- The naturally occurring elements like Uranium, Polonium, Radium etc. constantly undergo a spontaneous change with the emission of different radioactive rays. This phenomenon of emission of strong radioactive rays like α, β, ϒ and X-rays by such elements is called radioactivity.
- The substances which exhibit the properties of radioactivity are called radioactive elements.
Types of radioactivity:
- Natural radioactivity:
- Nuclear reactions which occur spontaneously in nature lead to natural radioactivity.
- The heavier radioactive elements in Uranium series, Thorium series and Actinium series decay naturally through a number of unstable nuclei emitting radiation, called natural radiation.
- Artificial radioactivity:
- Not all nuclear reactions are spontaneous.
- Some of the nuclear reactions only occur when stable isotopes are bombarded with particles such as neutrons and then they release radiation, called artificial radiation.
- This property of some elements to emit radiation is called artificial radioactivity.
- Transuranium elements (Neptunium, Lawrencium etc.) with atomic numbers more than 92 (Uranium), which have been artificially made, emit such radiation.
Types of radioactive rays and their properties;
There are different types of radioactive rays emitted from radioactive substances, but the frequently encountered are the following 3 types:
- Alpha rays:
- α-radiation is a heavy, very short-range particle.
- They have a charge of 3.202 x 10-19 coulomb (i.e. twice the charge of electron). Hence, they are actually doubly charged He nucleus.
- The velocity of α-particles ranges from 1/10 to 1/100 of the velocity of light.
- They produce intense ionization in the gas through which they pass. They have the strongest ionization power (about 100 times ionization power than β-particles and 10,000 times than ϒ-particles).
- They have less or no penetrating power. They can’t penetrate human skin, clothing or other materials.
- α- radiation emitting materials if inhaled, swallowed, or absorbed through open wounds can be harmful to humans.
- The presence of α-radiation can be detected by thin-window Geiger-Mueller (GM) probe.
- α- radiation cannot travel for a long distance in air and not hazardous.
- They are deflected by electric and magnetic field.
- Radium, Radon, Uranium, Thorium etc. emit α- radiation.
- Beta rays
- Beta radiation carries a light and short-range particle (β-particle) with a charge of 1.6 x 10-19C (equal to the charge of an electron). Hence, it is actually an ejected electron.
- They are fast moving radiation (several feet in air). The velocity of β-particles ranges from 33% to 99% of velocity of light.
- They are moderately penetrating. They can penetrate our skin up to the germinal layer. Prolonged exposure to high level β-radiation can cause skin injury.
- They can also ionize the gas through which they pass and are also deflected by electric and magnetic fields.
- Most high energy and moderately penetrating β-radiation emitting substances can be detected with the help of a thin-window GM and a survey instrument. However, low-energy and poorly penetrating radiation emitted by tritium, carbon-14, and sulfur-35, may be difficult or impossible to detect.
- Clothing provides some protection against beta radiation.
- Strontium-90, carbon-14, tritium, and sulfur-35 are some β-radiation emitting substances.
- Gamma rays:
- ϒ-radiation doesn’t carry any charged particles hence, it is charge less. But they are the most energetic of all the radioactive rays.
- They travel in a straight line with the velocity of light in vacuum and travel longer up to many feet in air.
- They are highly penetrating electromagnetic radiation and can penetrate through many inches in solid substances including human tissues. Hence, they are called penetrating radiation.
- They have the least ionization power of all the radioactive radiations.
- They are hazardous to our body. Hence, dense and thick materials are needed for shielding and protection from gamma radiation.
- Survey meters with a sodium iodide detector probe can easily detect Gamma radiation.
- They are not deflected by electric or magnetic fields, since they are electromagnetic waves.
- They are used in medicine (radiotherapy), industry (sterilization and disinfection) and the nuclear industry.
- Iodine-131, cesium-137, cobalt-60, radium-226, technetium-99m etc. emit ϒ-radiation.