The half - life for a given radioisotope indicates the time to undergo radioactive decay for half the radioactive nuclei in any sample. The remaining portion will be one fourth of the original sample after two half - lives. Similarly, one eighth of the original sample after three half - lives, and so on. In other words, radioactivity can also be defined as the time interval required to decay one - half of a radioactive sample's atomic nuclei or the time interval required to decrease by one - half the number of disintegrations per second of a radioactive material.
In a given sample, there is no disintegration of all atoms together. In the very beginning, some atoms may disintegrate for which lifetime is zero. Therefore, their lifetime may vary from zero to infinity in the case of atoms that disintegrate between them. The total lifetime of all the element atoms divided by the total number of atoms originally present in the element's sample is called the Mean life.
Alpha rays, beta rays, and gamma rays are the rays emitted during a substance's radioactivity. We will now discuss the properties of all the three one by one.
Alpha rays are the radiation that is positively charged. An alpha particle is a helium nucleus, denoted by the symbol α. The alpha particle is called a helium atom that has lost two electrons and carries a + 2e charge. Its weight is about four times that of an atom of hydrogen. It initially escapes from its parent atom's nucleus and is further repelled by electromagnetism as both the alpha particles and the nucleus are charged positively. The speed of α-particles ranges from 5% to 7% of the light speed. They have very little power to penetrate into a material medium but they have very high power to ionize. These rays are more massive than the beta rays and they contain a stream of positively charged particles, called alpha particles, which have the atomic mass of 4 and a positive charge of 2 (a helium nucleus). The emission from the nucleus of an alpha particle decreases the nucleus mass number by 4 and the atomic number decreases by 2. As in the reaction below, the alpha particle is the helium nucleus.
The alpha radiation consists of two neutrons and two protons attached to a helium atom's nucleus. These particles have an intense ionizing power which means that they have the ability to cause mutations when they come into contact with the atoms of a living tissue and sometimes such reactions can even lead to cancer. Despite their high ionization power, these rays are not very harmful. These are, in fact, the least dangerous of the three rays as long as they are not inhaled. A few centimeters of air can stop alpha rays from penetrating through the skin. Particles with alpha rays have the least power to penetrate. They cannot travel very far in the air and they cannot even get through a piece of paper. They just find and join two electrons to become an atom of helium before they can result in any kind of harm. However, it does not imply that they are not dangerous. When inhaled, they are quite risky. They stick to the lungs if inhaled and can become a cause of lung cancer.
Beta radiations consist of a stream of electrons termed as beta particles. β -particles are electrons moving at high speeds. They have greater penetrating power (in comparison with α-particles), but less ionizing power. Their speed of emission is almost the speed of light. They have a spectrum of energy as opposed to α-particles, i.e. beta particles have energy from a certain minimum to a certain maximum value. A β-particle is usually an electron, but it can be a positron as well. A positron is a positively charged particle which is the anti-matter equivalent of electron. On the emission of a beta particle, a neutron in the nucleus gets converted to a proton and hence the mass number remains unchanged but the atomic number increases by one unit. The most common example of beta emission is the degradation of radioactive copper which after emitting one electron leaves behind nickel atom:
Beta decline itself occurs in two types: one is positive β+ decline and the other is β- decline. β- emission occurs when one nucleus neutron is transformed into a proton, an electron and antineutrino. The fission resulting from nuclear reactors or by -products experience β-decline as the resulting product has an excess of neutrons.
Gamma rays comprise of ~10 - 12 m wavelength electromagnetic waves. Gamma radiation is a very powerful photon with a very short wavelength (0.0005 to 0.1 nm). Gamma emissions often accompany alpha and beta emissions as an excited nucleus falls into a lower and more stable state of energy. Gamma rays are high frequency electromagnetic waves without mass and without charge. This radioactive emission has the least power of ionization. These are emitted as a result of the transition from higher energy state to lower energy state of the excited nucleus. Once an alpha or beta - decay takes place in a nucleus, excess energy is often left in an excited state. Just as an electron can move to a lower energy state by emitting a photon to an infrared range somewhere in the ultraviolet range, by emitting a gamma ray, an atomic nucleus loses energy. The penetration power is the highest for gamma rays. They have the least power to ionize any material, but they are the most dangerous. Gamma rays have the highest penetrating power and only a few centimeters of lead sheet or a few meters of concrete can stop them. In certain cases they may even pass through them.
|Property||α ray||β ray||ray|
|Nature of particles||These are positively charged particles with 2He4nucleus||These are negatively charged particles (also known as electrons).||These are uncharged γ~0.01a, electromagnetic radiation|
|Charge on particles||They contain two units of positive charge||They contain one unit of negative charge||They contain no charge|
|Mass of particles||The mass of alpha particles is 6.6466 × 10–27 kg||The mass of beta particles is 9.109 × 10–31 kg||The gamma rays do not have any mass|
|Range of particles||~10 cm in air, can be stopped by 1mm of Aluminium metal sheet||Upto a few m in can be stopped by a few cms of Aluminium metal sheet||Several m in air stopped by a few cms of Lead metal sheet|
|Natural Sources of particles||They are radiated naturally by uranium radioisotopes e.g.92U236||They are radiated naturally by cobalt radioisotopes e.g.29Co68||They are emitted from the excited nuclei formed as a result of α, β decay|