Moseley's law describing characteristic X-rays is expressed as =...

Moseley's law is given by $\sqrt{\nu} = a(Z - b)$, where $\nu$ is the frequency of the characteristic X-ray and $Z$ is the atomic number of the target material.

The constants $a$ and $b$ depend only on the type of the spectral series (such as $K$-series, $L$-series, etc.) and the specific transition line within that series (like $K_{\alpha}$, $K_{\beta}$).

For example, for the $K_{\alpha}$ line, the screening constant is $b \approx 1$ and the proportionality constant is $a = \sqrt{\dfrac{3Rc}{4}}$. These values remain the same for all elements.

Thus, both $a$ and $b$ are independent of the target material.