Radioactive Decay (OCR Gateway GCSE Physics: Combined Science)

Unstable Nuclei

• Some atomic nuclei are unstable
• This is because of an imbalance in the forces within the nucleus
  • Forces exist between the particles in the nucleus

• Carbon-14 is an isotope of carbon which is unstable
  • It has two extra neutrons compared to stable carbon-12

Carbon-12 is stable, whereas carbon-14 is unstable. This is because carbon-14 has two extra neutrons

• Some isotopes are unstable because of their large size or because they have too many or too few neutrons

Radiation

• Unstable nuclei can emit radiation to become more stable
  • Radiation can be in the form of a high energy particle or wave

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

• The process of emitting radiation is called radioactive decay
• Radioactive decay is a random process
  • This means it is not possible to know exactly when a particular nucleus will decay

Types of Radioactive Decay

• When an unstable nucleus decays it emits radiation, called nuclear radiation
• There are different types of radiation that can be emitted:
  • Alpha
  • Beta
  • Gamma
  • Neutrons

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

Neutrons

• The symbol for a neutron is n
• Neutrons are one of the two particles found in the nucleus of atoms
• Neutrons are neutral, they have no charge

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

ANSWER:  C

• • Answer A is true. The number of neutrons in a nucleus determines the stability
  • Answer B is true. This is a suitable description of radioactive decay
  • Answer D is true. Radiation is about emissions. It is different to radioactive particles
  • Answer C is not true
    • Radioactive decay is a random process
    • It is not possible to predict precisely when a particular nucleus will decay

Activity

• Objects containing radioactive nuclei are called sources of radiation
• Sources of radiation decay at different rates which are defined by their activity
• The activity is defined as

The rate at which the unstable nuclei from a source of radiation decays

• Activity is measured in Becquerels
  • The symbol for Becquerels is Bq

• 1 Becquerel is equal to 1 nucleus in the source decaying in 1 second

Step 1: Determine the activity

• • The activity of the source is 2000 Bq
  • This means 2000 nuclei decay every second

Step 2: Determine the time period in seconds

• • The time period is 2 minutes
  • Each minute has 60 seconds
  • The time period in seconds is:

2 × 60 = 120 seconds

Step 3: Multiply the activity by the time period

Activity (Bq) × Time period (s) = 2000 × 120 = 240 000

• • Therefore, 240 000 unstable nuclei decay in 2 minutes

Detecting Radiation

• Radiation that is emitted from an unstable nucleus can be detected in different ways
  • For example, photographic film changes colour when exposed to radiation

• A Geiger-Muller tube is a device used to detect radiation

This Geiger-Muller Tube is connected to a Geiger Counter. This a common way of detecting radiation and measuring a count-rate

• Within the Geiger-Muller tube, ions are created by radiation passing through it
• The Geiger-Muller tube can be connected to a Geiger counter
• This counts the ions created in the Geiger-Muller tube
• Count-rate is the number of decays recorded each second by a detector

Step 1: Identify the different variables

• • The number of decays is 16 000
  • The time is 1 hour

Step 2: Determine the time period in seconds

• • 1 hour is equal to 60 minutes, and 1 minute is equal to 60 seconds

Time period = 1 × 60 × 60 = 3600 seconds

Step 3: Divide the total counts by the time period in seconds

Alpha Decay

• During alpha decay an alpha particle is emitted from an unstable nucleus
• A completely new element is formed in the process

Alpha decay usually happens in large unstable nuclei, causing the overall mass and charge of the nucleus to decrease

• An alpha particle is a helium nucleus
  • It is made of 2 protons and 2 neutrons

• When the alpha particle is emitted from the unstable nucleus, the mass number and atomic number of the nucleus changes
  • The mass number decreases by 4
  • The atomic number decreases by 2

• The charge on the nucleus also decreases by 2
  • This is because protons have a charge of +1 each

Beta Decay

• During beta decay, a neutron changes into a proton and an electron
  • The electron is emitted and the proton remains in the nuclei

• A completely new element is formed because the atomic number changes

Beta decay often happens in unstable nuclei that have too many neutrons. The mass number stays the same, but the atomic number increases by one

• A beta particle is a high-speed electron
• It has a mass number of 0
  • This is because the electron has a negligible mass, compared to neutrons and protons

• Therefore, the mass number of the decaying nuclei remains the same
• Electrons have an atomic number of -1
  • This means that the new nuclei will increase its atomic number by 1 in order to maintain the overall atomic number before and after the decay

Gamma Decay

• During gamma decay, a gamma ray is emitted from an unstable nucleus
• The process that makes the nucleus less energetic but does not change its structure

Gamma decay does not affect the mass number or the atomic number of the radioactive nucleus, but it does reduce the energy of the nucleus

• The gamma ray that is emitted has a lot of energy, but no mass or charge