Radiation alpha beta and gamma rays
According to the properties, three different types of radioactive rays, alpha, beta, and gamma-emitting from the radioactive substances in the radiation process.
Of these three particles, alpha and beta particles are deflected in the magnetic and electric field but gamma-rays are wave-motion of very small wavelength.
Discovery of nuclear particles
In 1896 the French scientist Henrey Becquerel, while investigating the nature of the x -rays discovered by Rontgen a few months earlier. He found that a photographic plate was affected by a sample of potassium – uranyl – sulfate placed.
Thus he obtains the conclusion that some radiation emitting from the uranium compound could attack the cover of the photographic plate. But the properties of these radiations were very similar to those of x -rays.
Marie Curie found the activity of mineral pitchblende. He shows that the activity of this compound greater than the uranium content. Thus in 1898 Pierre and Marie Curie isolated two new elements Polonium and Radium.
In 1900, Debierne and Giesel discovered actinium which was also radioactive.
Alpha-beta and gamma particles
The radiations emitted from naturally radioactive elements divided into three distinct parts which are
- An alpha particle (α)
- Beta particle (β)
- Gamma rays (γ)
Properties of alpha particles
Alpha particle consists of a stream of positively charged particle which carries +2 charge and mass number 4.
Rutherford showed that the alpha particle is identical to the nuclei of the helium atom. Thus alpha particle is doubly charged helium ion (He+2) with atomic number 2 and mass number 4.
But when an alpha particle ejected from within the nucleus of an atom, the mother element loses two units of atomic number and four units of mass number.
Properties of beta particles
The easy deflection of the beta particle in a magnetic or electric field proves that this ray made up of a stream of negatively charged particles. Thus the e/m values of beta ray identical to that of an electron.
e/m = 1.77 × 108 coulombs/gm
Hence the ejection of a beta particle with mass number 0 and charge 1, results in the transformation of a neutron into a proton.
Thus when a beta particle emitted from the nucleus, the daughter nucleus has an atomic number one unit greater than that of the mother nucleus.
Difference between the beta particle and electron
Although beta particles and electrons are identical in their electrical nature and charge/mass ratio, there is a fundamental difference between them.
- The ejection of an electron from an atom converts a neutral atom into a positively charged ion but leaves the nucleus undisturbed.
- But the ejection of a beta particle changes the very composition of the nucleus and produces an atom of the next higher atomic number.
Properties of gamma rays
Properties of gamma rays consist of electromagnetic radiation of very short wavelength (λ ∼ 0.005 – 1 Å). These are high energy photons.
During all nuclear reactions there occurs a change in the energy of the nucleus due to the emission of alpha or beta particles. The unstable, excited nucleus resulting from the emission of an alpha or beta particle gives off a photon and drops a lower and more stable energy state.
Thus gamma rays do not carry charge or mass, and hence emission of these rays cannot change the mass number or atomic nucleus properties.
Positron emission reaction example
Curies and Rutherford discover another mode of nuclear transformation. This involves the ejection of a positron from within the nucleus. Thus this ejection made by the conversion of a proton into a neutron.
The ejection of positron lowers the atomic number one unit but leaves the mass number unchanged.
Neutrino emission in beta decay
According to the principle of conservation of angular momentum, breaking down of a neutron into a proton, the beta particle creates a problem. Particles like neutron, proton, and electron have the spin angular momentum ± ½ (h/2π) each.
Angular momentum is not balanced on the radioactive reaction. If the angular momentum of the proton and the electron are +½ (h/2π) they exceed the angular momentum of the neutron.
Thus Pauli postulated that when a beta particle ejected another tiny neutral particle namely neutrino also ejected.
If the spin angular momentum of neutrino
Sum of angular momentum of the particles ejected from within the radioactive nucleus
This is the same as that of the neutron.
The mass of the neutrino = 0.00002 with respect to the oxygen scale. Thus the ejection of an electron from within the nucleus should be represented as
Neutron → proton + electron + neutrino
Mass and charge of alpha beta gamma
Comparison of nuclear and chemical reactions
Nuclear particles reactions are different from chemical reactions with the following respects
Chemical reactions involve some loss, gain or forming a chemical bond between the reactant atoms. Such reactions cannot alter the composition of the nuclei so that the atomic number of the chemical reactions unchanged.
CH4 + H2O → CO + 3H2
On the other hand cause of nuclear decay involves the emission of alpha, beta particles or gamma rays. Thus these reactions lead to a change in the atomic number of the nucleus.
In some artificially induced radioactive decay reactions, neutrons absorbed by target nucleus producing isotopes. Nuclear reactions, therefore, leads either to the birth of another element or produce radioactive isotopes of the parent element.
The nuclear reactions are accompanied by energy changes which far exceed the energy changes in chemical reactions.
The energy evolved in the radioactive transformation of one gram of radium five hundred thousand times as large as the energy released when one gram of radium combine with chlorine to form RaCl2.