For characteristic X-ray, electron should jump from
n=2 to n=1
the wavelength of X-ray is given by
1λ=Rα(Z−1)2[1−1n2]=Ehc
Ehc=R(Z−1)2(1−122)...(i)
Binding energy Eb=40 keV
Ebhc=R(Z−1)2(1−1∞2)...(ii)
EEb=[1−122][1−1∞2]
⇒EEb=[1−14]
E=34Eb=34×40,000=30,000 eV