CE 511 - May 31, 2006

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CE 511 - May 31, 2006

• Radiation - Decay, Activity, Dose, Effects
  Standards

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Objectives

• Be able to calculate half-life and decay
• Discuss sources of radiation
• Discuss dose - Roentgen, Gray, RBE, sievert
• Discuss effects of radiation - discern between
  acute and delayed effects

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Radioactive Decay Half-Life

• Radioactive decay is an example of a first-order
  process
• Depends only on the initial nuclei present
• Characterized by a unique decay constant (l) and
  half-life (T1/2)

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Activity

• Activity (A) is used to express the quantity of a
  radionuclide based upon the rate at which
  radiation is emitted during decay
• A ?N
• where ? is the decay constant and N is the
  number of atoms.
• Activity is measured in disintegrations per
  second (dps)
• One Bequerel equals 1 dps
• One curie equals 3.7 x 1010 dps
• Specific activity (S) is activity per unit mass

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Decay Chains - Daughter End-Products

• Decay progresses until a stable product results
  (End Product)
• Multiple daughter products are formed
• Lifetimes vary
• Long-lived parents may have daughters that are in
  secular equilibrium

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Sources of Radioisotopes

• Naturally occurring
• Artificially produced

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Natural Sources

• Most of 50 naturally occurring radioisotopes are
  primordial and result from
• Uranium series (238U)
• Thorium series (232Th)
• Actinium series (235U)
• Phosphorous series (40P)
• Half-lives of the parents are very long ( gt107
  yrs)
• Present when the earth was formed
• Cosmogenic nuclides
• Produced by collisions in the atmosphere with
  cosmic rays (14C, 3H, 7Be)

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Artificial Radioisotopes

• Accelerators or cyclotrons
• Bombard targets with protons, deuterons or alpha
  particles to produce isotopes
• Form 66Ga by bombarding 64Zn with deuterons
• Medical isotopes, 99Tc, 123I
• Reactors
• Fission produces thermal neutrons
• These interact with targets to form new products
• 59I, 60Co
• Fission products (89Sr, 95Zr, 131I) also result
  from the complete breakup of fuel (235U, 239Pu or
  233U )

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Radiation Dose

• Damage is a consequence of energy absorption
• Produce ion pairs
• Induce decomposition of cell molecules
• Units for Exposure and Absorbed Dose
• Exposure quantified by the roentgen
• For gamma- and X-rays
• Based on the ionization of air at STP
• Absorbed dose quantified by rad or gray (Gy)
• Energy imparted to the target by the radiation
• Gray equal to the adsorption of 1 J/kg
• 1 Gy 100 rad
• Need absorption coefficient to relate R rad

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Radiation Dose - continued

• Effects of ionizing radiation vary
• Relative Biological Effectiveness (RBE)
• gammas, x-rays, betas 1
• neutrons 5-10
• alphas 20
• Tissue weight factor (WT)
• Effective dose absorbed dose x RBE (rem or
  sieverts where 1 sievert 100 rem) x WT
• Provides for biological effects

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Biological Effects

• Sequential Pattern
• Latent Period - time lag between exposure and
  evidence of effect
• Short-term or acute - minutes to days
• Long-term or delayed - years to decades
• Demonstrable Effects Period
• Cell mitosis stops, chromosome breakage, motility
  changes..
• Recovery Period - typically after acute exposure
• Delayed effects can result from incomplete
  recovery

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Dose-Response Curve
No Threshold
Threshold

• No threshold used to establish radiation exposure
  guidance

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Determinants of Effect

• Rate of Absorption - most important in
  determining effect
• Area Exposed - the larger the area exposed the
  more extensive the damage
• Species and/or Individual Sensitivity -
• Plants, microorganisms more resistant
• Variation within species leads to a distribution
  of effects ( LD50 for humans is 450 R)
• Cell Sensitivity - rapidly dividing cells most
  sensitive

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Acute Dose - Radiation Syndrome

• Exposure to a large dose in a short amount of
  time produces a series of effects called
  Radiation Syndrome
• Prodrome - nausea, vomiting and malaise
• Latent Stage - analogous to the incubation period
  of a viral disease
• Manifest Illness Stage - Fever, infection,
  hemorrhage, severe diarrhea
• Recovery or death - depending on dose

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Acute Dose - Continued

• Dose targets specific cells - hematopoietic,
  gastrointestinal and cerebral
• Dose - effect
• lt 0.5 Gy - no indications
• 1 Gy - small show blood changes
• 2 Gy - injury evident, sensitive will perish
• 4.5 Gy - LD50
• 6 Gy - Gastrointestinal form - prognosis not good
• 8 - 10 Gy - nearly complete mortality

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Delayed Effects

• Manifest years to decades after exposure
• May occur from both acute and chronic exposure
• No one characteristic illness
• Cancer
• Cataracts
• Embryological effects
• Lifespan shortening
• Genetic mutations

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Delayed Effects

• Human Evidence
• Bone cancer - radium ingestion
• Lung cancer - uranium dust inhalation
• Leukemia - irradiated children, fetuses, A-bomb
  survivors and physicians
• Thyroid cancer - A-bomb survivors and irradiated
  children (x-rays)
• Skin cancer - physicians and dentists
• Cataracts - A-bomb survivors physicists
• Genetic effects - damage to sperm or egg cells
  (no direct evidence that it occurs in humans)

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Radiation Standards

• Nuclear Regulatory Commission (NRC) standards in
  10 CFR 20 for two distinct populations
• Occupational dose (for nuclear industry workers)
  - 0.05 Sv/yr
• Assumes healthy, knowledgeable adult at work 40
  h/wk
• Dose to the public - 0.001 Sv/yr
• No assumptions
• Excluded therapeutic and diagnostic doses

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Radiation Standards

• NRC discharge limits for radionuclides in air and
  water
• Specific for each radionuclide
• Account for natural background levels
• EPA established exposure limits for radon in
  indoor air of 4 pCi/L