· Web viewDIVISION of DIVISION of NUCLEAR and RADIOCHEMISTRY NUCLEAR and RADIOCHEMISTRY
Environmental Laboratory Accreditation Course for Radiochemistry: DAY THREE Presented by Minnesota...
-
Upload
daniel-madden -
Category
Documents
-
view
214 -
download
0
Transcript of Environmental Laboratory Accreditation Course for Radiochemistry: DAY THREE Presented by Minnesota...
Environmental Laboratory Accreditation Course for Radiochemistry: DAY THREE
Presented byMinnesota Department of HealthPennsylvania Department of Environmental ProtectionU.S. Environmental Protection AgencyWisconsin State Laboratory of Hygiene
Instrumentation & Methods: Laser Phosphorimetry, Uranium
Richard Sheibley
Pennsylvania Dept of Env Protection
Laser Phosphorimeter
UV excitation by pulsed nitrogen laser 337nm
Green luminescence at 494, 516 and 540
Excitation 3-4 X 10-9sec
Laser Phosphorimeter
Measure luminescence when laser is off
Use method of standard addition
Instrumentation & Methods: Alpha Spectroscopy, Uranium
Lynn West
Wisconsin State Lab of Hygiene
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
226
88Ra 22286
Rn + 42He + energy
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)
Cou
nts
4.5 5.5
Alpha spectrum at the theoretical limit of energy resolution
Instrumentation – Alpha Spectroscopy
Types of detectors Resolution Spectroscopy Calibration/Efficiency Sample Preparation Daily Instrument Checks
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)
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
Alpha Spectrometer Detector
An example of a passivated implanted planar silicon detector
600 mm2 active area
Resolution of 24 keV (FWHM)
Alpha Spectrometer
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)
Typical Alpha Spectrum
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
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
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
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
Sample Counting
Mounts with a small negative voltage can be used to help attract the recoil nucleus away from the detector
Reduces detector contamination
Sample Counting
Analyst can choose distance from detector
Trade off is between efficiency & resolution
Count performed slightly above atm. pressure to reduce contamination
Daily instrument checks
One hour background Pulser check
Stability check
Instrumentation & Methods:
Liquid Scintillation Counters & Tritium
Richard Sheibley
Pennsylvania Dept of Env Protection
Liquid Scintillation Counter
Principle Beta particle emission Energy transferred to Solute Energy released as UV Pulse Intensity proportional to beta
particle initial energy
Liquid Scintillation Counter
Low energy beta emitters Tritium – 3H Iodine – 125I, 129I, 131I Radon – 222Rn Nickel – 63Ni Carbon – 14C
Liquid Scintillation Counter
Energy Spectrum Isotope specific Beta particle Neutrino Total energy constant
Liquid Scintillation Counter
Components Vial with Sample + Scintillator Photomultipliers Multichannel Analyzer Timer Data collection & Output
Liquid Scintillation Counter
Variables Temperature Counting room Vial type glass vs. plastic Cocktail Energy window
Liquid Scintillation Counter
Other considerations Dark adapt Static Quenching
Liquid Scintillation Counter
Interferences Chemical
Absorbed beta energy Optical
Photon absorption
Liquid Scintillation Counter
Instrument Normalization Photomultiplier response Unquenched 14C Standard
Liquid Scintillation Counter
Performance assessment Carbon-14 Efficiency Tritium Efficiency Chi-square Instrument Background
Liquid Scintillation Counter
Method QC Background
Reagent background Efficiency
Method Quench correction
Tritium 3H (EPA 906.0 & SM7500-3H B)
Prescribed Procedures for Measurement of Radioactivity in Drinking Water
EPA 600 4-80-032 August 1980 Standard Methods 17th, 18th, 19th &
20th
Interferences
Non-volatile radioactive material Quenching materials Double distill – eliminate radium Static Fluorescent lighting
Tritium 3H Method Summary
Alkaline Permanganate Digestion Remove organic material
Distillation Collect middle fraction
Liquid Scintillation Counting
Calibration – Method
Raw water tritium standard Distilled Recovery standard
Background Distilled Deep well water
Distilled water tritium standard Distilled water to which 3H added Not distilled
Instrument Calibration
Calibrate each day of use Instrument Normalization Performance assessment
Carbon-14 Efficiency Tritium Efficiency Instrument Background
NIST traceable standards
Calculations
3H(pCi/L) = (C-B)*1000 / 2.22*E*V*FWhere:C = sample count rate, cpmB = background count rate, cpmE = counting efficiencyF = recovery factor2.22 = conversion factor, dpm/cpm
Calculations
Efficiency:E = (D-B)/G
Where:D = distilled water standard count rate, cpmB = background count rate, cpmG = activity distilled water standard, dpm
Calculations
Recovery correction factorF = (L-B) / (E*M)
Where:L = raw water standard count rate, cpmB = background count rate, cpmE = counting efficiencyM = activity raw water standard (before
distillation), dpm
Quality Control
Batch Precision: Sample duplicate OR Matrix spike duplicate Calculate relative percent difference Calculate control limits Should be < 20% Frequency 1 per 20
Quality Control, continued
Accuracy Laboratory fortified blank Matrix spike sample
2 – 10 Xs detection limit
Reagent background |reagent background|< detection limit
Instrument drift
Quality Control, continued
Daily control charts Acceptance limits Corrective action Preventative maintenance
Standard Operating Procedure
Written Reflect actual practice Standard format – EMMC or NELAC
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
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
Instrumentation & Methods: Strontium 89, 90
Lynn West
Wisconsin State Lab of Hygiene
Method Review
Strontium 89, 90 EPA 905.0, SM 7500-Sr B
Radiochemical Characteristics
Isotope T1/2 Decay Mode
MCLpCi/L
89Sr 50.55 days
Beta 80
90Sr 29.1 years Beta 8
90Y 64.2 hours Beta N/A
Strontium (EPA 905.0, SM 7500-Sr B)
Prescribed Procedures for Measurement of Radioactivity in Drinking Water
EPA 600 4-80-032 August 1980 Standard Methods 17th, 18th, 19th &
20th
Strontium Chemistry
Chemically similar to Ca +2 oxidation state in solution Insoluble salts include: CO3 & NO3
“Real Chemistry”
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
905.0 Method Summary
Isolate Strontium Measure total strontium Allow strontium to decay Isolate strontium 90 daughter –
yttrium 90 Measure yttrium 90
905.0 Method Summary
1 L acidified sample Isolate Strontium
Add stable Sr carrier Precipitate alkaline and rare earths as
carbonate Re-dissolve
905.0 Method Summary
Isolate Strontium(continued) Precipitate as nitrate Re-dissolve Precipitate as carbonate
Determine chemical yield
905.0 Method Summary
Measure total strontium activity
Determine 90Sr Yttrium in growth – 2 weeks Isolate yttrium Determine 90Y
905.0 Method Summary
Determine 89SrCalculatedTotal strontium minus 90Sr
905.0 Method Summary
Calculations includeRecovery correction In-growth correction – yttrium
Total strontium Strontium 90
Decay correction – yttrium Isolation of Y to end of count time
Calculation total strontium
Total strontium activity (D) D = C / 2.22*E*V*R
where:C = net count rate, cpmE = counter efficiency for 90SrV = sample volume, litersR = fractional chemical yield2.22 = conversion factor dpm/pCi
Calculations cont.
See handout
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
Instrumentation
Low background gas flow proportional counter P-10 counting gas (10% CH4 & 90% Ar)
Due to in growth and short half-life of 90Y, time is critical
Instrument Calibration
Isotope specific calibration 89Sr 90Sr 90Y
Use NIST traceable standards Perform yearly or after repairs
Quality Control
Batch Precision: Sample duplicate OR Matrix spike duplicate Calculate relative percent difference Calculate control limits Should be < 20% Frequency 1 per 20
Quality Control, continued
Batch Accuracy Laboratory fortified blank Matrix spike sample
2 – 10 Xs detection limit
Reagent background |reagent background|< detection limit
Instrument drift
Quality Control, continued
Daily control charts Acceptance limits Corrective action Preventative maintenance