State the use of radioactive isotopes in the industry?

Radioactivity is used in industries in the following ways:
1) Knowing the penetrating power of Beta particles, they are used to control the thickness of paper.
2) Radio isotopes such as Uranium 235 are used as a fuel in atomic energy reactors.
3) The ionising effect of radiations from radioactive isotopes is used in making certain luminescent signs.
4) Radio isotopes are used by engineers in factories to avoid the accumulation of charge on moving parts of machines due to friction.