Applications of Radioactivity (SL IB Physics)

• When selecting a radioactive isotope for use in industry, agriculture or medicine, the key factors to consider are
  • The penetrating power of the decay particle
  • The half-life of the decay particle

• Some key examples which require the use of radioactive isotopes are:
  • Nuclear power
  • In medicine e.g. radiotherapy, tracers and sterilising equipment
  • Carbon dating
  • Uranium-lead dating for ageing rocks
  • Detecting leaks in underground pipes
  • Controlling the thickness of materials
  • Smoke detectors

Carbon Dating

• The isotope carbon-14 is commonly used in radioactive dating
• It forms as a result of cosmic rays knocking out neutrons from nuclei, which then collide with nitrogen nuclei in the air:

• All living organisms absorb carbon-14, but after they die they do not absorb any more
• The proportion of carbon-14 is constant in living organisms as carbon is constantly being replaced during the period they are alive
• When they die, the activity of carbon-14 in the organic matter starts to fall, with a half-life of around 5730 years
• Samples of living material can be tested by comparing the current amount of carbon-14 in them and compared to the initial amount (which is based on the current ratio of carbon-14 to carbon-12), and hence they can be dated

Reliability of Carbon Dating

• Carbon dating is a highly reliable method for estimating the ages of samples between 500 and 60 000 years old
• This range can be explained by looking at the decay curve of carbon-14:

Carbon-14 decay curve used for radiocarbon dating

• If the sample is less than 500 years old:
  • The activity of the sample will be too high to measure small changes accurately
  • Therefore, the ratio of carbon-14 to carbon-12 will be too high to determine an accurate age

• If the sample is more than 60 000 years old:
  • The activity will be too low to distinguish between changes in the sample and background radiation
  • Therefore, the ratio of carbon-14 to carbon-12 will be too small to determine an accurate age

• Further sources of uncertainty arise with this dating method because:
  • The level of production of carbon-14 in the atmosphere varies slightly with location
  • The concentration of carbon-14 in the atmosphere may not have been constant over time

Uranium-Lead Dating

• For many years, scientists could not agree on the age of the Earth
• Until recently, the Earth was believed to be only millions of years old
• Over the last century, radiometric dating methods have enabled scientists to discover the age of the Earth is many billions of years old
• The most critical of these methods is uranium-lead dating

Uranium atoms decay whilst the number of lead atoms increases

• Initially, there is only uranium in the rock, but over time, the uranium decays via a decay chain which ends with lead-206, which is a stable isotope
• Uranium-238 has a half-life of 4.5 billion years
• Over time, the ratio of lead-206 atoms to uranium-238 atoms increases
• The ratio of uranium to lead in a sample of rock can then be used to determine its age

The decay chain of uranium-238 into lead-206 has been crucial for determining an accurate age of the Earth 

Detecting Leaks in Underground Pipes

• Leaks in underground pipes can be detected by introducing a gamma emitter to the fluid supply in the pipe
• By moving a detector along the ground above the pipe, the location of the leak can be identified at the point where an increased count rate is detected
• Gamma radiation is required as it is the most penetrating type of radiation
  • It is the only type of radiation that would be detectable after passing through several metres of ground
  • Beta radiation could be used if the pipe is not too thick and is near the surface

• The half-life of the isotope must be
  • Long enough for the activity of the source to remain at detectable levels
  • Short enough that the isotope does not stay present in the supply any longer than required

• The isotope sodium-24 is often used in leak detection
  • It emits both beta and gamma radiation and has a half-life of about 15 hours

The location of a leak in an underground pipe can be found at the point where a detector records a raised count rate compared to the other points along the pipe

Controlling the Thickness of Materials

• Beta radiation can be used to determine the thickness of aluminium foil, paper, plastic, and steel
• The thickness can be controlled by measuring how much beta radiation passes through the material to a Geiger counter
• Beta radiation must be used, because:
  • Alpha particles would be absorbed by all the materials
  • Gamma radiation would pass through undetected through the materials
• The Geiger counter controls the pressure of the rollers to maintain the correct thickness
• A source with a long half-life must be chosen so that it does not need to be replaced often

The pressure of the rollers can be adjusted to control the thickness of the aluminium foil depending on the amount of beta radiation detected

Smoke Detectors

• Smoke detectors contain a small amount of americium-241, an alpha emitter
• Within the detector, alpha particles are emitted and cause the ionisation of nitrogen and oxygen molecules in the air
• These ionised molecules enable the air to conduct electricity by allowing a small current can flow
• If smoke enters the alarm, it absorbs the alpha particles, hence reducing the current which causes the alarm to sound
• Americium-241 has a half-life of 460 years, so throughout the lifetime of a smoke detector, the activity of the source will not decrease significantly and it will not have to be replaced

The operation of a smoke detector

• Radionuclides are widely used in medical applications, such as
  • Radiotherapy
  • Radioactive tracers
  • Sterilising equipment

Radiotherapy

• Gamma radiation can be used to destroy cancerous tumours
  • The gamma rays are concentrated on the tumour to protect the surrounding tissue
• Less penetrating beta radiation can be used to treat skin cancer by direct application to the affected area

A radiotherapy machine. Powerful radiation is directed at the tumour and lead sheets can be used to prevent healthy tissue from being damaged

Radioactive Tracers

• Radioisotopes can be used as ‘tracers’ to monitor the processes occurring in different parts of the body
• Radioactive tracers with a short half-life are preferred because:
  • Initially, the activity is very high, so only a small sample needed
  • The shorter the half-life, the faster the isotope decays
  • Isotopes with a shorter half-life pose a much lower risk to the patient
  • The medical test doesn‘t last long so a half-life of a few hours is enough

A radioactive tracer must be injected into the patient in order to take PET scan images of brain activity

• One example is Iodine-131
  • This isotope is known to be specifically taken up by the thyroid gland making it useful for monitoring and treating thyroid conditions
  • It emits beta particles which means it will stay concentrated on the thyroid area and nowhere else in the body
  • It has a short half-life of 8 days meaning it will not be around too long to cause prolonged exposure

• Another isotope commonly used as a tracer is Technetium-99m
  • It is a gamma emitter with an energy of about 140 keV which is ideal for detection
  • It has a half-life of 6 hours so it is ideal for use as a tracer, but will not remain active for too long and can be tolerated by the body
  • Gamma radiation is ideal as it is the most penetrating so it can be detected outside the body
  • Also, gamma is the weakest ioniser and causes minimal damage
  • As well as this, technetium-99m may be prepared easily at the hospital when required making it a cost-effective treatment

Sterilising Medical Equipment

• Gamma radiation is widely used to sterilise medical equipment
• Gamma is most suited to this because:
  • It is the most penetrating out of all the types of radiation
  • It is penetrating enough to irradiate all sides of the instruments
  • Instruments can be sterilised without removing the packaging

• The general public might be worried that using gamma radiation in this way might cause the equipment itself to become radioactive, however, this is not the case because:
  • In order for a substance to become radioactive, the nuclei have to be affected
  • Ionising radiation only affects the outer electrons and not the nucleus
  • The radioactive material is kept securely sealed away from the packaged equipment so there is no chance of contamination