DrX %20(1).png)
DrX Whiz Niraj
Radiopharmaceuticals: Radioactivity & Isotopes
Comprehensive exam notes on Radioactivity, Properties of Alpha/Beta/Gamma rays, Half-life, Sodium Iodide I-131, and Safe Handling of radioactive substances.
Radioactivity is the spontaneous emission of particles or electromagnetic radiation from the unstable nucleus of an atom. The elements that exhibit this property are called Radioactive Isotopes (Radioisotopes).
The ultimate goal of radioactive decay is to reach a stable nuclear configuration.
Measurement of Radioactivity
Radioactivity is not measured by weight or volume, but by the Rate of Decay (disintegrations per second).
Units:
1. Curie (Ci): 1 Curie = 3.7 × 10¹⁰ disintegrations per second (dps).
2. Becquerel (Bq): 1 Bq = 1 disintegration per second (SI Unit).
Instruments Used: Geiger-Müller (GM) Counter, Proportional counter, and Scintillation counter.
When a radioactive nucleus decays, it typically emits three types of radiations. Their penetrating power and ionizing power are fundamentally different.
| Property | Alpha (α) Particles | Beta (β) Particles | Gamma (γ) Rays |
|---|---|---|---|
| Nature | Helium nucleus (2 protons, 2 neutrons) | High-speed electron | Electromagnetic radiation (like X-rays) |
| Charge | Positive (+2) | Negative (-1) | Neutral (No charge) |
| Penetrating Power | Lowest (Stopped by a sheet of paper) | Moderate (Stopped by aluminum/wood) | Highest (Stopped by thick lead/concrete) |
| Ionizing Power | Highest (Causes maximum tissue damage if swallowed) | Moderate | Lowest |
Definition: The time required for a radioactive substance to decay to exactly half of its original initial amount. Radioactive decay follows First-Order Kinetics.
Where λ (lambda) is the decay constant.
Iodine-131 is a highly important radioisotope used extensively in the diagnosis and treatment of thyroid gland disorders.
Properties
- It emits both Beta (β) particles and Gamma (γ) rays.
- Half-life: Exactly 8.04 days.
- Appearance: Clear, colorless solution (in water). It tends to oxidize to free iodine when exposed to light, turning pale brown.
Applications
- Diagnostic: Used in minute doses to test thyroid function (Thyroid uptake test) by tracing gamma emissions.
- Therapeutic: Used in large doses to treat Hyperthyroidism (Thyrotoxicosis) and Thyroid cancer. The emitted Beta particles selectively destroy overactive thyroid cells.
Handling radioactive materials requires extreme caution to prevent radiation exposure to the pharmacist, patient, and environment.
The ALARA Principle
ALARA stands for "As Low As Reasonably Achievable". The exposure to radiation must be minimized using three main rules:
- 1. Time: Minimize the time spent near the radioactive source.
- 2. Distance: Maximize the distance from the source (use long-handled tongs/forceps to pick up vials).
- 3. Shielding: Use appropriate protective shields. Gamma emitters require thick Lead (Pb) shielding, while Beta emitters require thick glass or plastic.
Storage Conditions:
- Store in an area exclusively designated for radioactive materials.
- Must be kept in Lead-lined containers (Lead pigs).
- Area must have a prominent "Radioactive Material" warning sign.
- Radioactive waste must be stored in shielded containers until its activity decays to background levels before disposal.
Clinical Alert: Radiation Sickness
Excessive accidental exposure to ionizing radiation causes severe cellular damage, especially to rapidly dividing cells (like bone marrow and intestinal lining). This results in Radiation Sickness, characterized by severe nausea, vomiting, hair loss, destroyed immunity (leukopenia), and an increased risk of long-term genetic mutations and cancer.