XIA UltraLo-1800 User manual
XIA LLC
UltraLo-1800 Alpha Particle
Counter
User’s Manual
Version: 1.1
December, 2021
UltraLo-1800 Alpha Particle Counter
Contents
I. Conventions of the Manual ..................................................................................................................5
II. Theory of Operation..............................................................................................................................6
A. Operation of the UltraLo-1800 .........................................................................................................6
1. Synopsis.........................................................................................................................................6
2. Electrostatic Description of the UltraLo-1800 ..............................................................................8
3. Signal Generation..........................................................................................................................9
4. Necessary Assumptions ..............................................................................................................12
B. Secondary Veto Channel.................................................................................................................13
1. Rejection .....................................................................................................................................13
2. Counting Efficiency .....................................................................................................................13
3. Induction .....................................................................................................................................14
C. Comparison to Gas Proportional Counters.....................................................................................15
D. Remaining Background Sources in the UltraLo-1800 .....................................................................17
1. Cosmogenics ...............................................................................................................................17
2. Radon ..........................................................................................................................................19
3. Undersized samples ....................................................................................................................22
4. When backgrounds become important......................................................................................22
III. Sample Handling .............................................................................................................................23
A. Standard Good Practices.................................................................................................................23
B. Interactions Between Sample Handling and Results ......................................................................23
C. Radon ..............................................................................................................................................23
D. Cleaning Procedure.........................................................................................................................24
1. Thorough.....................................................................................................................................24
2. Quick ...........................................................................................................................................24
IV. System Overview.............................................................................................................................25
A. Components....................................................................................................................................25
1. Complete System ........................................................................................................................25
2. Counting Module ........................................................................................................................26
3. Support Box.................................................................................................................................28
B. Requirements and Specifications....................................................................................................29
C. Counter Manipulations ...................................................................................................................30
UltraLo-1800 Alpha Particle Counter
1. Adjusting the Tray Height ...........................................................................................................30
2. Opening the Counter ..................................................................................................................30
3. Removing a Side Panel................................................................................................................31
4. Opening the Support Box............................................................................................................31
5. Opening the Electronics Box .......................................................................................................31
V. Maintenance/Calibration....................................................................................................................32
A. Calibration.......................................................................................................................................32
B. Weekly Maintenance ......................................................................................................................32
C. Monthly Maintenance ....................................................................................................................32
D. Annual Maintenance.......................................................................................................................32
VI. CounterMeasure.............................................................................................................................33
A. Application Requirements ..............................................................................................................33
B. Application Overview......................................................................................................................34
1. Main Panel ..................................................................................................................................34
2. Analysis Panel..............................................................................................................................39
3. Diagnostic Panel..........................................................................................................................41
4. Measurement History .................................................................................................................43
5. Start Run Dialog ..........................................................................................................................45
6. Options Dialog.............................................................................................................................47
7. Import Calibration Certification/Import Settings Dialogs...........................................................49
C. Common Interactions .....................................................................................................................50
1. Tooltips........................................................................................................................................50
2. Zoom and Pan .............................................................................................................................51
3. Splitters .......................................................................................................................................52
D. Making a Standard Measurement ..................................................................................................53
1. Measurement Flowchart.............................................................................................................53
2. Detailed Description ...................................................................................................................54
E. Generated Files ...............................................................................................................................56
1. Buffers.........................................................................................................................................56
2. Logs .............................................................................................................................................56
3. Parameters..................................................................................................................................56
4. Exports and Reports....................................................................................................................57
UltraLo-1800 Alpha Particle Counter
VII. Troubleshooting..............................................................................................................................59
A. General Steps..................................................................................................................................59
B. Noise Debugging .............................................................................................................................59
C. CounterMeasure Troubleshooting..................................................................................................61
1. Can’t Start CounterMeasure.......................................................................................................61
2. Can’t Start a Run .........................................................................................................................61
3. Status Icons Red..........................................................................................................................61
4. CounterMeasure is Frozen..........................................................................................................61
D. Counting Issues ...............................................................................................................................62
1. Can’t Connect to Counter ...........................................................................................................62
2. Counting Stops During Measurement ........................................................................................62
3. Noise on Traces...........................................................................................................................63
4. Excessively High Sample Emissivity.............................................................................................63
5. No Alphas....................................................................................................................................63
6. No Events ....................................................................................................................................65
7. Excess Events ..............................................................................................................................65
8. Non-Alpha Events........................................................................................................................67
9. Large Drop in Emissivity During a Run ........................................................................................67
VIII. Alpha Analysis Engine .....................................................................................................................68
A. Introduction ....................................................................................................................................68
B. Overview .........................................................................................................................................68
C. Classifications..................................................................................................................................69
IX. Counting Various Sample Types......................................................................................................71
A. Undersized Samples........................................................................................................................71
B. Dielectrics........................................................................................................................................71
C. Thick Samples..................................................................................................................................71
D. Bulk Sources....................................................................................................................................72
E. Tiling Samples .................................................................................................................................73
F. Powder Samples..............................................................................................................................74
X. Appendix .............................................................................................................................................75
A. Counter Efficiency...........................................................................................................................75
UltraLo-1800 Alpha Particle Counter
I. Conventions of the Manual
Before covering any technical issues, there are a few conventions used in this manual that should be
discussed.
First, when discussing interaction with the software, “click” will always mean “left-click”. Whenever
right-clicking is required, it will be denoted “right-click”. Additionally, when discussing the software
interface, menu names will be in italics, such as File, and options under the menu will be in quotations:
“Exit”.
In the Theory of Operation section, important new terms will be in bold: anode. This is to distinguish
them as terms that will be reused.
The manual has frequent references to other sections, these references will be actively linked (so
clicking on them in the PDF will take you to the correct section), and will be bolded and in light blue:
Theory of Operation. In contrast, hyperlinks that will open an e-mail client or internet browser are, per
the normal convention, underlined and in blue: [email protected]. Finally, references to figures aren’t
distinguished from normal text, but will link to the figure.
UltraLo-1800 Alpha Particle Counter
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.
UltraLo-1800 Alpha Particle Counter
reject all pulses except those from alpha particles emanating from the sample. This method allows the
UltraLo-1800 to approach “zero background” measurements.
The following sections will examine these processes in detail: how the signals are induced, what the
location-dependent characteristic shapes are, and how the pulse shapes are analyzed. Using this
information, we will then also examine two related topics: what the sources of residual background are
and when they need to be considered; and how to tell when the counter is not working properly and
troubleshooting is required. Finally, we will review counting statistics as they pertain to UltraLo-1800's
ability to provide estimates of the accuracy of its reported measurements.
Before moving on, we would like to re-emphasize the extremely strong correlation between output
pulse shape and point of alpha particle emanation within the counter. In particular, this correlation
leads to a shorthand verbal notation wherein we regularly refer to “a pulse resulting from an alpha
particle (or ‘alpha’ for short) emanating from the sample” as a “sample pulse”, thereby contrasting it
with “ceiling pulses” and “sidewall pulses”, whose initiating alphas emanated from the electrodes and
sidewall, respectively. To avoid confusion, pulses in the anode and guard processing channels will be
referred to as “anode” and “guard” pulses.
UltraLo-1800 Alpha Particle Counter
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 Vthroughout), 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:
(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 daway from the electrode (as in Figure II-2),
its drift time is d/veseconds, or . When dis the height of the entire chamber, D=15cm,
and V=1000V, then teis found to be approximately 70 μs in argon.
Figure II-2: Schematic overview of the important parts of the UltraLo-1800's active volume.
UltraLo-1800 Alpha Particle Counter
All of the electrons in the track drift at the same velocity, so the initial geometry is preserved from
the moment it is created (t0) until the first electron reaches the electrode (tS), as shown in Figure II-3.
(There is some diffusion of electrons out from the track, but for our purposes this effect is negligible). As
the track continues to drift, the electrode absorbs the electrons as they arrive, until they’re all absorbed
(tR). The time between t0and tRis called the risetime.
3. Signal Generation
As an electron moves toward the electrode it induces a current in the attached electronics. This may
be understood through Gauss’s Law, which states that the net electric flux through any closed surface is
proportional to the enclosed electric charge. Because the electrode is a conductor it does not support
internal electric fields, all external fields must terminate at its surface, which implies a net flux and, by
Gauss’s Law, the presence of charge on its surface. Further, any changes in the flux imply a change in the
charge present on the surface and therefore imply current in the circuitry connected to the electrode
that is proportional to the rate change of flux. This means that as the electron drifts toward the
electrode it induces a current in the attached electronics (this is also known as the Shockley-Ramo
theorem). This induction only happens while the electron is traveling; once it reaches the electrode its
electric field disappears, the changing flux goes to zero, and therefore the induced current goes to zero.
Figure II-3: The location of the electron track at the 3 important times.
UltraLo-1800 Alpha Particle Counter
In the UltraLo-1800, tracks consist of hundreds of thousands of electrons, how do many electrons
drifting at once behave? In that case the total observed current is the sum of the individually induced
currents. If the track isn’t parallel with the electrode, (and in general it won’t be), some electrons in the
track will induce current for longer than others, and once the track begins to hit the electrode (at tS) the
rate of induction slows. The rate at which induction slows will be proportional to the angle of the track
relative to the electrode; the further from parallel the longer it will take to slow. This slowing shows up
as a rounding of the pulse from tSto tR, as can be seen in Figure II-4.
What do these induced signals look like? In the UltraLo-1800 the electrode is connected to a charge-
sensitive preamplifier that integrates the current. The preamplifier outputs a signal (in volts) that is
proportional to the total current induced. As discussed above, for a track originating on the sample (α1
in Figure II-5) there are two distinct regions in time: one from t0until tswhere the electrons drift
unchanged, and another from tsuntil tRwhere the electrons are disappearing linearly in time. The
resultant integrated signal Ss(t) is thus linear until tsand then parabolic until tR, the chamber’s maximum
transit time:
where Cfis the detector capacitance, eis the charge of an electron, and:
where dsis the distance from the topmost electron of the track to the electrode (Figure II-5).
Figure II-4: Example of an alpha pulse. The anode is in red, the guard is in blue. The three lines are the three important times,
from left to right t0, tS, and tR. Note the rounding between tSand tR.
UltraLo-1800 Alpha Particle Counter
The resultant pulse is shown in Figure II-4. Note that SsMAX scales with N, so that the final pulse
amplitude is proportional to the energy of the alpha particle.
However, when a uniform charge track originates from the electrode, charge starts disappearing
linearly in time immediately, so that the resultant signal Sa(t) is a parabola given by:
The risetime taand maximum amplitude SaMAX are found to be:
dabeing the track length normal to the electrode.
The important lesson to be drawn from this comparison of the two track types is that electrode
pulses look different from sample pulses. Both the risetime and the maximum amplitude are much
longer for sample pulses. Because the ratio of electrode to sample risetimes (ta/tR) is da/D,if the sample
chamber is made several times the maximum range of an alpha particle, then the risetimes of the two
cases will always be separated. The UltraLo-1800 is designed so this ratio will be about 1/3 for a 5 MeV
alpha particle emitted perpendicular to the anode. Similarly, the ratio of maximum amplitudes
(SaMAX/SSMAX) for two identical alpha decays is . This ratio is dependent on the angle of
emission relative to the anode, but it will always be greater than or equal to the ratio between
risetimes. Since Dhas already been chosen to exaggerate the difference in risetimes, the difference in
amplitudes is exaggerated as well. Being able to differentiate pulses based on their risetime and
amplitude is a key feature of the UltraLo-1800.
Figure II-5: Demonstration of important distances in the UltraLo-1800's active volume.
UltraLo-1800 Alpha Particle Counter
4. Necessary Assumptions
There are several assumptions that we must make in order for our model to accurately describe the
physical counting system:
The first is that the electron drift speed is both uniform and known. Because electron mobility
changes with the type of counting gas, so does the drift speed. Further, it can also change by significant
amounts if the counting gas isn’t pure (e.g. concentrations of water of only 100 ppm can decrease
risetimes significantly). By selecting the correct operating gas in CounterMeasure and purging for
adequate amounts of time these assumptions should always be met (unless the material being counted
is itself a source of water vapor, see No Alphas).
We also assume that the charges are free to drift all the way across the active volume. This
assumption can break down if there are significant amounts of oxygen in the active volume, since O2has
a high electron affinity. As above, adequate purging also makes this condition easy to satisfy.
A third assumption is that the electric field is uniform, which is guaranteed by the design of our
active volume, but can be perturbed if, for example, materials with significant height or very high
resistivity are placed on the sample tray (see Counting Various Sample Types).
Finally, when we report energies we assume that all alpha energy is expended in the gas. If the alpha
isn’t emitted from the sample’s surface (surface emission) and passes through some amount of material
before entering the active volume, then its energy will be reduced by some unknown amount. Because
it’s impossible to know how much material an alpha passed through we cannot correct for this.
However, bulk sources (as these kinds of samples are called) show certain behaviors that are
understood, see Bulk Sources for more information.
UltraLo-1800 Alpha Particle Counter
B. Secondary Veto Channel
1. Rejection
The second key design feature that allows the UltraLo-1800 to be “zero-background” is its veto, or
guard electrode. The electrodes are arranged as shown in Figure II-2 and are read out simultaneously,
with the interior, active portion called the anode, and the exterior portion called the guard. As can be
seen from Figure II-6, tracks originating on the counter sidewalls or on the tray outside the sample
region will induce signals on the guard and can be rejected. When combined with the risetime
discrimination discussed earlier, we are capable of rejecting events that originate on any surface other
than the sample.
2. Counting Efficiency
Because the direction of emission of an alpha is random, some events that originate near the
boundary of the electrode, but still on the sample, may be rejected because part of their track crosses
the boundary (see α4in Figure II-6). The end result is that the UltraLo-1800 has a counting efficiency
that is less than 100%, and decreases with increasing energy (higher-energy alphas have longer track
length, and longer track lengths are more likely to cross the boundary). We have calculated this
efficiency loss both directly and with Monte Carlo simulations, and take it into account when calculating
an emissivity with CounterMeasure. For a 6 MeV alpha particle the overall efficiency in the 707 cm2
configuration is about 85%, and in the 1800 cm2configuration it is about 90%. When we report
emissivities they take this correction into account, see Emissivity Value for more details. For a table
showing the correction values, see Counter Efficiency.
Figure II-6: Alphas originating from various locations in the counter and their resulting pulses. α1 shows a sample alpha, α2 shows
a ceiling emission, and α3 shows a sidewall emission, and α4shows a sample alpha that travels under the guard. The anode pulse
is shown in red, the guard in blue.
UltraLo-1800 Alpha Particle Counter
3. Induction
The final phenomenon resulting from the split electrode is what we call charge induction (also
called Ramo induction). A pulse with significant charge induction is shown in Figure II-7. When electrons
drift near the boundary between the two electrodes, they will induce some charge on both electrodes,
even if they’re entirely located under only one. But as they move closer to the electrodes, that induction
will diminish and eventually disappear, except on the electrode where the electrons are finally collected.
This phenomenon is a result of the physics discussed in Signal Generation. When the charges are far
away the flux they cause on the surface of the electrodes is spread over a wide area. As they drift closer
this area decreases. If this area is initially shared by both electrodes, as it diminishes the rate change in
flux (and, by the Shockley-Ramo theorem, the induced charge) will itself change, increasing on one
electrode and decreasing on the other. The result is a pulse on the collecting electrode that has the
same final height as it would if it originated far from the boundary, but with significantly more curvature
in its rising edge, while the signal on the other channel will be at the same level at the beginning and the
end but with a significant bump that falls directly under the curved portion of the other trace. Our
algorithm looks for these signatures, and when we find them, we perform add-back, a specialized
procedure designed to provide a more accurate estimation of risetime and energy, and thus a more
accurate classification of the pulse.
Figure II-7: Pulse with significant induction. Note the negative curvature on the anode (red) and the hump on the guard (blue). The
yellow line is the “add-back” trace, discussed in the text.
UltraLo-1800 Alpha Particle Counter
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.
UltraLo-1800 Alpha Particle Counter
graphical depiction.) This means that pulses produced by a proportional counter contain no information
about the pulses’ point of origin in the detector, while an ionization counter’s pulses do. As discussed
earlier, knowing where pulses originate enables you to reject those which do not originate from the
sample. This fact allows the UltraLo-1800 to have a significantly lower intrinsic background than a
proportional counter made of similar materials.
In short, the important difference between the gas proportional counter and the ionization counter
in this context is that the ionization counter has poorer signal-to-noise, but saves information about
where pulses originated in the detector.
UltraLo-1800 Alpha Particle Counter
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/cm2in 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 our website for more details.
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 [talk available online] 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 tSand tR(second and third lines, respectively). The
anode is shown in red, the guard in blue.
UltraLo-1800 Alpha Particle Counter
ratio is worse. It is difficult to precisely measure this probability because of the inherent difficulty in
disentangling cosmogenics from true low-energy alphas. This difficulty is compounded by the fact that
the magnitude of the effect can vary greatly between different locations, with altitude, shielding
provided by building materials, and other factors contributing. Additionally the different electrode
configurations will see different rates. This is because the 707 cm2configuration has a much smaller
solid angle that a cosmic ray could pass through without leaving energy on the guard compared to the
1800 cm2configuration. The difference is greater than the difference in area alone, and therefore the
707 cm2configuration sees fewer rounds. We can say that at XIA (which is at about sea level with no
special shielding) we see approximately 8 rounds per hour in the 1800 cm2configuration, and that only a
small percentage of those can make it through our analysis (about 5%), which leads to a background
rate of about 0.0002 counts/hr/cm2. Additionally, virtually all of these are low-amplitude events, below
3.5 MeV. There are no common isotopes that emit alpha particles of that energy, so pulses in that range
can only be produced by bulk decays or rounds. Therefore this background can be virtually eliminated by
providing a stringent energy cut at around 3.5 MeV if one is only interested in surface activity.
Finally, it’s important to note that a cosmogenic background will be present in any gas-filled
counter, including proportional counters. And, as noted earlier, in proportional counters they cannot be
identified by shape as all pulses have the same shape.
UltraLo-1800 Alpha Particle Counter
2. Radon
The other source of background in the UltraLo-1800 is radon gas. Radon is a noble gas, and as such
is nonreactive and capable of diffusing some distance through materials (especially plastics). It’s also
radioactive (in fact, it’s the second-leading cause of lung cancer in the US). These two properties
combine to make it a pernicious source of background in any counting experiments. There are two
common isotopes of radon, 220Rn (also called “thoron” because it comes from the 232Th chain) and 222Rn
(sometimes the word “radon” is used solely in reference to this isotope, but as this is potentially
confusing in this manual “radon” will always mean the element with no specific isotope implied). The
decay chains of both are shown in Figure II-10. The two chains have very distinct alpha-decay energies,
and their spectra in equilibrium are shown in Figure II-11. Outside of equilibrium one or more of the
peaks may be missing, depending on conditions. A spectrum with a peak in any of those locations is
generally indicative of some kind of radon contamination on the sample.
In the UltraLo-1800 radon can show up in two different ways: in the “ongoing exposure” case, by
being continuously present in the counting gas itself; or in the “terminated exposure case” by decaying
Figure II-10: Decays series of 222Rn (top) and 220Rn (bottom).
UltraLo-1800 Alpha Particle Counter
near a sample and contaminating it with its daughter isotopes prior to insertion of the sample into the
Ultra-Lo, which “terminates” the exposure. The two cases can be distinguished with relative ease, since
all of radon’s daughters are either short-lived or very long-lived. If the exposure was terminated prior to
inserting the sample in the counter (case 2) there will be a distinctive drop in the emissivity through the
run (see Figure II-12). In contrast, in the ongoing exposure case no such drop will occur. Exposure prior
to measurement can be easily dealt with by simply waiting for the isotopes to decay (which takes a few
hours for 222Rn and several days for 220Rn). However, the problem of radon in the active volume has no
easy fix.
How does radon get into the active volume? There are many possible sources of exposure, including
the reservoir of counting gas, the tubing leading the gas into the counter, diffusion in through the
counter walls, or generation from minute 232Th/238U contamination in the materials of the counter.
Figure II-11: Simulated spectra from radon as they appear in the UltraLo-1800 based on the terminated exposure case. Given the purge time and
measurement windows, some peaks aren’t apparent. 220Rn (left) has small peaks at 6 and 8.6 MeV. These are small because of the split decay of
212Bi. The other decays in the chain are too fast to see through a purge. 222Rn (middle) has similarly-sized peaks at 6 and 7.7 MeV. As before, some
peaks are not visible because of the purge, but additionally 210Po is not shown because of the long half-life of 210Pb. A peak at 5 MeV is a giveaway
for 210Pb contamination in a sample, but that is a separate problem. Finally, there’s a combined spectrum (right).
Figure II-12: A run with significant terminated radon exposure. Note the similarities to the simulated combined spectrum above.
Other manuals for UltraLo-1800
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