Detecting Radiation

The Cloud Chamber

• • Soak the felt strip with a mixture of 50% alcohol and 50% water

  • Place few drops on metal base plate

  • Remove base cover and foam and place layer of dry ice evenly under the metal base plate

  • mount an alpha source on one side in one direction across the base plate

  • cover the lid and level the chamber using wedges

  • position a lamp at about the same level with the alpha source

  • Rub the top of the chamber with a clean cloth. This charges the plastic tip and by attraction removes dust particles and any existing ions from the chamber

As the air cools with the help of the dry ice, it becomes supersaturated with water molecules. The radiation cause ions to be formed in the air. Condensation of the water molecules occurs on the ions and is seen as visible as tracks along the path of the particle.

Alpha particles tracks are bold and straight. Many tiny water droplets form around the many ions produced along the alpha particle tracks. They are straight because of their large mass and momentum.

Beta particle tracks are much fainter than those of alpha particles because of their weaker ionising power. At slow speeds the very light particle is easily deflected by the electrons of atoms in the air. They suffer sudden changes in direction more frequently as they slow down.

X-Ray and Gamma Rays produce no tracks in the cloud chamber. However, an intense beam of either can cause an electron to be thrown out of an atom that absorbs it. This electron shows up as tracks similar to the beta particle.

Tracks shown for Alpha, Beta and Gamma radiation in Cloud Chamber

The Geiger Muller (G-M) tube is used to detect and count radioactive particles. The tube has two electrodes; a positive and a negative terminal. A central stiff wire forms the +ve terminal called the anode. The outer cylinder or metal tube forms the -ve terminal called the cathode. There is a voltage difference of about 300 – 500 V between the two electrodes. One end of the tube is sealed by a thin mica window. This window is transparent to alpha, beta and gamma radiation. An inert gas fills the tube at low pressure.

When radiation enters the tube (via the mica window) it produces ions. The electric field set up between its electrodes causes the electrons to be pulled to the anode (positive central wire). More ions are formed as these electrons collide with the argon gas and an avalanche of electrons arrive at the anode with a large quantity of charge which then flows around the circuit as a pulse. This pulse is counted.

Alpha and Beta radiation ionise the argon gas by removing electrons from the atom. Gamma radiation is absorbed by the outer metal shell and knock electrons into the argon gas which then produce more ions and an avalanche of electrons.

Scalers and Ratemeters are electronic pulse counters to count the radioactive particles when connected to a G-M tube.

Scaler: gives a reading which is the total or cumulative number of particles counted from the moment it was started.

Count rate = # counts / time of counting

Ratemeter give the reading in counts per second which is a count rate