Laboratory for Luminescent and Detector Materials

Scientific results:

1. Over the course of the implementation of the project, within the created concept of multiple-purpose scintillation materials we have conducted research to develop methods of controlling their properties for neutron detectors and to test materials in prototypes of detectors of various types. The experimental work to produce samples of materials  and research their properties, as a rule, was preceded by elaboration of ideas using nuclear-physics modeling (mainly in GEANT4).
2. The main group of the studied materials is a family of crystal scintillators based on complex oxides with the garnet structure. An important advantage of Gd as an absorbent of neutrons is the high cross-section of capture and the absence of the necessity for isotope enrichment. We proposed principles of detection of neutrons with various energies using these materials, from thermal to high-energy, that account, on the one hand, for the physics of the interaction of neutrons with gadolinium nuclei – the multitude of gamma quants emitted from the Gd(n,γ)Gd reaction and the presence of a significant share of soft gamma quants and conversion electrons and, on the other hand, the possibilities opened by the characteristics of the materials of this family – a fast   deexcitation kinetics not exceeding 100 ns and a high light yield up to ~50 thousand photons per MeV. These principles were reflected in proposed solutions for creating prototypes of detectors.
3. Our research of this group of materials was focused on determining the possibility of increasing the light yield of scintillation and accelerating the luminescence kinetics. We determined the optimal ratio of Gd and Y in scintillators with the (Gd,Y)₃Ga₃Al₂O₁₂ matrix to achieve a higher scintillation yield. For Gd₃Ga₃Al₂O₁₂:Ce³⁺ polycrystalline scintillators we established an enrichment of the boundaries of the grains by an activator and demonstrated that such enrichment leads to an increased intensity of deexcitation on the boundaries of grains. We produced samples of (Gd,Y)₃Ga₃Al₂O₁₂:Tb³⁺ ceramics, for which we detected an unprecedented high scintillation light yield that was close to the theoretically attainable maximum for this family of materials. Despite the slow deexcitation kinetics, for some applications such a material can turn out to be extremely interesting, for example, for detectors for neutron radiography with a frame accumulation time measured in seconds or minutes. Moreover, we have found a high radiation resistance of crystal materials based on oxides with the garnet stricture in flows of high-energy protons up to a fluence of about 10¹⁵ particles per cm², which is equivalent to a flow of neutral particles of about 10¹⁷ particles per cm². In the last year, as a result of a targeted optimization of the composition on the basis of previously determined laws, we produced samples of semitransparent and transparent compositionally disordered ceramics with the garnet structure (Gd,Y,Lu)₃Al₂Ga₃O₁₂, with a main component of the deexcitation kinetics that has a duration of less than 15 ns with a light yield of over 35 thousand photons per MeV, a result that is at the frontier of research in this field.
4. An equally important group of materials were scintillation glasses. The starting point were glasses based on Li₂O–SiO₂ activated with Ce³⁺, which is classical for neutron detection. Thee initial idea of additional moderation of neutrons by adding beryllium to glasses was abandoned following the results of research. We conducted studies of the stabilization of the activator in the required degree of oxidation of Ce³⁺ and proposed an original technique for producing glasses with characteristics that exceed  those of commercial samples. We also produced Tb‑activated Li₂O-SiO₂ glasses that possess a light yield of up to 30 thousand photons per MeV, which is unusual for glasses (this lead to an idea of creating Tb‑activated ceramics based on oxides with the garnet structure).
5. Moreover, we conducted a search for new potentially promising compositions of scintillation glasses. Interesting results were obtained for glasses in the Gd₂O₃-BaO-SiO₂:Ce system, for which we developed a laboratory methodology for the production of samples in the form of plates with an area of several square centimeters and Gd₂O₃-Al₂O₃-SiO₂:Ce glasses created on their basis that possess a fast deexcitation kinetics. These glasses can be viewed not only and not just for neutron detection, but for large-volume detectors in high-energy physics experiments.
6. Along with materials in the form of ceramics and glasses, within the project we have developed the idea of screens relying on scintillation powder pigment. We demonstrated the detection characteristics of screens based on GYAGG:Ce pigments, which are slightly inferior to classical screens relying on ZnS:Ag in terms of luminosity, but  have a detection efficiency that is not lower and have a significantly faster  deexcitation kinetics (patent No. RU 2781041 has been obtained by Kurchatov Institute). We refined a methodology for producing screens on the basis of various pigments. A screening of potential scintillation pigments among Li-containing scintillators with a light matrix lead to a discovery of the luminescent compound Li₂CaSiO₄:Eu²⁺, which had not been proposed for use as a scintillator earlier, in particular, for neutron detection.
7. During our work we have created prototypes of neutron detectors based on the researched materials (GAGG, GYAGG, Li glasses) and the modern element base, including silicon photomultipliers (SiPM) as the photoelectric detector. We demonstrated the operational capability of the detectors. To decrease sensitivity to the soft part of the gamma background, we used shielding, decreasing the thickness of the detecting element and amplitude discrimination of pulses. We demonstrated the multiplicity of gamma quants emitted in the Gd(n,γ)Gd reaction lead to the idea to use detectors to register neutrons for improved discrimination of background events. Additionally, the use of the principle of matching in combination with the use of a fast deexcitation dynamics of the scintillator with the garnet structure lead to the idea to create a segmented antineutrino  detector with multiple veto for selection of combinations of signals caused by the interaction in the proton target surrounding scintillation elements. The prototype of a cell of such a detector has been modeled and constructed to conduct tests with various scintillation signal readout types.
8. The fast deexcitation kinetics of Ce-activated scintillators made it logical to study the possibility of using the time-of-flight principle for the spectroscopy of neurons. The first experiments demonstrated that the use of neutron detectors based on such scintillators as, GAGG:Ce allows to implement time-of-flight measurements and, according to existing assessments this should, allow to use a flight base with a length of 5 meters (up to 0,5 meter), which is significantly less than the size of currently used similar devices. This can significantly simplify the assembly and functioning of time-of-flight neutron spectrometers.

Implemented results of research:

We are currently testing the produced materials in prototypes of detectors for various applications – for equipment of synchrotron stations, for the detection of neutrons, neutrinos and   high-energy particles in high-energy physics. In case of success, it will become possible to use the developed materials in these areas.

Education and career development:

• Two Candidate of Sciences dissertations, 7 master’s and bachelor’s degree theses have been prepared and defended.
  
• Employees of the Laboratory completed an internship at the home laboratory of the leading scientist.

Organizational and structural changes:

During development of the project we have created a basis for measurement and research of scintillation characteristics

Other results:

With the active participation of the leading scientist, in 2021 a road map was signed for collaboration between Kurchatov Institute and the National Academy of Sciences of Belarus until 2030, and in 2022 the Representative office of Kurchatov Institute was created in Belarus. At the Representative office we expect to conduct research in the area of competence of the Laboratory with the participation of our employees. 

Collaborations:

• European Organization for Nuclear Research – CERN: employees of the Laboratory participate in Crystal Clear Collaboration that is devoted to fundamental research involving the Large Hadron Collider.
  
• Moscow State University: employees of the Laboratory execute a collaborative research project within the Federal Research and Technology Program for the advancement of synchrotron and neutron research and scientific infrastructure for 2019–2027 with the topic «Advancement of synchrotron and neutron research and infrastructure for materials of next-generation power industry and safe storage of radioactive waste».
• Ural Federal University, named after the First President of Russia B. N. Yeltsin,: joint research to lay the groundwork in the field of the study of the physical properties of scintillators.