UltraLo-1800 Alpha Particle Counter
XIA LLC
Page 8
2.
Electrostatic Description of the UltraLo-1800
To understand how the UltraLo-1800 works we start with a description of its active volume. As
shown in Figure II-2, the active volume is set up as two parallel conductive plates separated by distance
D. The top plate, called the electrode, is held at positive voltage (denoted V throughout), which is
typically 1000 V. The bottom plate, called the sample tray, is held at ground potential and holds the
sample to be measured. This arrangement produces an electric field (E) between the plates that has the
value V/D. The volume between the plates is filled with a high-purity counting gas, in this case argon.
Finally, the sides of the chamber, called the sidewalls, hold the fieldshapers, which are PCBs containing
strips of copper separated by resistors that keep the electric field lines parallel throughout the volume.
Suppose a radioactive isotope on the sample tray decays, emitting an alpha particle in a random
direction. In Figure II-2 it leaves the atom and is stopped in the gas after traveling some distance. This
distance is called its range and is a function of both the alpha particle’s energy and the specific gas
species. As the alpha particle travels through the gas it loses energy by ionizing the gas molecules,
producing a track of ion-electron pairs. The range and number of pairs (N) created in the gas increase
with energy of the alpha particle. Being charged, these ions and electrons drift in the applied electric
field, the electrons toward positive voltage on the electrode, the ions toward the tray. Each drifts with a
velocity equal to its mobility (μ) in the gas times the electric field, or:
𝑣
𝑒
= 𝐸 ∙ 𝜇
𝑒
= 𝑉 ∙ 𝜇
𝑒
𝐷
⁄
(where the
subscript “e” indicates “electron”, and “i" would indicate “ion”). The mobility of electrons is thousands
of times higher than that of ions, thus they drift to the electrode faster than the ions drift to the tray.
How long does this take? If the electron is freed a distance d away from the electrode (as in Figure II-2),
its drift time is d/v
e
seconds, or
𝑡
𝑒
= 𝑑 ∙ 𝐷/𝑉 ∙ 𝜇
𝑒
. When d is the height of the entire chamber, D=15cm,
and V=1000V, then t
e
is found to be approximately 70 μs in argon.
Figure II-2: Schematic overview of the important parts of the UltraLo-1800's active volume.