What Is Radioactive Decay?
Inside every atom, the nucleus is held together by the strong nuclear force. For many nuclei, this arrangement is perfectly stable , but for others, the balance is off, and the nucleus will eventually spontaneously emit particles or energy to reach a more stable configuration. This process is called radioactive decay.
Radioactive Decay: The spontaneous and random disintegration of an unstable atomic nucleus, resulting in the emission of particles or electromagnetic radiation.
Two key characteristics define radioactive decay:
- Spontaneous , Decay occurs without any external trigger. No amount of heating, cooling, pressurising, or chemical reaction will speed it up or slow it down. The nucleus simply decays when it decays.
- Random , It is impossible to predict when any specific nucleus will decay. Each nucleus has a fixed probability of decaying in any given time interval, but the exact moment is entirely unpredictable.
Think of a large jar of coins, each with a 50% chance of landing heads when flipped. You can't predict which individual coin will land heads, but if you flip all of them, you can predict that roughly half will. Radioactive nuclei behave the same way , individual events are random, but the overall behaviour of a large sample is highly predictable.
The randomness of radioactive decay is a genuine quantum mechanical effect , not just a result of incomplete information. Even in principle, the exact moment of a single nuclear decay cannot be predicted.
Alpha, Beta, and Gamma Radiation
When an unstable nucleus decays, it releases energy in one (or more) of three forms: alpha particles, beta particles, or gamma rays. Each type has distinct properties that determine how far it travels and how much damage it can cause.
Alpha Radiation ()
Alpha Particle: A particle consisting of 2 protons and 2 neutrons, identical to a helium-4 nucleus (), emitted during alpha decay.
- Charge:
- Highly ionising , the large charge and relatively slow speed allow alpha particles to strip electrons from surrounding atoms very effectively.
- Poorly penetrating , stopped by a few centimetres of air or a thin sheet of paper.
- In alpha decay, both the mass number decreases by 4 and the atomic number decreases by 2.
Alpha decay of Uranium-238:
Check: Mass numbers balance (), atomic numbers balance (). ✓
Beta-Minus Radiation ()
Beta-Minus Particle: A high-energy electron () emitted when a neutron in the nucleus converts into a proton, releasing the electron and an antineutrino.
- Charge:
- Moderately ionising and moderately penetrating , stopped by a few millimetres of aluminium.
- The atomic number increases by 1; the mass number stays the same.
Beta-minus decay of Thorium-234:
An antineutrino () is also emitted to conserve lepton number and energy.
Gamma Radiation ()
Gamma Ray: A high-energy photon of electromagnetic radiation emitted from a nucleus following alpha or beta decay, carrying away excess energy.
- No charge, no mass
- Weakly ionising but highly penetrating , requires several centimetres of lead or metres of concrete to significantly reduce intensity.
- Gamma emission does not change the mass number or atomic number of the nucleus.
A quick memory aid for penetrating power: Alpha is stopped by A sheet of paper, Beta by a thin Block of aluminium, Gamma requires a Great thickness of lead.
