X Rays (2 questions)

1. How will look like the X ray emission spectrum from an anti-cathode of an X ray tube, if the electron acceleration voltage is gradually raised?2. Why is important the Moseley law: $1/\lambda =R(Z-\sigma)^2[(1/n^2)-(1/k^2)]$. What is the significance of the $\sigma$ constant?

1. At the beginning (low acceleration voltages) the continuum spectrum shows up. When the acceleration voltage is raised more the maximum frequency (upper limit) of the continuum spectrum moves towards higher values in accordance with the relation $h\nu_{max} =e U$. Next, the characteristic line spectrum of the anti-cathode material shows up.  The K series lines will be the last excited (and emitted). After all the discrete (characteristic) spectrum has been excited by rising more the acceleration voltage, only the intensities of the spectral lines will increase (and their position in energy will stay the same); at the same time the upper limit of the continuum spectrum will shift towards even more high frequencies.

Observation: The characteristic (discrete) spectrum is made up by principal spectral series denoted by the letters K, L, M,…. Inside a certain series, the individual lines are denoted by the subscript letters $\alpha$, $\beta$, … so that $K_{\alpha}$, $K_{\beta}$, … are the lines of the same series, having decreasing wavelengths  (increasing energies).

2. The importance of the Moseley law is that it has verified the correct order of the chemical elements in the chemical periodic system and it allowed for the first time the identification of some new elements. The “shielding constant” $\sigma$ shows how much of the nucleus charge is shielded by the atom electrons.