BMS1031 Lecture 12c&d: Biological Effects of Ionizing Radiation

Biological Effects of Ionizing Radiation

Learning Objectives

  • Understand that radioactive decay is a stochastic process.
  • Understand the common side-effects of exposure to ionizing radiation.
  • Explain the common deterministic and stochastic effects of ionizing radiation.
  • Understand how α\alpha, β\beta, γ\gamma, and neutron particles interact with matter.
  • Be familiar with the origins and magnitudes of background radiation.
  • Understand how dosimetry uses the concepts of absorbed dose, equivalent dose, and effective dose to quantify the effects of radiation exposure.
  • Be able to use the three principles of radiation protection to minimize the hazards associated with ionizing radiation.

Physiological Effects of Radiation

  • Dependent on the effective dose of radiation.
  • One Sievert = 5.5% chance of developing cancer.
Health Effects
  • Up to 10 mSv: No direct evidence of human health effects.
  • 10 - 1,000 mSv: No early effects; increased incidence of certain cancers in exposed populations at higher doses.
  • 1,000 - 10,000 mSv: Radiation sickness (risk of death); increased incidence of certain cancers in exposed populations.
  • Above 10,000 mSv: Fatal.
Health Risks Arising from Low Doses
  • Risk of cancer from 1 mSv of radiation: 1 in 17,000
  • Normal Incidence: 57 in 17,000
  • Risk of severe hereditary effect from 1 mSv of radiation: 1 in 77,000
  • Normal Incidence: 1,770 in 77,000
  • The risk of obtaining cancer from 1 mSv of radiation exposure is equivalent to the risk of getting cancer from smoking approximately 100 cigarettes.

Personal Radiation Monitor

  • Thermoluminescent dosimetry (TLD).
  • Radiation knocks electrons from the material.
  • Electrons are trapped in the material impurities.
  • Released after heating up the dosimeter - produces light.

Radiation Damage

  • Direct and indirect damage.
  • Production of free radicals by ionizing water in the body.
  • Cell damage.
  • DNA damage.
Mechanisms of Cell Damage
  • Cell death (apoptosis).
  • Bases chemically modified.
  • Breakage to one or both of sugar-phosphate backbones.
  • Hydrogen bonds broken.
  • Cells responsible for immunity (lymphocytes) decrease.
  • Cells with damaged DNA either die (cell death) or attempt to repair the damage (repair).
  • If the DNA damage is misrepaired, a cell may develop into a cancer depending on the site of DNA damage.
  • Recovery is quantitatively incomplete, and lymphocytes with impaired functions form (accelerated aging).
  • These lymphocytes are less able to respond to new antigens and develop and maintain memory.
  • As a result, resistance to such pathogens as viruses and germs decreases, resulting in persistent inflammation.

Types of Effects: Deterministic

  • Cause and effect relationship between ionizing radiation and the known side-effects.
  • Threshold below which no effects occur.
  • Varies between people.
  • Above threshold severity increases with dose.
  • Common side effects:
    • Skin erythema
    • Hair loss
    • Sterility
    • Cataracts
    • Acute radiation sickness
    • Chronic radiation sickness

Types of Effects: Stochastic

  • Occur by chance and without a threshold level.
  • Severity can be independent of dose (risk is not independent).
    • Cancer
    • Induced hereditary effects

Interactions with Matter

  • The probability of an interaction occurring depends on the mass and the charge of the incident particle.
Alpha ($\alpha$) Radiation
  • Heavy particle (∼10−27\sim 10^{-27} kg) with large charge (2e).
  • High probability of interactions
    • Both mechanical collisions and Coulomb interaction.
  • Typical energy of a particle: ~35 MeV.
  • Energy per interaction ~ 30-100 eV.
  • Lots of atoms ionized in short distance (volume).
  • Result: Large amount of damage on short distance scale.
Beta ($\beta^-$ and \beta^+$) Radiation
  • Light particles (\sim 10^{-30}kg),smallercharge(+/−1e).</li><li>Coulombinteractionbetweenkg), smaller charge (+/- 1e).</li> <li>Coulomb interaction between\beta^-particleandelectronsinthematerialdominates.</li><li>WeakerCoulombforce(cfparticle and electrons in the material dominates.</li> <li>Weaker Coulomb force (cf\alpha)−lowerprobabilityofinteractions(collisions)sotheparticletravelsfurther.</li><li>BrakingradiationmayalsoproduceX−rays.</li><li>SecondaryelectronsmayalsoproduceX−rays.</li><li>Result:Muchlessdamage(cf) - lower probability of interactions (collisions) so the particle travels further.</li> <li>Braking radiation may also produce X-rays.</li> <li>Secondary electrons may also produce X-rays.</li> <li>Result: Much less damage (cf\alpha),mediumpenetrationdepth,heating(burns)ofmatter.</li></ul><h5id="ddbetaddradiation">), medium penetration depth, heating (burns) of matter.</li> </ul> <h5 id="ddbetaddradiation">\beta^+ Radiation
    • An anti-particle (positron) and particle (electron) collide and annihilate one another.
    Neutrons
    • Heavy but neutral.
    • Interact only with nuclei.
    • Can eject proton from hydrogen atom.
    • Can eject light nuclei from atoms — creates ions and electrons ($\beta$).
    • Slow neutrons can be absorbed by nuclei.
    • Photon is released and transmutation to radioactive nuclei.
    • Short lifetime - decay to proton, electron, and anti-neutrino.
    • Result: Very low probability of interactions, usually long range (except water).
    Photons: \gamma and X-rays
    • Photoelectric effect – inner shell electrons < 30 keV
    • Pair production (>1.02 MeV)
    • Compton scattering – outer shell electrons > 30 keV

    Summary of Radiation Types

    TypeNature of RadiationPenetrating PowerIonizing Power - Ability to Remove Electrons from Atoms
    Alpha2 neutrons & 2 protonsLowVery High
    Charge = +2Stopped by a few cm of air, or a sheet of paper
    BetaElectron/positronModerateModerate
    Charge = +/-1Stopped by a few mm of metal. Eg. Aluminum
    GammaPhotonsVery HighLow
    Charge = 0 (no charge)Stopped by a thick layer of concrete or lead.no energy)

    Natural and Man-Made Sources of Radiation

    Natural sources
    • Terrestrial – isotopes of Thorium, Uranium, Polonium, Radium etc.
    • Cosmic radiation – produces radioactive isotopes, e.g. 14C used in carbon dating.
    Man made sources:
    • Induced radioactivity and artificial isotopes, e.g., nuclear reactors, cyclotrons, radionuclide generators, medical imaging, fallout.

    Dosimetry

    • Dosimetry is the measurement of the amount of radiation dose and its effect.
    • Dose depends on:
      • Amount of energy absorbed by the body.
      • Type of radiation.
      • How tissues react to the radiation.
    Absorbed Dose
    • Energy of ionizing radiation deposited in 1kg of material.
    • Unit (SI): Gray.
    • 1 Gy = 1 J/kg
    • Old unit: Rad.
    • 1 Gy = 100 rad
    Equivalent Dose
    • Equivalent Dose = Absorbed Dose x {w_R}</li><li>Takesthetypeofradiationintoaccount.</li><li>Unit(SI):Sieverts(Sv).</li><li>Oldunit:rem(Roentgenequivalentman).</li><li>1Sv=100rem</li></ul><h5id="effectivedose">EffectiveDose</h5><ul><li>EffectiveDose=EquivalentDosex</li> <li>Takes the type of radiation into account.</li> <li>Unit (SI): Sieverts (Sv).</li> <li>Old unit: rem (Roentgen equivalent man).</li> <li>1 Sv = 100 rem</li> </ul> <h5 id="effectivedose">Effective Dose</h5> <ul> <li>Effective Dose = Equivalent Dose x{w_T}
    • Takes the biological effect of radiation on different organs or tissues into account.
    • Unit (SI): Sieverts (Sv)

    Average Yearly Radiation Exposure in Australia

    • Melbourne average: 2.2 mSv/yr
    • World: 2.4 mSv/yr
      • Cosmic (0.3 mSv)
      • Terrestrial (0.6 mSv)
      • Radon and progeny (0.2 mSv)
      • Potassium-40 in the body (0.2 mSv)
      • Uranium/Thorium in the body (0.2 mSv)
      • Atmospheric weapons testing (<0.005 mSv)
      • Medical (1.7 mSv)

    Typical Radiation Doses

    • Natural Radiation (Terrestrial and Airborne): 1.2 mSv per year
    • Natural Radiation (Cosmic radiation at sea level): 0.3 mSv per year
    • Total Natural Radiation: 1.5 mSv per year
    • Seven Hour Aeroplane Flight: 0.05 mSv
    • Chest X-Ray: 0.04 mSv
    • Nuclear Fallout (From atmospheric tests in 50's & 60's): 0.02 mSv per Year
    • Chernobyl (People living in Control Zones near Chernobyl): 10 mSv per year
    • Cosmic Radiation Exposure of Domestic Airline Pilot: 2 mSv per year

    Use of Radiation: Risk Benefit

    • Legal and ethical responsibilities require the lowest possible doses to be used: ALARA = As Low As Reasonably Achievable.
    • Radiation workers are monitored for both short interval dose rates, as well as total integrated yearly dose
      • Radiation workers <20mSv/yr (avg = 0.12mSv/yr)
      • General public < 1 mSv/yr
      • Natural background level ~1.5mSv/yr
      • Monash has an action limit of 10% of the above International Commission on Radiation Protection (ICRP) limits.

    Radiation Protection Principles

    • Shielding
    • Distance
    • Time
    X-ray Attenuation
    • Shielding removes (absorbs) a fraction of the incident X-rays: \Delta N = -\mu N_0 \Delta t<ul><li>where<ul> <li>where\muisthelinearattenuationcoefficient(units:cm−1)</li></ul></li><li>is the linear attenuation coefficient (units: cm-1)</li></ul></li> <li>\muisfixedforagivenmaterialcomposition(density)andX−rayenergy</li><li>Conventionallyweuseintensity,thenumberofphotonsperunitareaperunittimeinsteadofN:is fixed for a given material composition (density) and X-ray energy</li> <li>Conventionally we use intensity, the number of photons per unit area per unit time instead of N:\Delta I = -\mu I_0 \Delta t</li><li>Thenthefinalintensityisgivenas:</li> <li>Then the final intensity is given as:I(t) = I_0exp(-\mu t)</li></ul><h5id="distance">Distance</h5><ul><li>Theintensityofradiationfollowsaninversesquarelaw:</li> </ul> <h5 id="distance">Distance</h5> <ul> <li>The intensity of radiation follows an inverse square law:I = \frac{I_0}{r^2}$$