Some nuclei are unstable. An unstable nucleus becomes more stable by giving out radiation. This is called radioactive decay. It is random, so we cannot say when a particular nucleus will decay.
There are four things an unstable nucleus can emit. An alpha particle is made of two protons and two neutrons, which is the same as a helium nucleus. A beta particle is a fast-moving electron that comes from the nucleus. A neutron can also be emitted by some nuclei. Gamma rays are electromagnetic radiation with no mass and no charge, unlike alpha and beta, which are particles.
A Geiger-Müller tube can be used with radioactive sources to investigate activity. The tube detects each radiation particle or ray that enters it and the counter records it. The count rate is the number of counts in a given time, for example 180 counts in 3 minutes is a count rate of 60 counts per minute.