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

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

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

2 Instrumentation & Methods: Laser Phosphorimetry, Uranium Richard Sheibley Pennsylvania Dept of Env Protection

3 Laser Phosphorimeter UV excitation by pulsed nitrogen laser 337nm Green luminescence at 494, 516 and 540 Excitation 3-4 X sec

4 Laser Phosphorimeter Measure luminescence when laser is off Use method of standard addition

5 Instrumentation & Methods: Alpha Spectroscopy, Uranium Lynn West Wisconsin State Lab of Hygiene

6 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

7 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 Alpha spectrum at the theoretical limit of energy resolution

8 Instrumentation – Alpha Spectroscopy Types of detectors Resolution Spectroscopy Calibration/Efficiency Sample Preparation Daily Instrument Checks

9 Types of detectors (Alpha Spectroscopy) Older technology Diffused junction detector (DJD) Surface barrier silicon detectors (SSB) Ion Implanted Layers Fully depleted detectors State-of-the-art technology Passivated implanted planar silicon detector (PIPS)

10 PIPS Good alpha resolution due very thin uniform entrance window Surface is more rugged and can be cleaned Low leakage current Low noise Bakable at high temperatures

11 Alpha Spectrometer Detector An example of a passivated implanted planar silicon detector 600 mm 2 active area Resolution of 24 keV (FWHM)

12 Alpha Spectrometer

13 Resolution Broadening of peaks is due to various sources of leakage current – Noise Low energy tails result from trapping of charge carriers which results from the incomplete collection of the total energy deposited Good resolution increases sensitivity (background below peak is reduced) Resolution of 10 keV is achievable with PIPS (controlled conditions)

14 Typical Alpha Spectrum

15 Calibration/Efficiency Energy calibration Efficiency can be determined mathematically using Monte-Carlo simulation Efficiency can be determined using a NIST traceable standard in same geometry as samples Efficiency determination not always needed with tracers

16 Sample Preparation Final sample must be very thin to insure high resolution and minimize tailing. Also should stable & rugged The following mounting techniques are commonly used: Electrodeposition Micro precipitation Evaporation from organic solutions Organics must be completely removed

17 Sample Preparation Chemical and radiochemical interferences must be removed during preparation Nuclides must be removed which have energies close to the energies of the nuclide of interest, ie 15 to 30 keV Ion exchange Precipitation/coprecipitation techniques Chemical extractions Chemicals which might damage detector must be elimanted

18 Sample Preparation A radioactive tracer is used to determine the recovery of the nuclide of interest Since a tracer is added to every sample, a matrix spiked sample is not required

19 Sample Counting Mounts with a small negative voltage can be used to help attract the recoil nucleus away from the detector Reduces detector contamination

20 Sample Counting Analyst can choose distance from detector Trade off is between efficiency & resolution Count performed slightly above atm. pressure to reduce contamination

21 Daily instrument checks One hour background Pulser check Stability check

22 Instrumentation & Methods: Liquid Scintillation Counters & Tritium Richard Sheibley Pennsylvania Dept of Env Protection

23 Liquid Scintillation Counter Principle Beta particle emission Energy transferred to Solute Energy released as UV Pulse Intensity proportional to beta particle initial energy

24 Liquid Scintillation Counter Low energy beta emitters Tritium – 3 H Iodine – 125 I, 129 I, 131 I Radon – 222 Rn Nickel – 63 Ni Carbon – 14 C

25 Liquid Scintillation Counter Energy Spectrum Isotope specific Beta particle Neutrino Total energy constant

26 Liquid Scintillation Counter Components Vial with Sample + Scintillator Photomultipliers Multichannel Analyzer Timer Data collection & Output

27 Liquid Scintillation Counter Variables Temperature Counting room Vial type glass vs. plastic Cocktail Energy window

28 Liquid Scintillation Counter Other considerations Dark adapt Static Quenching

29 Liquid Scintillation Counter Interferences Chemical Absorbed beta energy Optical Photon absorption

30 Liquid Scintillation Counter Instrument Normalization Photomultiplier response Unquenched 14 C Standard

31 Liquid Scintillation Counter Performance assessment Carbon-14 Efficiency Tritium Efficiency Chi-square Instrument Background

32 Liquid Scintillation Counter Method QC Background Reagent background Efficiency Method Quench correction

33 Tritium 3 H (EPA & SM H B) Prescribed Procedures for Measurement of Radioactivity in Drinking Water EPA August 1980 Standard Methods 17 th, 18 th, 19 th & 20 th

34 Interferences Non-volatile radioactive material Quenching materials Double distill – eliminate radium Static Fluorescent lighting

35 Tritium 3 H Method Summary Alkaline Permanganate Digestion Remove organic material Distillation Collect middle fraction Liquid Scintillation Counting

36 Calibration – Method Raw water tritium standard Distilled Recovery standard Background Distilled Deep well water Distilled water tritium standard Distilled water to which 3 H added Not distilled

37 Instrument Calibration Calibrate each day of use Instrument Normalization Performance assessment Carbon-14 Efficiency Tritium Efficiency Instrument Background NIST traceable standards

38 Calculations 3 H(pCi/L) = (C-B)*1000 / 2.22*E*V*F Where: C = sample count rate, cpm B = background count rate, cpm E = counting efficiency F = recovery factor 2.22 = conversion factor, dpm/cpm

39 Calculations Efficiency: E = (D-B)/G Where: D = distilled water standard count rate, cpm B = background count rate, cpm G = activity distilled water standard, dpm

40 Calculations Recovery correction factor F = (L-B) / (E*M) Where: L = raw water standard count rate, cpm B = background count rate, cpm E = counting efficiency M = activity raw water standard (before distillation), dpm

41 Quality Control Batch Precision: Sample duplicate OR Matrix spike duplicate Calculate relative percent difference Calculate control limits Should be < 20% Frequency 1 per 20

42 Quality Control, continued Accuracy Laboratory fortified blank Matrix spike sample 2 – 10 Xs detection limit Reagent background |reagent background|< detection limit Instrument drift

43 Quality Control, continued Daily control charts Acceptance limits Corrective action Preventative maintenance

44 Standard Operating Procedure Written Reflect actual practice Standard format – EMMC or NELAC

45 Demonstration of Proficiency Initial Method detection limit – MDL 40 CFR 136, Appendix B Alternate procedure 4 reagent blanks < Detection limit (DL) 4 laboratory fortified blanks (LFB) DL < LFB < MCL Evaluate Recovery and Standard Deviation against method criteria

46 Demonstration of Proficiency Ongoing Repeat initial demonstration of proficiency Alternate procedure 4 Reagent blanks and laboratory fortified blanks Different batches Non-consecutive days Blank < Detection limit (DL) LFB met method precision and accuracy criteria

47 Instrumentation & Methods: Strontium 89, 90 Lynn West Wisconsin State Lab of Hygiene

48 Method Review Strontium 89, 90 EPA 905.0, SM 7500-Sr B

49 Radiochemical Characteristics IsotopeT 1/2 Decay Mode MCL pCi/L 89 Sr50.55 days Beta80 90 Sr29.1 yearsBeta8 90 Y64.2 hoursBetaN/A

50 Strontium (EPA 905.0, SM 7500-Sr B) Prescribed Procedures for Measurement of Radioactivity in Drinking Water EPA August 1980 Standard Methods 17 th, 18 th, 19 th & 20 th

51

52 Strontium Chemistry Chemically similar to Ca +2 oxidation state in solution Insoluble salts include: CO 3 & NO 3 Real Chemistry

53 Interferences Radioactive barium and radium Precipitated as carbonate Removed using chromate precipitation Non-radioactive strontium Cause errors in recovery Calcium Precipitated as carbonate Removed by repeated nitrate precipitations

54 905.0 Method Summary Isolate Strontium Measure total strontium Allow strontium to decay Isolate strontium 90 daughter – yttrium 90 Measure yttrium 90

55 905.0 Method Summary 1 L acidified sample Isolate Strontium Add stable Sr carrier Precipitate alkaline and rare earths as carbonate Re-dissolve

56 905.0 Method Summary Isolate Strontium(continued) Precipitate as nitrate Re-dissolve Precipitate as carbonate Determine chemical yield

57 905.0 Method Summary Measure total strontium activity Determine 90 Sr Yttrium in growth – 2 weeks Isolate yttrium Determine 90 Y

58 905.0 Method Summary Determine 89 Sr Calculated Total strontium minus 90 Sr

59 905.0 Method Summary Calculations include Recovery correction In-growth correction – yttrium Total strontium Strontium 90 Decay correction – yttrium Isolation of Y to end of count time

60 Calculation total strontium Total strontium activity (D) D = C / 2.22*E*V*R where: C = net count rate, cpm E = counter efficiency for 90 Sr V = sample volume, liters R = fractional chemical yield 2.22 = conversion factor dpm/pCi

61 Calculations cont. See handout

62 Calculations cont. Verify computer programs 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

63 Instrumentation Low background gas flow proportional counter P-10 counting gas (10% CH 4 & 90% Ar) Due to in growth and short half-life of 90 Y, time is critical

64 Instrument Calibration Isotope specific calibration 89 Sr 90 Sr 90 Y Use NIST traceable standards Perform yearly or after repairs

65 Quality Control Batch Precision: Sample duplicate OR Matrix spike duplicate Calculate relative percent difference Calculate control limits Should be < 20% Frequency 1 per 20

66 Quality Control, continued Batch Accuracy Laboratory fortified blank Matrix spike sample 2 – 10 Xs detection limit Reagent background |reagent background|< detection limit Instrument drift

67 Quality Control, continued Daily control charts Acceptance limits Corrective action Preventative maintenance


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