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