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Environmental Laboratory Accreditation Course for Radiochemistry: DAY TWO Presented by Minnesota Department of Health Pennsylvania Department of Environmental.

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Presentation on theme: "Environmental Laboratory Accreditation Course for Radiochemistry: DAY TWO Presented by Minnesota Department of Health Pennsylvania Department of Environmental."— Presentation transcript:

1 Environmental Laboratory Accreditation Course for Radiochemistry: DAY TWO Presented by Minnesota Department of Health Pennsylvania Department of Environmental Protection U.S. Environmental Protection Agency Wisconsin State Laboratory of Hygiene

2 Instrumentation & Methods: Alpha Scintillation Counter Ra226, Ra228 Lynn West Wisconsin State Lab of Hygiene

3 Method Review Radium 226 (EPA 903.1) Radium 228 (EPA 904.0) Alpha-Emitting Radium Isotopes (EPA 903.0)

4 Radium Chemistry Chemically similar to Ca & Ba +2 oxidation state in solution Insoluble salts include: CO 3, SO 4, & CrO 4 Forms a complex with EDTA Property used extensively in analytical procedures

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6 Radiochemical Characteristics IsotopeT 1/2 Decay Mode Series 223 Ra11.1 DAlphaActinium ( 235 U) 224 Ra3.6 DAlphaThorium ( 232 Th) 226 Ra1622 AAlphaUranium ( 238 U) 228 Ra5.8 ABetaThorium ( 232 Th)

7 Radium 226 (EPA 903.1) Prescribed Procedures for Measurement of Radioactivity in Drinking Water EPA 600 4- 80-032 August 1980

8 Interferences No radioactive interferences The original method does not use a yield correction

9 238 U decay series

10 903.1 Method Summary 1 L acidified sample Ra co-precipitated with stable Ba as SO 4 Precipitate is separated from sample matrix & supernate is discarded

11 Method summary cont. (Ba-Ra)SO 4 is dissolved in EDTA Solution Transferred to a bubbler After a period of ingrowth, 222 Rn is purged for sample & collected in scintillation cell.

12 A typical radon de- emanation system Bubbler Scintillation Cell Vacuum System & gauge Avoid using Hg manometer if possible

13 Scintillation Cell 222 Rn from sample is collected in the cell Progeny establish secular equilibrium in about 4 hrs The alpha counts from 222 Rn & its progeny are collected Zn(Tl)S Quartz Window

14 Alpha Scintillation Cell Counter Sample counted 4 hrs after de-emanantion Alpha particles interact with Zn(Ag)S coating & emit light Light flashes are counted on a scaler

15 Radon Cell Counters

16 Instrument Calibration Each instrument system & scintillation cell needs to be calibrated Calibration samples should be prepared in the same manner as the samples. The entire de- emanation system effects the calibration measurement Use NIST traceable standards Perform yearly or after repairs

17 Calculations

18 Calculations cont. Computer programs should be hand verified Decay constants and time intervals must be in the same units of time Minimum background count time should be equal to the minimum sample count time

19 Method Quality Control Per each batch of 20 samples, analyze the following: Method blank Laboratory control sample Precision sample Matrix spike sample Established action limits for each

20 Method Quality Control, cont. Instrument operating procedure should describe Daily control charts and acceptance limits Required action Preventative maintenance

21 Method SOP main sections SCOPE AND APPLICATION SUMMARY OF METHOD REGULATORY DEVIATIONS METHOD PERFORMANCE SAFETY SAMPLE HANDLING & PRESERVATION INTERFERENCES DEFINITIONS EQUIPMENT REAGENTS METHOD: DETERMINATION OF 226 RA CALIBRATION OF SCINTILLATION CELLS CALCULATIONS QUALITY CONTROL WASTE DISPOSAL POLLUTION PREVENTION REFERENCES FIGURES

22 Radium 228 (EPA 904.0) Prescribed Procedures for Measurement of Radioactivity in Drinking Water EPA 600 4- 80-032 August 1980

23 Interferences The presence of 90 Sr in the water samples gives a positive bias to the measured 228 Ra activity. Due to the short half-life of 228 Ac, a emitter of similar energy is substituted during instrument calibration. A high or low bias may result depending on which isotope is selected. Natural Ba may result in falsely high chemical yield.

24 232 Th- decay series

25 904.0 Method Summary 228 Ra in a drinking water sample is co-precipitated with Ba & Pb as SO 4 The (Ba-Ra)SO 4 precipitate is dissolved in basic EDTA. The progeny, 228 Ac, is chemically separated from its parent by repeatedly forming the (Ba-Ra)SO 4 Allow at least 36 hrs for the ingrowth of 228 Ac & secular equilibrium

26 904.0 Method Summary, cont. 228 Ac is then separated from 228 Ra by precipitation as a OH -. (Save supernate) This is the end of ingrowth & the beginning of 228 Ac decay 228 Ac is co-precipitated with Y as (Ac-Y 2 (C 2 O 4 ) 3 )

27 904.0 Method Summary, cont. Transferred to a planchet & counted on a low-background / proportional counter The Ba carrier yield is found by precipitating the Ba from the supernatant as BaSO 4

28 Instrumentation Low background gas flow proportional counter P-10 counting gas (10% CH 4 & 90% Ar) Due to short half-life of 228 Ac, a multi-detector system is desirable 6.13 hr Processing time from start of decay to count is about 250 m

29 Gas flow proportional counter window assembly

30 Instrument Calibration Each instrument system needs to be calibrated Calibration samples should be prepared in the same manner as the samples. Use isotope with beta energy approximately equal to 0.404 keV Use NIST traceable standards Perform yearly or after repairs

31 Calculations

32 Method Quality Control Per each batch of 20 samples, analyze the following: Method blank Laboratory control sample Precision sample Matrix spike sample Established action limits for each

33 Method Quality Control, cont. Instrument operating procedure should describe Daily control charts and acceptance limits Required action Preventative maintenance

34 Method SOP main sections SCOPE AND APPLICATION SUMMARY OF METHOD REGULATORY DEVIATIONS METHOD PERFORMANCE SAFETY SAMPLE HANDLING & PRESERVATION INTERFERENCES DEFINITIONS EQUIPMENT REAGENTS METHOD: DETERMINATION OF 228 RA CALIBRATION OF INSTRUMENT CALCULATIONS QUALITY CONTROL WASTE DISPOSAL POLLUTION PREVENTION REFERENCES FIGURES

35 Alpha-Emitting Radium Isotopes (EPA 903.0) Prescribed Procedures for Measurement of Radioactivity in Drinking Water EPA 600 4- 80-032 August 1980

36 Interferences (EPA 903.0) Natural Ba may result in falsely high chemical yield Ingrowth of progeny must be corrected for Method only corrects for 226 Ra progeny Does not accurately measure 226 Ra if other alpha emitting isotopes are present Calibration based only on 226 Ra

37 Ac-228 6.13 hours Th-232 1.4×10 10 y Ra-228 5.75 y Th-228 1.90 y Ra-224 3.64 days Rn-220 54.5 s Tl-208 3.1 m Po-212 300 ns Po-216 158 ms Pb-208 stable Pb-212 10.6 hours Bi-212 60.6 m Mass number (N) Atomic number (Z) 67% 33% beta decay alpha decay 232 Th- decay series

38 Pa-234 1.18 m 238 U decay series beta decay Atomic number (Z) Mass number (N) alpha decay Po-210 138.4 d U-238 4.4×10 9 y Th-234 24.1 d U-234 2.48×10 5 y Th-230 8.0×10 4 y Ra-226 1622 y Rn-222 3.825 d Pb-206 stable Po-214 1.6×10 -4 s Po-218 3.05 m Bi-210 5.0 d Pb-210 22 a Pb-214 26.8 m Bi-214 19.7 m

39 Pa-231 3.48×10 4 y 235 U decay series beta decay Atomic number (Z) Mass number (N) alpha decay U-235 7.3×10 8 y Th-231 25.6 h Th-227 18.17 d Ra-223 11.7 d Rn-219 3.92 s Po-211 0.52 s Po-215 1.83×10 -3 s Bi-210 5.0 d Pb-207 stable Pb-211 36.1 m Bi-211 2.15 m Ac-227 22.0 y Fr-223 22 m At-219 0.9 m Bi-215 8 m At-215 10 -4 s Tl-207 4.79 m

40 903.0 Method Summary 1 L acidified sample Ra co-precipitated with stable Ba & Pb as SO 4 223 Ra 224 Ra 226 Ra Precipitate is separated from sample matrix & supernate is discarded

41 903.0 Method Summary, Cont. Progeny ingrowth starts with the final (Ba-Ra)SO 4 precipitation. Since a correction factor is applied to correct for ingrowth, care needs to be taken to avoid disturbing the radon progeny ingrowth after this step Transfer to tared planchet & dry under infra-red heat lamp

42 Instrumentation (EPA 903.0) Low background gas flow proportional counter P-10 counting gas (10% CH 4 & 90% Ar) Alpha scintillation counter

43 Instrument Calibration (EPA 903.0) Each instrument system needs to be calibrated Calibration samples should be prepared using 226 Ra Use NIST traceable standards Perform yearly or after repairs

44 Calculations (EPA 903.0)

45 Method Quality Control (EPA 903.0) Per each batch of 20 samples, analyze the following: Method blank Laboratory control sample Precision sample Matrix spike sample Established action limits for each Demonstration of capability

46 Method Quality Control, Cont. (903.0) Instrument operating procedure should describe Daily control charts and acceptance limits Required action Preventative maintenance

47 Method SOP main sectionsMethod SOP main sections (903.0) SCOPE AND APPLICATION SUMMARY OF METHOD REGULATORY DEVIATIONS METHOD PERFORMANCE SAFETY SAMPLE HANDLING & PRESERVATION INTERFERENCES DEFINITIONS EQUIPMENT REAGENTS METHOD: DETERMINATION OF 228 RA CALIBRATION OF INSTRUMENT CALCULATIONS QUALITY CONTROL WASTE DISPOSAL POLLUTION PREVENTION REFERENCES FIGURES

48 Instrumentation & Methods: Gamma Spectroscopy Lynn West Wisconsin State Lab of Hygiene

49 Instrumentation – Gamma Spectroscopy/Alpha Spectroscopy Quick review of Radioactive Decay (as it relates to σ & spectroscopy) Interaction of Gamma Rays with matter Basic electronics Configurations Semi-conductors Resolution Spectroscopy Calibration/Efficiency Coincidence summing Sample Preparation Daily instrument checks

50 Review of Radioactive Modes of Decay Properties of Alpha Decay Progeny loses of 4 AMU. Progeny loses 2 nuclear charges Often followed by emission of gamma

51 Review of Radioactive Modes of Decay, Cont. Properties of Alpha Decay Alpha particle and progeny (recoil nucleus) have well- defined energies spectroscopy based on alpha-particle energies is possible Energy (MeV) Counts 4.55.5 Alpha spectrum at the theoretical limit of energy resolution

52 Review of Radioactive Modes of Decay, Cont. Properties of beta (negatron) decay No change in mass number of progeny. Progeny gains 1 nuclear charge Beta particle, antineutrino, and recoil nucleus have a continuous range of energies no spectroscopy of elements is possible Often followed by emission of gamma

53 Review of Radioactive Modes of Decay, cont. Counts Ar-36 Cl-36 Energy (MeV) Beta Emission from Cl-36. From G. F. Knoll, Radiation Detection and Measurement, 3rd Ed., (2000).

54 Review of Radioactive Modes of Decay, Cont. Properties of Positron decay No change in mass number of progeny Progeny loses 1 nuclear charge Positron, neutrino, and recoil nucleus have a continuous range of energies no spectroscopy of elements is possible Positron is an anti-particle of an electron

55 Review of Radioactive Modes of Decay, Cont. Properties of Positron decay When the positron comes in contact with an electron, the particles are annihilated Two photons are created each with an energy of 511 keV (the rest mass of an electron) The annihilation peak is a typical feature of a spectrum

56 Review of Radioactive Modes of Decay, Cont. Other modes of decay Electron Capture Neutron deficient isotopes Electron is captured by the nucleus from an outer electron shell Vacancy left from captured electron is filled in by electrons from higher energy shells X-rays are released in the process

57 Review of Radioactive Modes of Decay, Cont. Other modes of decay Auger electrons Excitation of the atom resulting in the ejection of an outer electron Internal conversion electrons Excitation of the nucleus resulting in the ejection of an outer electron Bremsstrahlung Braking radiation Photon emitted by a charged particle as it slows down Adds to the continuum

58 Review of Radioactive Modes of Decay, Cont. Gamma Emission No change in mass, protons, or neutrons Excess excitation energy is given off as electromagnetic radiation, usually following alpha or beta decay Gamma emissions are high-energy, short-wave-length

59 Source: http://lasp.colorado.edu

60 Review of Radioactive Modes of Decay, Cont. Gamma Emission Decay Schemes

61 Pb Shielding e-e- e 511 γ γ γ Pb X Ray e-e- PE e-e- e-e- CS γ γ Source γ γ γ e 511 γ e-e- e-e- e-e- PP CS KEY PE Photoelectric absorption CS Compton scattering PP Pair production γ gamma-ray e - Electron e Positron

62 Gamma Spectrum Features Source: Practical Gamma-Ray Spectrometry, Gilmore & Hemingway

63 Resolution

64 Basic Electronic Schematic – Gamma Spectroscopy Detector Bias Supply DetectorPreamplifier Multichannel Analyzer (MCA) Amplifier Low Voltage Supply

65 Configurations of Ge Detectors Electrical contact True coaxialClosed-end coaxial Holes Electrons + Holes Electrons p-type coaxial,-type n-type coaxial, v-type p+ contact n+ contact

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70 Nature of Semi-conductors Good conductors are atoms with less than four valence electrons atoms with only 1 valance electron are the best conductors examples copper silver gold

71 Nature of Semi-conductors, Cont. Good insulators are atoms with more than four valence electrons atoms with 8 valance electron are the best insulators examples noble gases

72 Nature of Semi-conductors, Cont. Semiconductors are made of atoms with four valence electrons they are neither good conductors nor good insulators examples germanium silicon

73 Nature of Semi-conductors, Cont. Energy Band Diagram VALENCE BAND FORBIDDEN BAND VALENCE BAND FORBIDDEN BAND CONDUCTION BAND Insulator Semiconductor Conductor

74 Nature of Semi-conductors, Cont. Covalent bonds are formed in semiconductors the atoms are arranged in definite crystalline structure the arrangement is repeated throughout the material each atom is covalently bonded to 4 other atoms

75 Nature of Semi-conductors, cont. Pure Semi-conductor Each atom has 8 shared electrons there are no free electrons or no electrons in the conduction band however, thermal energy can cause some valence electrons to gain enough energy to move in to the conduction band this leads to the formation of a hole

76 Nature of Semi-conductors, cont. Pure Semi-conductor Both holes (+) & free electrons (-) are current carriers a pure semi conductor has few carriers of either type more carriers lead to more current doping is the process used to increase the number of carriers in a semiconductor

77 Nature of Semi-conductors, cont. Pure Semi-conductor Impurities can be added during the production of the semiconductor, this is called doping The impurities are either trivalent or pentavalent trivalent examples indium, gallium, boron pentavalent examples arsenic, phosphorus, antimony

78 n-type Semiconductor An impurity with 5 valence electrons (group V) will form 4 covalent bonds with the atoms of the semiconductor One electron is left over & loosely held by the atom This type of impurity is known as donor impurities. There are more negative carriers

79 n-type Semiconductor VALENCE BAND CONDUCTION BAND Valence electron forbidden band Donor electron forbidden band Donor electron Energy level

80 p-type semiconductors An impurity with 3 valence electrons (group III) will form 3 covalent bonds with the atoms of the semiconductor The absence of the fourth electron leaves a hole This type of impurity is known as acceptor impurities. There are more positive carriers

81 p-type Semiconductor, cont. VALENCE BAND CONDUCTION BAND Valence electron forbidden band Acceptor hole forbidden band Acceptor hole Energy level

82 Depletion Zone In the depletion zone the charge carriers have canceled each other out voltage is developed across the depletion zone due to the charge separation + - p-type n-type VcVc Depletion zone V + + + + + + + + + + + - - - - - - - - - + + + + + + + + + + + + - - - - - - - - - -

83 Calibration/Efficiency Ideally, calibration sources would be prepared such that a point calibration is performed for each nuclide reported this is totally impractical for analyzing routine unknown samples Sources should be prepared to have identical shape and density as the sample

84 Calibration/Efficiency Differences in density are less important than differences in geometry Newer software packages allow the user to create different efficiencies mathematically Source strength should not be so great as to cause pile-up

85 Calibration/Efficiency The calibration energies should cover the entire range of interest For close to the detector geometries, choose a multi-lined source made from a combination of nuclides which do not suffer from True Coincidence Summing (TCS). See Table 7.2 pg 153 Gilmore, G. and Hemingway, J. 1995. Practical Gamma-Ray Spectrometry. John Wiley & Sons, New York

86 Coincidence Summing True Coincidence Summing (TCS) The summing of gamma rays emitted almost simultaneously from the nucleus resulting in a negative bias from the true value Larger detectors suffer more from TCS than do smaller detectors TCS can be expected whenever samples contain nuclides with complicated decay schemes

87 Coincidence Summing True Coincidence Summing (TCS) TCS can be expected whenever samples contain nuclides with complicated decay schemes The degree of TCS is not dependent on count rate TCS is geometry dependent and is worse for close to the detector geometries

88 Coincidence Summing True Coincidence Summing (TCS) TCS is geometry dependent and is worse for close to the detector geometries Summed pulses will not be rejected by the pile-up rejection circuitry because the pulses will not be misshapen For detectors with thin windows X-rays that would normally be absorbed in the end cap may contribute to TCS Well detectors suffer the worst from TCS

89 Coincidence Summing True Coincidence Summing (TCS) Newer software packages have systems for reduces this problem

90 Coincidence Summing Random Coincidence Summing Also known as pile-up Two or more gamma rays being detected at nearly the same time Counts are lost from the full-energy peaks in the spectrum Affected by count rate Pile-up rejection circuitry reduces problem

91 Sample Preparation Acidify water samples Note: Iodine is volatile in acidic solutions Active material should be distributed evenly throughout the geometry Samples should be homogenous Calibration materials should simulate samples (actual or mathematical)

92 Daily Instrument Checks Short background count Linearity check Resolution check Additionally, a long background count is needed for background subtraction

93 Instrumentation & Methods: Gamma Emitting Radionuclides USEPA 901.1 Jeff Brenner Minnesota Department of Health

94 EPA Method 901.1 Gamma Emitting Radionuclides Gamma Emitting Radionuclides

95 EPA Method 901.1 What well cover Scope of the method Summary of the method Calibration Determining energy calibration Determining efficiency calibration Determining system background Quality control Interferences Application Calculations Activity

96 EPA Method 901.1 Scope The method is applicable for analyzing water samples Measurement of gamma photons emitted from radionuclides without separating them from the sample matrix. Radionuclides emitting gamma photons with the following energy range of 60 to 2000 keV.

97 EPA Method 901.1 Gamma Emitting Radionuclides Summary Water sample is preserved in the field or lab with nitric acid Homogeneous aliquot of the preserved sample is measured in a calibrated geometry.

98 EPA Method 901.1 Gamma Emitting Radionuclides Summary Sample aliquots are counted long enough to meet the required sensitivity.

99 EPA Method 901.1 Gamma Emitting Radionuclides Summary

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101 EPA Method 901.1 Calibrations Gamma Emitting Radionuclides Library of radionuclide gamma energy spectra is prepared Use known radionuclide concentrations in standard sample geometries to establish energy calibration. Single solution containing a mixture of fission products emitting Low energy Medium energy High energy Example (Sb-125, Eu154, and Eu-155)

102 EPA Method 901.1 Gamma Emitting Radionuclides Summary 86.54 Eu-155 105.31 Eu-155 123.07 Eu-154 176.33 Sb-125 247.93 Eu-154 427.89 Sb-125 463.38 Sb-125 591.76 Eu-154 600.56 Sb-125 635.90 Sb-125 692.42 Eu-154 723.30 Eu-154 756.86 Eu-154 873.20 Eu-154 996.30 Eu-154 1004.76 Eu-154 1274.51 Eu-154 1596.45 Eu-154

103 EPA Method 901.1 Gamma Emitting Radionuclides Counting efficiencies for the various gamma energies are determined from the activity counts of those known standard values. A counting efficiency vs. gamma energy curve is determined for each container geometry and for each detector.

104 EPA Method 901.1 Gamma Emitting Radionuclides Summary 86.54 Eu-155 105.31 Eu-155 176.33 Sb-125 427.89 Sb-125 463.38 Sb-125 600.56 Sb-125 996.30 Eu-154 1004.76 Eu-154 1274.51 Eu-154

105 EPA Method 901.1 Calibrations Gamma Emitting Radionuclides FWHM used to monitor peak shape Smaller tolerance for low energy Greater tolerance for high energy Document a few FWHM to determine instrument drift

106 EPA Method 901.1 Gamma Emitting Radionuclides Summary 86.54 Eu-155 105.31 Eu-155 123.07 Eu-154 176.33 Sb-125 247.93 Eu-154 427.89 Sb-125 463.38 Sb-125 591.76 Eu-154 600.56 Sb-125 635.90 Sb-125 692.42 Eu-154 723.30 Eu-154 756.86 Eu-154 873.20 Eu-154 996.30 Eu-154 1004.76 Eu-154 1274.51 Eu-154 1596.45 Eu-154

107 EPA Method 901.1 Gamma Emitting Radionuclides Summary

108 EPA Method 901.1 (Determine System Background) Contribution of the background must be measured Measure under the same conditions, counting mode, as the samples Background determination is performed every time the liquid nitrogen is filled

109 EPA Method 901.1 (Batch Quality Control) Instrument efficiency check Analyzed daily Control chart Establish action limits Low background check Analyzed weekly Control chart Establish action limits Analytical Batch Sample Duplicates at a 10% frequency Sample Spikes at a 5% frequency Control chart Establish action limits

110 EPA Method 901.1 Interferences Significant interference occurs when counting a sample with a NaI(Tl) detector. Sample radionuclides emit gamma photons of nearly identical energies. Sample homogeneity is important to gamma count reproducibility and counting efficiency. Add HNO 3 to water sample container to lessen the problem of radionuclides adsorbing to the container

111 EPA Method 901.1 Application The limits set forth in PL 93-523, 40 CFR 34324 recommend that in the case of man- made radionuclides, the limiting concentration is that which will produce an annual dose equivalent to 4 mrem/year. If several radionuclides are present, the sum of their annual dose equivalent must not exceed 4 mrem/year.


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