  This paper presents an absolute X-ray photon energy measurement method that
uses a Bond diffractometer. The proposed system enables the prompt and rapid
in-situ measurement of photon energies in a wide energy range. The
diffractometer uses a reference silicon single crystal plate and a highly
accurate angle encoder called SelfA. We evaluate the performance of the system
by repeatedly measuring the energy of the first excited state of the
potassium-40 nuclide. The excitation energy is determined as 29829.39(6) eV. It
is one order of magnitude more precise than the previous measurement. The
estimated uncertainty of the photon energy measurement was 0.7 ppm as a
standard deviation and the maximum observed deviation was 2 ppm.
