Class 10 > Unit # 20:Nuclear Structure > Radioactivity

Talha's Physics Academy

Explain the Phenomenon of Radioactivity

Phenomenon of Radioactivity

Introduction

Henri Becquerel discovered that Uranium atoms ($Z = 92$) emit highly penetrating radiations capable of penetrating paper, glass, and even aluminum. Based on his experimental results, he explained the phenomenon of natural radiation.

"Radioactivity is defined as the phenomenon of spontaneous disintegration of the atomic nucleus."

Explanation

Radioactivity is a self-disrupting activity exhibited by naturally occurring heavy elements. It has been observed that elements with an atomic number greater than $83$ are unstable and spontaneously emit certain types of radiations. Such substances (e.g., Uranium, Radium, Thorium) are called radioactive substances, the emitted rays are called radioactive radiations, and the phenomenon is known as radioactivity. Rutherford and his co-workers proved that these radiations consist of three distinct types.

Experiment

Radioactive radiations can be separated by applying an electric or magnetic field across the beam. A small amount of radioactive substance is placed at the bottom of a cavity drilled in a heavy lead block. When a narrow beam of radioactive rays is allowed to pass through the space between two oppositely charged plates, the paths of the rays bend under electrostatic forces. A similar deflection is observed in the presence of a magnetic field.

Results and Conclusions

The analysis of the deflected rays yields three distinct components:

1. $\alpha$ (Alpha) Particles: Rays bending towards the negative plate indicate that they consist of positively charged particles, designated as $\alpha$-rays. An alpha particle is identical to a helium nucleus ($_2^4\text{He}$), consisting of two protons and two neutrons.
2. $\beta$ (Beta) Particles: Rays bending towards the positive plate indicate that they consist of negatively charged particles, designated as $\beta$-rays. Beta particles are a stream of high-energy electrons originating from nuclear decay.
3. $\gamma$ (Gamma) Rays: Rays that pass straight through undeflected indicate zero electrical charge. These are highly energetic photons or $\gamma$-rays, which are high-frequency electromagnetic radiations.

Fig: Magnetic and electric field separation of Alpha ($\alpha$), Beta ($\beta$), and Gamma ($\gamma$) radiations from a radioactive source.

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