Scintillation of liquid nitrogen

TL;DR

This study demonstrates for the first time that liquid nitrogen exhibits measurable scintillation, with a very faint light yield, which could impact background considerations in rare event physics experiments.

Contribution

It provides the first measurement of scintillation light in liquid nitrogen and estimates its yield relative to gaseous nitrogen.

Findings

Liquid nitrogen shows measurable scintillation with a yield of about 2.39 photons per MeV.

The scintillation yield is approximately 1.42% of gaseous nitrogen at standard conditions.

This faint scintillation could influence background levels in cryogenic particle detectors.

Abstract

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 backgrounds for rare event searches such as the direct detection of dark matter. Using a coincidence-tagged alpha decay, we demonstrate that liquid nitrogen exhibits measurable, albeit very faint, scintillation. Assuming the same scintillation wavelength spectrum as gaseous nitrogen, we estimate a relative scintillation yield of $Y_{\text{LN}}/Y^{\text{STP}}_{\text{GN}} = 0.0142 \pm 0.0005\text{(stat)} \pm 0.0030\text{(sys)}$ with respect to gaseous nitrogen at standard temperature and pressure. Considering the average scintillation yield from alpha decays in gaseous nitrogen, this implies a scintillation yield for alpha decays in liquid nitrogen of $Y_{\text{LN}} = 2.39 \pm 0.56$ photons per MeV. To our knowledge this is the first measurement of scintillation in liquid nitrogen.