  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.
