Accurate determination of gamma-ray energies for E≤2 MeV

A flat crystal spectrometer facility has been established at the 57 MW reactor at the Institut Laue-Langevin (ILL). The high flux reactor with the associated source changing facilities produces the intense capture gamma-rays needed for the high-resolution low-efficiency spectrometer. Initial measurements of gamma-ray energies up to 2 MeV from the reaction 35Cl(n,γ) have clearly demonstrated that sub-ppm measurements of intense sources are possible in the 2 to 4 MeV region. Energy values for the 517, 786, 788, 1165, 1951, 1959 keV lines are available with uncertainties of ∼1 ppm. Three of these lines (786+1165 = 1951) satisfy the sum rule to better than 1 ppm. Future prospects for high energy capture γ-ray measurements which impact on the neutron mass and the fundamental constants are briefly discussed