Radioactivity

Radioactivity

Bacquerel (1886) discovered that uranium salts emit some active rays called radioactive rays and the property itself was known as radioactivity. Potassium uranyl sulphate was the first compound in which radioactivity was observed. Madame Curie et al. (1898) found that thorium and radium are also radioactive substances.

Rutherford (1899) proved that there are two types of radiation i.e. $\alpha$ -and $\beta$ -rays carrying (+) ve and (-) ve charge respectively. Willard (1900) discovered the third type of radiation, i.e. $\gamma$ -rays which is neutral. Characteristics of these three types of radiation are given below.

Characteristics of $\alpha-, \beta- \, \, and \, \, \gamma$ -rays

(a) $\alpha$ -rays
Nature: particle
Symbol : $\alpha, He^{2+}$
Charge : +2 unit
Mass: $6.65 \times 10^{-27}kg$ (4 units)
Velocity : $2 \times 10^7m s^{-1}$
Ionizing power : maximum
Penetrating power : checked by $10^{-4}m$ thin Al-foil (least)
Effect on photographic plate ZnS screen : effect more
Kinetic energy : maximum

(b) $\beta$ -rays
Nature: particle
Symbol : $\beta, -1\beta, -1e^0$
Charge : -1 unit
Mass: $9.10 \times 10^{-31}kg$ (no mass)
Velocity : $24- 28 \times 10^8m s^{-1}$
Ionizing power :less
Penetrating power : checked by $5 \times 10^{-3}m$ thin Al-foil (less)
Effect on photographic plate ZnS screen : very little effect
Kinetic energy : less

(c) $\gamma$ -rays
Nature: electromagnetic radiation
Symbol : $\gamma$
Charge : 0
Mass: 0
Velocity : $3 \times 10^8m s^{-1}$
Ionizing power : least
Penetrating power : passed through even $1.5 \times 10^{-2}m$ thin steel sheet
Effect on photographic plate ZnS screen : nearly no effect on ZnS
Kinetic energy : ×

Theory of nuclear disintegration

This theory was proposed by Rutherford and Soddy. According to this theory:

(i) A nucleus of low binding energy emits $\alpha$ -particles to get stable nucleus.

(ii) A neutron transforms into proton by the emission of $\beta$ -particle (i.e. electron)
$^1_0n \to ^1_1p + ^0-1e$

(iii) The resultant nucleus after the emission of $\alpha$ and $\beta$ -particles still possesses higher energy level than required for its stability, the difference of energy comes out in the form of $\gamma$ -rays.

Therefore emission of $\alpha$ and $\beta$ -particles is called primary emission, while that of $\gamma$ -rays is called secondary emission. In other words $\gamma$ -emission takes place only after the emission of $\alpha$ or $\beta$ -particles, i.e., when nucleus is excited.

Stability of nuclei with respect to neutron proton ratio

The stability of a nucleus depends upon the ratio of number of neutrons (n) and protons (p) present in it. It has been found that most of the stable nuclei (non-radioactive nuclei) liens in zone of stable nuclei (figure). Nuclei whose n/p ratio lies outside the zone are unstable and therefore,
number of protons
Undergo spontaneous radioactive disintegration from one atom to another giving $\alpha$ or $\beta$ -particles. This process of disintegration continues until a stable nucleus is obtained.

It is clear from the figure that for nuclei with atomic numbers equal to 20 or less, the ratio n/p is one for stable nuclei. As the atomic number increases the ratio n/p increases up to 1.6. If the n/p ratio is too high, it tends to emit $\beta$ -particles (to change neutron into proton and the ratio n/p gets lowered and comes closer to the zone of stability), e.g.

$_{36}^{87}Kr \to ^{87}_{37} Rb + ^0_{-1} e \\ ^{32}_{15} P \to ^{32}_{16} S + ^0_{-1}$

On the other hand if the n/p ratio is low; it tends to emit $\alpha$ -particle or lose the positron to come closer to the zone of stability, e.g.,

$_{84}^{212}Po\to ^{208}_{82} Rb + ^4_2 H e \\ ^{13}_{7} N \to ^{13}_{6} C + ^0_{-1}$

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