Radioactive decay happens when an unstable atomic nucleus changes into a more stable form by releasing particles or energy. The three major types are alpha decay, beta decay, and gamma decay. Each type sends out a different kind of radiation and changes the nucleus in a different way.
Understanding these decays helps explain nuclear energy, medical imaging, radiation safety, and how scientists date ancient materials.
Alpha decay releases a heavy particle made of two protons and two neutrons, so it strongly changes the element. Beta decay changes a neutron into a proton or a proton into a neutron, so it changes the atomic number by one. Gamma decay releases high-energy electromagnetic radiation, usually after another decay leaves the nucleus excited.
The main differences among alpha, beta, and gamma radiation are what is emitted, how far it penetrates matter, and how the nucleus changes.
Understanding Physics: Types of Radioactive Decay
The balance inside a nucleus is delicate. Protons repel each other electrically, yet the strong nuclear force pulls nearby protons and neutrons together. In large nuclei, the electrical repulsion becomes harder to control.
A nucleus may lower its total energy by ejecting a cluster of particles. Alpha emission is possible because of quantum tunnelling. The alpha particle does not need enough ordinary energy to climb out through the nuclear barrier.
It has a small chance of appearing outside that barrier. This chance is different for each isotope, which helps explain why some alpha emitters last fractions of a second while others last billions of years.
Beta decay is controlled by the weak nuclear force. This force can change one type of nucleon into another, helping a nucleus move toward a more suitable neutron to proton ratio. In beta minus decay, a neutron becomes a proton.
A fast electron and an antineutrino carry away energy. The antineutrino is important because energy, momentum, and electric charge must all be conserved. The electron does not always leave with the same energy.
Its energy forms a range because the antineutrino takes a varying share. There is another process called beta plus decay, where a proton becomes a neutron and releases a positron and a neutrino. Some nuclei achieve a similar change by capturing one of their own inner electrons.
Radiation affects material by transferring energy to atoms. If enough energy is transferred, electrons are removed from atoms. This is ionisation, and it can damage cells or alter electronic equipment.
Alpha particles lose energy quickly because they are heavy and carry a double positive charge. They are stopped by the outer dead layer of skin, but they can be very harmful if an alpha emitting substance is breathed in or swallowed. Beta particles travel farther and can pass into skin.
Plastic or thin metal is often used for shielding. Gamma rays have no electric charge and no rest mass, so they can travel much farther before interacting.
Thick concrete, water, or lead reduces their intensity. Distance and exposure time matter as much as shielding.
Decay is random for any one nucleus. Scientists cannot predict when a particular atom will decay. They can predict the behaviour of a huge sample using its half life.
After one half life, about half of the original unstable nuclei remain. The word about matters because decay follows probability. Detectors use the ionisation caused by radiation to count events.
A Geiger counter gives separate clicks, showing the random nature of decay. When studying nuclear equations, track the mass number and atomic number separately. Check what is emitted, then use conservation to find the daughter nucleus.
This habit prevents a common mistake of treating gamma emission as though it changes the element. Gamma emission changes nuclear energy, not the number of protons.
Key Facts
- Alpha decay emits an alpha particle: 4/2 He, which has 2 protons and 2 neutrons.
- In alpha decay, the mass number decreases by 4 and the atomic number decreases by 2: A/Z X -> A-4/Z-2 Y + 4/2 He.
- Beta minus decay emits an electron and an antineutrino: n -> p + e- + antineutrino.
- In beta minus decay, the atomic number increases by 1 while the mass number stays the same.
- Gamma decay emits a gamma ray photon: excited nucleus -> lower-energy nucleus + gamma.
- Penetrating power usually increases in the order alpha < beta < gamma, while ionizing power usually decreases in the order alpha > beta > gamma.
Vocabulary
- Radioactive decay
- Radioactive decay is the spontaneous change of an unstable atomic nucleus into a more stable nucleus by emitting particles or energy.
- Alpha particle
- An alpha particle is a helium nucleus made of two protons and two neutrons.
- Beta particle
- A beta particle is a high-speed electron or positron emitted during beta decay.
- Gamma ray
- A gamma ray is a high-energy photon emitted from an excited atomic nucleus.
- Penetrating power
- Penetrating power is the ability of radiation to pass through materials before being absorbed or stopped.
Common Mistakes to Avoid
- Confusing alpha particles with single protons is wrong because an alpha particle contains 2 protons and 2 neutrons, the same as a helium nucleus.
- Changing the mass number during beta decay is wrong because beta decay changes a neutron into a proton or a proton into a neutron, so the total number of nucleons stays the same.
- Thinking gamma decay changes the element is wrong because gamma emission releases energy but does not change the atomic number or mass number.
- Ranking alpha radiation as the most penetrating is wrong because alpha particles are large and highly charged, so paper or skin can often stop them.
Practice Questions
- 1 A uranium-238 nucleus undergoes alpha decay. What are the mass number and atomic number of the daughter nucleus?
- 2 Carbon-14 undergoes beta minus decay. If carbon has atomic number 6, what element is produced and what is its mass number?
- 3 A nucleus emits a gamma ray after beta decay. Explain why gamma emission changes the energy of the nucleus but not the identity of the element.