A proportional counter spectrometer, hereinafter denoted as a p.c.s., of unique design has been constructed for the purpose of measuring beta-spectra whose energies lie below a few hundred Kev. The p.c.s. is well suited for the problem in that thin and uniform radioactive sources which are essential for accurate low energy spectral measurements can be employed. The utilization of radioactive sources of low surface densities is made possible by an allowable source area of a few square centimeters and an ~100 per cent geometrical efficiency for particle detection by the p.c.s. In addition, the p.c.s. obviates a Geiger-Huller counter window of finite thickness which is often associated with the magnetic spectrometers. The penetration of beta-particles through the Geiger-Muller counter window can lead to distortion of the beta-spectrum at low energies
