Improving the light collection using a new NaI(Tl)crystal encapsulation

TL;DR

This paper introduces a new encapsulation method for NaI(Tl) crystals that enhances light yield by over 30%, enabling more sensitive low-energy particle detection for dark matter and neutrino experiments.

Contribution

A novel encapsulation technique that directly attaches photo sensors to NaI(Tl) crystals, significantly increasing light yield and lowering energy detection thresholds.

Findings

Light yield improved by over 30%

Achieved up to 22 photoelectrons per keV

Enhanced sensitivity for sub-keV energy events

Abstract

NaI(Tl) crystals are used as particle detectors in a variety of rare-event search experiments because of their superb light-emission quality. The crystal light yield is generally high, above 10 photoelectrons per keV, and its emission spectrum is peaked around 400 nm, which matches well to the sensitive region of bialkali photocathode photomultiplier tubes. However, since NaI(Tl) crystals are hygroscopic, a sophisticated method of encapsulation has to be applied that prevents moisture from chemically attacking the crystal and thereby degrading the emission. In addition, operation with low energy thresholds, which is essential for a number of new phenomenon searches, is usually limited by the crystal light yield; in these cases higher light yields can translate into lower thresholds that improve the experimental sensitivity. Here we describe the development of an encapsulation technique that simplifies the overall design by attaching the photo sensors directly to the crystal so that light losses are minimized. The light yield of a NaI(Tl) crystal encapsulated with this technique was improved by more than 30%, and as many as 22 photoelectrons per keV have been measured. Consequently, the energy threshold can be lowered and the energy resolution improved. Detectors with this higher light yield are sensitive to events with sub-keV energies and well suited for low-mass dark matter particle searches and measurements of neutrino-nucleus coherent scattering.