Radioactivity

Study Summary: Lecture 8 - Radioactivity

1. Introduction to Radioactivity

Radioactivity is the disintegration or decay of unstable atoms, accompanied by the emission of radiation.

• Unstable atoms: Elements like Uranium and Radium are considered radioactive due to their unstable nature.
• Radiations: The emitted particles/rays are primarily Alpha (α) particles, Beta (β) particles, and Gamma (γ) rays.

2. Properties of Radiation

The table below summarizes the key properties of alpha, beta, and gamma radiations:

3. Units of Radioactivity

The following table lists common units of radioactivity and their definitions:

• Quality factor:
  • α ray is 20
  • β ray is 1
  • γ ray is 1
• The quality factor indicates how dangerous a particular radiation is; a higher value signifies more dangerous radiation.
• It is a dimensionless quantity (i.e., it has no unit).

4. Nuclear Reaction

• Nuclear reaction involves nuclear changes within atoms.
• It results in changes in the numbers of protons or neutrons, leading to the formation of different atoms.
• This is distinct from chemical reactions, which involve the donation, acceptance, or sharing of extra-nuclear electrons.

5. Examples of Nuclear Reactions

In a nuclear reaction, the sum of the mass number and the atomic number must be conserved on both sides of the equation.

• 2713Al + 42He → 3015P + 10n       (summary: 2713Al (α,n) 3015P)
• 147N + 42He → 178O + 11p       (summary: 147N (α,p) 178O)
• 23892U + 10n → 23992U + γ       (summary: 23892U (n,γ) 23992U)
• 147N + 10n → 146C + 11p       (summary: 147N (n,p) 146C)
• 105B + 42He → 137N + 10n       (summary: 105B (α,n) 137N)

6. Detection and Measurement of Radiation

Radiation can be detected and measured using various equipment:

• Equipment examples: Spinthariscopes, scintillation counters, Geiger-Muller counters, cloud chambers, and bubble chambers.
• These devices generally operate by detecting and measuring ions created by α, β, and γ radiations present in chambers filled with liquid or a particular kind of gas (such as air, hydrogen, argon, etc.).

7. Half-life

• The half-life of a radioactive isotope is the time it takes for half of its radioactivity to decay.
• Each radioactive isotope possesses a unique characteristic half-life. For instance:
  • Sodium-24 (24Na): 15 hours
  • Carbon-14 (14C): 5.73 × 103 years
  • Uranium-238 (238U): 4.47 × 109 years
• The value of half-life is independent of the amount of the isotope. Whether you have 1g or 1000g of 24Na, its half-life remains 15 hours.

8. Radioactive Decay Law

Every radioactive decay follows the equation:

Nt = N0 × exp(-λt)   or   Nt = N0 × e-λt

Where:

• Nt = count at time t
• N0 = count at time 0 (initial count)
• λ = decay constant
• t = time

The relationship between half-life (t1/2) and the decay constant (λ) is given by:

t1/2 = 0.693 / λ

9. Types of Radioactive Decay

There are two primary types of radioactive decay:

• Alpha (α) decay
• Beta (β) decay

In radioactive decay, the original nucleus is called the parent nucleus, and the resulting nucleus is called the daughter nucleus.

9.1. Alpha (α) Decay

• Alpha decay occurs when a parent nucleus transforms into a daughter nucleus by emitting an alpha particle (42α, which is a helium nucleus).
• Examples:
  • 22688Ra (parent) → 22286Rn (daughter) + 42α
  • 23492U (parent) → 23090Th (daughter) + 42α
• In α decay, the atomic mass (mass number) of the parent nucleus is reduced by 4, and the atomic number is reduced by 2 in the daughter nucleus.

9.2. Beta (β) Decay

• Beta decay occurs when a parent nucleus transforms into a daughter nucleus by emitting a beta particle (0-1β, which is an electron).
• Examples:
  • 21482Pb (parent) → 21483Bi (daughter) + 0-1β
  • 23490Th (parent) → 23491Pa (daughter) + 0-1β
• In β decay, the atomic number of the parent nucleus is increased by 1, while the mass number remains unchanged in the daughter nucleus.

10. Applications of Radioactivity

Radioactivity has various important applications:

• Detection of metal fatigue: Useful in aircraft design and maintenance.
• Food preservation: Used to extend the shelf life of food by killing microorganisms.
• Medicine: Employed in cancer treatment (radiotherapy) and diagnostic imaging.
• Radiocarbon dating: Used to determine the age of ancient artifacts and geological formations.
• Preparation of artificial elements: For example:

  24796Cm + 42H → 25098Cf + 10n (artificial element Californium)