Types of Radiations and its properties
Nuclear reaction
** A nuclear
reaction is different from a chemical reaction.
** In a chemical reaction, atoms of the reactants combine by
a rearrangement of extranuclear electrons but the
nuclei of the atoms remain unchanged.
**
In a nuclear reaction, on the other
hand, it is the nucleus of the atom which is involved. The number of protons or neutrons in the nucleus changes to
form a new element.
**
A study of the nuclear changes in atoms is termed Nuclear Chemistry.
Radioactivity
**
A number of elements such as uranium and radium are unstable. Their atomic
nucleus breaks of its own accord to form a smaller atomic nucleus of another
element. The protons and neutrons in the unstable nucleus regroup to give the
new nucleus. This causes the release of excess particles and energy from the original
nucleus, which we call radiation.
**
The elements whose atomic nucleus emits radiation are said to be radioactive.
**
The spontaneous breaking down of the unstable atoms is termed radioactive
disintegration or radioactive decay.
**
The disintegration or decay of unstable atoms accompanied by emission of
radiation is called Radioactivity
Types of Radiations
**
The radioactive radiations are of three types. These were sorted out by
Rutherford (1902) by passing them between two oppositely charged plates (Fig).
**
The one bending towards the negative plate carried positive charge and were
named α
(alpha) rays. Those bending towards the positive plate
and carrying negative charge were called β (beta)
rays. The third type of radiation, being uncharged, passed straight through the
electric field and were named γ (gamma) rays.
**
α, β
and γ rays could be easily detected as they cause
luminescence on the zinc sulphide screen placed in their path.
Properties of Radiation
Alpha
(α), beta (β) and gamma (γ) rays differ from each other in nature and
properties. There chief properties are : (a) Velocity; (b)
Penetrating power; (c) Ionization.
Alpha rays (α)
(1)
Nature: They consist of streams of α-particles.
By measurement of their e/m, Rutherford showed that they have a mass of 4 amu
and charge of +2. They are helium nuclei and may be represented as 4α2
or 4He2 .
(2)
Velocity: α-particles are ejected from radioactive nuclei with very high
velocity, about one-tenth that of light.
(3)
Penetrating power: Because of their charge and relatively large size, α-particles
have very little power of penetration through matter. They are stopped by a
sheet of paper, 0.01 mm thick aluminium foil or a few centimeters of air.
(4) Ionization: city and attraction for electrons, α-particles
break away electrons from gas molecules and convert them to positive ions.
Beta
rays (β)
(1)
Nature: They are streams of β-particles
emitted by the nucleus. From their deflection electric and magnetic fields,
Becquerel showed that β-particles are identical with electrons. They have very small
mass (1/1827 amu) and charge of – 1. A β-particle
is symbolized as 0β–1 or
0e–1.
(2)
Velocity: They travel about 10 times faster than α-particles.
Their velocity is about the same as of light.
(3)
Penetrating power: β-Particles are 100 times more penetrating in comparison to α-particles.
This is so because they have higher velocity and negligible mass. β-particles
can be stopped by about 1 cm thick sheet of aluminium or 1 m of air.
(4) Ionization: The ionization produced by β-particles
in a gas is about one-hundredth of that of α-particles.
Though the velocity of β-particles is higher but the mass being smaller, their kinetic energy
is much less than α-particles. Hence they are poor ionizers.
Gamma rays (γ)
(1)
Nature: Unlike α and
β-rays, they do not consist of particles of matter. γ-Rays
are a form of electromagnetic radiation of shorter wavelength than X-rays. They
could be thought of as high-energy photons released by the nucleus during α-
or β-emissions. They have no mass or charge and may be symbolized as
0γ0
(2)
Velocity: Like all forms of electromagnetic radiation, γ-rays
travel with the velocity of light.
(3) Ionizing power: Their ionizing power is very weak in comparison to α-
and β-particles. A γ-photon displaces an electron of the gas molecule to
yield a positive ion. Since the chances of photon-electron collisions are
small, γ-rays are weak ionizers.
(4)
Penetrating power: Because of their high velocity and non-material
nature, γ-rays are most penetrating. They cannot be stopped even by a 5
cm thick sheet of lead or several meters thick layer of concrete.
Comparison of properties of α, β and γ-rays
Artificial Radioactivity
**
Many stable nuclei when bombarded with high speed particles produce unstable
nuclei that are radioactive.
**
The radioactivity produced in this manner by artificial means is known as artificial
radioactivity or induced radioactivity.
**
The artificial isotopes disintegrate in a definite fashion and have specific
half-life.
**
For example, aluminium-27 when bombarded with a neutron emits an alpha particle
and forms sodium-24 which is radioactive. It disintegrates spontaneously by
emission of a beta particle (0e–1)
and the product is magnesium-24. Sodium-24 has half-life of 24 hours.
Nuclear Isomerism
**
Sometimes α
and β-decays may produce a pair of nuclei that have the
same number of protons and neutrons but different radioactive properties.
**
A pair of nuclei having same number of protons and neutrons but different
half-lives are called nuclear isomers. The phenomenon is called nuclear
isomerism.
**
The nuclear isomers may be isotopic or isobaric
Example of Nuclear isomerism
Uranium-Z
and Uranium-X2 constitute a pair of nuclear isomers. Both nuclei
contain 91 protons and 143 neutrons, and are isotopes. They exhibit β-ray
activity with half-lives 6.7 hr and 1.14 min respectively.
Explanation
The α- or β-decay of a radionuclide first leaves it in an excited state.
This is then converted into the ground state nucleus. The excited and the
ground state nuclei thus produced are called nuclear isomers. The nuclear
isomers may be isotopic or isobaric.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
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