Alpha, Beta & Gamma Particles (Cambridge (CIE) AS Physics): Revision Note

Conservation of nucleon number & charge

• Nuclear processes such as fission and fusion are represented using nuclear equations (similar to chemical reactions in chemistry)

• The number of protons and neutrons in atom is known as its constituents

• For example:

${}_{92}{}^{235}\mathrm{U}+{}_0{}^1\mathrm{n}\to {}_{38}{}^{90}\mathrm{Sr}+{}_{54}{}^{144}\mathrm{Xe}+2{}_0{}^1\mathrm{n}$

• The above equation represents a fission reaction in which a uranium nucleus is hit with a neutron and splits into two smaller nuclei – a Strontium nucleus and Xenon nucleus, releasing two neutrons in the process

• In nuclear equations, the nucleon number and charge are always conserved

• This means that:

  • the sum of the nucleons on the left hand side must equal the sum of the nucleons on the right hand side

  • the sum of the charge on the left hand side must equal the sum of the charge on the right hand side

• In the above equation, the sum of the nucleon (top) numbers on both sides are equal

$235+1=90+144+\left(2\times 1\right)$

$236=236$

• The same is true for the proton (bottom) numbers

$92+0=38+54+\left(2\times 0\right)$

$92=92$

• By balancing equations in this way, you can determine the nucleon number, proton number or the number of missing elements

• Let's consider another example:

${}_{92}{}^{235}\mathrm{U}+{}_0{}^1\mathrm{n}\to {}_{37}{}^{96}\mathrm{Rb}+{}_{55}{}^{137}\mathrm{Cs}+\mathrm{N}{}_0{}^1\mathrm{n}$

• Determine the total nucleon number

  • This is determined from the side of the equation where all the values are known

  • In this example, from the reactants

    ${}_{92}{}^{235}\mathrm{U}+{}_0{}^1\mathrm{n}$

  • The total nucleon number = 235 + 1 = 236

• Equate the total nucleon number to the total nucleon number of the products including the unknown N

  ${}_{37}{}^{96}\mathrm{Rb}+{}_{55}{}^{137}\mathrm{Cs}+\mathrm{N}{}_0{}^1\mathrm{n}$

  • Total nucleon number of reactants = 96 + 137 + (N ×1) = 236

• Rearrange to solve for N

$\mathrm{N}=\frac{236-96-137}{1}$

$\mathrm{N}=3$

• Balancing the equation shows that 3 neutrons must be released in the reaction

Alpha, beta & gamma particles

• Some elements have nuclei that are unstable

  • This tends to be when the number of nucleons does not balance

• 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 radioactive emission: alpha, beta and gamma

Alpha particles

• 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

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

• The nuclide notion for an alpha particle is:

${}_2{}^4\mathrm{\alpha }$ or ${}_2{}^4\mathrm{He}$

• Alpha is a highly ionising form of radiation

  • It has a large charge of +2e

• Alpha is a weakly penetrating form of radiation

  • This is because it is so ionising, it readily interacts with any object instead of passing through it

  • Alpha particles have a range of a few cm in air

• Alpha particles can be blocked by skin or a piece of paper

Beta particles

• Beta (β⁻) particles are high energy electrons emitted from the nucleus

  • β⁻ particles are emitted by nuclei that have too many neutrons

During beta-minus decay, a neutron in a parent nucleus becomes a proton in a daughter nucleus by emitting a beta-minus particle (an electron) and an anti-electron neutrino

• The nuclide notion for a beta minus particle is:

${}_{-1}{}^0\mathrm{\beta }$

• Beta (β⁺) particles are high energy positrons emitted from the nucleus

  • β⁺ particles are emitted by nuclei that have too many protons

During beta-plus decay, a proton in a parent nucleus becomes a neutron in a daughter nucleus by emitting a beta-plus particle (a positron) and an electron neutrino

• The nuclide notion for a beta plus particle is:

${}_{+1}{}^0\mathrm{\beta }$

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

Gamma rays

• Gamma (γ) rays are high energy electromagnetic waves

• They are emitted by nuclei that need to lose some energy

• The nuclide notation for a gamma particle is:

${}_0{}^0\mathrm{\gamma }$

• If these particles hit other atoms, they can knock out electrons, ionising the atom

• This can cause chemical changes in materials and can damage or kill living cells

Ionisation by radiation

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

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

Properties of types of radiation

• u is the atomic mass unit (see “Atomic Mass Unit (u)”)

• e is the charge of the electron: 1.60 × 10^-19 C

• c is the speed of light: 3 × 10⁸ m s^-1