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This consists of a thin EJ-212 plastic scintillator with a layer
of silver activated zinc sulfide phosphor (ZnS:Ag) applied to
one side.
The primary use of EJ-444 is the detection of alpha and beta
particles in health physics instrumentation.
A non-coated plastic scintillator may also be used for this
purpose, but the amplitudes of the alpha and beta pulses are
sufficiently similar to make it difficult to separate the two
by simple counting electronics making use of counting windows.
In contrast, the ZnS:Ag layer on the EJ-444 is made just thick
enough to fully stop all common alpha particles while allowing
many low energy betas to pass into the plastic scintillator layer
for detection there.
C-14 betas may be detected with moderate efficiency.
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The plastic scintillator component is usually kept quite thin
in order to minimize gamma sensitivity, and the standard
thickness in EJ-444 is 0.25mm (0.010 inch).
Hence, even energetic betas such as from P-32 and SrY-90
generate pulses grouped in a fairly narrow amplitude region
that is below the amplitudes of over 95% of the alpha pulses
generated in the ZnS:Ag layer.
The EJ-444 alpha detection efficiency is essentially the same
as achieved with bare plastic scintillator.
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To give an indication of the beta detection efficiencies of
EJ-444, counting comparisons were made between bare 0.25mm
EJ-212 and EJ-444 made with an identical EJ-212 component.
The bare plastic scintillator is considered to be a reliable
reference for maximum detection of the radioactive particles
investigated.
The results are highly uniform from batch to batch, and the
typical values are presented below:
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