4.3 - Alpha, Beta & Gamma Radiation

Radioactive decay and the types of nuclear radiation

• Some isotopes have unstable nuclei. An unstable nucleus tends to break down, or decay, releasing radiation as it changes into a more stable arrangement.
• This process is called radioactive decay, and a substance whose nuclei decay this way is radioactive.
• A nucleus is unstable when there's an imbalance between the numbers of protons and neutrons it contains (for example, too many neutrons), or when it is too heavy.
• The radiation comes from the nucleus itself.

The three types of radiation

1. Alpha particle (α) - Two protons and two neutrons, the same as a helium nucleus. Relative mass 4, relative charge +2. Comparatively large, heavy and slow-moving.
2. Beta particle (β) - A fast-moving electron created in, and thrown out of, the nucleus when a neutron turns into a proton. Virtually no mass and a relative charge of -1. Small and travels quickly.
3. Gamma ray (γ) - A high-frequency electromagnetic wave released by the nucleus. Carries away energy but has no mass and no charge.

Effect on the element

• Emitting an alpha or beta particle usually changes the nucleus into a different element.
• Gamma radiation carries only energy, so it leaves the element unchanged.

Neutron emission

• An unstable nucleus with too many neutrons can get rid of the excess by emitting a neutron (n).
• A neutron carries no charge.
• The number of protons doesn't change, so the nucleus stays the same element, but with a different mass number - making it a different isotope.
• Neutron radiation is ionising.

Ionising radiation and ionising power

• Ionising radiation knocks electrons off atoms it passes, turning those atoms into positively charged ions.
• Ionising power measures how readily a source does this.

What affects ionising power

How strongly a type of radiation ionises depends on its charge and its energy:

• Charge - A more highly charged particle attracts or repels an atom's electrons more strongly, so it's more likely to remove electrons.
• Energy - A particle carrying more energy can transfer more energy to an atom it collides with, making ionisation more likely.

Ionising power of each type

• Alpha - Strongly ionising. It carries much more mass and energy than beta and has the largest charge (+2), so it collides with and ionises many atoms over a short distance.
• Beta - Moderately ionising.
• Gamma - Weakly ionising. With no mass and no charge, it tends to pass straight through atoms rather than collide with them.

Penetration and range of radiation

The more strongly a type of radiation ionises, the more it collides with atoms, so the shorter its range and the more easily it's absorbed.

Range and what stops each type

• Alpha - Travels only a few centimetres in air and is stopped by a sheet of paper.
• Beta - Travels up to a few metres in air and is stopped by a thin sheet of aluminium (around 5 mm thick).
• Gamma - Travels a long distance through air and is only absorbed by thick lead or metres of concrete. Gamma is never completely stopped; thick lead and concrete reduce its intensity rather than blocking it entirely.

Uses of radiation

Alpha - Smoke detectors

• The alpha source ionises the air inside the detector, allowing a small current to flow.
• When smoke enters, it absorbs the alpha particles and stops the current, which triggers the alarm.

Beta - Monitoring metal thickness

• Beta penetrates further than alpha (which would be stopped completely) but not as far as gamma (which would pass through unaffected).
• The amount of beta radiation passing through changes with the sheet's thickness, allowing rollers to adjust pressure accordingly.