Why is it necessary to use the Ka transition (2p → 1s) in copper (see Problem 88) to generate X-rays? Why not use, for example, the 4s → 3p transition?

Problem 88

X-ray diffractometers often use metals that have had their core electrons excited as a source of X-rays. Consider the 2p → 1s transition for copper, which is called the Ka transition. Calculate the wavelength of X-rays (in Å) given off by the Ka transition if the energy given off by a mole of copper atoms is 7.77 * 10⁵ kJ.