ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES OF RADIONUCLIDES

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ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO
INTAKES OF RADIONUCLIDES

• Indirect Monitoring Methods

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Indirect Measurements Unit Objectives

• The objective of this unit is to provide a review
  of the principles and techniques that are used
  for the indirect measurements for assessment of
  radionuclide intake by the human body, including
  the use of workplace measurements for assessment
  of internal exposure.
• At the completion of the unit, the student should
  understand indirect measurement principles and
  limitations and how select the appropriate
  measurement techniques and assessment methods.

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Indirect Monitoring Methods - Unit Outline

• Introduction
• Biological Samples
• Physical Samples
• Sample Preparation and Analysis
• Detection Methods
• Sample Activity

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Introduction
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Basic principles

• Determination of activity concentration in
• Biological materials separated from the body,
• Physical samples taken at the workplace.
• Most applicable for radionuclides
• Non-penetrating radiation (e.g. ?, ?),
• Low energy photons (detection sensitivity),
• Accessibility (lack of WBC facility).

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Principal radiometric methods
Available emissions a a a ß ß ß ? ?
Sample Urine Faeces or physical sample Breath Urine Faeces or physical sample Breath Urine Faeces or physical sample
Source preparation requirements Isolation of radioelements from matrix Chemical separation of elements Isolation of radioelements from matrix Chemical separation of elements None None or concentration Separation from matrix None None None
Methods of detection Gross a counting a spectrometry Gross a counting a spectrometry Gross a counting a spectrometry Liquid scintillation counting (limited energy discrimination) Gross ß counting Gross ß counting Gross ? counting ? spectroscopy Gross ? counting ? spectroscopy
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Principal non-radiometric methods
Sample Urine Faeces or physical sample
Source preparation requirements None or separation of elements None or separation of elements
Methods of detection Fluorometry ICP/MS Delayed neutron assay Fluorometry ICP/MS Delayed neutron assay
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GI fractional absorption
TABLE II. TYPE CHARACTERISTICS OF SELECTED
RADIONUCLIDES - INGESTION
Element Gut transfer factor, f1 Compounds
Hydrogen 1.00 Tritiated water
1.00 Organically bound tritium
Carbon 1.00 Labelled organic compounds
Sulphur 0.80 Inorganic compounds
0.10 Elemental sulphur
1.00 Organic sulphur
Calcium 0.30 All compounds
Iron 0.10 All compounds
Cobalt 0.10 All unspecified compounds
0.05 Oxides, hydroxides and inorganic compounds
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Lung absorption types
TABLE III. TYPE CHARACTERISTICS OF SELECTED
RADIONUCLIDES - INHALATION
Element Absorption type Gut transfer Factor, f1 Compounds
Hydrogen Va Tritium gas, methane
Vb Tritiated water, organic compounds
Carbon Va Monoxide
Carbon Vb Dioxide, organic compounds (vapour)
F 0.80 Sulphides and sulphates determined by
combining cation
M 0.80 Elemental sulphur. Sulphides and sulphates
determined by combining cation
Fa 0.80 Vapour
Calcium M 0.30 All compounds
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Radiological characteristics
TABLE IV. RADIOLOGICAL CHARACTERISTICS OF
SELECTED INTERNAL CONTAMINANTS
Radionuclide Half-life Principal emission energies (MeV) and yields (fraction/transformation)a
H-3 12.3 a ß 5.683 x 10-3 (1.0)
C-14 5730 a ß 4.945 x 10-2 (1.0)
P-32 14.3 d ß 6.947 x 10-1 (1.0)
S-35 87.4 d ß 4.883 x 10-2 (1.0)
Ca-45 163 d ß 7.723 x 10-2 (1.0)
Ca-47 4.53 d ß 2.411 x 10-1 (0.819)
? 8.169 x 10-1 (0. 18)
? 1.297 (0.749)
Fe-55 2.7 a Auger 6.045 x 10-4 (1.39)
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Physical samples

• Physical samples air, surface wipes, etc.
• Very specific to the situation
• For low doses (lt 1 mSv) only, information
• Physico-chemical form (except radon)

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Methods of analysis

• Radiometric (detection and quantification of
  radionuclide emissions) or non-radiometric
• The choice of method depends on
• equipment available,
• samples,
• activity levels (expected and needed),
• skill and experience
• Main criteria (set by the regulatory authority)
  adequacy, sensitivity and accuracy

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Methods of analysis

• Method of analysis depends on the emissions.
• If only a or ß decay, separation from the matrix
  may be necessary.
• For urine, direct dispersion of a small volume of
  the sample into a liquid scintillant may be
  sufficient.
• Radionuclides emitting penetrating photons can
  usually be quantified in bulk samples.

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Beta emitters are difficult to resolve
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Methods of analysis

• a particles are monoenergetic
• a spectroscopy can be performed after separation
  from the sample matrix
• Many alphas have similar energies and cannot
  always be resolved.
• Respective elements must then be separated
  chemically.
• Non-radiometric methods can also used.

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Biological Samples
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Biological samples

• Radionuclide concentrations in body materials
• Usually uses excreta to assess intakes
• Main sources of bioassay data
• Urine
• Faeces
• Breath
• Blood
• Nose blows or nasal swabs provide early
  radionuclide identification and estimates.

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Biological sample selection

• Determining factors
• major excretion route (absorption, solubility),
• the ease of sample collection, preparation,
  analysis and interpretation.
• Urine easily obtained, analysed, well absorbed
  radionuclides (chemical forms)
• Faeces insoluble radionuclides (chemical forms)

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Biological sample selection

• Practical guidance for biological samples
• urine inhalation V, F, M ingestion
• faeces inhalation S (ingestion, low absorption)
• others very specific to the situation

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Routes of intake, transfer and excretion
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Biological samples - Urine

• Urine contains waste and other materials
• Nominal daily output of urine is 1.4 L
• Output depends on physiological and environmental
  conditions
• Total 24h output should be collected to estimate
  accurately the daily excretion rate
• If 24 h samples cant be collected, the first
  morning voiding is preferable for analysis

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Biological samples - Urine

• Creatinine - a metabolic product in muscle
  metabolism - can be used to estimate 24 h
  excretion of radionuclides
• 3H2O
• 24 h collection not necessary
• Any sample provides an accurate estimate
• Frequent sampling improves the time resolution
  needed for biokinetic modeling.

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Biological samples - Urine

• It takes time for many materials to get in urine
• Results from samples obtained soon after an
  incident must be interpreted cautiously.
• Sampling strategy soon after the intake, several
  hours later, at time intervals specified by
  biokinetic and physical parameters
• Proper sampling intervals depend on excretion
  rate and radionuclide half life

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137Cs in urine
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137Cs in urine
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Urine bioassay sample handling

• Some chemical forms of the radioisotopes can be
  degraded in the urine sample
• Radioactive material can be adsorbed on the
  surfaces of some containers.
• Samples must often be stabilized until analysis
  by
• refrigeration or freezing,
• addition of a carrier
• addition of a preservative, e.g. thymol

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Biological samples - Faeces

• Waste products through the GI tract and cellular
  debris from the intestine walls
• Materials cleared from the lung, systemic
  material excreted into the GI tract, and material
  passing unabsorbed through the GI tract following
  ingestion.
• Nominal transit time is about 2 days
• Mass and composition of individual voidings is
  quite variable

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Biological samples - Faeces

• Usually necessary to collect total faecal
  elimination over a period of 3 to 4 days
• Samples should be analysed promptly, ashed or
  preserved by deep freezing.
• Samples can contain significant levels of
  potentially toxic biological contaminants.
• Wet or dry ashing procedures can be used
• Radionuclides may be analysed either in the
  volatile fraction or residual ash

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60Co in faeces
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Biological samples - breath

• Few materials produce significant exhalation
• Tritiated water
• 226Ra - 222Rn
• 232Th - 220Rn
• Sample collection takes about 30 minutes
• Direct analysis or extraction of specific
  components
• Water vapour in a low temperature trap,
• 14C02 on ethanolamine based absorbers.

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Biological samples - blood

• Provides direct information on the circulating
  radionuclides
• Rapid clearance makes it a doubtful indicator of
  total systemic content
• May be useful in exceptional circumstances
• Tritiated water
• 51Cr labelled red blood cells
• May need preservatives to prevent clotting
• White blood cells and chromosome aberrations used
  after accidents

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Biological samples - others

• Saliva, sweat, teeth, nails, nose blows, etc.
• Indicators of intakes
• Nose blows screening for ?, ? emitting
  radionuclides,
• Hair plutonium, ? time pattern of blood
  concentration,
• Teeth e.g. Sr exposures,
• Biopsy or autopsy tissues special radionuclides
  and dose reconstruction

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90Sr in teeth
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Sample collection and transport

• Biological hazard - handle with care!
• Collection is a critical part - training of
  workers (co-operation is essential)
• Collection after weekend may be wise
• Urine
• Clean containers with tight fitting lids
• Record identification and time of collection

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Sample collection and transport

• Be aware of any interfering medical treatments
• Faecal sampling - Clinical sampling kits are
  available and can be provided to the worker
• Explain the importance to the worker
• Line faecal collection containers with plastic
  bags that can be ashed with the sample

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Sample collection and transport

• Sample transport procedures
• Appropriate sample identification
• Need for sample pre-treatment
• Appropriate packaging and labelling
• Prevention of biological or radiological
  contamination
• Maintain 'chain of custody'

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Physical Samples
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Physical samples

• Air sampling
• Static air samplers
• Personal air samplers
• Impactors
• Surface contamination
• Survey instruments
• Swipe (smear) sampling
• Other materials

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Physical samples

• Provide identification, characterization
  (particle size and chemical characteristics) and
  rough intake estimate
• Cant be used for individual biokinetics
• Useful for determining the need for individual
  monitoring
• For some radionuclides, physical samples may
  provide the best intake estimates

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Physical samples - air samplers

• Air samplers provide
• Identification of radionuclides
• Relative concentrations
• Airborne particle size
• Particle size is important
• For low exposures, an assumed AMAD of 5 µm is
  sufficient
• If the workplace exposure can be high, specific
  parameters may be needed

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Physical samples - air samplers

• Bioassay technique may depend on particle size
• Cascade impactors are commonly used for particle
  size characterization
• Personal cascade impactors more common
• Each impactor stage is analysed
• A calibration factor is applied to obtain
  particle size distribution

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Cascade impactors
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Static air samplers

• Static air samplers
• Sampling at a preset flow rate
• 1.5m height
• Filter or impactor
• Indicator of workplace conditions
• Intake, exposure estimation for radon

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Personal air samplers

• Personal air sampling
• Portable equipment
• Used in the workers breathing zone
• More realistic results (air concentration
  inhomogenities)
• Additional data is needed (breathing rate)

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Physical samples - surface contamination

• Indicators of radioactivity levels
• Sampling technique is very specific (physical,
  chemical form, surface, etc.)
• Commonly used method swiping of a defined area
  direct analysis or sample preparation,
  radionuclide separation
• Collection efficiency should be determined (about
  10 typically)

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Physical samples - external contamination of
worker

• Contamination on workers clothes or skin ?
  indication of a possible intake (monitors)
• Intake (rough) estimations only in special cases
• Nose swabs, ? 10 (4 ?m AMAD) (breathing rate,
  nose breathing, AMAD, time, nose blowing, etc.)
• Indication of the magnitude and possible severity
  of intakes, rather than quantification

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Sample Preparation and Analysis
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Analysis techniques

• Considerations for sample preparation and
  measurement
• Radionuclides
• Chemical forms
• Physical characteristics
• Interfering radionuclides
• Sensitivity
• Preliminary information and/or screening
• Identify a, ß, or ? emissions

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Analysis techniques

• Sources of interference may include
• Naturally occurring radionuclides
• Radionuclides from previous intakes
• Radiopharmaceuticals, and
• Prescription or non-prescription drugs that
  affect the chemical processing of the sample
• Workplace air samples may include
• Natural radon and thoron progeny, or
• High levels of dust loading

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Analysis techniques

• Measurement sensitivity depends on the detectable
  level of intake necessary to demonstrate
  regulatory compliance
• The MDA lt Derived Recording Level (DRL) for the
  method chosen
• Intake at Recording Level (RL) ? DRL

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Radiochemistry

• a and ß emitters usually separated from
• Bulk of the sample matrix
• Interfering radionuclides
• Separation generally involves
• Sample preparation and preconcentration,
• Chemical separation of the radionuclides,
• Preparation of the source for counting.
• Little preparation for pure emitters in a simple
  matrix (e.g. 3H or 14C in urine)

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Sample preparation

• Reduction of organic components (dry and wet
  ashing)
• Avoid loss of volatile elements!
• Some samples need vigorous chemistry
• For highly insoluble matrices, treatment with
  hydrofluoric acid may be needed.

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Preconcentration

• Separation of by co-precipitation, e.g.
• Iron hydroxide or lanthanum fluoride in phosphate
  is used for actinides,
• Calcium oxalate in phosphate for calcium,
  strontium and actinides,
• Bismuth phosphate for americium.
• Stable carrier element followed by precipitation
  is often used for 32P and 90Sr.
• Specific methods are for organics containing 14C
  and 3H

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Radiochemical separation

• Dissolution of ash or precipitate
• Chemical separation by
• Solvent extraction (? organic phase),
• Ion exchange separation (e.g., column),
• Co-precipitation (addition of stable carrier)

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Preparation of source for counting

• ? and X-ray normally none or minimal
• ? emitters precipitation of anions, filter or
  planchette
• ? emitters electrodeposition (SS disc),
  co-precipitation (filter), solvent extraction

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Detection Methods
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Detection methods - ? counting

• Gross ? gas flow proportional, scintillation
  (ZnS), counting efficiency ? 45
• LSC (higher activity levels)
• Nuclide specific ? spectrometry, Si detectors in
  vacuum chambers (very long counting time for mBq
  activities)
• Radon track detectors (? tracks on a plastic
  film are etched in NaOH, counting)

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Detection methods - ? spectrum
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Detection methods - ? counting

• LSC, esp. for low energy ? emitters (3H, 14C,
  241Pu, etc.)
• Dispersion in organic liquid scintillator, light
  photons, PM tubes
• Quenching effect (interference, absorption)
• Energy discrimination is limited (low, medium and
  high energy regions)
• Background from 40K is a problem in urine
• Gas flow, GM, proportional counters for energies
  gt 100 keV (calibration!)

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Detection methods - ? spectrum (LSC)
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Detection methods - Photon counting

• Counting in bulk samples (for ?? keV energies
  attenuation will be significant)
• Primarily HPGe or NaI(Tl)
• Low background shielding is crucial (natural ?
  emitting radionuclides)
• ? spectrometry fast (preparation and
  measurement) and sensitive equipment is rather
  expensive, but it can be shared with
  environmental monitoring

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Detection methods - HPGe ? spectrum
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Detection methods - NaI(Tl) ? spectrum
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Non-radiometric methods

• Luminescence, e.g. fluorimetry for U in urine
• Alkaline fluoride fusion
• Process 1 mL of urine
• Read on a fluorimeter
• MDA 3 µg U/L
• Kinetic phosphorescence analysis (KPA)
• 10 mL urine
• Laser used to stimulate uranium ions
• Emission of photoluminescence photons
• MDA 7 ng U/L

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Non-radiometric methods

• Delayed neutron counting
• Uranium content of urine, etc
• Delayed neutrons from U fission after irradiation
  in a reactor.
• ICP/MS
• Chemical separation of radionuclides of the same
  mass number (A) is needed
• Preparation is similar to ? spectrometry
• Normally used for A gt 60
• MDA very low, but equipment expensive

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Sample Activity
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Determination of sample activity

• Various methods of sample analysis yield measured
  levels of activity, e.g. cpm
• Sample counts minus background are converted
  absolute activity, e.g. Bqkg-1
• The method chosen depends on the method of
  analysis

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Determination of sample activity

• The methods used include
• Comparison of ? counts from the sample with those
  from standards with known amounts of the specific
  radionuclide
• Yield determination for ? or ? emitters using
  tracers of the radionuclide under analysis
• Internal and external standardization for ?
  emitters analysed by liquid scintillation
  counting.

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Calculation of sample activity

• The activity in the sample is calculated as
• where Ns is the sample counts
• Nb is the counts in the same region
  from the blank
• F is the efficiency of the method
• and ts and tb are the counting times

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Calculation of sample activity

• When a tracer is added to determine yield, F may
  be calculated using
• where Nt is the counts in the tracer region,
• Nb is the counts in the tracer region from
  the blank,
• ts and tb are the counting times for the
  tracer b (and sample) and the blank, and, At
  is the activity of the tracer added (Bq)

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References

• FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED
  NATIONS, INTERNATIONAL ATOMIC ENERGY AGENCY,
  INTERNATIONAL LABOUR ORGANISATION, OECD NUCLEAR
  ENERGY AGENCY, PAN AMERICAN HEALTH ORGANIZATION,
  WORLD HEALTH ORGANIZATION, International Basic
  Safety Standards for Protection against Ionizing
  Radiation and for the Safety of Radiation
  Sources, Safety Series No. 115, IAEA, Vienna
  (1996).
• HEALTH PHYSICS SOCIETY, Performance Criteria for
  Radiobioassy, An American National Standard, HPS
  N13.30-1996 (1996).
• INTERNATIONAL ATOMIC ENERGY AGENCY, Occupational
  Radiation Protection, Safety Guide No. RS-G-1.1,
  ISBN 92-0-102299-9 (1999).
• INTERNATIONAL ATOMIC ENERGY AGENCY, Assessment of
  Occupational Exposure Due to Intakes of
  Radionuclides, Safety Guide No. RS-G-1.2, ISBN
  92-0-101999-8 (1999).
• INTERNATIONAL ATOMIC ENERGY AGENCY, Indirect
  Methods for Assessing Intakes of Radionuclides
  Causing Occupational Exposure, Safety Guide,
  Safety Reports Series No. 18, ISBN 92-0-100600-4
  (2000).
• INTERNATIONAL COMMISSION ON RADIOLOGICAL
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  Exposure of Workers Replacement of ICRP
  Publication 54, ICRP Publication 78, Annals of
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• NATIONAL COUNCIL ON RADIATION PROTECTION AND
  MEASUREMENTS, Use of Bioassay Procedures for
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  Rep. 87, NCRP, Bethesda, MD (1987).