The non-radiative shifting of vacancy from L2 to L3 sub-shell, Coster–Kronig yield f23, is measured for W, Pt, Pb and U using the selective excitation technique for L2 and L3 sub-shell electrons in a compact tri-axial double-reflection geometrical setup with minimum scattered background. The setup consists of a low-power x-ray tube with Rh anode as a photon source, primary exciter for L2 and L3 sub-shells of an experimental target and the experimental target itself in one plane and a Si-PIN detector to record the emitted x-rays from the experimental target in the perpendicular plane. A separate experimental procedure has been adopted in a 90° single-reflection setup both to determine the exciting photon flux that strikes at the experimental target and to nullify exactly the irrelevant L x-rays produced by tube photons which are scattered from the primary exciter. The procedure leads to reduction of probable errors in the measurements and authenticity of determined values.