Write the relation for distance of closest approach.

Explanation

To derive the relation for the distance of closest approach between an alpha particle and a target nucleus, we can follow these steps:

### Step 1: Understand the Concept of Closest Approach
The distance of closest approach refers to the minimum distance between two charged particles, in this case, an alpha particle and a nucleus. At this point, the kinetic energy of the alpha particle is completely converted into potential energy due to the electrostatic interaction between the two charges.

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