Atomic Interactions and Generation of Radiation

Atomic Dimensions and Structure

Imagine scaling an atom up to the size of a football stadium; the nucleus would be just a single marble sitting in the centre.

• The radius of an atom is approximately $1 \times 10^{-10}$ metres.
• The nucleus is incredibly small in comparison, measuring less than 1/10,000 of the atomic radius (approximately $1 \times 10^{-15}$ metres).
• Almost all the mass of an atom is concentrated in this central nucleus, while the orbiting electrons have a negligible mass.
• Electrons orbit the nucleus at specific, fixed distances called energy levels (or shells).

Absorption and Emission of Radiation

Electrons can jump between energy levels like climbing rungs on a ladder, but they must gain or lose exactly the right amount of energy to make the leap.

• Absorption: When an atom takes in electromagnetic (EM) radiation, an electron gains energy and undergoes excitation, moving to a higher energy level further from the nucleus.
• Emission: When an atom releases EM radiation, an electron undergoes de-excitation, losing energy and dropping to a lower energy level closer to the nucleus.
• The energy of the absorbed or emitted radiation corresponds exactly to the difference in energy between the two levels.

The relationship between energy and frequency is shown by the equation:

$$\Delta E = hf$$

Where $\Delta E$ is the energy change, $h$ is Planck's constant, and $f$ is the frequency. A large electron energy drop produces a high-frequency photon (such as an X-ray), while a small drop produces a low-frequency photon (such as visible light).

Where Does Radiation Come From?

Not all radiation is born in the same place; some types come from the deep core of the atom, while others are generated in the outer electron clouds.

• Gamma rays: These are generated by nuclear rearrangements. When a nucleus has excess energy (an excited state), particles shift into a more stable configuration and release a high-energy gamma photon. This involves no change to the mass or atomic number.
• X-rays, Ultraviolet (UV), and visible light: These are generated entirely by electron transitions when electrons lose energy and drop to lower energy levels, completely independent of the nucleus.

A classic example of a nuclear rearrangement equation is:

$${^{60}_{27}Co^*} \rightarrow {^{60}_{27}Co} + {^0_0\gamma}$$

(Note: The asterisk denotes the nucleus is in an excited state. Emitting a gamma ray allows it to lose energy and become stable without changing its particles).

Additionally, UV radiation interacts with our atmosphere. It is absorbed by oxygen to produce ozone ($O_3$). The resulting ozone layer then absorbs further incoming UV radiation, protecting life on Earth.

Ionisation by High-Energy Radiation

If an electron absorbs enough energy, it does not just jump to a higher energy level—it breaks free from the atom entirely.

• Ionising radiation includes high-energy EM waves at the high-frequency end of the spectrum, specifically gamma rays, X-rays, and high-energy ultraviolet.
• Ionisation occurs when an atom becomes a positively charged ion by losing one or more outer electrons.
• The radiation transfers enough energy to an outer electron to overcome the electrostatic attraction of the nucleus, effectively knocking it out of the atom.

The risk of harm to the human body from this process depends on both the type of radiation and the size of the dose. Radiation dose is measured in Sieverts (Sv) or millisieverts (mSv).