The detection of helium in a deuterium gas environment (both mass 4) is an important task in the field of nuclear fusion physics. To fulfill this purpose a detector has been developed which is based on spectroscopic measurement techniques. For the excitation of the helium and deuterium atoms different electron sources have been tested. Most sources had to be rejected because they either produce a disturbingly high background light level or cannot be operated reproducibly after venting or are mechanically not stable enough. The best line emitting light source has been found to be a Penning gauge. The light is collected in an optical fiber bundle, transferred away from the tokamak experiment to an area more easily accessible, split by interference filters intoD&agr;light and the light of a strong He iline, and detected by photomultipliers. Unfortunately broad spectral lines from the deuterium molecule superimpose the strongest He ilines and have nearly the same intensity as the helium lines at a He concentration of about 5%. Therefore, some effort is necessary to deduce the partial pressures of helium and deuterium. A method is described which yields the calibration factors for the observed nonlinear pressure response of the spectral lines. The lower limit for the determination of the helium concentration presently amounts to about 1%–2%; the time resolution of the system is of the order of a few milliseconds.