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Etching the external surface of a glass vial improves LSC detection efficiency and LS-spectroscopy resolution.
Why ?
Because with clear vials, at the interface between air and the external enveloppe of the vial, there is a
refractive index step (1.5 to 1). This causes internal reflexion if the incidence angle of the photon, relative
to the surface normal, is greater than a critical angle (about 42 °). Two main consequences :
- less light can escape the vial, so the detection efficiency is decreased
- all parts of the scintillator are not equivalent : light created in the middle of the vial can escape because
its incidence is always normal to the surface. This is not the case for light emitted near the glass enveloppe.
This cause an excess variance in the distribution of photons emitted by the vial and lower the energy resolution
of the spectrum. This is specially evident for LS alpha spectroscopy.
This problem can easily be solved by etching the vial surface. At a microscopic level, the surface incidence is
random and this increases the transmission probability.
How to etch glass surfaces ?
Wrap a translucent adhesive tape around the vial. That's it !
This works fine but can cause some mechanical problems in some LS-counters...
Grind in a mortar (not a glass mortar !) the following chemicals :
15 g of sodium fluoride
10 g of ammonium sulfate
and 15 g of barium sulfate.
Add :
8 g of oxalic acid
12 g of water
and 40 g of glycerol.
Soak the glass vials in the mixture and let them stay there for 2 to 4 days.
Wash the vials with plenty of water and
dry.
The etching mixture can be stored in plastic bottle in refrigerator.
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