A calibration proved necessary to quantify the momentum releases, due to the laser control system (AUTOSCAN) operating in fixed frequency mode. In practice, this fixes the laser frequency but the data allocation software still records the input channels as a function of wavelength. To obtain an accurate calibration of the momentum abscissa, the control (ramp) voltage of the magnetic sector was recorded simultaneously with the momentum release, and the starting and finishing points of the ramp voltage taken for each release. To calibrate the ramp wave in terms of momentum, a mass spectrum of GeH+ was recorded using the same scan control unit.