Neutral Bremsstrahlung spectroscopy in Argon

Neutral Bremsstrahlung (NBrS) is a radiative process resulting from the interaction of low-energy electrons with neutral atoms or molecules. Following its recent discovery in argon and xenon, NBrS has become increasingly relevant to rare-event detection, particularly in experiments using electroluminescence Time Projection Chambers (TPCs). However, experimental data on the NBrS emission spectrum remains scarce, hindering validation and refinement of its theoretical framework. In this work, we present an experimental study of the NBrS emission spectrum in gaseous argon across the 150–550 nm wavelength range under varying electric fields. We employed a unique experimental setup combining an 241 Am alpha-particle source with a cascaded gas electron multiplier (GEM) system to overcome the inherent low photon yield of NBrS. Our spectral measurements provide the first experimental observation of the distinctive low-wavelength cutoff predicted by theory. These measurements also enabled us to resolve the electric field dependence of the NBrS emission and excitation-based electroluminescence independently. Our results indicate that the NBrS emission is present at reduced electric fields as low as 96 V cm − 1 bar − 1 , well below the Ar excitation threshold. These findings have implications for the development of low-field NBrS-based detector readouts, enable more accurate simulations for optical TPCs, and advancing our understanding of electroluminescence processes in noble gases. • New method to characterize the low-yield Neutral Bremsstrahlung spectrum. • First observation of the Neutral Bremsstrahlung low-wavelength cutoff. • Neutral Bremsstrahlung yield characterized in argon at low electric fields. • Results may validate and refine Neutral Bremsstrahlung models. • Neutral Bremsstrahlung offers potential for new technologies.