1. Radiation Basics
Understanding the fundamental principles of radioactivity and nuclear decay is essential for nuclear medicine practice.
Learning Objectives
By the end of this chapter, you will be able to:
- Describe atomic structure and nuclear stability
- Explain different types of radioactive decay
- Calculate radioactive decay and half-life
- Understand decay schemes and branching ratios
Atomic Structure
The Nucleus
The nucleus consists of:
- Protons (Z): Positively charged particles; determines the element
- Neutrons (N): Neutral particles; contributes to nuclear stability
- Mass number (A): Total nucleons (A = Z + N)
Notation: Nuclides are written as ^A_Z X, e.g., ^99m_43 Tc (Technetium-99m with 43 protons and 56 neutrons)
Nuclear Stability
Nuclei are stable when they have an optimal neutron-to-proton ratio:
- Light elements: N/Z ≈ 1
- Heavy elements: N/Z ≈ 1.5
Unstable nuclei undergo radioactive decay to achieve stability.
Types of Radioactive Decay
Alpha (α) Decay
- Emission of helium nucleus (2 protons + 2 neutrons)
- Z decreases by 2, A decreases by 4
- Example: Ra-226 → Rn-222 + α
- Clinical relevance: Ra-223 (Xofigo) for bone metastases
Beta-Minus (β⁻) Decay
- Neutron converts to proton with emission of electron and antineutrino
- Z increases by 1, A unchanged
- Example: I-131 → Xe-131 + β⁻
- Clinical relevance: I-131 therapy, Y-90 microspheres
Beta-Plus (β⁺) Decay
- Proton converts to neutron with emission of positron and neutrino
- Z decreases by 1, A unchanged
- Positron annihilates with electron → two 511 keV photons
- Clinical relevance: F-18, Ga-68 for PET imaging
Electron Capture (EC)
- Inner orbital electron captured by nucleus
- Proton converts to neutron
- Characteristic X-rays emitted
- Example: I-123 decays primarily by EC
Isomeric Transition (IT)
- Metastable nuclear state releases energy as gamma ray
- No change in Z or A
- Example: Tc-99m → Tc-99 + γ (140 keV)
Key Point: Tc-99m decays by isomeric transition, making it ideal for imaging due to the pure gamma emission without particulate radiation.
Radioactive Decay Mathematics
Activity
Activity (A) = rate of decay = λN
Where:
- λ = decay constant (ln2 / t₁/₂)
- N = number of radioactive atoms
Units:
- Becquerel (Bq) = 1 disintegration per second
- Curie (Ci) = 3.7 × 10¹⁰ Bq
Decay Equation
Half-Life Calculations
| Radionuclide | Half-life | Primary Use |
|---|---|---|
| Tc-99m | 6.0 hours | General SPECT imaging |
| I-123 | 13.2 hours | Thyroid imaging |
| I-131 | 8.0 days | Thyroid therapy |
| F-18 | 110 minutes | PET imaging |
| Ga-68 | 68 minutes | PET imaging |
Practice Problem
Question: A Tc-99m dose is calibrated at 30 mCi at 7:00 AM. What is the activity at 1:00 PM?
Solution:
- Time elapsed = 6 hours = 1 half-life
- Activity = 30 mCi × (1/2)¹ = 15 mCi
Summary
Key Takeaways:
- Radioactive decay occurs when unstable nuclei seek stability
- Different decay modes emit different types of radiation
- Half-life is the time for activity to decrease by half
- Tc-99m (IT decay) is ideal for imaging; I-131 (β⁻ decay) for therapy
Content coming soon: Decay schemes, secular and transient equilibrium, production methods