Neutral Bremsstrahlung studies in Xenon Optical TPCs

For decades, secondary scintillation in noble gases was assumed to be solely due to VUV emission from excimers, created in a three-body collision of two neutral atoms and one excited atom produced by electron impact - the so-called electroluminescence (EL) process.
Neutral Bremsstrahlung (NBrS) is non-excimer-based secondary scintillation, emitted when drifting electrons scatter on neutral atoms and, unlike EL, occurs even for electric field values below the gas excitation threshold.
We have unambiguously measured NBrS in pure Xe, in well-controlled conditions in a dedicated setup based on a Gas Proportional Scintillation Counter. We reveal, as well, the presence of NBrS in the NEXT-White TPC, at present the largest optical HPXe-TPC in operation worldwide.
For field values above 1 kV/cm/bar, the typical intensities used for EL-based signal amplification, there is solid evidence that NBrS is emitted with an intensity that is about two orders of magnitude lower than conventional, excimer-based EL.
Our experimental results for NBrS yield are in excellent agreement with the values obtained with Pyboltz, a recently developed simulation toolbox based on Magboltz and Python.
The complete study and results are being published in Physical Review X.