Promising technologies for high-resolution X-ray contact imaging of biological objects with an innovative LiF-based detector

Extreme ultraviolet (EUV) radiation and soft X-rays (XUV) are currently used by physicists and biologists to obtain images of living biological samples at a spatial resolution lower than 100 nm. The (2.2-4.4) nm wavelength interval (known as “water-window”) is generally preferred, because of the strong absorption of carbon (as compared with that of water) which allows to obtain a natural contrast in cell imaging. X-ray contact microscopy (the sample is located in tight contact with the imaging detector) is a simple technique, it does not require monochromatic and coherent radiation and it allows to record single-shot in vivo images. The spatial resolution of the images obtained by contact microscopy is essentially limited by the resolution of the used detector. We proposed and tested a novel X-ray imaging detector based on optically stimulated luminescence (OSL) of stable color centers (CCs) in lithium fluoride (LiF) [1, 2, 3] that has very peculiar performances without requiring any development process. LiF is a radiation-sensitive material, well known in dosimetry and used in optolectronic devices. Photoluminescent CCs can be produced at the surface of LiF crystals and films by different kinds of ionizing radiation, such as soft X-rays and EUV light. The readout process of the X-ray LiF-based detector consists of measuring the efficient OSL of the CCs in the visible spectral range under blue-light pumping. The OSL signal intensity is locally proportional to the X-ray transparency of the specimen that was placed in contact with the LiF surface during the exposure. By using advanced optical microscopes in fluorescence mode, such as a Confocal Laser Scanning Microscope or a Scanning Near Field Microscope (SNOM), OSL images with submicron spatial resolution can be obtained. The peculiarities of the LiF detector, like its high spatial resolution (in principle limited only by the CC dimension, that is, at atomic scale) over a large field of view, wide dynamic range, simplicity of use, and efficiency of the reading technique, can be exploited for X-ray microscopy in different configurations, even for in vivo and lensless imaging. Results will be presented concerning images of biological specimens obtained in absorption contrast by using laser-plasma X-ray sources. Improvements of LiF detector performances and of its readout technique are currently under development.