Ionising Power & Deflection

• The relative ionising effects of nuclear radiation depend upon their:

  • kinetic energy
  • electric charge
• Ionisation is when an atom becomes negatively or positively charged by gaining or losing electrons
• Nuclear radiation can ionise the atoms that it hits

  • This is mostly done by removing an electron so the atom loses a negative charge and is left with an overall positive charge
• When radiation passes close to atoms it can knock out electrons, ionising the atom

  A) radiation

  B) electron

  C) electron

  
• The greater the charge of the radiation, the more ionising it is

  • This means alpha radiation is the most ionising as it has a charge of +2
  • A beta particle has a charge of –1 so it is moderately ionising
  • This means gamma radiation is the least ionising as it has a charge of 0 (no charge)
• The higher the kinetic energy of the radiation, the more ionising it is

  • This means the alpha particle is still the most ionising because it has the greatest mass
  • However, a beta particle is very light (it is an electron) but travels at high speeds, therefore, it has a lot of kinetic energy and is still moderately ionising
  • Gamma radiation has virtually no mass so it is weakly ionising
• Remembering the properties of alpha, beta and gamma radiation helps to deduce how much ionising power they have

  • E.g. An alpha particle is a helium nucleus which contains two protons and two neutrons. It therefore has a charge of +2 since each proton has a charge of +1 and a neutron has no charge.
  • Kinetic energy is defined by the equation 1/2mv^2 therefore it depends on the mass m of the particle and its velocity v
• α-particles, β-particles and γ-radiation are deflected differently in electric and magnetic fields
• A particle is deflected in an electric field if it has charge
• A particle is deflected in a magnetic field if it has charge and is moving perpendicular to it

  • Therefore, since gamma (γ) particles have no charge, they are not deflected by either electric or magnetic fields
  • Only alpha (α) and beta (β) particles are deflected
• Alpha particles have a charge of +2 (the charge of a helium nucleus)
• Beta particles have a charge of −1 (the charge of an electron)
• In an electric field created between negatively and positively charged plates

  • 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 can be deflected by electric fields

  A) alpha

  B) beta

  C) negative

  D) positive

  E) gamma

  
• Alpha particles are heavier than beta particles

  • Therefore, beta particles are deflected more in the electric field
• Similarly, alpha and beta particles are deflected by magnetic fields whilst they are moving

  • They are deflected in opposite directions due to their opposite charges
• Alpha and beta particles can also be deflected by magnetic fields

  A) beta

  
• It is important to note that because of their opposite charges, alpha and beta particles will deflect in opposite directions.

  • You do not need to know which direction alpha and beta particles are deflected in a magnetic field (this is covered at A-level) but you should know that they are deflected, whilst gamma is not because they are charged and they deflect in opposite directions.