Proton Excitation of Continuous Emission in the Noble Gases*

Protons were used to excite the noble gases, He, Ne, Ar, Kr, and Xe, and the spectra of light emitted in the vacuum-ultraviolet (vuv) region were measured. The same gases were excited with a gas-discharge source operating in the spark mode. While it is well known that the spectral shape depends on the discharge conditions, we found the spectra of purified noble gas to be quite stable and reproducible when excited with protons. In most instances, the continuous spectra excited by the charged particles differ greatly from the discharge spectra at the same pressure. In both cases the spectra depend on the pressure, but in a much less complicated way in the case of proton excitation. With fast protons, we also determined the total number of photons emitted per second per centimeter of proton track and per microampere of proton beam current. The energy radiated in the vuv region per centimeter of proton track length is an appreciable fraction of the proton stopping power for all the stable noble gases.