UltraLo-1800 Alpha Particle Counter
XIA LLC
Page 12
4.
Necessary Assumptions
There are several assumptions that we must make in order for our model to accurately describe the
physical counting system:
The first is that the electron drift speed is both uniform and known. Because electron mobility
changes with the type of counting gas, so does the drift speed. Further, it can also change by significant
amounts if the counting gas isn’t pure (e.g. concentrations of water of only 100 ppm can decrease
risetimes significantly). By selecting the correct operating gas in CounterMeasure and purging for
adequate amounts of time these assumptions should always be met (unless the material being counted
is itself a source of water vapor, see
We also assume that the charges are free to drift all the way across the active volume. This
assumption can break down if there are significant amounts of oxygen in the active volume, since O
2
has
a high electron affinity. As above, adequate purging also makes this condition easy to satisfy.
A third assumption is that the electric field is uniform, which is guaranteed by the design of our
active volume, but can be perturbed if, for example, materials with significant height or very high
resistivity are placed on the sample tray (see
Finally, when we report energies we assume that all alpha energy is expended in the gas. If the alpha
isn’t emitted from the sample’s surface (surface emission) and passes through some amount of material
before entering the active volume, then its energy will be reduced by some unknown amount. Because
it’s impossible to know how much material an alpha passed through we cannot correct for this.
However, bulk sources (as these kinds of samples are called) show certain behaviors that are
understood, see
