When voltage applied to an X-ray tube increases from $10\text{kV}$ to  $20\text{kV}$  the wavelength difference between the $k_{\alpha }$  line and the short wave cut-off of continuous X-ray spectrum increases by a factor of 3.0. Identify the  target element. Take $\frac{4}{3R}\simeq 1200\stackrel{0}{ \text{A}}$  where R is Rydberg’s constant and  $\text{hc}=12.4\text{keV}\stackrel{0}{\text{A}}$