  The krypton electroluminescence yield was studied, at room temperature, as a
function of electric field in the gas scintillation gap. A large area avalanche
photodiode has been used to allow the simultaneous detection of the
electroluminescence pulses as well as the direct interaction of x-rays, the
latter being used as a reference for the calculation of the number of charge
carriers produced by the electroluminescence pulses and, thus, the
determination of the number of photons impinging the photodiode. An
amplification parameter of 113 photons per kV per drifting electron and a
scintillation threshold of 2.7 Td ( 0.7 kV/cm/bar at 293 K ) was obtained, in
good agreement with the simulation data reported in the literature. On the
other hand, the ionisation threshold in krypton was found to be around 13.5 Td
(3.4 kV/cm/bar), less than what had been obtained by the most recent simulation
work-package. The krypton amplification parameter is about 80% and 140% of
those measured for xenon and argon, respectively. The electroluminescence yield
in krypton is of great importance for modeling krypton-based double-phase or
high-pressure gas detectors, which may be used in future rare event detection
experiments.
