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Abstract: Lithium fluoride, LiF, is a well-known dosimeter material. In thin films the direct use of optical absorption spectra to individuate the stable formation of different kinds of point defects is often precluded by the presence of interference fringes due to the refractive index difference between film and substrate. Photoluminescence measurements are more sensitive. Recently a very effective investigation method is developing: it is often called combined excitation–emission spectroscopy (CEES) in the literature. In this work we present the basic characteristics of this technique and the first results of the investigation of polycrystalline LiF films grown by thermal evaporation on fused silica substrates and gamma irradiated at several doses up to 106 Gy in air. [Copyright &y& Elsevier]

The white beam (∼6–80 keV) available at the TopoTomo X-ray beamline of the ANKA synchrotron facility (KIT, Karlsruhe, Germany) was used to perform edge-enhancement imaging tests on lithium fluoride radiation detectors. The diffracted X-ray image of a microscopic boron fibre, consisting of tungsten wire wrapped by boron cladding, was projected onto lithium fluoride thin films placed at several distances, from contact to 1 m . X-ray photons cause the local formation of primary and aggregate colour centres in lithium fluoride; these latter, once illuminated under blue light, luminesce forming visible-light patterns—acquired by a confocal laser scanning microscope—that reproduce the intensity of the X-ray diffracted images. The tests demonstrated the excellent performances of lithium fluoride films as radiation detectors at the investigated photon energies. The experimental results are here discussed and compared with those calculated with a model that takes into account all the processes that concern image formation, storing and readout. [ABSTRACT FROM AUTHOR]

• Photoluminescence behavior of color centers in LiF irradiated by 18 MeV protons. • Proton beam imaging by visible photoluminescence of color centers in LiF crystals. • Photoluminescence saturation dose from Bragg curve profiles reconstruction. • Photoluminescence saturation dose from proton beam transversal intensity maps. • Reliable method for proton-beam diagnostics and LiF dose response investigation. The paper concerns the analysis of spatial distributions of visible photoluminescence of optically active F 2 and F 3 + color centers created in lithium fluoride crystals by irradiation with proton beams. Spectrally integrated photoluminescent maps are detected in a fluorescence microscope and numerically elaborated to gather pieces of information regarding both the beam characteristics and the material response. A method based on two independent measurements (Bragg curve and beam transversal map) is developed to evaluate the saturation dose defined as the value above which the photoluminescence intensity begins saturating up to an asymptotic maximum. The paper describes the procedure and demonstrates its application to the experimental case of LiF crystals irradiated by a proton beam of 18 MeV nominal energy. [ABSTRACT FROM AUTHOR]

Solid-state radiation detectors based on the visible photoluminescence of radiation-induced F 2 and F 3 + color centers in lithium fluoride crystals have been used for advanced diagnostics of 18 and 27 MeV proton beams, produced by the TOP-IMPLART linear accelerator, which is under development for protontherapy applications at ENEA C.R. Frascati, Italy. Visible fluorescence microscopy was successfully used to read the latent proton beam images stored in the lithium fluoride crystals, thanks to high emission efficiency of the F 2 and F 3 + color centers obtained by simultaneous optical excitation in the blue spectral range. High intrinsic spatial resolution and wide dynamic range of these novel LiF detectors allow obtaining two-dimensional images of both the beam transverse intensity distribution and of the Bragg curve. By using a simple defect formation model that takes into account the energy released in the material and the saturation of defect concentration, not only the two-dimensional dose map was obtained from the beam transverse intensity distribution image, but also the full Bragg curve was reconstructed, allowing the thorough characterization of proton beams produced by the accelerator. • Photoluminescence of color centers in LiF crystals used for proton beam imaging. • High-resolution imaging of the Bragg curve with a fluorescence microscope. • Estimation of the proton beam parameters from Bragg curve best-fitting. • High-resolution imaging of the transversal proton beam intensity distribution. • Dose mapping obtained from the transversal proton beam intensity imaging. [ABSTRACT FROM AUTHOR]

Abstract Lithium fluoride (LiF) crystals were irradiated with a proton beam. With nominal beam energies of 3 and 7 MeV, several irradiations were performed which delivered doses ranging from 103 to 107 Gy. The ionisation produced by the protons in the LiF samples induced the stable formation in the crystalline lattice of colour centres, two types of which, the F 2 and F 3 + ones, possess broad photoluminescence bands in the red and green, respectively, when optically pumped in the blue at wavelengths close to 450 nm. At both proton energies, measurements of the distinct F 2 and F 3 + contributions to the emission spectra showed a decrease of the F 3 + integrated photoluminescence intensity at large doses, say from ∼106−107 Gy on. We try to explain such a behaviour by assuming the formation of absorption and/or quenching centres, and compare the predictions of an ad hoc developed model with experimental data. Highlights • Photoluminescence by colour centres generated in LiF crystals by 3&7 MeV protons. • Photoluminescence of F 3 + colour centres in LiF diminishes at high doses. • Reabsorption and quenching possible causes of photoluminescence decay. • Theoretical model for photoluminescence intensity vs. dose that includes quenching. • Reabsorption of photoluminescence in LiF possibly caused by F 4 colour centres. [ABSTRACT FROM AUTHOR]

Colour centres (CCs) in lithium fluoride (LiF) are well known for application in tuneable lasers and dosimeters. The visible photoluminescence (PL) of radiation-induced, broad-band light-emitting aggregate CCs in LiF crystals and films has been proposed for high spatial resolution X-ray imaging; use of LiF-based detectors has been recently successfully extended to advanced diagnostics of low-energy proton beams. After exposure, transversal dose mapping was obtained on LiF films by acquiring the visible PL image of the irradiated spots in a fluorescence microscope under blue-light pumping. Irradiation of thermally evaporated LiF thin films with a proton beam of 3 MeV nominal energy, produced by a linear accelerator, in the fluence range of 10 11 –10 15 protons/cm 2 , induces the formation of stable CCs, mainly the primary F centre and the aggregate F 2 and F 3 + defects. A comparison with irradiations performed at 7 MeV shows that the spectrally integrated PL as a function of the absorbed dose is independent on the selected beam energy, at least as far as typical LiF film thicknesses are concerned. The PL behaviour vs. dose can be described by a linear growth which covers up to three order of magnitude, followed by saturation at high values (> ≈10 5 Gy). The spectral contributions of F 2 and F 3 + CCs to the detected PL, in the red and in the green respectively, under laser pumping were carefully analysed in order to investigate behaviour differences of these defects at high doses. [ABSTRACT FROM AUTHOR]

Solid-state radiation detectors based on the photoluminescence of stable point defects in lithium fluoride crystals have been used for advanced diagnostics during the commissioning of the segment up to 27 MeV of the TOP-IMPLART proton linear accelerator for proton therapy applications, under development at ENEA C.R. Frascati, Italy. The LiF detectors high intrinsic spatial resolution and wide dynamic range allow obtaining two-dimensional images of the beam transverse intensity distribution and also identifying the Bragg peak position with micrometric precision by using a conventional optical fluorescence microscope. Results of the proton beam characterization, among which, the estimation of beam energy components and dynamics, are reported and discussed for different operating conditions of the accelerator. [ABSTRACT FROM AUTHOR]

Proton beams of 7 MeV energy, produced by a linear accelerator, were used to irradiate LiF crystals and thin films thermally evaporated on glass substrates in the dose range from 10 3 to 4 × 10 6 Gy, inducing the formation of stable photoluminescent colour centres (mainly F 2 and F 3 + ), emitting in the visible spectral range. Using a conventional fluorescence microscope, the transversal proton beam intensity was mapped by acquiring the photoluminescence image of the irradiated spots. Image analysis allowed measuring the integrated photoluminescence intensity as a function of the irradiation dose: a linear optical response was obtained up to different maximum dose values, after which a quenching was observed, depending on the nature of the samples (crystals or films). The colour centres formation was investigated by optical absorption spectroscopy at room temperature and the Principal Component Analysis was applied to the absorption spectra of irradiated LiF crystals. In samples irradiated at highest doses, it allowed clearly identifying the formation of more complex aggregate defects, which appears strictly related to the observed photoluminescence quenching effect. [ABSTRACT FROM AUTHOR]

Luminescence properties of point defects in insulating materials are successfully used for solid state light sources and radiation detectors. Among them, colour centres in lithium fluoride, LiF, are well known for their application in tuneable lasers and dosimeters. Broad-band light-emitting F 2 and F 3 + electronic defects, stable at room temperature, are produced in LiF crystals and films by different kinds of radiation. Under blue optical pumping in their overlapping absorption bands, the efficient photoluminescence spans over the green–red visible spectral range. Novel LiF thin-film radiation imaging detectors based on the exploitation of the peculiar spectral characteristics of F 2 and F 3 + defects and of the optical properties and radiation sensitivity of the LiF material have been proposed and successfully tested for extreme ultraviolet radiation, soft and hard X-ray imaging, including micro-radiography of biological objects, even for in vivo samples. After exposure to X-rays, the latent images stored in the LiF thin layers by local formation of active defects are read with conventional and advanced optical fluorescence microscopes. Among the main advantages of the LiF thin-film imaging detectors, there are intrinsic very high spatial resolution, large field of view and wide dynamic range. Moreover, these solid state radiation detectors are easy to handle, as insensitive to ambient light, and no development process is needed. Recently their use has been extended to proton beam advanced diagnostics. In this paper, a short review of their properties, results and applications in X-ray biological imaging and proton dose-mapping is presented, underlying the great versatility and potentialities offered by LiF thin films, whose photoluminescence response can be improved through the choice of suitable substrates and growth conditions. [ABSTRACT FROM AUTHOR]

Proton beams of 3 and 7 MeV energies, produced by a linear accelerator, were used to irradiate lithium fluoride crystals and thermally evaporated LiF thin films in the fluence range of 10 11 –10 15 protons/cm 2 . The irradiation induces the formation of stable colour centres, mainly the primary F centre and the aggregate F 2 and F 3 + defects. By optical pumping in the blue spectral region, the F 2 and F 3 + centres emit broad photoluminescence bands in the visible spectral range. By conventional fluorescence microscopy, the integrated photoluminescence intensity was carefully measured in LiF crystals and thin films as a function of the irradiation fluence: a linear optical response was obtained in a large range of fluence, which is dependent on the used LiF samples and the selected beam energy. It was possible to record the transversal proton beam intensity profile by acquiring the photoluminescence image of the irradiated spots on LiF films by a standard optical microscope. Using LiF films grown on silicon substrates irradiated in a particular geometry, the same optical reading microscopy technique allowed one to measure the distribution of colour centres photoluminescence along the depth and direct imaging the Bragg peak position, which gives a rough estimation of the initial proton beam energy. [ABSTRACT FROM AUTHOR]

Abstract: Lithium fluoride is a well-known dosimeter material and it is currently under investigation also for high-resolution radiation imaging detectors based on colour centre photoluminescence. In order to extend their applications, proton beams of 3MeV energy, produced by a linear accelerator, were used to irradiate LiF crystals and thin films in the fluence range of 1010–1015 protons/cm2. The irradiation induces the formation of colour centres, mainly the primary F centre and the aggregate F2 and defects, which are stable at room temperature. By optical pumping in the blue spectral region, the F2 and centres emit broad photoluminescence bands in the visible spectral range. By conventional fluorescence microscopy, the integrated photoluminescence intensity was carefully measured in LiF crystals and thin films as a function of the irradiation fluence: a linear optical response was obtained in a large range of fluence, which is dependent on the used LiF samples. Colour centres concentrations were estimated in LiF crystals by optical absorption spectroscopy. It was possible to record the transversal proton beam intensity profile by acquiring the photoluminescence image of the irradiated spots on LiF films. [Copyright &y& Elsevier]

Abstract: In this work, we present preliminary in-line X-ray lensless projection imaging results at a synchrotron facility by using novel solid-state detectors based on non-destructive readout of photoluminescent colour centres in lithium fluoride thin films. The peculiarities of LiF radiation detectors are high spatial resolution on a large field of view, wide dynamic range, versatility and simplicity of use. These properties offered the opportunity to test a broadband X-ray synchrotron source for lensless projection imaging experiments at the TopoTomo beamline of the ANKA synchrotron facility by using a white beam spectrum (3–40 keV). Edge-enhancement effects were observed for the first time on a test object; they are discussed and compared with simulations, on the basis of the colour centre photoluminescence linear response found in the investigated irradiation conditions. [Copyright &y& Elsevier]

Abstract: Novel X-ray imaging detectors based on photoluminescence of colour centres in lithium fluoride (LiF) have been proposed and tested for extreme ultraviolet, soft and hard X-rays up to 10keV. For the first time we present the optical characterisation of LiF crystals and thin films irradiated at the TOPO–TOMO beamline of synchroton light source Anka (Karlsruhe, Germany) in the energy range 6–40keV for different exposure times. Absorption and photoluminescence spectra were analysed to study the optical response of the LiF-based detectors. High resolved X-ray imaging of commercial test patterns has been obtained on LiF crystals and films by optical readout with a confocal laser scanning fluorescence microscope. [Copyright &y& Elsevier]

Abstract: Among insulating materials containing point defects, lithium fluoride (LiF) is a radiation-sensitive alkali halide well known in dosimetry and as active medium in light-emitting devices and lasers. Electronic point defects can be produced in LiF crystals and films by different kinds of radiation. Some of these electronic defects, known as colour centres, are optically active, with broad absorption and emission bands in the visible spectral range. Novel thin-film imaging detectors for extreme ultraviolet and soft X-rays, based on photoluminescence from aggregated colour centres in LiF, have been proposed and are currently under development, successfully extending their operation also in the hard X-ray region, up to 10keV. Among the main peculiarities of LiF-based detectors, there are intrinsic high spatial resolution, in principle limited only by the point defect size, large field of view and wide dynamic range. These imaging detectors feature great versatility, as they can be grown in the form of thin films on different substrates by well-assessed physical deposition techniques. We present the promising results obtained using different X-ray laser–plasma sources in the field of nanotechnology, biology and in material science in view of future perspectives of research and development. [ABSTRACT FROM AUTHOR]

Abstract: We report about the light-emitting properties of colour centres in lithium fluoride thin films and crystals treated with low-penetrating extreme-ultraviolet radiation and soft X-rays. Experimental fluorescence-imaging results show that, under the same irradiation conditions, stronger photoluminescence, up to a factor eight, is released by colour centres in a film thermally grown on silicon substrate than in a crystal. By using a classical-electromagnetism model, we take into account the role of the silicon substrate in the enhancement of spontaneous emission over a broad spectral range to analyze such behaviour. [ABSTRACT FROM AUTHOR]

Passive solid-state radiation detectors, based on the radiophotoluminescence of stable color centers in nominally pure lithium fluoride (LiF) crystals, have been used for advanced diagnostics of the segment up to 35 MeV of the TOP-IMPLART proton linear accelerator for protontherapy applications, under development at ENEA Frascati. In LiF, proton beams generate a spatial distribution of F 2 and F 3 + aggregate color centers, which efficiently emit radiophotoluminescence in the red-green spectral range under optical pumping around 450 nm. LiF crystals were irradiated with 35 MeV nominal energy protons at several doses of interest for protontherapy (from 0.5 to 50 Gy). For the first time is shown here the possibility of measuring the radiophotoluminescence spectra of proton irradiated LiF even at such low doses and how the signal showed a linear behavior with dose with a good signal-to-noise ratio. A Bragg-curve image, stored as color-center distribution in a LiF crystal after a 50 Gy-dose irradiation in a particular geometry, was acquired in a conventional fluorescence microscope. After appropriate image processing, the signal intensity resulted to be proportional to the linear energy transfer along the whole Bragg curve. By best-fitting it with an ad-hoc analytical formula, the depth-dose distribution was reconstructed and the proton beam energy and energy spread were estimated. • RPL spectra of 35 MeV proton-irradiated LiF crystals measured from 0.5 to 50 Gy. • RPL response linear vs dose with good signal-to-noise ratio under laser excitation. • RPL Bragg curve image obtained by a fluorescence microscope at low doses. • RPL response along the whole Bragg curve reproduces well the LET curve in LiF. [ABSTRACT FROM AUTHOR]