Optical Diagnostics Using Lithium Fluoride Color Centers

Lithium fluoride (LiF) crystals host point defects known as colour centres, which form when ionising radiation displaces fluorine ions and creates vacancy aggregates. Under optical excitation, these defects emit visible photoluminescence whose intensity and spatial distribution are directly related to the deposited radiation dose and particle energy. Two aggregate defects dominate the emission spectrum: the red-emitting F₂ centre and the green-emitting F₃⁺ centre. By imaging these emissions with microscopes or wide-field detectors, researchers can reconstruct three-dimensional maps of radiation tracks, profile charged-particle beams and measure dose distributions with micrometre resolution. Applications span from advanced diagnostics of proton and X-ray beams at large-scale facilities to passive dosimetry in medical radiotherapy and neutron field monitoring. The combination of high spatial resolution, wide dynamic range and simple read-out optics makes LiF-based detectors a versatile platform for optical diagnostics in physics, biology and materials science.

Research from Nature Portfolio

Recent foundational work has demonstrated in situ three-dimensional visualisation of X-ray free-electron laser (XFEL) beam profiles by direct irradiation of a LiF crystal. High photoluminescence sensitivity and a large dynamic range enabled mapping of beam intensity both near and far from the focal plane with sub-micrometre resolution. Data confirmed theoretical models of photoelectron cloud expansion within LiF and allowed quantitative evaluation of beam quality and source size. The methodology shows promise for single-shot characterisation of femtosecond X-ray pulses and optimisation of focusing optics at XFEL and synchrotron facilities.

Research from all publishers

Optical read-out of LiF has been extended to low-energy proton imaging, where individual proton tracks at energies around 1–2.6 MeV were detected and counted using fluorescence microscopy. By focusing excitation light at defined depths, researchers estimated proton range and validated fluence measurements against conventional plastic detectors, opening avenues for microscopic track dosimetry in radiobiology.

Thin-film LiF detectors deposited on silicon substrates have been used to image full Bragg curves of proton beams up to 35 MeV. Radiophotoluminescence images acquired under blue-light excitation revealed depth profiles of F₂ and F₃⁺ centres corresponding to energy deposition in the film and substrate. Careful comparison with Monte Carlo simulations elucidated the roles of multiple Coulomb scattering and film density, paving the way for compact on-chip beam diagnostics in proton therapy research.

Studies of dose linearity and dynamic range in bulk LiF crystals irradiated with 2.3 MeV and 26 MeV protons have shown that F₂ centre emission exhibits a linear response over more than five orders of magnitude in dose, with a dynamic range exceeding 110 dB. Such broad linearity and stable luminescence under varying dose rates underscore the suitability of LiF for quantitative, high-resolution dosimetry in clinical and experimental settings.

Optical Diagnostics Using Lithium Fluoride Color Centers publication trend

The graph below shows the total number of articles in optical diagnostics using lithium fluoride color centers across all publications each year (not limited to Nature Index journals).

Technical terms

Colour centre: A point defect in a crystal lattice that absorbs and emits light when activated by radiation.

Radiophotoluminescence: Visible light emission from colour centres under optical excitation following ionising radiation exposure.

Bragg curve: A graph showing the variation in energy deposition of charged particles as they travel through matter, featuring a pronounced peak near the end of their range.

F₂ centre: An aggregate defect in LiF consisting of two adjacent fluorine vacancies; emits red photoluminescence around 670 nm.

F₃⁺ centre: A tri-vacancy aggregate defect in LiF that emits green photoluminescence around 525 nm.

References

1. 3D visualization of XFEL beam focusing properties using LiF crystal X-ray detector. Scientific Reports (2015).
2. Detection of fluorescent low-energy proton tracks in lithium fluoride crystals. Radiation Measurements (2024).
3. Proton Bragg peak imaging by colour centre radiophotoluminescence in lithium fluoride thin film radiation detectors on silicon. Journal of Materials Science: Materials in Electronics (2023).
4. Dynamic range and dose linearity of the radiophotoluminescence intensity in lithium fluoride crystals irradiated with 2.3 and 26 MeV protons. Journal of Luminescence (2023).