chamber Black Coating
For a scattering chamber to function properly
and give accurate results, its surface must be ultra-flat
black and must not reflect light into the cone of darkness
(in the center). The original process used to blacken the
conical surfaces and lens end plate surfaces of the sampling
chamber was to break off a small piece of camphor block and
ignite it with a match. The black sooty smoke that was emitted
from the flaming block was used to coat both the conical surfaces
and the lens plate surface with an ultra-flat black sooty
coating. This became a safety hazard due to an open flame
and tighter controls on the environment and is no longer recommended.
In scattering chambers manufactured prior
to 1988, the surfaces were not textured but were machine smooth.
If this type scattering chamber is employed, the unit can
be disassembled and the surfaces cleaned with denatured alcohol
or lacquer thinner. The surfaces of the conical area and the
lens endplate, where the phototube is attached and the small
pick-up lens is secured, must also be cleaned and blackened.
Care must be exercised to avoid blackening the lens. The denature
alcohol or lacquer thinner only needs to be used when the
surfaces are shiny and oily. If the surfaces are clean and
dry but have foreign matter attached, the foreign matter can
usually be dislodged by spraying compressed air over the surfaces.
In the scattering chamber design used between
1988-95, ATI employed a new process which gave a textured
finish to the inner surface of the conical areas of the scattering
chamber and the small lens endplate. The textured finish eliminated
smooth surfaces that reflect light. Although ATI used a special
Bausch and Lomb ultra-black photo finish, the straylight was
still rather high. ATI found that if the surfaces were sprayed
with Krylon #1602 ultra-flat black paint, background or straylight
reading were very low. The black anodized electroplated finish
provides an excellent primer for the paint to adhere to and
results in few maintenance problems.
The current method recommended consists of
cleaning both the conical surface and lens endplate surface
of all foreign matter. If the scattering chamber is the older
style, it can be sprayed with Krylon® #1602 ultra flat
black paint. This should work fine in any pre-1995 ATI units.
Post-1995, scattering chambers do not require any coating
to operate correctly.
|
