Tl₂ZrCl₆ and Tl₂HfCl₆ Intrinsic Scintillators for Gamma Rays and Fast Neutron Detection

Thallium zirconium chloride (Tl2ZrCl6) and its hafnium analog (Tl2HfCl6) are intrinsic scintillators that have been recently reported, primarily for gamma-ray detection and spectroscopy. These crystals are especially useful for dual-mode detection, gammas exploiting the highly effective Z of the materials, and fast neutrons using the (n, p) and (n, α) reactions with chlorine. Z
_eff
(70.7) and high photoelectric conversion efficiency of the zirconium variant without any activator makes it particularly attractive in this regard. We report here on the growth of undoped Tl
₂
ZrCl
₆
and Tl
₂
HfCl
₆
crystals using vertical Bridgman technique. The X-ray emission spectrum shows a broadband in the range of 400-600 nm with a peak around 470 nm for both crystals. The light yields (LYs) of these crystals were measured under irradiation by a
¹³⁷
Cs gamma source (662 keV) and compared with that from the commercial Tl-doped NaI crystal. Normalized for the quantum efficiency of the photomultiplier (PMT) at the relevant emission wavelengths, the respective LYs of Tl
₂
ZrCl
₆
and Tl
₂
HfCl
₆
are estimated to be 40 000 and 18 000 photons/MeV with corresponding energy resolution of 5% and 9%, respectively. We have demonstrated for the first time the neutron detection capability of these materials showing excellent pulse shape discrimination (PSD) with figure of merit on the order of 2.4. PSD measurement of Tl
₂
ZrCl
₆
using a
²⁴¹
AmBe source demonstrates well-separated gamma, recoiled proton, and alpha particle bands.