Why is the alpha efficiency of a pancake detector considered low?

This has a lot to do with the alpha source itself. Starting with the source, not all particles are headed in the direction of the detector. The next challenge for the alphas is to make it off of the surface of the source. Alpha standards *must* be protected from the outside world to prevent the radioactive material from coming off. Typically, electroplated sources are used, but there are ones that exist generally using a thin sheet of Mylar and adhesive. Also, many of the alpha particles are absorbed prior to escaping the source.

Efficiency measurements are specified at a fixed distance from the source. We recommend using a source to detector distance of 1 cm for all efficiency measurements. So the attenuation of 1 cm of air must also be figured in. This can be easily achieved using the Wipe Test Plate and the Ranger.

In addition to this, the screen protects the detector face as well, so some of the incident alpha particles are stopped there.

Finally, the absorption of the mica window is considered. Once an alpha particle makes it into the active volume of the detector the intrinsic efficiency is essentially 100%.

It is possible to determine a different efficiency, using the alpha emission rate from the source (hence a 2-pi efficiency) instead of the total activity deposited. Since we have to use one number, we go with the activity-based, 4-pi efficiency, as that is most representative of how the majority of measurements are taken in the working environment. This is generally counting wipe samples and direct measurements on surfaces.

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