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Nucleus — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsNucleus5 Marks
Question
What is radioactivity? State the laws of $\alpha$-rays and $\beta$-rays.
✓

Answer

Radioactivity : In 1896 a French physicist Henri Becquerel while investigating the relationship between X-rays and fluorescence, accidently found that photographic plate wrapped in thick black paper was blackened when a uranium compound was placed near it. Careful investigations showed that this property had no relation with fluorescence of X-rays, but was a property of the element uranium. The photographic plate was affected by some type of radiations emitted by uranium. The emission of these radiations was spontaneous not affected by external agencies like temperature pressure, electric field and magnetic field. These radiations were named Becquerel rays and the phenomenon was named as radioactivity. Hence :
"The phenomenon of spontaneous disinte-gration of heavy nuclei with the emission of certain radiations is called radioactivity and the radiations are called radioactive rays.
The radioactivity found in naturally occurring elements is called natural radioactivity.
Properties of $\alpha$-rays :
(a) $\alpha$-rays Consist of doubly ionised helium atoms.
(b) $\alpha$-rays are deflected by the electric field and magnetic field.
(c) $\alpha$-rays Carry positive charge of $3.2\times10^{-19}C$.
(d) The Velocity of $\alpha$-rays of about 10% the velocity of light speed.
(e) $\alpha$-rays affect the photographic plate.
(f) $\alpha$-rays are absorbed by thin foils. For example, an aluminium foil 0.01 cm thick is sufficient to absorb them. In air at normal pressure they may penetrate up to 3 to 4 cm.
(g) $\alpha$-rays ionise the gas through which they pass. A single $\alpha$-particle can ionise even up to 20000 gas atoms.
(h) $\alpha$-rays fluorescence in zinc sulphide, barium platinocyanide etc.
(i) $\alpha$-rays get scattered by the thin foils.
Properties of $\beta$-rays :
(a) $\beta$-rays consist of electrons.
(b) $\beta$-rays are deflected by the electric field and the magnetic field.
(c) $\beta$-rays Carry negative charge of $1.6\times10^{-19}C$.
(d) $\beta$-rays velocity is upto 90% the velocity of light.
(e) $\beta$-rays affect the photographic plate.
(f) Penetrating power of $\beta$-rays is about 1000 times more than the $\alpha$-particles.
(g) $\beta$-rays can cause ionisation of the gases. But their ionisation power is about 1/100th of the $\alpha$-particles.
(h) $\beta$-rays cause fluorescence in zinc sulphide and other materials.
(i) $\beta$-rays are scattered by the metallic sheet more than the $\alpha$-particles.
(j) Mass of $\beta$-particles depends upon their velocity. If rest mass of $\beta$-particle is $m_0$, and its mass is '$m$' when it is in state of motion with velocity in then according to Einstein theory of relativity :
$m=\frac{m_0}{\sqrt{1-\left(\frac{v^2}{c^2}\right)}}$, where $c=$ velocity of light.

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