Detection of Radioactivity

Detection of Radioactivity

Detection of Radioactivity

• Ionising nuclear radiation is measured using a detector connected to a counter

Detection of Alpha Particles

• α-particles are detected using a cloud chamber or spark counter

Cloud Chambers

• There are several different types of cloud chambers

• A cloud chamber contains:

  • A metal plate on the bottom, cooled using dry ice

  • A transparent viewing film at the top with a light source at one side, allowing the observer to identify what is happening

  • A thin film of alcohol evaporated inside the chamber forming a vapour

  • A radioactive source placed in the chamber

  • A magnetic field at right angles to the chamber

A Simple Cloud Chamber Set-Up

A cloud chamber contains vapour obtained from the cooling of alcohol using dry ice

• When charged particles pass through the cloud chamber:

  • They collide with gas molecules

  • Creating ions as the alcohol condenses 

  • Forming a path of condensation within the chamber

• α-particles can be identified because they produce thick short tracks and curl slowly in one direction

An Alpha Particle Path in a Cloud Chamber

Alpha particles can be identified by their short thick tracks that bend slightly in one direction

 Spark Counters

• A spark counter consists of a thin metal gauze mounted very close (about a mmm) to a thin wire

• A large voltage (around 5000 V) is applied across the gauze and the wire to cause sparking between them

• The voltage is then reduced until no more sparking occurs

• When an alpha radiation source is brought close to the gauze it will ionise the air around it

  • Creating sparks between the gauze and the wire

A Spark Counter Set-Up

A spark counter contains a high voltage placed between a gauze and a wire to create sparking

Count Rate

• The count rate is the number of decays per second recorded by a detector and recorded by the counter

  • It is measured in counts/s or counts/min

• The count rate decreases the further the detector is from the source

  • This is because the radiation becomes more spread out the further away it is from the source

Detection of Beta Particles and Gamma Radiation

• The Geiger-Müller tube is the most common device used to measure and detect β-particles and γ-radiation

• Each time it absorbs radiation it transmits an electrical pulse to a counting machine called a counter 

  • This makes a clicking sound or displays the count rate on a screen

• The greater the frequency of clicks heard, or the higher the count rate seen, the more radiation the Geiger-Müller tube is absorbing

  • Therefore, it matters how close the tube is to the radiation source

  • The further away from the source, the lower the count rate detected

A Geiger-Müller Counter

A Geiger-Müller tube (or Geiger counter) is a common type of radiation detector