Alpha, Beta & Gamma Particles (HL IB Physics)

• Some isotopes of elements are unstable
  • This can happen when a nucleus has an imbalance of protons and neutrons or too much energy

• To become more stable, a nucleus can emit particles or radiation by the process of radioactive decay
• The three main types of radioactive particle or radiation are:
  • Alpha particles
  • Beta particles
  • Gamma radiation

Alpha Particles

• An alpha (α) particle is a high-energy helium nucleus
  • It contains 2 protons and 2 neutrons
  • It has a mass of 4u and a charge of +2e

• The nuclear notation for an alpha particle is:

Nuclear notation for an alpha particle (a helium nucleus)

• Alpha particles are usually emitted by large, unstable nuclei with too many nucleons (protons and neutrons)
• When an unstable nucleus decays, its composition changes
• 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

• As there is a change in proton number, the parent nucleus is a different element to the daughter nucleus

During alpha decay, a parent nucleus becomes a daughter nucleus by emitting an alpha particle (helium nucleus)

Beta-Minus Decay

• A beta-minus (β⁻) particle is a high-energy electron
  • It has a mass of 0.0005u and a charge of −1e

• The nuclear notation for a beta-minus particle is:

• Beta-minus particles are usually emitted by unstable nuclei with too many neutrons

• Beta-minus decay is when a neutron turns into a proton and emits an electron and an anti-electron neutrino
• 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

Beta-Plus Decay

• A beta-plus (β⁺) particle is a high-energy positron
  • It is the antimatter particle of the electron
  • It has a mass of 0.0005u and a charge of +1e

• The nuclear notation for a beta-minus particle is:

• Beta-plus particles are usually emitted by unstable nuclei with 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

Gamma Radiation

• Gamma (γ) rays are a type of high-energy electromagnetic radiation
• They are emitted by nuclei that need to lose some energy
• The nuclear notation for gamma radiation is:

Nuclear notation for gamma rays

• 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

• Alpha, beta and gamma radiation can be characterised by 
  • Ionising ability - a measure of the amount of ionisation caused when nuclear radiation passes through a material
  • Penetrating power - a measure of the distance nuclear radiation will travel before losing all its energy 

• The greater the ionising ability of a type of radiation, the lower its penetrating power, and vice versa

Ionising ability

• If any type of radiation collides with an atom, it can knock out electrons, ionising the atom
• This can cause chemical changes in materials and damage to living cells

• The ionising ability of radiation can be quantified by the number of ion pairs it produces per cm of air
  • Highly ionising radiation may produce 10⁴ ion pairs per cm of air
  • Weakly ionising radiation may produce 1 ion pair per cm of air

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

Penetrating power

• The distance radiation can travel before losing most, or all, of its energy, is described by its penetrating power
• The lower the penetrating power of a type of radiation, the shorter its range in air
  • Highly ionising radiation has a low penetrating power
  • Weakly ionising radiation has a high penetrating power

Deflection in Electric and Magnetic Fields

• When a charged particle enters an electric field it will undergo a deflection
  • Alpha particles are deflected towards the negative plate
  • Beta particles are deflected towards the positive plate
  • Gamma radiation is not deflected and travels straight through between the plates

Alpha and beta particles are deflected by an electric field whereas gamma rays are not

• When a charged particle moves in a magnetic field, it will also undergo a deflection
• Faster-moving particles move in larger circular paths according to the equation:

• The larger the circular path, the greater the deflection
• The amount of deflection of a particle depends on:
  • The speed of the particle, 
  • The mass of the particle, 
  • The charge on the particle, 

Comparing Alpha, Beta & Gamma

• The ionising abilities and penetrating powers of alpha, beta and gamma can be investigated by
  • Measuring the count rate of a radioactive source using a Geiger counter
  • Placing different materials between the source and the detector
  • Measuring the count rate again to see if the material causes a significant reduction

• Alpha particles can be stopped by a single sheet of paper
• Beta particles can be stopped by a few millimetres of aluminium foil
• The intensity of gamma radiation can be reduced by several metres of concrete or several centimetres of lead

Alpha particles are highly ionising and easily absorbed by atoms whereas gamma radiation is highly penetrating and requires very thick lead to reduce its intensity

• The properties of the different types of radiation are summarised in the table below:

Comparison of alpha, beta and gamma radiation

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 cm in air
  • This also means it can 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 particles can be deflected slightly in strong electric and magnetic fields
  • Alpha particles have the highest charge, but also the greatest mass, so their high momentum means they deflect less than a beta particle (in a given field)

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 can 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 particles can be deflected through large angles by electric and magnetic fields
  • Beta particles typically travel at much greater speeds than alpha particles, but have much less mass, so they deflect significantly more than an alpha particle (in a given field)

Properties of Gamma Radiation

• 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 cm 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 rays are not deflected in magnetic and electric fields as they are electrically neutral
  • However, they can transfer their energy to atomic electrons which can be deflected