A New Generation of Scintillators
"Single Crystal Aluminates: A New Generation of Scintillators"
R. Autrata et. al.
Scanning Electron Microscopy II, 489-500 (1983).

SEM Applications

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.