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UltraLo-1800 Alpha Particle Counter

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XIA LLC

Page 72

risetime, see

Theory of Operation

). This is especially problematic when the sample is not flat as any

features will cause local distortions.

The only solution to samples with features on their surfaces is careful measurement. However, the

UltraLo-1800 does have the ability to adjust the height of the tray in order to measure thicker samples.
Adjusting the blocks under the sample tray to move thick samples closer to the intended distance from
the electrode makes measurements on them very simple. See

Adjusting the Tray Height

for details on

how to adjust the blocks.

D.

Bulk Sources

Bulk sources are materials whose alpha emitters are distributed throughout their entire volume.

This results in a broad tail of events going down to low energy from the main peak, the main peak being
from  the  alpha  emissions  at  or  near  the surface.  The UltraLo-1800  will count these  types  of  materials
well, but it is only capable of measuring the emissivity, that is the amount of radiation leaving a surface.
This  is  not  the  same  as  the  rate  of  decays  throughout  the  entire  sample.  Additionally,  as  with  any
counter there is a cutoff energy below which the UltraLo-1800 isn’t sensitive. You can see this in Figure
IX-1, where no alphas below approximately 1.5 MeV (or 750 ADC) register in the detector. The energy at
which  about  half  of  events  register  is  around  2.5  MeV.  The  cutoff  energy  is  dependent  on  the  anode
trigger  threshold,  the  cutoffs  cited  here  are  based  on  measurements  taken  at  the  recommended
threshold value of 175.  Additionally,  the exact  cutoff at a given threshold will vary per counter.  With
surface sources this isn’t an issue as no alpha particle has an energy that low, but with bulk sources it
means that the detector won’t see events originating below a certain depth.  Finally, with a bulk source
the alpha energy spectrum will be “smeared” down to the cutoff energy due to alpha particles emitted
from the bulk losing energy as they pass through the sample material.

Figure IX-1: A bulk source with no alphas below about 1.5 MeV (750 ADC).

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Содержание UltraLo-1800

Страница 1: ...XIA LLC UltraLo 1800 Alpha Particle Counter User s Manual Version 1 1 December 2021...

Страница 2: ...ncy 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 i...

Страница 3: ...tenance 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 Optio...

Страница 4: ...ect 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 Dro...

Страница 5: ...le 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 manua...

Страница 6: ...rticle emitted from the sample s 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 s...

Страница 7: ...y 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 o...

Страница 8: ...igure 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 speci...

Страница 9: ...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...

Страница 10: ...d 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...

Страница 11: ...uch 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 t...

Страница 12: ...s the active volume This assumption can break down if there are significant amounts of oxygen in the active volume since O2 has a high electron affinity As above adequate purging also makes this condi...

Страница 13: ...may be rejected because part of their track crosses the boundary see 4 in Figure II 6 The end result is that the UltraLo 1800 has a counting efficiency that is less than 100 and decreases with increa...

Страница 14: ...es 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 d...

Страница 15: ...iplication 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 multiplicat...

Страница 16: ...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 proportio...

Страница 17: ...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 th...

Страница 18: ...herefore the 707 cm2 configuration 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 cm2 configuration a...

Страница 19: ...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...

Страница 20: ...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 232 Th 238 U cont...

Страница 21: ...er is set up and maintained correctly Counters have been run in underground labs with external radon activities of over 500 Bq m3 with no increase in midairs If you see more activity than around 5 mid...

Страница 22: ...ity of a sample is significantly above this number say around 0 005 or higher then the background can be safely ignored as it will only have a minor impact on the final results But as sample emissivit...

Страница 23: ...t the cleanest of environments more sample handling means more contamination Because of this it s best to handle samples as little as possible In addition more exposure to atmospheric air will lead to...

Страница 24: ...g formula from Biodex available here Radiacwash contains mild acids and will slightly etch some metals including copper After several moments remove the Radiacwash with distilled or deionized water us...

Страница 25: ...body of the UltraLo 1800 2 Support Box Houses several necessary components that are separated from the counter for noise reasons 3 Rigid Gas Tube Connects Support Box gas output to Counting Module ga...

Страница 26: ...e values to the Control Laptop not shown 2 Upper Chamber contains a Preamplifiers Analog amplification of the signals coming from the Active Volume b Moisture meter Measures the moisture in the Active...

Страница 27: ...internal pressure to ensure that gas exit and the door seal are working as intended 6 Gas outlet Provides a controlled exit of argon from the Active Volume 7 Check valve Provides overpressure exhaust...

Страница 28: ...nclosure Top View Cover Removed 9 Power supplies AC DC converters to supply power to all systems 10 Flow control Electronically controls flow rate 11 Optical isolation Isolates motor when it s not in...

Страница 29: ...0 7 A Typical Power Counter Laptop 0 08 kW Max Power 0 100 kW Gas Type Argon Pressure 21 PSI 150 kPa 5 Tubing OD metal tubing Component Width in Depth in Height in Weight lbs Counter Module closed 34...

Страница 30: ...he numbers indicate the height of that face of the block all 3 pairs of faces are different The tallest is 1 440 which corresponds to a full height tray Rotating the blocks so 1 315 faces up will lowe...

Страница 31: ...ody to serve as a vibration reducer 5 Set the panel and screws aside 6 To replace the panel you will need to carefully align the first few screws as the panel sits slightly above the surface 4 Opening...

Страница 32: ...ground in the detector as low as possible If you change samples often you may want to do this more frequently than once a month In addition doing a run with a standard sample of your choosing to ensur...

Страница 33: ...ully it must be launched from the UltraLo user account set up on the laptop when it ships if you wish to run it from a different user account on the laptop please contact XIA at support xia com Counte...

Страница 34: ...lectronics See Can t Start CounterMeasure If there s an issue at startup CounterMeasure will fail to start and throw an error message in most cases offering to send an error report to XIA It s best pr...

Страница 35: ...nted This is the number of alpha particles that CounterMeasure has observed The number is shown as Corrected Observed The corrected alpha number takes into account efficiency losses See Counting Effic...

Страница 36: ...he user to look for systematic changes over time For instance a slow rise in emissivity could indicate that there were problems with moisture early in the run and a sharp drop could indicate radon exp...

Страница 37: ...tton indicated as j in Figure VI 1 You can remove the ROI selection either by sliding the bars back double clicking inside the ROI control window hitting Ctrl A or going to the View menu and selecting...

Страница 38: ...e is no ROI set n Tools Tools provides access to the Diagnostic Panel the log window see Generated Files for more details on logs the Analysis Panel and the Options Dialog The Diagnostics Panel will b...

Страница 39: ...Event Trace Plot This plot shows the raw digitized waveforms for the selected event and the fits from their analysis The anode trace is red the guard trace is blue and the fits are light red and blue...

Страница 40: ...kipped while scrolling This is a convenient way to closely inspect the traces in a certain event class without having to scroll through all the rest All event classes are initially toggled on The even...

Страница 41: ...ut the choice of 175 corresponds to a cutoff of about 1 5 MeV with higher thresholds cutting off at higher energies and lower threshold cutting off at lower energies Typically the guard is set to 0 me...

Страница 42: ...ectively d Fast Fourier Transform Plot The Fast Fourier Transform FFT plot shows the FFT of the raw ADC trace and what frequencies are present in it The four largest peaks are identified and highlight...

Страница 43: ...ration if the measurement is stopped early for some reason see below 4 Stopped Notes the reason measurement ended Reasons include a Criteria Met The measurement went for the planned duration b User Re...

Страница 44: ...e The location of the data file generated Clicking the link will open a Windows Explorer window with the data file selected In addition to the defaults listed above the Measurement History Dialog disp...

Страница 45: ...a purge Occasionally you may want to start a run without purging in order to remeasure the current sample In this case simply uncheck the purge box and the run will begin immediately after hitting the...

Страница 46: ...Dialog Sample serial number or ID and sample description are two that XIA has found useful Unlike duration these values will not be remembered from run to run This section will only appear once custom...

Страница 47: ...out a deep understanding of their functions can easily lead to erroneous pulse analyses and measurement results b Background As discussed in Remaining Background Sources in the UltraLo 1800 there is a...

Страница 48: ...ew parameter adds that column to the Measurement History dialog the column will be empty for any preceding measurements The parameter is also added as a field in the Start Run Dialog These fields can...

Страница 49: ...m the update XIA will provide a update file Save that file in a convenient location on the computer running CounterMeasure then navigate to File Import Calibration Certification This will launch the d...

Страница 50: ...Tooltips Tooltips are very common in CounterMeasure Most buttons and text entry locations have tooltips that explain their purpose To see a tooltip simply hover the cursor over the control that you r...

Страница 51: ...ift key and right click If you ve zoomed in multiple times you will need to zoom back out multiple times To pan the plot you simply hold down the Ctrl key which causes the cursor to turn into a hand t...

Страница 52: ...d Fast Fourier Transform on the Diagnostics Panel To adjust the relative heights press down your left mouse button on the bar between the plots The cursor will change into the one pictured in Figure V...

Страница 53: ...xia com XIA LLC Page 53 D Making a Standard Measurement This section details the steps to take in order to make a typical measurement first using a flowchart and subsequently via text 1 Measurement F...

Страница 54: ...ot changing samples please skip the next three steps and begin at Start Run Eject tray To change samples hit the Tray Control Button to eject the tray Swap Samples Remove the old sample put on the new...

Страница 55: ...should stay relatively constant from run to run Excessive numbers of any kind of events can indicate a problem Scrolling through some traces can show any abnormal noise that may have cropped up such...

Страница 56: ...ly ask to see this log file When a CounterMeasure log gets too large 52 MB it is renamed and a new one is generated CounterMeasure log will always be the most recent log CounterMeasure 1 log the secon...

Страница 57: ...r exports available besides the Measurement Report To export one of them go to the Main Panel and navigate to File and click on Export There will be a list of five files available for export Alpha Spe...

Страница 58: ...UltraLo 1800 Alpha Particle Counter support xia com XIA LLC Page 58 Figure VI 22 Sample of a run report...

Страница 59: ...e the counter is operating well is to take traces at several time steps from high to low to see if any obvious oscillations appear in the trace or any frequency peaks appear in the FFT Taking traces a...

Страница 60: ...Lo 1800 Alpha Particle Counter support xia com XIA LLC Page 60 Figure VII 1 TOP An ADC trace with 60 Hz peak and harmonics MIDDLE A typical trace at 400 s time step BOTTOM A typical trace at 1 s time...

Страница 61: ...oot this The next most likely cause is that the tray is either all the way open or not fully closed If the tray is open close it If it appears to be closed try running it out then back in For informat...

Страница 62: ...me disconnected and cause interruptions Further temporary power outages even if they re very fleeting can also cause communication issues If a stable power line is available using it will help prevent...

Страница 63: ...com and we will assist you with debugging 4 Excessively High Sample Emissivity If a sample has a higher emissivity than you expected there are a few things to check First look at the emissivity versus...

Страница 64: ...ounts of moisture and then outgas into environments with lower moisture this is often observed in vacuum experiments including the counting environment in the UltraLo 1800 If you re attempting to meas...

Страница 65: ...he counter is run filled with room air for instance it will likely see no events 7 Excess Events There are a number of causes of extra events The following sections include reasons for excesses of all...

Страница 66: ...c radon peaks it could be that excess moisture is causing lowered risetimes and alphas are showing up as midairs For details on troubleshooting this scenario see Moisture Finally some percentage of co...

Страница 67: ...ons The computed energy and risetime of those events will sometimes fall within the typical energy and risetime bands occupied by our various classifications but most often they won t Vibrations are e...

Страница 68: ...following steps 1 Prescreener A quick fit to both traces that classifies them individually based on amplitude if the resulting classification pair indicates a potential alpha pass trace to Level One...

Страница 69: ...igger signal Noise These traces are also quite flat but have some feature which caused the channel to trigger These can be caused by random fluctuations in the baseline These fluctuations are inherent...

Страница 70: ...ds are events whose peak is too rounded to be caused by alpha particle These are the result of particles that can cause longer ionization tracks most likely cosmic rays o Ceiling An alpha decay emanat...

Страница 71: ...for it to finish How long this takes depends on many factors including the plastic type the humidity it has been exposed to and the temperature Outgassing can be done either in the counter itself or...

Страница 72: ...well but it is only capable of measuring the emissivity that is the amount of radiation leaving a surface This is not the same as the rate of decays throughout the entire sample Additionally as with...

Страница 73: ...or rectangles as they can simply be pushed together to cover the surface of the tray For other shapes or samples where each piece is slightly different other solutions are required Often overlapping...

Страница 74: ...terested in counting powders and would like a tray of your own contact support xia com we can either provide the drawings for this tray at no charge or if you prefer manufacture one and ship it to you...

Страница 75: ...is clearly zero Since track length varies with alpha particle energy counting efficiency is clearly energy dependent For a given energy track length the counting efficiency at each location within the...

Страница 76: ...e may or may not have been a missed event as implied by the correction factor However the corrected number of alphas is still the best unbiased estimate that we can produce of the true emissivity It i...

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