Unraveling xenon primary scintillation yield for cutting-edge rare event experiments

Xenon scintillation has been widely used in rare event detection experiments such as neutrinoless double beta decay, double electron captures and dark matter searches. Nonetheless, experimental values for primary scintillation yield in gaseous xenon (GXe) remain scarce and dispersed. The mean energy required to produce a scintillation photon, wsc, in GXe in the absence of recombination has been measured to be in the range of 34-111 eV. Lower values were reported for alpha-particles when compared to electrons produced by gamma- or x-rays. Since wsc is expected to be similar for x-, gamma-rays or electrons and alpha-particles, the above difference cannot be understood. In addition, one may pose the question of a dependence of wsc on photon energy. We carried out a systematic study on the absolute primary scintillation yield in GXe for electric fields in the 70-300 V/cm/bar range and for 1.2 bar supported by a robust geometrical efficiency simulation model. We were able to clear-out the above standing problems: we determined wsc for x/gamma-rays in the 5.9-60 keV range and alpha-particles in the 1.5-2.5 MeV range; no significant dependency neither on radiation type nor on energy was observed. Our values agree well with both state-of-art simulations and literature data obtained for alpha-particles. The discrepancy between our results and experimental values in the literature for x/gamma-rays is discussed in this work and attributed to unaddressed large systematic errors in previous studies. These findings can be extrapolated to other gases and have impact on experiments such as double beta decay, double electron capture and directional dark matter searches while also on potential future detection systems such as DUNE-Gas. Neglecting the 3rd continuum emission, as is the case of most of the literature values, a mean wsc-value of 38.7 [+- 0.6 (sta.)] [(- 7.2) (+ 7.7) (sys.)] eV was obtained.