Determination of the Low-Energy Region of the Tritium Beta Spectrum

The tritium beta spectrum has been determined in a proportional counter spectrometer fitted with field-adjusting electrodes down to an energy of 200 ev. Below 1.2 kev the experimental spectra exhibit positive deviation from the theoretical spectrum which was corrected for the screening effect and for counter resolution. Various possible explanations for the deviation, which amounts to +6.5% at 0.3 kev, are examined. A possible explanation is that $w$, the energy to produce an ion pair in the counting gas, increases by several percent in the energy interval 1.2 to 0.25 kev.