The influence of temperature on the photoluminescence of lithium fluoride crystals

Research project NCN PRELUDIUM 2020/37/N/ST5/01975

Realized in years 2021-2023

Research team:

Małgorzata Sankowska (project manager)

Paweł Bilski (research supervisor)

The goal of the project is the optimization of photoluminescence properties of lithium fluoride (LiF) crystals for their application as radiation detectors and gaining a better understanding of temperature aspects of colour center physics.

With the rapid development of technology, ionizing radiation becomes more and more prevalent in human life. It gives a motivation to introduce new and to improve already existing methods of detecting, measuring, and imaging of ionizing radiation that could be used in various areas e.g. medicine, radiation protection, industry, research, and many others. The Fluorescent Nuclear Track Detection method (FNTD) is one of the most revolutionary techniques developed over the last years in the field of radiation dosimetry.

FNTD technique exploits photoluminescence (PL) of radiation-induced crystal lattice defects called colour centers (CC). Such centers, when excited by light of an appropriate wavelength, emit photons that make it possible to see the track of a particle while using a fluorescence microscope. Thanks to lithium fluoride (LiF) nuclear track detectors it is possible to image tracks of heavy ions such as helium, carbon, neon, silicon, and iron. They can also be used to detect and measure neutron doses or to establish the energy of the alpha particles interacting with the crystal. LiF crystals were also exploited to register tracks of cosmic radiation at the Earth’s orbit.

While it is apparent that this technique has great potential it, unfortunately, suffers due to the low signal-to-noise ratio. The enhancement of signal-to-noise ratio and photoluminescence intensity is extremely important as it could allow imaging tracks not visible now and open new directions of applications. The thermal treatment of LiF crystals seems to be the most feasible way to improve the signal-to-noise ratio.

Although LiF is a very well-known luminescent and optical material studied for several decades there are still some gaps in our knowledge about it. One of them is the influence of thermal treatment on colour centers and their photoluminescence. Concentrations of various colour centers in LiF and their photoluminescence spectra were found to be significantly influenced by temperature.  Under thermal treatment, some centers begin to disintegrate, while others interact with each other creating new species. All these have an impact on PL spectrum shape and intensity. While it is known that such dependencies exist, the reported data are often contradictory or incomplete. Our goal is therefore to gather missing information on this subject.

Within the project, we plan to investigate the temperature effects at each part of the stage of the process (before irradiation, during irradiation, after irradiation and during PL measurement). We intend to measure absorption spectra, PL emission spectra and PL excitation spectra as well as to register and analyze microscopic images of nuclear tracks in various spectral windows. In all experiments, not only temperature but also the duration of treatment, as well as heating and cooling rates will be varied. We also intend to check if observed effects are universal or are dependent on the specific properties of crystals. To establish that, we plan on using in our experiments crystals grown with different methods, with different growth parameters, starting materials etc.

The project results have so far been presented in the form of a poster (poster) at the 11th International Conference on Luminescent Detectors and Transformers of Ionizing Radiation (The poster received the “Best Poster Award”), International Conference on Individual Monitoring of Ionising Radiation (IM2022) and Neutron and Ion Dosimetry Symposium (NEUDOS-14), 9th International Conference on Optical, Optoelectronic and Photonic Materials and Applications & 14th Europhysical Conference on Defects in Insulating Materials, and The 7th International Workshop on Advanced Spectroscopy and Optical Materials (IWASOM) The results were also published in the Radiation Measurements and Materials journals.