XIA UltraLo-1800 User manual

XIA LLC
UltraLo-1800 Alpha Particle
Counter
User’s Manual
Version: 1.1
December, 2021

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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

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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

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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

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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.

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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.

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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.

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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.

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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.

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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.