Biology

They emit radiation as they decay.

That radiation is not generally detectable by human senses (unless it glows in a color range we can see).

We need lab instruments to detect the radiation such as:

1. Geiger Counter, with Geiger-Mueller (GM) Tube or Probe-A GM tube is a gas-filled device that, when a high voltage is applied, creates an electrical pulse when radiation interacts with the wall or gas in the tube. These pulses are converted to a reading on the instrument meter.
2. MicroR Meter, with Sodium Iodide Detector-A solid crystal of sodium iodide creates a pulse of light when radiation interacts with it. This pulse of light is converted to an electrical signal by a photomultiplier tube (PMT), which gives a reading on the instrument meter.
3. Ionization (Ion) Chamber-This is an air-filled chamber with an electrically conductive inner wall and central anode and a relatively low applied voltage. When primary ion pairs are formed in the air volume, from x-ray or gamma radiation interactions in the chamber wall, the central anode collects the electrons and a small current is generated. This in turn is measured by an electrometer circuit and displayed digitally or on an analog meter.
4. Neutron REM Meter, with Proportional Counter-A boron trifluoride or helium-3 proportional counter tube is a gas-filled device that, when a high voltage is applied, creates an electrical pulse when a neutron radiation interacts with the gas in the tube. The absorption of a neutron in the nucleus of boron-10 or helium-3 causes the prompt emission of a helium-4 nucleus or proton respectively. These charged particles can then cause ionization in the gas, which is collected as an electrical pulse, similar to the GM tube.