When the colliding electron remove an electron from innermost $K$ - shell (corresponding to $n=1$ ) of atom and electron from some higher shell jumps to $K$ - shell to fill up this vacancy, characteristic $X$ - ray of $K$ - series are obtained
$\therefore K_{\alpha}$ and $K_{\beta} X$-rays are emitted when there is transition of electron between the levels $n=2$ to $n=1$ and $n=3$ to $n=1$ respectively.