Problem 6.138

When the voltage applied to an X-ray tube increased from $V_{1}=10 \mathrm{kV}$ to $V_{2}=20 \mathrm{kV}$, the wavelength interval between the $K_{\alpha}$ line and the short-wave cut-off of the continuous X-ray spectrum increases by a factor $n=3.0 .$ Find the atomic number of the element of which the tube's anticathode is made.