  Proportional electroluminescence (EL) is the physical effect used in
two-phase detectors for dark matter searches, to optically record (in the gas
phase) the ionization signal produced by particle scattering in the liquid
phase. In our previous work the presence of a new EL mechanism, namely that of
neutral bremsstrahlung (NBrS), was demonstrated in two-phase argon detectors
both theoretically and experimentally, in addition to the ordinary EL mechanism
due to excimer emission. In this work the similar theoretical approach is
applied to all noble gases, i.e. overall to helium, neon, argon, krypton and
xenon, to calculate the EL yields and spectra both for NBrS and excimer EL. The
relevance of the results obtained to the development of two-phase dark matter
detectors is discussed.
