Detection of ultracold neutrons with powdered scintillator screens

TL;DR

The paper tackles the limitations of ZnS:Ag/10B ultracold neutron detectors, namely long decay times and phosphorescence, by evaluating two faster scintillators, YAP:Ce and LYSO:Ce, fabricated as powder-based screens with a thin $^{10}$B coating. YAP:Ce exhibits a decay time of about $28$ ns and yields roughly $20 ext{%}$ higher UCN counts per unit area than ZnS:Ag, with $60 ext{%}$ less phosphorescence after $2$ days, while LYSO:Ce shows a $32$ ns decay but significantly higher phosphorescence and about $20 ext{%}$ fewer UCN counts due to lower light output linked to its emission spectrum and Lutetium-related radioactivity. The authors attribute the differences in performance to emission spectra and convolution with the photomultiplier’s quantum efficiency, demonstrating that YAP:Ce is the more favorable replacement for high-rate UCN experiments, whereas ZnS:Ag remains suitable for lower-rate scenarios and LYSO:Ce is less advantageous due to phosphorescence. These findings inform detector design choices for precision UCN measurements and high-count-rate applications.

Abstract

Zinc sulfide (ZnS:Ag) scintillators coated with a thin 10B layer are widely used for ultracold neutron (UCN) detection, but their application is limited by long decay times and significant phosphorescence. We investigated two possible replacement scintillators: yttrium aluminum perovskite (YAP:Ce) and lutetium ttrium orthosilicate (LYSO:Ce). Both exhibit decay times on the order of 30-40 ns, which can help reduce dead time in high-count-rate experiments. YAP:Ce showed approximately 60% lower phosphorescence than ZnS:Ag after 2 days and detected about 20% more UCN. In contrast, LYSO:Ce exhibited higher phosphorescence and produced fewer UCN counts compared to both ZnS:Ag and YAP:Ce. While both tested scintillators are capable UCN detectors, YAP:Ce consistently outperformed LYSO:Ce across all measured performance metrics.

Figures (8)

• Figure 1: Cross-sectional scan of a LYSO:Ce screen. The scintillator crystal surface layer is indicated between the red arrows; the numbers adjacent to the arrows denote the layer thickness. The layer beneath the crystals corresponds to the adhesive sheet onto which the crystals are deposited.
• Figure 2: Pulse height of an average waveform from ZnS:Ag (blue), YAP:Ce (red), and LYSO:Ce (green). Solid lines represent scintillator activation with 5.5 MeV alphas from $^{241}\mathrm{Am}$ while dashed lines represent UCN capture on $^{10}\mathrm{B}$.
• Figure 3: Phosphorescence counts from ZnS:Ag (blue), YAP:Ce (red), and LYSO:Ce (green).
• Figure 4: Experimental setup used in Sec. \ref{['sec:ucn_detector']}. The gray-filled region inside the stainless-steel guide indicates where UCN are present when the gate valve is open. The dotted lines in front of the PMTs mark the positions of the tempered-glass windows with attached scintillators.
• Figure 5: ZnS:Ag pulse-integral spectra for signal (blue) and background (violet) measurements. Events below 300 ADC$\cdot$ns are dominated by background and ZnS:Ag retriggering, while the increasing counts above 300 ADC$\cdot$ns correspond to genuine UCN events.