To observe the characteristic K lines of the X-ray spectrum, One of the n = 1 electrons

Question:

To observe the characteristic K lines of the X-ray spectrum, One of the n = 1 electrons must be ejected from the atom. This is generally accomplished by bombarding the target material with electrons of sufficient energy to eject this tightly bound electron. What is the minimum energy required to observe the K lines of

(a) Tungsten,

(b) Molybdenum, and