Radioactivity and Nuclear Reactions

Radioactivity

Nuclear process involving only the nucleus.
Spontaneous emission of radiation from an unstable nucleus.
Emitted radiations include α, β, and γ particles.
Nucleons (protons and neutrons) are the particles inside the nucleus.

Alpha Particles (α)

Resemble a helium nucleus (2 protons and 2 neutrons).
Carry a +2e charge, where e = 1.6 × 10^{-19} C.
Represented in decay equations: _{92}^{236}X \rightarrow _{90}^{232}Y + \alpha + Energy
Range in air: up to 5 cm.
Stopped by a paper sheet.
During α-decay:
- Proton number decreases by 2.
- Neutron number decreases by 2.
- Nucleon number decreases by 4.

Random and Spontaneous Nature of Radiation

Radioactivity is random; decay can be shown in a graph.
Spontaneous: Emission is independent of environmental conditions (temperature, pressure).

Ionizing Power of Radiations

α-particles: Greatest ionizing power.
β-particles: Moderate ionizing power.
γ-radiations: Least ionizing power.

Beta Particles (β)

Resemble an electron.
Carry a -1e charge, where e = 1.6 × 10^{-19} C.
Can be represented as _{-1}^0\beta or _{-1}^0e
Smaller than α-particles.
Moderate penetrating power; pass through paper but are stopped by aluminum.
β-particles have a range of kinetic energies and speeds (up to 99% of c).

Beta Decay

When a β-particle is emitted:
- Proton number increases by 1.
- Neutron number decreases by 1.
- Nucleon/mass number remains unchanged.
Decay equation: _{39}^{100}X \rightarrow _{40}^{100}Y + _{-1}^0\beta + Energy
Graphically, there's a horizontal change towards the right.

Gamma Rays (γ)

Charge less and massless electromagnetic radiation.
Not deflected by electric or magnetic fields.
Least ionizing power.
High-frequency γ-rays are stopped by a block of lead.
Low-frequency γ-rays are blocked by thick aluminum or thin lead sheets.
Follow wave properties.

Energy of Radiations

Measured in electron volts (eV) or mega-electron volts (MeV).
Conversion: 1 eV = 1.6 × 10^{-19} J, 1 MeV = 1.6 × 10^{-13} J
Example calculation: Calculating the number of ions created by an α-particle with a given kinetic energy.

Geiger-Marsden Experiment (Gold Foil Experiment)

Most α-particles detected in the direct path (0 degrees).
Few particles detected when the GM tube was moved to an angle.
As the angle increased, the count rate decreased.

Observations and Conclusions

Most α-particles passed straight through the gold foil, suggesting that most of the atom consists of empty space.
Some α-particles deflected more than 90 degrees, suggesting a dense, positively charged nucleus.
Some α-particles deflected less than 90 degrees, corresponding to α-particles passing near the nucleus.

Nuclear Reactions

Reactions involving the nucleus, including fission and fusion.
Gold is used as foil because its inert, malleable and has high Ar (relative atomic mass).

Nuclear Fission

A large, unstable nucleus splits into two or more fragments when bombarded with a neutron.
Large amount of energy is evolved.
Example: _{92}^{235}U + _0^1n \rightarrow _{56}^{141}Ba + _{36}^{92}Kr + 3_0^1n + Energy
Nucleon number and atomic number remain conserved.

Nuclear Fusion

Two lighter nuclei combine to form a heavier nucleus.
A large amount of energy is released; requires initial energy to start.
Example: _{1}^{2}H + _{1}^{3}H \rightarrow _{2}^{4}He + _0^1n + Energy
Mass is not conserved alone; some mass is converted into energy.

Naturally Occurring vs. Artificially Triggered Nuclear Reactions

Naturally occurring reactions do not require external energy.
Artificially triggered reactions require external energy to start.
Reactants are less stable and have more mass-energy than products in artificially triggered reactions that release energy.

Supplying Energy to Reactants

In reactions requiring energy, supplying energy to the smaller particle makes it easier to accelerate the reaction.