The x-ray beam coming from an x-ray tube will be

An X-ray beam coming from an X-ray tube, contain more than one wavelength, 

Hence, not monochromatic. 

Let the potential difference between accelerating plates of tube $=V$

maximum  $K·E$ gained by electron $=e. V$

This maximum  $K.E$ is equal to maximum energy of  $x$-ray photon $=$ E_max
${\mathrm{E}}_{\max }=\frac{\mathrm{hc}}{\mathrm{\lambda }}=\mathrm{ev}$
$\Rightarrow \mathrm{\lambda }=\frac{\mathrm{hc}}{\mathrm{ev}}$
$X$-ray beam has energy less than or equal to $E_{\text{max}}$

and since wavelength is inversely proportional to energy, the X-rays will have larger than a certain minimum wavelength.