An SEM user will never get an image that is better that the scintillator!
So while one might cut back on the quality of other aspects of their
SEM's system's operation, the quality of the scintillator is paramount in
terms of getting the best possible image from your SEM.
Basically it is the classic battle of the trade offs! The SPI P-47
powder scintillator is the one with the fastest response time, which
is well under 80 ns. However, the system itself, which consists of
the powder scintillator material dispersed on a high optical quality
substrate designed specifically for this application, and a proprietary
polymer acts as the "glue" that binds the scintillator layer to the
special substrate. In the earlier days of SEM, scintillators tended
to fail by peeling off, but today's technology permits one to make
scintillators where this mode of failure just does not happen anymore.
SPI guarantees its scintillators against failure by peeling or delaminating.
Like all other powder scintillators, the SPI P-47's are a phosphor powder
on a substrate. High uniformity of the powder size is needed for maximum
smoothness in the coating, since a rougher surface will result in an inferior
signal-to-noise ratio. When new, although it is difficult for most users to
measure accurately performance of scintillators from different sources, in
today's world, most reputable suppliers offer P47 power scintillators that
perform more or less equivalently.
However, nothing is forever, and that is certainly true for the P47
powder scintillators, and as time goes on, with exposure to the
ionizing radiation of the type being generated in the typical modern
SEM, there is a gradual deterioration of the response of the scintillator
leading to the time when the scintillator has to be replaced. Just how
fast that time comes depends to some degree on the type of work your are
doing (e.g. EDS work uses higher beam fluxes, generating more radiations)
and also to a lesser degree, the cleanliness of your vacuum system.
But it also depends on the combination of the right P47 powder formulation,
the optically clear substrate and how the powder coating is attached. And
in this respect, it is well appreciated by those who have tried different
scintillators, that there is indeed an observable difference in lifetimes.
The SPI P-47 powder scintillators are therefore formulated with not only
new scintillator performance in mind, but also longevity in the SEM.
The single crystal scintillators of YAG and YAP in comparison indeed do last
forever and will outlive the lifetimes of most scanning electron microscopes.
However, the single crystal scintillators of YAG and YAP exhibit slightly
slower response times than P-47. Note that in the graph the caption
"2 mg/cm" means 2 mg of scintillator powder per square cm of scintillator
area. The biggest improvement of a YAG or YAP over powder scintillators
will be for those with relatively clean vacuum systems. If the vacuum
system is an especially dirty one, one would have to conduct a periodic
cleaning in order to maintain the favorable characteristics of the single
crystal scintillator material.
We hope this brief introduction to scintillators will help you to make
the right choice for your SEM. Generally speaking, if you want to be
forever freed of the concern about replacing your scintillator, you
should think seriously about a YAG or YAP. For more advanced reading,
consider some of the numerous
publications from the scientific literature covering various aspects of
scintillator technology as applied specifically to applications in an SEM.
Reference: "Single Crystal Aluminates:
A New Generation of Scintillators", R. Autrata et. al., Scanning Electron Microscopy II, 489-500 (1983).
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