Atomic mass knowledge: 1999 status

An overview is given of developments of interest for the determination of atomic masses of atoms around nuclear ground-states. There are also developments that will affect the mass value for the neutron. It is derived essentially by combining the mass spectroscopic results for H and D with the energy of the γ-rays emitted in the capture of thermal neutrons in hydrogen. The most precise value for the latter is derived from measurements of its wavelength, by diffraction in a silicon crystal (5). Unfortunately, there was some uncertainty about the value of its lattice constant. It is hoped, that the current evaluation by Taylor and Cohen of data, important for determination of fundamental physical constants, will improve the situation. Also, it has been reported to us that a new measurement is in progress. Somewhat less accurate γ-energy values are obtained by measurements in semicon- ductor diodes. These are calibrated, essentially, with the 411 keV γ-ray of 198Au, which