Scintillation of liquid nitrogen

TL;DR

Liquid nitrogen scintillation was measured for the first time using a thoron-based BiPo tagging approach to isolate alpha-induced scintillation from Cherenkov light and radioluminescence. The study finds LN scintillation is measurable yet very faint, with a relative yield to GN at STP of about $0.014$ and an absolute yield near $2.39$ photons per MeV. The method combines Geant4 simulations, careful background subtraction, and AIRFLY yield corrections to quantify the LN response and assess potential backgrounds for Oscura-like cryogenic detectors. The results explain prior non-detections and highlight the need for mitigation strategies to address LN scintillation as a background in dark matter searches.

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.

Figures (14)

• Figure 1: Overview of the detector system. Panel A shows the cryostat with the muon panels on the bottom arranged one perpendicular to the other. Inside the cryostat there is the PTFE cup coupled to the PMT. Panel B shows a zoomed section view of the PTFE cup. The positions of the temperature sensors used to monitor the filling status of the detector are also shown.
• Figure 2: The decay chain of $^{220}$Rn following its emanation from a $^{228}$Th source PhysRevD.95.072008Audi_2012NNDC. Prompt alphas are highlighted in blue, and the $^{212}$BiPo coincidence in green.
• Figure 3: Example of a $^{212}$BiPo candidate event showing the full acquired waveform (black) and the calculated baseline (red). The inset shows a zoomed in view of the identified $Q_0$ and $Q_1$ pulses, marked with colored boxes. The other small pulses in the waveform are consistent with single photoelectrons.
• Figure 4: Tagged coincidence event rate as a function of time since the beginning of the filling process. The source was closed at 72.6h with the detector full of liquid. The "signal" period is defined as 7385, while the "background" period is defined as 202240.
• Figure 5: $\Delta t$ distribution for tagged coincidence events in the liquid signal dataset, after subtracting out the distribution from the background dataset. The residual distribution is fit with an exponential decay yielding a decay time compatible with the $^{212}$BiPo coincidence (431.4ns).