A 12.5 MeV `alpha`- particle approaching a gold nucleus is deflected `180^@` . What is the closest distance to which it approaches the nucleus ?

Explanation

To solve the problem of finding the closest distance an alpha particle approaches a gold nucleus when it is deflected by 180 degrees, we can follow these steps:

### Step-by-Step Solution

1. **Understanding the Problem**:
- An alpha particle with a kinetic energy of 12.5 MeV is approaching a gold nucleus and is deflected by 180 degrees. We need to find the closest distance (R) it approaches the nucleus.

2. **Convert Kinetic Energy to Joules**:
...