Question 1: What is meant by natural radioactivity? How are the natural radioactive radiations classified into three types?
The phenomena of spontaneous disintegration of heavier elements (Z>82) into lighter elements with the emission of three types of radiations is called radioactivity.
The element is said to be radioactive element. Atom which disintegrates is called parent atom and the new atom formed is called daughter atom.
Becquerel first observed this phenomena in 1896.
It is observed that the radiations emitted by a radioactive element has three types of characteristics. This is the base of the classification of radioactive radiations.
- α-particles: These are positively charged helium nuclei having two protons and two neutrons. Large unstable nuclei emit these radiations.
ZXA → Z-2XA-4 + α-particle + Q
Where ZXA is the parent atom, z-2XA-4 is the daughter atom and Q is the energy released in disintegration.
92U238 → 90Th234 +2He4 + Q
Properties of α-particles
- They are positively charged helium nuclei.
- They have limited penetrating power and can be easily stopped by cardboard or thin metal.
- They are deflected by electric and magnetic field.
- They have large ionization power.
In the process of β particles emission, electron is emitted. There is no change in the atomic mass number A, however, the atomic number Z changes by -1 or +1. It depends whether the nucleus emits electron or positron. The process is
ZXA______→ Z+1XA + -1β0 + antineutrino + Q
90Th234 ——–→ 91Pa234 + -1β0 + antineutrino + Q
6C11 ———-→ 5C11 + +1β0 + neutrino + Q
Properties of β particles
- β-particles are either electrons or positron.
- They are deflected by electric and magnetic fields.
- They are more penetrating than α-particles.
- Its ionizing power is less than α-particles.
With the emission of α or β-particles, the daughter nucleus is in the excited state. Such a nucleus may go to a more stable state by emitting one or more γ-rays. γ-rays are photons with no mass and, therefore, mass number A or atomic Z are not changed.
ZXA ——–→ (zXA) ———-→ zXA + γ
Here (ZXA) is the excited state nucleus.
Properties of γ radiations
- It consists of high energy photons.
- Gamma rays have high penetrating power.
- They have weak ionizing power.