An alpha particle is made to move through two plates apart. The speed with which it enters the gap…

An alpha particle is made to move through two plates $10\text{ cm}$ apart. The speed with which it enters the gap in the first plate is $2.5\times10^6\text{ m s}^{-1}$. The particle exits the second plate with a velocity of $5\times10^6\text{ m s}^{-1}$. Name a device where this principle is used.