Phase I Rinal Report: Ultra-Low Background Alpha Activity Counter Page: 2 of 35
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In certain important physics experiments that search for rare-events, such as neutrino or double beta
decay detections, it is critical to minimize the number of background events that arise from alpha particle
emitted by the natural radioactivity in the materials used to construct the experiment. Similarly, the
natural radioactivity in materials used to connect and package silicon microcircuits must also be
minimized in order to eliminate "soft errors" caused by alpha particles depositing charges within the
microcircuits and thereby changing their logic states. For these, and related reasons in the areas of
environmental cleanup and nuclear materials tracking, there is a need that is important from commercial,
scientific, and national security perspectives to develop an ultra-low background alpha counter that would
be capable of measuring materials' alpha particle emissivity at rates well below 0.00001 a/cm2/hour.
This rate, which corresponds to 24 alpha particles per square meter per day, is essentially impossible
to achieve with existing commercial instruments because the natural radioactivity of the materials used to
construct even the best of these counters produces background rates at the 0.005 a/cm2/hr level. Our
company (XIA) had previously developed an instrument that uses electronic background suppression to
operate at the 0.0005 0.005 a/cm2/hr level. This patented technology operates as follows. First, it sets up
an electric field, in a large parallel plate geometry, between a large planar sample and a large planar
anode, and fills the gap between them with pure Nitrogen. When an alpha particle enters the chamber it
ionizes the Nitrogen, producing a "track" of electrons, which are attracted to the anode by the electric
field. Because alpha particles emitted from the anode produce tracks close to the anode, it takes less than
10 microseconds (ps) to the electrons to be collected, producing a signal with a 10 ps risetime from the
instrument's preamplifier. Tracks produced by alphas emitted from the sample, on the other hand, have
to drift across the full gap between sample and anode, which takes about 35 ps and produces a signal
with a 35 ps risetime. By digitizing the preamplifier signals and analyzing them in a digital signal
processor, we can easily distinguish between these two risetimes and thereby count only the alpha
particles emitted by the sample. Because the sheet of sample absorbs any alpha particle emitted from the
sample tray before they reach the counter's active volume, the tray's emissivity does not contribute to the
counter's background either. In addition, extensions of the method to the counter's sidewalls similarly
allow us to reject alpha particles emitted from the sidewalls. We are thus able to obtain background rates
that are up to a factor of 1000 lower than in conventional instruments that do not employ this active
Attempting to extend this principle to count at the 0.00001 a/cm2/hour, level encounters difficulties,
however. In particular, at this emissivity, there will typically be only 2.4 alpha particles per square meter
per day! Since approximately 6 true counts are required to measure activity at the 95% confidence level,
large sample areas are required if measurements are to proceed in reasonable time periods.
Unfortunately, increasing the counter's anode area to a square meter raises its capacitance so much that
the preamplifier noise levels swamp the alpha particle signals and make counting impossible.
In this SBIR we proposed to solve this dilemma by segmenting the single large area electrode into
several smaller independent electrodes whose individual capacitances would be small enough so that we
could still detect the alpha particles reliable. Since each electrode would then have its own electronics,
we would have to capture signals from all of them in coincidence (since an alpha track might well deposit
charge on more than one electrode), but we had already developed simple techniques for doing so for
XIA's gamma-ray spectrometers. Therefore, in Phase I we proposed to show proof of principle by
subdividing our original 1,800 cm2 electrode into 4 square segments, each 625 cm2 and demonstrating
that signal noise on individual channels has been reduced as expected. Because the Phase II counter with
a 1 m2 segmented anode would require 16 such segments plus a segmented guard as well, we also
proposed to design low cost signal processing electronics to instrument it in Phase II.
Our Phase I effort met our major proof of principle goals by carrying out our stated research plan. In
particular, we showed that reducing the anode size by a factor of 4 in area reduced electronic noise by 3.
Noise reduction did not scale exactly with electrode area because a significant fraction of electrode
capacitance is to its neighboring electrodes and this term does not drop as fast as the area term. We also
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Warburton, W.K. Phase I Rinal Report: Ultra-Low Background Alpha Activity Counter, report, July 22, 2005; United States. (digital.library.unt.edu/ark:/67531/metadc782708/m1/2/: accessed August 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.