Alpha, Beta & Gamma Particles (HL IB Physics)

• Some elements have nuclei that are unstable
  • This tends to be when the number of protons and neutrons in a nucleus is unbalanced

• In order to become more stable, they emit particles and/or electromagnetic radiation
  • These nuclei are said to be radioactive

• There are three main types of radiation:
  • Alpha
  • Beta (minus & plus)
  • Gamma

Alpha Decay

• Alpha (α) particles are high-energy particles made up of 2 protons and 2 neutrons (the same as a helium nucleus)
• They are usually emitted from nuclei that are too large

• Alpha decay is common in large, unstable nuclei with too many protons
• The decay involves a nucleus emitting an alpha particle and decaying into a different nucleus

Alpha decay produces a daughter nucleus and an alpha particle (helium nucleus)

• When an unstable nucleus (the parent nucleus) emits radiation, the composition of its nucleus changes
• As a result, the isotope will change into a different element (the daughter nucleus)
• Alpha decay can be represented by the following radioactive decay equation:

• When an alpha particle is emitted from a nucleus:
  • The nucleus loses 2 protons: proton number decreases by 2
  • The nucleus loses 4 nucleons: nucleon number decreases by 4

Beta-Minus Decay

• Beta-minus (β⁻) particles are high-energy electrons emitted from the nucleus
  
  • Beta-minus particles are emitted by nuclei that have too many neutrons

• Beta-minus decay is when a neutron turns into a proton and emits an electron and an anti-electron neutrino
  • Therefore, the nucleon number of the decaying nuclei remains the same

• Electrons have a proton number of -1, so overall:
  • The proton number increases by 1
  • The nucleon number remains the same

Beta-minus decay often happens in unstable nuclei that have too many neutrons. The nucleon number stays the same, but the proton number increases by one

• When a β^- particle is emitted from a nucleus, such as carbon-14, the decay equation is as follows:

• The new nucleus formed from the decay is called the “daughter” nucleus (nitrogen in the example above)

Beta-Plus Decay

• Beta-plus (β⁺) particles are high-energy positrons (antimatter particle of electrons) emitted from the nucleus
  • Beta-plus particles are emitted by nuclei that have too many protons

• Beta-plus decay is when a proton turns into a neutron and emits a positron and an electron neutrino
• Positrons have a proton number of +1, so overall:
  • The proton number decreases by 1
  • The nucleon number remains the same

Beta-plus decay often happens in unstable nuclei that have too many protons. The nucleon number stays the same, but the proton number decreases by one

• When a β⁺ particle is emitted from a nucleus, such as carbon-6, the decay equation is as follows:

Gamma Radiation

• Gamma (γ) rays are high energy electromagnetic waves
• They are emitted by nuclei that need to lose some energy

• Gamma particles are photons, so they have a proton number of 0, so overall:
  • The proton number remains the same
  • The nucleon number remains the same

Properties of Alpha Radiation

• Alpha is the most ionising type of radiation
  • This is due to it having the highest charge of +2e
  • This means it produces the greatest number of ion pairs per mm in air
  • This also means it is able to do more damage to cells than the other types of radiation

• Alpha is the least penetrating type of radiation
  • This means it travels the shortest distance in air before being absorbed
  • Alpha particles have a range of around 3-7 cm in air

• Alpha can be stopped by a single piece of paper

Properties of Beta Radiation

• Beta is a moderately ionising type of radiation
  • This is due to it having a charge of ±1e
  • This means it is able to do some slight damage to cells (less than alpha but more than gamma)
• Beta is a moderately penetrating type of radiation
  • Beta particles have a range of around 20 cm - 3 m in air, depending on their energy

• Beta can be stopped by a few millimetres of aluminium foil

Properties of Gamma Radiation

• If gamma radiation collides with an atom, it can knock out electrons, ionising the atom
• This can cause chemical changes in materials and can damage or kill living cells

When radiation passes close to atoms, it can knock out electrons, ionising the atom

• Gamma is the least ionising type of radiation
  • This is because it is an electromagnetic wave with no charge
  • This means it produces the least number of ion pairs per mm in air
  • It can still cause damage to cells, but not as much as alpha or beta radiation. This is why it is used for cancer radiotherapy

• Gamma is the most penetrating type of radiation
  • This means it travels the furthest distance in air before being absorbed
  • Gamma radiation has an infinite range and follows an inverse square law

• Gamma can be stopped by several metres of concrete or several centimetres of lead

Different types of radiation are stopped by different materials

• The properties of the different types of radiation are summarised in the table below
• Note that charge is often described as 'relative charge'

• Where:
  • u = atomic mass unit
  • e = charge of the electron: 1.60 × 10^-19 C
  • c = speed of light: 3 × 10⁸ m s^-1