MAP.17 Radiation safety and dosimetry

Introduction to Radiation Safety and Dosimetry

• Presenter: Dr. Andy Ma

Learning Outcomes

• Discuss the interactions of decay products with biological tissue.

• Differentiate between:

  • Photoelectric effect

  • Compton effect

  • Pair production

  • Pair annihilation

• Demonstrate the role of the radioactive decay equation in calculations of half-life.

• Differentiate between:

  • Activity

  • Becquerel and Curie

  • Absorbed dose

  • Quality factor

• Differentiate between equivalent dose and effective dose.

• Describe the physiological effect of radiation.

• Restate commonly used maximum permissible doses.

• Discuss environmental radioactivity.

Ionizing Radiation & Its Interaction with Matter

• Types of Radiation:

  • Ionizing Radiation: Has sufficient energy to free electrons within tissue.

  • Common types include:

    • Alpha (α) particles

    • Beta (β) particles

    • X-rays

    • Gamma (γ) rays

• Ionizing radiation can leave secondary ionization trails, potentially disrupting sensitive biological systems.

Categories of Radiation in Biological and Medical Sciences

• Positive ions (e.g., α-particles)

• Electrons (e.g., β-particles)

• Photons (e.g., X-rays and γ-rays)

• Neutrons

• All radiation types can cause biological damage through ionizing effects.

Characteristics of Alpha Particles

• α-Particles:

  • Short range in matter (~1/density).

  • A 5 MeV α-particle travels ~4 cm in air but can't penetrate paper.

  • High ionization efficiency due to larger mass and double charge.

• Energy deposition: ~100 keV/mm in tissue.

Characteristics of Beta Particles

• β-Particles:

  • Energies range from a few keV to ~1 MeV.

  • Much smaller mass than α-particles leading to greater speed.

• Energy loss is less efficient, approximately 0.25 keV/mm.

• Range in tissue is larger (e.g., a 1 MeV β-particle can penetrate about 4 mm).

Characteristics of Photons (X-rays and Gamma Rays)

• Photons:

  • Originates from nuclear processes (γ-rays) vs. atomic processes (X-rays).

  • Photon energy is transferred to electrons, causing secondary ionization.

  • Penetration depth is greater than α and β-particles.

• Primary interactions include:

  • Photoelectric effect (<0.1 MeV)

  • Compton scattering (up to 1 MeV)

  • Pair production (>1.02 MeV)

Characteristics of Neutrons

• Neutrons are uncharged and do not cause ionization directly.

• Interact primarily with atomic nuclei, causing recoil, which can lead to interactions with electrons.

• At low energy (~1 eV), neutrons are likely to be captured by nuclei.

Radiation Penetration

• Penetration depth varies by the type of radiation:

  • α-particles: stopped by a few cm of air or paper.

  • β-particles: can traverse a few meters in air and several mm in aluminum.

  • γ-rays: can penetrate several cm of high-density materials like lead.

Radioactive Half-Life

• Defined as the period it takes for half of a radioactive sample to decay.

• Example: 131I has a half-life of 8 days.

• Variability: Half-lives can range from seconds to billions of years.

Biological Half-Life

• The biological half-life indicates how long it takes for half of a substance to be excreted from the body.

• Effective Half-Life combines both radioactive and biological half-lives using the formula:

  [ T_{Effective} = \frac{1}{ \frac{1}{T_{1/2\ (radioactive)}} + \frac{1}{T_{1/2\ (biological)}}} ]

• Example calculation for 131I demonstrates effective half-life.

Radioactive Decay Rate

• The radioactive decay rate (l) indicates the decay frequency of unstable nuclei:

  • Equation: ( A = lN ) where ( A ) is activity, ( N ) is number of undecayed nuclei.

• Decreases exponentially: ( N = N_0 e^{-lt} )

• Half-life relates to decay constant: ( T_{1/2} = \frac{0.693}{l} )

Radiation Exposure and Biological Effects

• Hazard from ionization: can cause cellular damage, mutations, and affect reproduction.

• Absorbed Dose (RAD): energy absorbed per unit mass.

• Measurement: 1 rad = 0.01 J/kg.

Radiation Weighting Factor (wR)

• wR accounts for different biological effects from various radiation types.

• Example values:

  • X-rays and γ-rays: 1

  • β-particles: 1

  • Protons (1-10 MeV): 2

  • α-particles: 20

Equivalent Dose Calculation

• Equivalent Dose = Absorbed Dose × wR

• Unit: Sievert (Sv), with 1 Sv = 100 rem

Effective Dose

• Effective Dose incorporates tissue weighting factors (wT) for different organs:

• Example calculation for lungs, liver, and bones demonstrated a total Effective Dose of 18.5 mSv.

Maximum Permissible Dose (MPD) Guidelines

• Public: 1 mSv/year

• Radiation workers: 20 mSv/year

• NOTE: Aim to keep exposure ALARA (As Low As Reasonably Achievable).

Linear-Non-Threshold Theory (LNT)

• ICRP recommends a linear relationship for risk at low radiation doses.

• Risk extrapolation is based on higher dose data.

Clinical Application Exposures

• Average doses:

  • Chest X-ray: ~0.017 mSv

  • CT scan: ~8 mSv

  • Diagnostic (Tc99): 1 - 7 mSv

Risks from Radiation Exposure

• Average world risk factor: 0.05 Sv-1.

  • 1 Sv dose: 5% cancer risk.

  • Reduced to 1 mSv: 0.005% risk.

Immediate Effects of Radiation Exposure

• Doses and effects:

  • 0-10 rem: No observable effects

  • 10-100 rem: Slight decreases in white blood cell counts

  • 100-200 rem: Nausea, risk of cancer

  • 200-500 rem: Severe symptoms

  • 2000 rem: Likely fatal.

Natural Radiation Exposure

• Average exposures from natural sources:

  • Cosmic radiation: ~1 mSv/year

  • Radon: significant contributor.

Conclusion

• Understanding radiation safety is crucial for healthcare and industry workers to mitigate risks.