Speaker
Description
High-energy-resolution scintillators are demanded for food/environmental gamma-ray monitoring systems in Fukushima or for other applications. Generally, halide scintillators have high light output due to small band-gap energy, and therefore high energy resolutions are expected [1]. However, almost all halide materials have hygroscopic nature, which makes them difficult to handle.
In 2015, Cs2HfCl6 (CHC) has been reported as non-hygroscopic halide scintillator [2]. CHC has a high light output of up to 54,000 photons/MeV, and its energy resolution is estimated to be 3.3%, from full width at half maximum (FWHM), at 662 keV. In order to improve the energy resolution, we focused on its non-proportional response. In the case of LaBr3, the non-proportional response improved by Sr2+-doping [3]. Therefore non-proportional response and energy resolution for CHC might be improved by doping alkaline earth metals as well. In this study, we report the effect of AE2+-doping (AE2+ is alkaline earth metals; Mg2+, Ca2+, Sr2+ and Ba2+) into Hf4+ site on scintillation properties.
Non-doped and AE2+-doped CHC crystals were synthesized from 99.9%-pure (Zr-free) HfCl4, 99.999%-pure CsCl, 99.999%-pure MgCl2, 99.99%-pure CaCl2, 99.998%-pure SrCl2 and 99.99%-pure BaCl2 from a nominal composition of Cs2(Hf0.995AE0.005)Cl5.99 by the vertical Bridgman method. Crystal phases were identified by powder X-ray diffraction. Excitation/emission wavelengths were evaluated from photo- and X-ray excited radio-luminescence spectra. Light output, its non-proportionality, energy resolution and scintillation decay constant were evaluated using a 137Cs gamma-ray source.
Finally, we succeeded in growing non-doped and AE2+-doped CHC single crystals. The crystal structure of all specimens was determined as Fm-3m. No other phase was observed. Non-doped CHC showed broad emission around 400 nm under X-ray excitation. The light output and energy resolution were estimated to be 42,000 photons/MeV and 5.2% at 662 keV (FWHM), respectively. The scintillation decay constant was estimated using double exponential fitting, and fast component and slow component were determined to be 0.27 µs (4.5%) and 5.52 µs (95.5%), respectively.
On the other hand, radio-luminescence emission spectrum of Mg:CHC was the same as for the non-doped CHC. Its light output and FWHM energy resolution were estimated to be 45,000 photons/MeV and 6.0% at 662 keV, respectively. The scintillation decay constant consisted of fast 0.69 µs (7.5%) and slow 5.99 µs (92.5%) components. In presentation, we show the results of other AE2+-doped CHC and discuss the relationship between their scintillation properties and co-doped elements.
References
[1] P. Dorenbos, Nucl. Instrum. Meth. in Phys. Res. A, 486 (2002) 208
[2] A. Burger et. al., Appl. Phys. Lett., 107 (2015) 143505
[3] S. Alekhin et al., J. Appl. Phys., 113 (2013) 224904
