Novel Glass Ceramic Scintillator for Detection of Slow Neutrons in Well Logging Applications

One of the neutron detection techniques is based on the scintillation glasses enriched with the ⁶Li isotope. A limitation to increase the light yield (LY) in glass scintillators is imposed by the absence of long-range order in their atomic structure. As a result, the mean free path of excitons in glasses is very small and the delivery of electronic excitations to luminescent centers through the exciton migration has very low efficiency. The only way toward creating effective scintillation in glasses is to use direct excitation of the luminescent centers, which are dopants in the form of Ce³⁺ ions. However, the synthesis of the glasses heavily doped with cerium possesses significant technological challenge due to heterovalent properties of cerium. These limitations can be overcome by the impregnation of the glass matrix with scintillator nanocrystals with average size less than the emitted light wavelength. The controlled recrystallization of the glass enables control of the nanocrystal dimension and their distribution in the glass matrix. Details of the method are described in US patent application 13/242 839. By using this approach we have synthesized glass ceramic scintillation material obtained through partial crystallization of Ce-doped lithium-silica glass with the formation of the petalite nanocrystals in the glass matrix. Samples containing 16.5 at.% of lithium demonstrate bright radio-luminescence with maximum at 410 nm, fast scintillation with an average decay time constant of 70 ns, and LY up to 240% of LY of GS20 scintillation glass. LY of the obtained material is almost constant between 20° C and 70 °C and depends weakly on the temperature decreasing by ~30% with the temperature increase from 70 °C to 170 °C. Scintillation detectors based on this material can replace He3 neutron counters in many applications, including wireline and logging-while-drilling (LWD) “sourceless” neutron por- sity measurement that requires the neutron detectors capable to operate at temperatures above 150 °C.