Investigating scintillation in liquid nitrogen: measurement and implications

Liquid nitrogen is commonly used in cryogenic applications and is a promising medium for the direct immersion cooling of sensors used for nuclear and particle physics experiments. The scintillation properties of gaseous nitrogen are well-documented, but little is known about the scintillation of liquid nitrogen. If present, scintillation light from interactions of ambient radioactivity could produce background events for rare event searches (e.g. direct detection of dark matter).

In this study, we present results demonstrating that liquid nitrogen exhibits measurable, albeit very faint, scintillation. Measurements were performed at Pacific Northwest National Laboratory (PNNL), using a general-purpose cryogenic detector test facility. We used a 228Th radioactive source to emanate 220Rn into a nitrogen gas stream. The gas was then condensed and collected in a chamber where progeny decay products were monitored by a photomultiplier tube. Utilizing the unique time coincidence of 212Bi-212Po decays, we identified high energy alpha particles and measured the scintillation yield of liquid nitrogen relative to nitrogen gas.