Nuclear Energy: Effects and Uses of Radiation (Dosimetry, Biological Damage, and Medical Applications)

Passage of Radiation Through Matter: Biological Damage

Ionizing Radiation and Its Effects

Ionizing radiation refers to particles or electromagnetic waves energetic enough to remove electrons from atoms, thereby ionizing them. This process can cause significant damage to materials, especially biological tissues.

• Types of Ionizing Radiation: Includes alpha (α), beta (β), and gamma (γ) rays, X-rays, as well as particles such as protons, neutrons, and pions.

• Biological Damage: Ionization can disrupt molecular structures in cells, potentially leading to mutations, cancer, or cell death.

Measurement of Radiation — Dosimetry

Units and Concepts in Radiation Measurement

Measuring radiation is essential for assessing exposure risks and for medical applications. The amount of radiation is quantified in terms of activity, absorbed dose, and effective dose.

• Source Activity: The number of nuclear disintegrations per second.

• Units of Activity:

  • Curie (Ci): $1\ \text{Ci} = 3.70 \times 10^{10}$ decays per second

  • Becquerel (Bq): $1\ \text{Bq} = 1$ decay/s (SI unit)

• Absorbed Dose: The energy deposited by radiation per unit mass of material.

  • Rad: $1\ \text{rad} = 1.00 \times 10^{-2}\ \text{J/kg}$

  • Gray (Gy): $1\ \text{Gy} = 1\ \text{J/kg} = 100\ \text{rad}$ (SI unit)

• Effective Dose: Accounts for the type of radiation and its biological effect, using a quality factor (QF).

  • Rem: $\text{Effective dose in rem} = \text{dose in rad} \times \text{QF}$

  • Sievert (Sv): $\text{Effective dose in Sv} = \text{dose in Gy} \times \text{QF}$

Quality Factor Table

The quality factor (QF) reflects the relative biological effectiveness of different types of radiation:

Type	QF
X- and γ-rays	1
β (electrons)	≈ 1
Fast protons	≈ 3
Slow neutrons	≈ 3
Fast neutrons	Up to 10
α particles and heavy ions	Up to 20

Radiation Exposure Guidelines

• Natural background radiation: ~0.3 rem/year

• Maximum for radiation workers: 5 rem in any one year, or below 2 rem/year averaged over 5 years

• Acute exposure: 1000 rem is almost always fatal; 400 rem has about a 50% fatality rate

Example: Radioactivity Taken Up by Cells

In an experiment, $0.016\ \mu\text{Ci}$ of $^{32}_{15}\text{P}$ (half-life 14.26 days) is injected into a bacterial culture. After 1 hour, a 70% efficient detector records 720 counts per minute. The percentage of $^{32}_{15}\text{P}$ taken up by the cells is calculated as follows:

• $A_0 = 0.016 \times 10^{-6} \times 3.7 \times 10^{10} = 592\ \text{s}^{-1}$

• $A_{1h} = 720/60 = 12\ \text{s}^{-1}$

• Percentage taken up: $\% = 100 \times \frac{A_{1h}}{0.7 A_0} = 2.9\%$

Example: Whole-Body Dose Calculation

For a 70-kg worker exposed to a 40-mCi $^{60}_{27}\text{Co}$ source (1.5 m2 cross-section, 4 m distance, 4 h/day):

• Radiation received in 4 hours:

  • $(4 \times 10^{-2} \times 3.7 \times 10^{10}) \times (1.5/(4\pi 4^2)) \times (4 \times 3600) = 1.59 \times 10^{11}$

• Dose received:

  • $(1.59 \times 10^{11} \times 2.5 \times 10^6 \times 1.6 \times 10^{-19}) / (70 \times 2) = 4.5 \times 10^{-4}\ \text{Gy}$

Example: Radon Exposure

To estimate the mass of radon ($^{222}_{86}\text{Rn}$, half-life 3.8 days) that emits 4.0 pCi:

• $A = \lambda N$

• $N = A/\lambda = (4 \times 10^{-12} \times 3.7 \times 10^{10}) \times (3.8 \times 86400/\ln(2))$

• $N = 7.05 \times 10^4$

• $m = 222 N / N_A = 2.6 \times 10^{-17}\ \text{g}$

Radiation Therapy

Medical Use of Radiation

Radiation therapy is a common treatment for cancer. The goal is to destroy cancer cells while minimizing damage to healthy tissue.

• Technique: The radiation source is often rotated around the patient so that the tumor receives the maximum dose, while healthy tissues receive less exposure.

• Application: Used for various cancers, especially when tumors are localized.

Tracers in Research and Medicine

Diagnostic and Research Applications

Radioactive isotopes are used as tracers in medicine and research to study processes in living organisms and to diagnose diseases.

• Noninvasive Scans: Tracers can be used to detect abnormal concentrations in the body, such as tumors.

• Detection: Gamma-ray detectors (e.g., scintillation counters) are used to measure the radiation emitted by tracers.

• Example: Technetium-99m is commonly used in imaging the heart, bones, and other organs.

Summary of Key Points

• Radiation damage is measured using dosimetry, which quantifies the absorbed and effective dose.

• The biological effect of radiation depends on the type and energy of the radiation, as well as the tissue exposed.

• Radiation has important medical applications, including cancer therapy and diagnostic imaging using radioactive tracers.