In an $X$-ray tube, the accelerating voltage is $20 \mathrm{kV}$. Two targets $A$ and $B$ are used one by one. For $A$ the wavelength of the $K_{\mathrm{\alpha }}$  line is $62 \mathrm{pm}$. For $\mathit{‘}B\mathit{’}$ the wavelength of the $L_{\mathrm{\alpha }}$ line is $124 \mathrm{pm}$. The energy of the $B$ ion with vacancy in $M$ shell is $5.5 \mathrm{keV}$ higher than the atom of $B$. [Take $\mathrm{hc} = 12400 \mathrm{eV}\stackrel{\circ }{\mathrm{A}}$]