consider an x-ray tube with a Cun anode and a monochromator consisting of a Ni single crystal with (111) planes parallel to surface of crystal. what is the lowest voltage capable of producing X-rays trough a monochromator ?

Question

consider an x-ray tube with a Cun anode and a monochromator consisting of a Ni single crystal with (111) planes parallel to surface of crystal. what is the lowest voltage capable of producing X-rays trough a monochromator ?
answer in V.