UltraLo-1800 Alpha Particle Counter
XIA LLC
Page 17
D.
Remaining Background Sources in the UltraLo-1800
While the UltraLo-1800 is capable of filtering out pulses originating from its various internal surfaces
(and is in that sense “zero-background”) there still remain several known sources of rarer events that
produce traces that our software classifies as sample alphas but that do not originate from the sample.
Thus these sources contribute to a background rate that we presently estimate to be about 0.0005
counts/hr/cm
2
in the UltraLo-1800 (for a counter at sea level with no significant overhead shielding). The
following background sources are under active R&D at XIA, see
1.
Cosmogenics
Historically, ionization counters were most commonly used for measuring fluxes of cosmic radiation,
and we’ve discovered that the UltraLo-1800 still works for that purpose. Early on in the development of
the detector we observed pulses that didn’t look quite like alphas. While alpha pulses will have some
rounding at their peak (due to the absorption of the electrons into the electrode), the rounding
observed on these pulses was much more significant (see Figure II-9). We created a new class of pulse
(the “round”) in order to separate them out while we pursued their origins. From early on we suspected
they were caused by cosmic rays, but proving it experimentally was difficult. The reason we suspected
cosmic rays is that in order to observe a very rounded peak the ionization track would need to be very
long, but in order to deposit energy equivalent to a 1-5 MeV alpha particle it would have to have a fairly
high dE/dx. Alpha tracks are too short and dense, and beta and gamma rays are far too weak. This leaves
some kind of cosmic ray, most likely a light baryon such as a proton.
A recent experiment conducted by XIA [
] showed a strong dependence between
elevation/depth and observed round rate. These findings clearly indicated that rounds are indeed
caused by cosmogenics.
While we can identify and filter out cosmogenic events with some efficiency a few will still pass
through our analysis and show up as alphas, particularly at low alpha energies where our signal-to-noise
Figure II-9: A "Round" pulse. Note the extremely large time between t
S
and t
R
(second and third lines, respectively). The
anode is shown in red, the guard in blue.
