Radioactive emissions can be detected using the following common detectors:
When a radioactive substance is placed near a photographic film coated with silver halide, the latter will produce a similar effect as seen with exposure to visible light. After the exposure is completed, this latent record of the accumulated exposure can be made visible through a chemical development process.
Geiger-Muller Tube (GM-tube)
Main article here: https://www.miniphysics.com/geiger-muller-tubecounter.html
A GM-tube is the most useful radiation detector used for determining the intensity of a beam of radiation or for counting individual charge particles.
When ionizing radiation enters the tube through the thin mica window, it collides with the large argon atoms and ionized them.
These free electrons will then accelerate towards the fine wire anode placed along the axis of the cylindrical cathode. These accelerated electrons will cause further ionization of the argon atoms by colliding with them, thus producing an ‘avalanche’ of electrons which are collected almost at once by the anode.
The positively charged ions will be attracted towards the cathode. The collection of the electrons and the argon ions at the electrodes produces a current pulse. This current pulse is then amplified and fed to a ratemeter.
Diffusion Cloud Chamber
When emissions from the radioactive substance are allowed to come into contact with air molecules, it will tend to knock out the electrons of the molecules along its paths, causing ionization (i.e., the air molecules are electrically imbalanced and become ions).
The diffusion cloud chamber is the device for making visible the paths of ionizing emissions.
It consists of a container containing air. At the top of the chamber, a row of felt strips is soaked in ethanol (alcohol). The lower part of the chamber is cooled by solid carbon dioxide or liquid helium. The alcohol evaporates into vapour and continuously diffuses downwards. It then becomes supersaturated at the centre of the chamber (i.e., the air holds more vapour than it normally could).
When emissions from the radioactive substances are placed in the chamber, the emissions will cause ionization along its paths. The ionized air molecules will attract the vapour molecules and become too heavy to stay in suspension and thus, condense into tiny liquid droplets along the paths of the emissions.
Video of a diffusion cloud chamber: