A training overview of basic theory and application of alpha discrimination

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1 A training overview of basic theory and application of alpha discrimination 1

Physics Alpha Spectrometer Calibration and Maintenance

2 Outline Enabling Objectives What is Alpha Spectrometry? Purpose and Applications Background Radon and Aerosol Physics Alpha Spectrometer Calibration and Maintenance Quality Assurance WIPP Examples Review Conclusions 2

3 Enabling Objectives Understand the radioactive decay chains which generate radon Understand the alpha and beta radiation that is emitted during the decay process Understand radionuclide equilibrium Understand basics of aerosol physics which effect measurable radon alpha and beta activity 3

4 Enabling Objectives (2) Understand the impact of meteorological conditions on Radon concentration Understand basics of instrumentation for measuring alpha spectra. Understand basics of signal processing required to generate alpha and beta spectra. Understand how dust impacts radionuclide assay and identification 4

5 Enabling Objectives (3) Understand how radon can mask TRU alpha activity Understand implications for measuring RAF using alpha and beta spectrometry Understand traditional alpha spectrometry performed by WIPP labs 5

gamma decay in that single energy emission

6 What is Alpha Spectrometry? Alpha decay of isotopes is similar to gamma decay in that single energy emission takes place. Beta decay has a spectrum and is not as simple to discriminate beta emitting isotopes Measuring the energy of alpha particles can identify the isotope Counting emission rates can assay the isotope in terms of Curies. 6

discrete peaks Semi log plots are often used to show

7 Beta Spectrometry Similar to alpha spectrometry but beta spectra are a continuum Beta spectra do not show discrete peaks Semi log plots are often used to show both at the same time Linear scale shows detail of large peaks only 7

8 Maximum and Average Energy The maximum energy tends to have few particles The average energy is simply the arithmetic average energy each particle has The mean energy is the most likely energy and often is near the average 8

Radiochemistry is the best method, but takes days to process.

9 Purpose and Applications of Alpha Spectrometry Discrimination of radon progeny from TRU activity. Radiochemistry is the best method, but takes days to process. Assay of alpha emitting isotopes. Used for effluent or contamination assay. Best method of assaying radionuclides and distinguishing between NORM and TRU 9

century Radon gas is breathed out Radon progeny can stay in the

10 History Serially decaying nuclides were first identified by Bateman in 1908 Impact of radon progeny on health identified in mid 20 th century Radon gas is breathed out Radon progeny can stay in the lungs USA background radiation dose now considers radon progeny exposure 10

11 Radon Radon Atomic number 86 Nobel gas (non reactive) Naturally occurring Decays to stable lead via release of alpha or beta particles, which are charged. Progeny behave like aerosols Both uranium and thorium decay to radium, which decays to radon Radium only decays to d 11

12 Uranium Atomic number 92 Abundance in the earth s crust: 2.7 ppm by mass Dominant isotope U 238 at 99.3% abundance with half life of 4.5E9 yr U 238 decays to Rn 222 through Ra 226 U 235 decays to Rn 219 through Ra

13 Thorium (break) Atomic number 90 Crustal abundance 9.6 ppm by mass Only isotope Th 232 is naturally occurring and radioactive with half life of 1.4E10 yr Th 232 decays to Rn 220 through Ra

14 Radium U 238 eventually decays to produce Ra 226 Radon (Rn 222) is the only decay product of Ra 226 Th 232 eventually decays to produce Ra 224 Thoron (Rn 220) is the only decay product of Ra 224 U 235 eventually decays to produce Ra 223 Actinon (Rn 219) is the only decay product of Ra 223 The only natural source of radon on the planet is from radium. Without radium, there will be no radon, without radon, there will be no radon progeny. 14

15 Radon Isotopes Radon is found in three isotopic forms: Rn 220 Referred to as thoron Comes from the decay of naturally occurring Thorium Half life of less than one minute Rn 222 Referred to as radon Comes from the decay of naturally occurring Uranium Half life of almost 4 days Rn 219 (negligible) Referred to as actinon Comes from the decay of U second half life, doesn t escape its radium matrix 15

not reactive Oxidation number of zero There are no free

16 Radon Properties All isotopes of radon chemically behave the same and have the same chemical properties Radon is not reactive Oxidation number of zero There are no free electrons in its electron shells Passes readily through a HEPA 16

17 Kinetic theory of Gasses cont Pressure is generated from molecules impacting vessel walls. Increasing their velocity is done by increasing temperature Velocity increase causes an increase in pressure Decreasing volume will increase impact rate 17

18 Kinetic theory of Gas cont. Pressure = P, Volume = V, number of molecules =n and temperature = T PV=nRT (R=gas constant) Increase T, you increase P(at constant V) Decrease V at constant T, you increase P etc 18

19 Diffusion The random motion of molecules causes mixing by adjacent volumes until a uniform distribution is attained throughout all attached volumes. Gaseous diffusion tends to happen much faster than liquid diffusion by volume 19

20 Gas behavior in a TRUPACT All internal containers have a HEPA filter intended to pass H2 but not aerosols (such as Pu) Kinetic theory of gas dictates radon will diffuse into volumes outside of drums until an equilibrium (equal volumetric concentration) is attained 20

21 Radon in a TRUPACT II When RAF vacuum is applied to inner lid, vacuum is transferred slowly to drum interior. Drum radon at that moment is HEPA filtered so only pure radon should pass. Progeny comes only from aged radon outside the drums 21

22 Gas behavior in the WHB Gas will diffuse and follow general flow patterns at the same time This same process would occur with a Pu aerosol release Ventilation flow will occur along with diffusion Exhaust will compete with diffusion to remove all contaminants prior to full equilibrium 22

23 Radon and operations (break) Is some decay enough? What if the TRU has the same beta/alpha ratio as NORM? Can we know TRU is below limits? Radon can exist at levels of health concern Typically at levels 1,000 to 10,000 times > Pu in activity y = m1 + m2*exp(-m3*x) Value Error m m m Chisq NA R NA y = * e^( x) R= Time (minutes) 23

24 Radon Decay Chain from Thorium 1.8E Rn 55 s α 6.3 MeV 1.6E E E E E E E E E Bi & 218 Po 214 Po 212 Po Energy (kev) 2.2 MeV β 64.06% 212 Po 3E 7 s α 8.8 MeV 216 Po.15 s α 212 Pb 10.6 hr 6.8 MeV β 0.3, 0.6 MeV 35.94% 212 Bi 60.6 min α 208 Tl 3.1 min 6.1 MeV β MeV 208 Pb Stable 24

25 Radon Decay Chain from Uranium 226 Ra 1600 y α 222 Rn 3.82 d α 5.5 MeV 218 Po 3.1 m α 6 MeV 214 Pb 26.8 m 4.9MeV β 0.7, 1.0 MeV MeV β 214 Bi 19.9 min 214 Po μs α 7.7 MeV 210 Pb 21 y β 210 Bi 5.0 d < 0.1 MeV 1.8E E E E E E E E E MeV β 212 Bi & 218 Po 210 Po 138 d 5.3 MeV 206 Pb Stable α 214 Po 212 Po 0.0E Energy (kev) 25

26 Interim review (break) What is the only source of radon? What would the time evolution of a pure radon source onto an air filter look like in terms of activity? What would a time evolution of aged radon (or thoron) and its progeny activity look like on a filter? Why don t we see radon emissions in our detectors? What is secular equilibrium? What are the driving forces causing radon emission from our waste packages? How does the radon get out of the waste? What is diffusion? 26

27 Radon Decay (and its progeny) Different equilibrium factors mean different relative amounts of progeny being attached or unattached Thoron progeny have long half lives Radon (Uranium) progeny have shorter half lives (~30 min) y = m1 + m2*exp(-m3*x) Value Error m m m Chisq NA R NA y = * e^( x) R= Time (minutes) 27

28 Attached and unattached fractions Each time a beta or gamma emission takes place, a new element is created from an old one The new element is an ion and interacts through collisions and Coulomb attractions/repulsions The ion can interact through dipole interactions when inducing polarization in dust particles in the air. This sets up an attractive potential to adhere certain portions of the airborne radionuclides to the dust (organic and mineral) in the air. Filter efficiency depends not only on particle size but flow rate 28

29 Dielectric polarization A point charge next to a grounded flat metal conductor will induce a charge distribution on the surface of that conductor to force the lines of flux to be normal to the conductor surface. This equates to a phantom source of equal charge behind the surface of the conductor If the planar material is a dielectric, only a polarization is induces so the phantom source is normalized smaller in amplitude based on the dielectric constant of the two media on either side of the plane This occurs in a more complicated fashion if the separating surface is not a plane (such as a sphere or dust particle) 29

30 Equilibrium factors A time series average has been reported for outdo0r air as 1/0.9/0.6/0.4 for the radon series Rn222/Po218/Pb214/Bi214 and estimated at 1/0.4/0.3 for the thoron series Pb212/Bi212/Tl218 The source terms in radon and thoron are dynamic as must then be the progeny Plate out on surfaces depends on particle size, humidity, surface area in the immediate vicinity, dust loading in the air and material type When fresh/filtered air comes in (as in the WHB), progeny must build up and mix with ambient air 30

31 Radon in a TRUPACT II revisited Only pure radon gets through the HEPA filters on the interior waste containers Progeny has to build up from radon decay in volume sampled by RAF Progeny plate out and aerosol attachment physics take place 31

32 Radon in the Environment The equilibrium distribution of radon and its progeny depends on environmental conditions: Relative humidity holds, wind disperses, and barometric pressure can either hold or disperse radon and progeny Rain and condensed vapor can concentrate radon and progeny Dust can capture the progeny and disperse it 32

33 Radon progeny scrubbing from rain Gamma dose rate follows radon progeny Dose rates increase on the ground when it rains Progeny plates out on surfaces at higher rate Fixed and surface removable will be increased on TRUPACT receipts and people passing through portal monitors 33

Equilibrium Equilibrium is established when the activity of each progeny are equivalent to the activity of the parent (radon) Radon is dynamic, meaning it can be blown away after arising from soil,

34 Equilibrium Equilibrium is established when the activity of each progeny are equivalent to the activity of the parent (radon) Radon is dynamic, meaning it can be blown away after arising from soil, and is also dependent on environmental conditions such as barometric pressure and soil moisture Radon progeny is also dynamic as it can plate out, be suspended or resuspended, attach and detach from aerosols and particles, and be blown away So attaining equilibrium all in one Radon leaches from Soil Resuspension rocks! 34

35 Heat transfer basics (break) Conduction When two items are in contact and have different temperatures, heat flow will occur. Radiation Infrared radiation occurs between separated regions of different temperature This is the same radiation from items which are red hot or otherwise glowing from heat Convection When air is heated through conduction, it rises and is replaced by cooler air. This creates an air current transferring heat from the hotter item. The earth is heated through radiation and can cool through convection, conduction and radiation. When the air is still, the dominant cooling is through radiation which interacts with the cooler air, heating it up. 35

The inversion prevents mixing with upper atmosphere air which would dilute

36 Meteorological Dependencies Radon typically is more concentrated in the mornings due to temperature inversions. The inversion prevents mixing with upper atmosphere air which would dilute surface concentrations This gives rise to diurnal variations CAM spectra tend to follow the radon source term with a small time lag. 36

37 Radon Concentrations Radon typically is more concentrated in the mornings due to temperature inversions. The inversion prevents mixing with upper atmosphere air which would dilute surface concentrations. Humidity and dust loading effect unattached fraction When windy conditions are present, thoron to radon ratios tend to be higher. Why? When you walk your dog to what are you more likely to be exposed radon or thoron and why? 37

Radon and Aerosol Physics When radon or its progeny decay, the result is an ion which will be attracted to ambient dust through because they are both charged particles.

38 Radon and Aerosol Physics When radon or its progeny decay, the result is an ion which will be attracted to ambient dust through because they are both charged particles. Impaction will result in some ions stripping or giving charge to ambient particles or being attached This results in both free and attached components of the progeny. Attached fraction tends to be in the respirable range Particles can stay in the lung constantly irradiating living tissue tumors, cancers 38

Radon Discrimination Radon progeny has multiple discrete alpha emissions

removable alpha activity Discrimination of TRU and radon progeny alpha

Radiochemistry often takes multiple days, but is very good at

39 Radon Discrimination Radon progeny has multiple discrete alpha emissions This progeny can and does plate out on surfaces generating surface removable alpha activity Discrimination of TRU and radon progeny alpha activity requires either alpha spectroscopy or radiochemical separation. Radiochemistry often takes multiple days, but is very good at discriminating TRU from radon progeny and the only validated assay method for this discrimination 39

atmospheric temperature inversion has just lifted) Why?

40 CAM Alarms The most common cause of a false CAM alarm occurs when a large shift in radon concentration has occurred (such as when an atmospheric temperature inversion has just lifted) Why? RADOS CAMs have two set points. FASs are counted twice with decay time in between 40

41 Alpha Spectrometry System Solid State Detector Displays spectrum and allows for analysis of peaks Vacuum Pump Detector Electronics PC Analysis Software Sample Optional Used for Higher Resolution Filter, swipe or other medium Amplifier, MCA, and other signal changing electronics 41

42 Alpha Particle Interaction An alpha particle has approximately 4000 times more mass than orbital electrons in matter. High energy alphas tend to travel in straight lines It is not highly likely that they will come close to a nucleus Initial energy deposition is almost constant 42

43 Alpha Spectrometry Requires measuring the energy of each alpha particle detected. Typically done with solid state detectors but can be done with ion chambers. Solid state detectors include GeLi, NaI, HPGe Total ionizations created are correlated with incident particle energy The energy of the alpha particle identifies the radionuclide 43

Detector Output 1.8E+05 1.6E+05 1.4E+05 214 Po 1.2E+05 1.0E+05 8.0E+04 6.0E+04 212 Bi & 218 Po 4.0E+04 2.0E+04 212 Po 0.

44 Detector Output 1.8E E E Po 1.2E E E E Bi & 218 Po 4.0E E Po 0.0E Energy (kev) From tomultiplier tubes count photons 44

45 Pulse Height Histogram An ion chamber collects the amount of charge generated in its controlled volume The height of the pulse determines how much activity you have in your sample By placing a single count at a given amplitude for each time a charge is collected at that amplitude, the histogram is built up. The resultant histogram is the alpha spectrum This is your sample assay 45

46 What happens with low counts? Preliminary radon spectrum (short count time) Aged radon with long count time (only Thoron progeny) 46

47 What happened to the isolo? WIPP Measurements isolo blank filter (dpm) Rn loaded filter with source (dpm) % TRU detected % % % % % % % (break) WHC (Hanford) Measurements Table 6 (WCH 172 Rev. 0) 308W Hood TH/U/Ra/Rn Test Time post sample (min) Percent detected, Radon compensated Count length % % % % 30 Table 7 (WCH 172 Rev. 0) Percent detected, Source/Nuclide Radon compensated Sample ID Pu/Am 93.50% 1 Unat 47 mm 90.2% 2 Pu % 3 Unat 43.9% 4 Unat 8.1% 5 (30 min) Unat 15.3% 6 (30 min) Unat 72.7% 8 Pu % 9 Pu % 10 Pu % 12 47

48 TRU Region of interest Noise sources Counting Relative Activity Algorithm assumptions Spectral shape Activity levels Sample size Sample geometry Relative activities From DRAFT Standard N Performance Specifications for Instrumentation Systems Designed to Measure Radon Progeny in Air, Final Draft 48

credit is given to 6 MeV tails into TRU ROI Result is very useful

49 Do not use isolo printout values Attribute all counts in the TRU ROI to being actual TRU Result is very conservative because no credit is given to 6 MeV tails into TRU ROI Result is very useful as only 10 to 15 % of alpha activity now can be partly due to TRU 49

50 New paradigm in control limits 20 dpm α TRU 200 dpm β ( 90 Sr) 1000 dpm NORM Only gross α & β on isolo 50

51 Basics of algorithms (i.e., CAM Alarms) Constant spectral shape assumption Constant activity assumption Changes assumed due only to Pu content Time resolution limited Sensitivity related to time resolution 51

52 Radon Masking Effects 92 cpm estimate 3.4 cpm estimate Channel number Spectra acquired from used PAS filters having a small TRU count rate contribution present due to a source being placed behind the filter (Figure 1). The upper spectra had a total accumulation time of 3.5 minutes whereas the lower spectrum had a 16 minute accumulation time. No amplitude normalization was used, intensity similarities reflect radon progeny activity on the filters. 52

RAF If 3 dpm (1 cpm) falls in the TRU ROI on an RAF count, concern is warranted. NUREG 1400 gives a resuspension factor of 0.01, whereas we use a factor of 0.

53 RAF If 3 dpm (1 cpm) falls in the TRU ROI on an RAF count, concern is warranted. NUREG 1400 gives a resuspension factor of 0.01, whereas we use a factor of 0.001, meaning that 3000 dpm of surface removable activity is credible within the TRUPACT 4 dpm was enough to ID a leaky drum from an RAF What would you do if you saw one or more counts on an RAF in the TRU ROI? What if you saw no counts in the peak channel location but many on either side? 53

Signal Detection at low activity When radon or TRU has low

Channels with zero counts can occur even in the peak channel

It is best to have sample with more activity on it than very

54 Signal Detection at low activity When radon or TRU has low activity, few counts are seen and the signal grows fuzzy Channels with zero counts can occur even in the peak channel location. It is best to have sample with more activity on it than very little activity because you can t always see peaks clearly when there is only a little activity on a sample being analyzed. 54

55 Efficiency and Energy Efficiency requires using the whole peak (not just a channel or two). Use of only one channel for an assay measurement drastically reduces efficiency Energy requires one or more channels at known discrete energies 55

56 Quality of Signal is Important Low activity samples produce noisy spectra Noise is relative Produced by electronics, background, and other factors The signal to noise ratio is the true metric for quality Filter loading, geometry and overlapping signals can degrade quality Desire higher signal, lower noise Can be done with longer counts or more sensitive instrumentation or more sample activity 56

57 Full Width at Half Maximum Used to determine the resolution of the spectrum Is the width of the peak at half its maximum height The smaller it is, the better the spectrum. See next slide for examples of good and bad spectra. 57

58 58

59 Particulate deposition on filters There are four primary particle deposition mechanisms on filter media The dominant mechanisms are impaction and interception for aerosols >0.2 um. Diffusion is the dominant mechanism <0.2 um 136/ c.html#fig3 59

60 Filter efficiency Not all filters behave the same Typically filters have higher efficiency when wet You can even charge the filter to increase efficiency 136/ c.html#fig3 60

61 Efficiency tends to increase with loading As the filter loads with particulate, the efficiency tends to increase Does not address flow, spectra or DP 136/ c.html#fig3 61

Impurities make assay very difficult Assay can be affected by water content and dust loading due to masking of the alpha emissions

62 Sample Preparation If high resolution is needed, impurities must be removed from the sample. Impurities include filter media, dust and any chemical constituents not being assayed. Impurities make assay very difficult Assay can be affected by water content and dust loading due to masking of the alpha emissions Alpha spectroscopy has its limitations. Sometimes only radiochemistry can be used to identify TRU activity There is no simple go/no go system for identifying TRU activity when radon and its progeny are present. 62

Calibration and Maintenance Detectors must be calibrated and properly maintained. Some detectors may have to be replaced if exposed to light or salt or humidity over time.

63 Calibration and Maintenance Detectors must be calibrated and properly maintained. Some detectors may have to be replaced if exposed to light or salt or humidity over time. Calibration requires NIST traceable calibrated sources in the same configuration as samples to be assayed. Equipment must operate properly to achieve accurate assay. RCT must know if equipment is operating properly and if the sample is appropriate for alpha spectroscopy 63

Could your analysis be defended in court and accepted?

64 Quality Assurance Rigorous QA is required for regulatory compliance activities which rely on radiological assay. how certain are you that your alpha spec interpretation is correct? Could your analysis be defended in court and accepted? Accurate and complete documentation must demonstrate each component relied upon for accuracy and precision is working properly and was properly considered in the analysis. If it was not written down, then it did not occur. 64

WIPP Examples Radon at intake shaft less at station A WIPP salt is a

Station A detected small amounts of Plutonium Plutonium in

nearby Challenge is to discriminate between Plutonium from waste

65 WIPP Examples Radon at intake shaft less at station A WIPP salt is a big filter Shaft scraping with galloway released fallout when Station A detected small amounts of Plutonium Plutonium in environment due to Gnome site explosion which released fallout nearby Challenge is to discriminate between Plutonium from waste versus from the environment. Radon at WIPP is typical for the US averaging 0.1 to 0.4 pci/l 65

66 Review Part 1 What is the initial decay product of all radium isotopes? What is final decay product of all radium isotopes? What kinds of radioactivity comes from radon progeny? What are some meteorological parameters that effect radon progeny distributions? What is the difference between attached and unattached progeny? What effect does a temperature inversion have on radon and its progeny? 66

67 Review Part 2 What is an alpha spectrometer? What is a pulse height distribution? What does it tell you? When is your sample not appropriate for alpha spec? What does dust and salt on an air filter do to the alpha spectrum? How can radon mask TRU alpha activity? What do a few counts on an RAF in the TRU ROI imply? 67

Radon progeny have predicable qualities that can be leveraged in mitigation

68 Conclusions Alpha assay has many similarities to gamma assay Radon discrimination is a problem that has been with the nuclear community since the Manhattan project. Radon progeny have predicable qualities that can be leveraged in mitigation solutions Current state of the art method of discerning radon from TRU is radiochemistry 68

69 Photo Credits Citation Nougat_Gnome_Marker.jpg

70 Photo Credits (2) /images/wipp4_pix.jpg content/uploads/2008/07/courtroom.jpg to paper.jpg /Lecture7 hchp.ppt. 70

71 Photo credits (3) n.gif / c.html#fig / c.html#fig3 71

Experiment Radioactive Decay of 220 Rn and 232 Th Physics 2150 Experiment No. 10 University of Colorado

Experiment Radioactive Decay of 220 Rn and 232 Th Physics 2150 Experiment No. 10 University of Colorado Experiment 10 1 Introduction Radioactive Decay of 220 Rn and 232 Th Physics 2150 Experiment No. 10 University of Colorado Some radioactive isotopes formed billions of years ago have half- lives so long