Write the expression for distance of closest approach for a `alpha`-particle.

Explanation

To derive the expression for the distance of closest approach for an alpha particle, we can follow these steps:

### Step 1: Understand the Concept
The distance of closest approach refers to the minimum distance between the alpha particle and the nucleus when the alpha particle is approaching the nucleus. At this point, the kinetic energy of the alpha particle is converted into potential energy due to the electrostatic repulsion between the positively charged alpha particle and the positively charged nucleus.

### Step 2: Write the Kinetic Energy of the Alpha Particle
The kinetic energy (KE) of the alpha particle can be expressed as:
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