UltraLo-1800 Alpha Particle Counter
XIA LLC
Page 15
C.
Comparison to Gas Proportional Counters
As previously mentioned, the UltraLo-1800 is an ionization counter and most other low-rate, large-
area alpha counters are proportional counters. There are some important differences between the two,
and if you’re familiar with proportional counters some, but not all, of that familiarity will be able to
transfer over.
First we’ll look at similarities. Both types of counters are filled with inert gas, both use the electron-
ion pairs formed by alpha particles, both use electrodes to collect the signal, and the pulse processing
equipment is largely similar.
The main difference is the electric field in the counter. For ionization counters the field’s value stays
fairly low and is uniform (the UltraLo-1800’s field is around 66 V/cm) while gas proportional counters
have fields high enough to create electron avalanches. An electron avalanche occurs when drifting
electrons accelerate to energies high enough to create electron-ion pairs themselves. The created
electrons can then accelerate and create more pairs, causing an “avalanche” of electrons. In
proportional counters this process is called gas multiplication. To create avalanches proportional
counters typically have an anode composed of thin wires, this creates a region of very high electric field
close to the wire, which is called the multiplication region.
These small changes have large practical consequences. The most immediately obvious is that the
proportional counter has a large internal gain, and so the resulting signals are much larger and easier to
process. This means less electronic noise and fewer design constraints on detector parts such as
preamplifiers. The higher noise and more difficult design for ionization counters is why proportional
counters were long the favored device for low-rate, large-area counting.
However, the tradeoff for this internal gain is that proportional counters lose any information about
where the pulse originated. Pulses from the counting tray look the same as those from the sidewalls, the
ceiling, even the electrode wires themselves. This is because the vast majority of the signal comes from
the time between the first electrons of a track entering the multiplication region and the final
multiplication ending. The duration of that signal is only dependent on the orientation of the track
relative to the anode wires, and not on where the track originated in the detector. (See Figure II-8 for a
Figure II-8: Events and the resulting pulse from a proportional counter. All events result in the same pulse shape.