Radioactivity
Category : JEE Main & Advanced
“Radioactivity is a process in which nuclei of certain elements undergo spontaneous disintegration without excitation by any external means.’’ and the elements whose atoms disintegrate and emit radiations are called radioactive elements.
Henry Becquerel (1891) observed the spontaneous emission of invisible, penetrating rays from potassium uranyl sulphate \[{{K}_{2}}U{{O}_{2}}{{(S{{O}_{4}})}_{2}}\], which influenced photographic plate in dark and were able to produce luminosity in substances like ZnS.
Later on, M.M. Curie and her husband P. Curie named this phenomenon of spontaneous emission of penetrating rays as, Radioactivity.
Curies also discovered a new radioactive element Radium from pitchblende (an ore of U i.e. \[{{U}_{3}}{{O}_{8}}\]) which is about 3 million times more radioactive than uranium. Now a days about 42 radioactive elements are known.
The radioactivity may be broadly classified into two types,
(1) If a substance emits radiations by itself, it is said to possess natural radioactivity.
(2) If a substance starts emitting radiations on exposure to rays from some natural radioactive substance, the phenomenon is called induced or artificial radioactivity.
Radioactivity can be detected and measured by a number of devices like ionisation chamber, Geiger Muller counter, proportional counter, flow counter, end window counter, scintillation counter, Wilson cloud chamber, electroscope, etc.
Nature and characteristics of radioactive emissions
The phenomenon of radioactivity arises because of the decay of unstable nuclei or certain element. The nature of the radiations emitted from a radioactive substance was investigated by Rutherford (1904) by applying electric and magnetic fields. When these radiation were subjected to electric or magnetic field, these were split into three types a, b and g –rays.
Characteristics of radioactive rays
|
a-Ray |
b-Ray |
g-Ray |
|
Charge and mass : It carries +2 charge and 4 unit mass. |
It carries -1 charge and no mass. |
It has no charge and negligible mass. |
|
Identity : Helium nuclei or helium ion\[{{\,}_{2}}H{{e}^{4}}\] or He2+. |
Electron \[-1{{e}^{0}}\] |
High energy raditons. |
|
Action of magnetic field : Deflected towards the cathode. |
Deflected to anode. |
Not deflected. |
|
Velocity : 1/10th to that of light. |
Same as that of light. |
Same as that of light. |
|
Ionizing power : Very high nearly 100 times to that of b-rays. |
Low nearly 100 times to that of g-rays. |
Very low. |
|
Effect on ZnS plate : They cause luminescence. |
Very little effect. |
Very little effect. |
|
Penetrating power : Low |
100 times that of a-particles. |
10 times that of b-particles. |
|
Range : Very small. |
More than a-particles. |
More |
|
Nature of product : Product obtained by the loss of 1 a-particle has atomic number less by 2 units and mass number less by 4 units. |
Product obtained by the loss of 1 b-particle has atomic number more by 1 unit, without any change in mass number. |
There is no change in the atomic number as well as in mass number. |
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