UltraLo-1800 Alpha Particle Counter
XIA LLC
Page 6
II.
Theory of Operation
A.
Operation of the UltraLo-1800
1.
Synopsis
The diagram of the UltraLo-1800 in Figure II-1 shows its major components. It is basically a
specialized ionization counter comprising an active volume filled with argon, a lower grounded
electrode that is a conductive tray holding the sample (called the sample tray) and an upper pair of
positively charged electrodes. Of these two electrodes, the anode sits directly above the sample, while
the guard electrode surrounds and encloses the anode. Both electrodes are connected to charge-
integrating preamplifiers whose output signals are digitized and then processed by a digital pulse shape
analyzer.
An alpha particle emitted from the sample (αs) creates an ionization track of argon ions and
electrons. As the electrons drift in the counter's electric field, they induce a time varying charge on the
anode that is seen as a current by the preamplifier, which integrates it to produce an output pulse that
is digitized and then analyzed to extract its risetime, amplitude and shape. Similarly, ionization tracks
produced by alpha particles emanating from other counter components (e.g. the anode (αa) and the
sidewalls (αw)) also produce digitized output pulses in the anode or guard processing channels or both.
The UltraLo-1800's geometry is designed specifically so that the output pulses associated with these
different points of alpha particle emanation are substantially different – having, specifically, different
risetimes, amplitudes and shapes. The UltraLo-1800's analysis software uses this shape information to
Figure II-1: Schematic overview of the UltraLo-1800.