The Three Types of Emission (Cambridge O Level Physics)

• Unstable nuclei can emit radiation to become more stable
• Radiation can be in the form of a high-energy particle or wave
• The process of emitting radiation is called radioactive decay

Radioactive Decay

Unstable nuclei decay by emitting high-energy particles or waves

• As the radiation moves away from the nucleus, it takes some energy with it
  • This reduces the overall energy of the nucleus
  • This makes the nucleus more stable

• Radioactive decay is a spontaneous and random process
• A random process means:

It is not possible to know exactly when, or in which direction, a particular nucleus will decay

• A spontaneous process means:

Radioactive decay cannot be affected by environmental factors such as temperature, pressure, or chemical conditions

• When an unstable nucleus decays, it emits radiation called nuclear radiation
• There are different types of radiation that can be emitted:
  • Alpha (α) particles
  • Beta (β^-) particles
  • Gamma (γ) radiation
• These changes are spontaneous and random

Alpha Particles

• The symbol for alpha is α
• An alpha particle is the same as a helium nucleus
  • This is because they consist of two neutrons and two protons

• Alpha particles have a charge of +2
  • This means they can be affected by an electric field

Beta Particles

• The symbol for beta is β^-
• Beta particles are fast-moving electrons
• They are produced in nuclei when a neutron changes into a proton and an electron
• Beta particles have a charge of -1
  • This means they can be affected by an electric field

Gamma Rays

• The symbol for gamma is γ
• Gamma rays are electromagnetic waves
• They have the highest energy of the different types of electromagnetic waves
• Gamma rays have no charge

Alpha Particles, Beta Particles and Gamma Rays

Alpha particles, beta particles and gamma waves can be emitted from unstable nuclei

• α, β and γ radiation can be identified by the emission from a nucleus by recalling their:
  • Nature (what type of particle or radiation they are)
  • Their relative ionising effects (how easily they ionise other atoms)
  • Their relative penetrating abilities (how far can they travel before they are stopped completely)

• The properties of alpha, beta and gamma are given in this table and then described in more detail

Different Properties of Nuclear Radiation

• The trend down the table shows:
  • The range increases
  • Penetrating power increases
  • Ionisation decreases

Penetrating Power

• Alpha, beta and gamma radiation penetrate materials in different ways
  • This means they are stopped by different materials

   

Penetrating Power of Alpha, Beta and Gamma Radiation

Alpha, beta and gamma are different in how they penetrate materials. Alpha is the least penetrating, and gamma is the most penetrating

• Alpha is stopped by paper, whereas beta and gamma pass through it
• Beta is stopped by a few millimetres of aluminium
  • Gamma can pass through aluminium

• Gamma rays are only partially stopped by thick lead

• 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

Electric Fields

• Alpha particles have a charge of +2 (charge of a helium nucleus)
• Beta particles have a charge of −1 (charge of an electron)
• Therefore, between an electric field created between a negatively charged and positively charged plate
  • 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

 The Deflection of Alpha, Beta and Gamma Particles in a Magnetic Field

Alpha and Beta particles can be deflected by electric fields

• Alpha particles are heavier than beta particles
  • Therefore, beta particles are deflected more in the electric field and alpha is deflected less

Magnetic Fields

• 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

The Deflection of a Beta Particle in a Magnetic Field

Alpha and Beta particles can also be deflected by magnetic fields