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Solid-state radiation detectors based on the visible photoluminescence of radiation-induced F2 and F3+ 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 F2 and F3+ 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.

An analytical model that approximates Bragg curves due to protons in water is extended to lithium fluoride (LiF), both in bulk and lower-density homogeneous thin film form. To that purpose, Monte Carlo simulations of proton range for energies from 5 to 65 MeV are best fitted so that optimal values of the model parameters are deduced for LiF. Tests are performed by comparing analytically computed Bragg curves with corresponding ones simulated by Monte Carlo software in LiF, bulk and lower density, at two example proton energies and energy-straggling widths. The model is then applied to the reconstruction of the energy spectra of proton beams by best fitting experimental Bragg profiles due to visible photoluminescence of optically-active F2 and F 3 + color centers created in LiF crystals by the ionizing interaction of protons with the material.

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 F2 and F3+ 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.

Surface treatments were performed on single-crystal semi-insulating 6H–SiC by femtosecond pulsed laser irradiation, aimed at analyzing the effect of the laser-induced periodic surface structures (LIPSSs) on the films’ optical properties. The surface morphology study of the laser-induced nanostructures allows determining the modification threshold fluence of about 0.7 J cm−2 as well as detecting fine (160 nm) and coarse (450–550 nm) ripples according to different values of the laser pulse fluence (ΦP) released to the material. Micro-Raman spectroscopy allows determining the presence of undesired amorphous structural phases when ΦP exceeds 2.15 J cm−2, whereas no compositional variations occur for lower values of ΦP. Samples treated on the entire surface with the pulse fluence conditions to obtain fine ripples were optically tested. Although the long-range order is progressively lost as the accumulated laser fluence increases, the heaviest treated samples show solar absorptance values > 75% and spectral selectivity up to 1.7 projected at the operating temperature of 1000 K, thus pointing out the suitability of fs-laser surface textured 6H–SiC to act as a selective solar absorber for energy conversion devices operating at high temperature.

Lithium fluoride crystals were gamma irradiated at three different clinical doses (5, 10 and 20 Gy) with the 60Co reference beam of the Italian National Institute of Ionizing Radiation Metrology. The irradiations created optically-active point defects in the crystal lattice, among which the aggregate F2 and F3+ color centers, whose visible photoluminescence spectra were subsequently measured for increasing power densities of a 445 nm laser excitation in stationary conditions. The careful analysis of the spectra best fits vs. the excitation laser power suggests the presence of an emission band spectrally overlapped with the F3+ one and ascribable to F3 (R2) defects. Finally, a suitable elaboration of the resulting data for the emission band of the F3+ centers allows estimating a parameter of the energy triplet state of these defects, whose value well agrees with the literature.

We present the study of optical and spectral properties of radiation-induced stable point defects, known as color centers (CCs), in lithium fluoride (LiF) for the detection of 10 keV XFEL beam at Spring-8 Angstrom Compact free electron LAser (SACLA) in Japan. A thick LiF crystal was irradiated in four spots with 10 keV XFEL beam (pulse duration = 10 fs) with different number of accumulated shots. After irradiation the colored-LiF spots were characterized with an optical microscope in fluorescence mode and their photoluminescence intensity and spectra were analyzed.

This contribution to ECPD2019 is dedicated to the memory of Anatoly Faenov. During a period of approximately thirteen years 1994–2006, Anatoly and his wife Tatiana Pikuz (simply “Tania” for friends), accepting the frequent invitations of the National Institute for Nuclear Physics (INFN) and of the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), cooperated with many Italian research laboratories dedicated to EUV and soft X-ray generation, spread in different towns (L'Aquila, Frascati, Milano, Padova, Pisa, Roma, etc.). In spite of the fact that they could stay in Italy only about one or two months per year, their activity was so intense that more than 50 peer- reviewed publications were generated from their experimental and theoretical work (just considering only the results obtained at L'Aquila and Tor Vergata—Rome Universities and at the ENEA Research Center of Frascati), without mentioning the cultural atmosphere that they stimulated in the field of Science and Humanity. The numerous experimental spectra obtained at ENEA by means of their spherically bent mica spectrometers, together with the corresponding theoretical simulations performed in Moscow, allowed to study the changing role of different excitations mechanisms for various plasma conditions, and to characterize at best the ENEA laser-plasma source for different applications: polychromatic and monochromatic micro-radiography of dried biological samples at 1 keV, soft X-ray contact microscopy (SXCM) of living cells in the water-window spectral region, spectroscopy of hollow atoms, etc. In this memorial paper, the main results of biological samples imaging on lithium fluoride (LiF) detectors, obtained with the ENEA and Tor Vergata University laser-plasma sources, are presented. In particular, the improvement of the micro-radiography and of the SXCM techniques obtained after moving from photoresist detectors and photographic films to lithium fluoride (LiF) detectors are discussed, for both dried and wet biological samples.

In a recent paper, an analytical expression for proton Bragg curves was introduced and utilized to best fit photoluminescent depth distributions of optically active F 2 and F 3 + color centers in lithium fluoride, with the purpose of evaluating the energy spectrum of a proton beam, assumed to consist of the superposition of Gaussian components. Here, an original alternative method is introduced by means of which the same energy spectrum analysis can be performed by iteratively applying a suitable mathematical operator to the photoluminescent depth distribution, no best fitting operation being needed.

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.

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 F2 and F3+ 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 F2 and F3+ 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. © 2015 Elsevier B.V. All rights reserved.

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 1011–1015 protons/cm2, induces the formation of stable CCs, mainly the primary F centre and the aggregate F2 and F3 + 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 (> ≈105 Gy). The spectral contributions of F2 and F3 + 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. © 2018 Elsevier B.V.

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 10 keV. 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 6-40 keV 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. © 2012 Elsevier B.V.

Summary In this paper, the use of lithium fluoride (LiF) as imaging radiation detector to analyse living cells by single-shot soft X-ray contact microscopy is presented. High resolved X-ray images on LiF of cyanobacterium Leptolyngbya VRUC135, two unicellular microalgae of the genus Chlamydomonas and mouse macrophage cells (line RAW 264.7) have been obtainedutilizingX-rayradiationinthewaterwindowenergy rangefromalaserplasmasource.Theusedmethodisbasedon loading of the samples, the cell suspension, in a special holder wheretheyareinclosecontactwithaLiFcrystalsolid-stateXrayimagingdetector.Afterexposureandsampleremoval,the imagesstoredinLiFbythesoftX-raycontactmicroscopytechniquearereadbyanopticalmicroscopeinfluorescencemode. The clear image of the mucilaginous sheath the structure of the filamentous Leptolyngbya and the visible nucleolus in the macrophage cells image, are noteworthiness results. The peculiaritiesoftheusedX-rayradiationandoftheLiFimaging detectorallowobtainingimagesinabsorptioncontrastrevealing the internal structures of the investigated samples at high spatialresolution.Moreover,thewidedynamicrangeoftheLiF imagingdetectorcontributestoobtainhigh-qualityimages.In particular, we demonstrate that this peculiar characteristic of LiF detector allows enhancing the contrast and reveal details even when they were obscured by a nonuniform stray light.

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 1011-1015 protons/cm2. The irradiation induces the formation of stable colour centres, mainly the primary F centre and the aggregate F2 and F 3 + defects. By optical pumping in the blue spectral region, the F2 and F3 + 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.© 2014 Published by Elsevier B.V.

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. © 2017 Elsevier B.V.

Planar active waveguides were produced in lithium fluoride crystals by implantation with 1.5 MeV He + ions at several doses. The colored samples have been characterized by optical absorption, photoluminescence and m-line spectroscopy. By comparing the measured guided-mode effective indices with the ones calculated by means of the Chandler–Lama approach, the depth profile of refractive index was derived, showing that there are two competitive physical mechanisms, associated with different processes of energy deposition along the ion track, responsible for positive and negative modifications of the refractive index in the irradiated volume.

Irradiation of lithium fluoride (LiF) crystals by low energy (few keV) electron beams gives rise to the efficient formation of stable laser active colour centres, mostly F2 and F3+ aggregate defects, on their surface. The presence of primary and aggregate point defects induces a local increase of the refractive index of the thin irradiated layer in the green-red spectral range, where the F3+ and F2 visible emissions are located. The use of electron-beam lithography techniques allows for the definition of coloured strips, so miniaturised optical active channel waveguides may be fabricated by a direct-writing process. We report on the observation of amplified spontaneous emission of the red light from LiF:F2 centres in active waveguides realised with different irradiation doses in a LiF crystal, in order to determine the influence of this fundamental parameter on the amplification properties of these optical confining structures. Low pumping power densities have been used in quasi-continuous-wave regime at room temperature; the appreciable values of the gain coefficient, several cm−1, with an exciting power density of few hundreds mW/cm2, make this material a good candidate for the realisation of active integrated optical devices.

Nuclear instruments & methods in physics research / A 833, 68 - 76(2016). doi:10.1016/j.nima.2016.07.005, 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 1m. 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., Published by Elsevier, Amsterdam

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.

A lithium fluoride crystal slab was irradiated in air with a proton beam of nominal 7 MeV energy at a linear accelerator that is under development at ENEA C.R. Frascati. The irradiation generated a spatial distribution of stable colour centres in the crystal that could be detected as a luminescent image under blue-light excitation in a fluorescence microscope. The reconstruction of the whole Bragg curve from the luminescent image is here demonstrated for the first time by taking into account finite proton-energy bandwidth and saturation of defect concentration due to high absorbed dose. The obtained results confirm lithium fluoride detectors based on visible photoluminescence of radiation-induced aggregate colour centres as reliable candidate devices for advanced proton-beam diagnostics. © CopyrightEPLA, 2018.

The use of LiF thin films and single crystals was proposed and tested as X-ray imaging detector based on optically stimulated luminescence from visible-emitting color centers. The main peculiarities of this detector – i.e. high spatial resolution on a large field of view, wide dynamic range, versatility and simplicity of use – make it very promising as X-ray imaging plate for applications in photonic devices, biology and material science, as well as in the characterization of intense X-ray sources. In order to investigate the response and the sensitivity of this detector to soft X-rays, a suitable characterization of colored thin layers is performed by means of optical spectrophotometry and photoluminescence spectroscopy for samples colored under different irradiation conditions.

We present experimental results on simultaneous surface nanostructuring and optical activation of lithium fluoride crystals by 800 eV off-normal Ar + sputtering. Our data demonstrate that the formation of periodic nanostructures is accompanied by the efficient production of stable electronic defects, optically active in the green and red parts of the visible spectra, thus providing the possibility to conceive and fabricate advanced insulating substrates.

A confocal laser scanning microscope has been used in a non-conventional approach to reconstruct the depth distribution of stable, optically active color centers induced at the surface of a lithium fluoride crystal by a low energy electron beam. The formation of F2 and F 3 + aggregate electronic defects was limited to the penetration depth of the electrons in the material, as obtained from Monte Carlo simulations. This novel measurement technique could be easily used for spatial characterization of luminescent optical waveguides in transparent dielectrics.

Broad-band visible-emitting micro-strips were induced in Lithium Fluoride crystals by direct writing with a monochromatic soft X-ray beam. They were successfully characterized via fluorescence imaging, photoluminescence and optical transmittance. The irradiation gave rise to stable primary and aggregate color centers within a thin surface layer of the crystals. Their volume concentrations were evaluated together with the induced modification of the complex refractive index in the irradiated volume.

Processes of radiation creation and annealing of Frenkel defects, as well as their transformation processes, have been studied in γ -irradiated LiF single crystals containing various impurities, by means of measurements of thermally stimulated changes in absorption and emission spectra. Important information about the density of colour centres in LiF, which depends on γ -irradiation doses, impurities and temperatures, has been obtained for small crystal contents of oxygen, hydroxyl and metal ions. A comparison of annealing parameters of the LiF crystals subjected to different doses of irradiation, but to the same thermal bleaching procedures, has been performed and the results are critically discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Light emitted when heating crystalline materials previously exposed to ionizing radiation is commonly known as thermoluminescence. The resultant glow curve is related to the material, the impurities and the defect centers generated by the irradiation. Several efforts have been made in the past to associate glow peaks with specific defect centers. Recently, in the case of LiF we established a link between F3+ and F3 centers and the temperature region of the glow curve below 200 °C. This result was obtained by using especially treated samples with known concentration of color centers in first place, and annealing measurements in second place. By continuing these studies, it has been possible to establish a further link between F2 centers and a prominent feature of the glow curve near 260 °C. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Permanent bleaching of F and F2 color centers in gamma irradiated lithium fluoride crystals has been obtained under low power, CW laser illumination at 244 nm. Optical absorption and photoluminescence measurements reveal a selective change in the concentrations of these kinds of electronic defects. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The use of efficient wavelength-shifters from the vacuum-ultraviolet to the photo-sensor's range of sensitivity is a key feature in detectors for Dark Matter search and neutrino physics based on liquid argon scintillation detection. Thin film of Tetraphenyl-butadiene (TPB) deposited onto the surface delimiting the active volume of the detector and/or onto the photosensor optical window is the most common solution in current and planned experiments. Detector design and response can be evaluated and correctly simulated only when the properties of the optical system in use (TPB film + substrate) are fully understood. Characterization of the optical system requires specific, sometimes sophisticated optical methodologies. In this paper the main features of TPB coatings on different, commonly used substrates is reported, as a result of two independent campaigns of measurements at the specialized optical metrology labs of ENEA and University of Tor Vergata. Measured features include TPB emission spectra with lineshape and relative intensity variation recorded as a function of the film thickness and for the first time down to LAr temperature, as well as optical reflectance and transmittance spectra of the TPB coated substrates in the wavelength range of the TPB emission.© 2013 IOP Publishing Ltd and Sissa Medialab srl.

A thin film of Tetraphenyl-butadiene (TPB) deposited onto the surface delimiting the active volume of the detector and/or onto the photosensor's optical window is the most common solution to down convert argon VUV scintillation light in current and planned liquid argon based experiments for dark matter searches and neutrino physics. Characterization of the main features of TPB coatings on different, commonly used substrates is reported, as a result of measurements at the specialized optical metrology labs of ENEA and University of Tor Vergata. Measured features include TPB emission spectra with lineshape and relative intensity variation recorded as a function of the film thickness and for the first time down to LAr temperature, as well as optical reflectance and transmittance spectra of the TPB coated substrates in the wavelength range of the TPB emission.© 2013 IOP Publishing Ltd and Sissa Medialab srl.

LiF films grown on glass and Si(100) substrates by thermal evaporation were used as radiation imaging detectors to evaluate the Bragg peak position of a 3 MeV proton beam produced by a linear accelerator. The proton irradiation of LiF films induces the formation of stable color centers, mainly the primary F centers and the aggregate F2 and F3 + defects, whose local concentrations are proportional to the deposited energy. Under optical pumping in the blue spectral region, F2 and F3 + color centers emit broad photoluminescence bands located at 678 and 541 nm, respectively, which can be detected by conventional fluorescence microscopy. Photoluminescence images of the top surface of the exposed LiF films allowed obtaining the Bragg peak position of the proton beam, whose evaluation is in satisfactory agreement with software simulations.

Among insulating materials, lithium fluoride (LiF) has been successfully used as ionizing radiation dosemeter for more than 60 years. Thermoluminescence (TL) has been the most commonly used reading technique to evaluate the absorbed dose. Lately, optically stimulated luminescence (OSL) of visible emitting color centers (CCs) has also been explored in pure and doped LiF. This work focuses on the experimental behaviour of nominally pure LiF crystals dosemeters for 6 MV x rays at low doses based on photoluminescence (PL) of radiation induced CCs. Polished LiF crystals were irradiated using 6 MV x rays produced by a clinical linear accelerator. The doses (absorbed dose to water) covered the 1-100 Gy range. Optical absorption spectra show stable formation of primary F defects up to a maximum concentration of 2 1016 cm-3, while no significant M absorption band at around 450 nm was detected. On the other hand, under Argon laser excitation at 458 nm, PL spectra of the irradiated LiF crystals clearly exhibited the characteristic F2 and F+ 3 visible broad emission bands. Their sum intensity is linearly proportional to the absorbed dose in the investigated range. PL integrated intensity was also measured using a conventional fluorescence optical microscope under blue lamp illumination. The relationship between the absorbed dose and the integrated F2 and F+ 3 PL intensities, represented by the net average pixel number in the optical fluorescence images, is also fairly linear. Even at the low point defect densities obtained at the investigated doses, these preliminary experimental results are encouraging for further investigation of CCs PL in LiF crystals for clinical dosimetry. © Published under licence by IOP Publishing Ltd.

Eight nominally pure lithium fluoride crystals, obtained from a single LiF crystal containing less than 100 ppm impurities, were irradiated by gamma-rays from a 60Co source at room temperature, with doses from 8.4 to 2.5 105 Gy, but one at -60 °C. Optical density measurements were performed to investigate the radiation-induced color centres (CCs) and to evaluate their concentrations. Thermoluminescence (TL) glow curves were collected and simulated by a first-order kinetics approach, and from the best-fit procedure ten glow peaks (GPs) spanning from 100 to 450 °C were highlighted. A comparative analysis of GP intensities and CC concentrations as a function of the irradiation dose has questioned their association as obtained in previous measurements, showing the impurities, less than 1018 cm-3, still playing a predominant role in TL spectra. New measurements on LiF crystals more pure, at least an order of magnitude, are required to establish for sure the association of GPs to CCs. © Published under licence by IOP Publishing Ltd.

Ion beam induced luminescence (IBIL) spectra of pure LiF under irradiation by a 2 MeV proton beam were analyzed as a function of the dose in order to deepen the kinetic mechanisms underlying the formation of luminescent point defects. The intensity evolution with dose at several emission wavelengths has been studied within a wide spectral interval, from ultraviolet (UV) to near infrared (NIR), and their different change rates have been correlated to the electronic defect formation processes. The intensity at few selected wavelengths was analyzed with a multiple linear regression (MLR) method in order to demonstrate that a linear calibration curve can be obtained and that an on-line optical dose monitor for ion beams can be realized. © 2015 Elsevier B.V. All rights reserved.

The depth refractive index profiles of broadband visible-emitting planar waveguides produced in LiF crystals with 1.5- and 2-MeV He+ ions at different doses have been derived from mode analysis. They show that there are two competitive mechanisms responsible for positive and negative modifications of the refractive index in the irradiated volume associated with different processes of energy deposition of the incident ions, so as to induce a complex coloration profile along the penetration direction, which is strongly dependent on the irradiation dose.

Point defects in lithium fluoride (LiF) have recently attracted renewed attention due the exciting results obtained in the realisation of miniaturised optical devices. Among light-emitting materials, LiF is of particular interest because it is almost not hygroscopic and can host, even at room temperature, stable color centers (CCs) that emit light in the visible and in the near infrared spectral range under optical excitation. The increasing demand for low-dimensionality photonic devices imposes the use of advanced irradiation methods for producing luminescent structures with high spatial resolution. An innovative irradiation technique to produce luminescent CCs in LiF crystals and films by using an extreme ultra-violet and soft X-ray laser-plasma source will be presented. This technique is capable to induce colored patterns with submicrometric spatial resolution on large areas in a short exposure time as compared with other irradiation methods. Luminescent regular arrays produced by this irradiation technique will be shown. Recently, the idea of using a LiF film as image detector for X-ray microscopy and micro-radiography based on optically-stimulated luminescence from CCs has been developed.

The spectral identification of several photoluminescent defects produced in LiF by ionising radiation is still uncertain because many optical transitions are possible and their different spectral features are often overlapping. Performing a conventional spectroscopic characterization requires a lot of time and intuition, but recently a more effective investigation method has been developed: combined excitation-emission spectroscopy (CEES). In this approach a large number of excitation (or emission) spectra is recorded at a sequence of emission (or excitation) wavelengths. The resulting three-dimensional data set of excitation intensities as a function of excitation and emission wavelengths, or energies, is visualized using a contour plot, where lines of equal emission intensities are drawn. For the first time this method has been successfully applied to coloured LiF crystals and films. As an example, the case of visible-emitting colour centres, absorbing in the M band spectral region in a gamma irradiated LiF crystal is reported and discussed. Spectroscopic features of well-known active defects have been confirmed, more complex centres clearly identified and other properties better clarified. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A great effort in X-ray imaging studies has been dedicated to the design of novel optics-detector solutions that aims in creating a compact laboratory X-ray microscopy apparatus. Based on our experience in the use of both capillary/polycapillary optical systems and LiF imaging detectors we have recently tested a new combination of these techniques. The potential of the optics both to concentrate and to shape X-ray radiation enforced by the high performances in terms of spatial resolution/dynamic range of LiF imaging detectors allows us to use very simple imaging techniques, like the contact one, particularly suitable for compact imaging systems. In this paper we present first results about high spatial resolution images of transmitted X-rays through a polycapillary semi-lens. Submicron X-ray imaging of thick geological samples was also presented and discussed. These results demonstrate the potentialities of these novel X-ray imaging system for characterization of X-rays optics and for applications in material, life and heart sciences. © 2012 Elsevier B.V.

The extreme ultraviolet radiation emitted by a laser-plasma source or by a capillary discharge laser is applied to the generation of luminescent patterns in lithium fluoride. This novel technique is able to produce colored patterns with high spatial resolution on large (more than 10 cm/sup 2/) areas in a short exposure time compared with other irradiation methods like the electron beam writing. The potentials of this technique for applications in photonics are commented. This work reviews the activity performed during the past four years at the ENEA Frascati Center and at L'Aquila University, Italy. Preliminary images of microradiography or X-ray contact microscopy using lithium fluoride as an imaging detector are presented. The advantages of this new detector compared with photographic films or with photoresists are discussed.

Light produced when heating materials which have been previously exposed to ionizing radiation is commonly known as thermoluminescence (TL). The structure of TL glow curves is characteristic of the material and is influenced by the impurities it contains as well as the type of defect centers generated by the irradiation. An extensive body of knowledge exists concerning TL. In the case of LiF, several efforts have been made to associate glow peaks with defect centers responsible for optical absorption, but up to now there is no certain attribution. In this work nominally pure LiF samples have been irradiated with γ rays, then treated thermally and optically to change the concentration of electron color centers in a known way, and the resulting TL has been studied in conjunction with optical absorption. By comparing the glow curves from ambient temperature to 450 °C and the optical absorption in the purple-blue spectral region, it has been possible to establish a link between F 3 + color centers and the low temperature region of the glow curve below 200 °C. Accurate spectroscopic measurements and theoretical fittings of the absorption spectrum in the UV–VIS spectral region have also been made in order to ascertain the possible role of other aggregate color centers in TL.

Stable color center production in lithium fluoride crystals has been applied to single-shot high-spatial-resolution imaging of a 2 ns long, 46.9 nm capillary discharge soft-X-ray laser pulse. Images of the unfocused beam and of the beam focused by a multi-layer mirror were recorded. The LiF crystal sensitivity was sufficient to impress high-contrast photo-luminescent patterns with single-pulse irradiation on an area up to 40 mm2. Interference fringes due to the soft-X-ray laser coherence were observed. The results demonstrate the potential of LiF crystals and films as a sub-micrometer resolution two-dimensional imaging tool for single-shot soft-X-ray laser applications.

Thermally grown thin film and bulk crystals of lithium fluoride (LiF) were irradiated by swift heavy ions to understand the color center production mechanism and their transformation to more complex defects. These samples have been characterized by optical absorption and photoluminescence (PL) under 442 nm excitation at room temperature by HeCd laser. We observed that bulk crystals show relatively very intense PL bands at around 530 and 665 nm , which correspond to the emission of F3+ and F2 centers, respectively, as compared to the thin films at the same electronic excitation and ion fluence. Detailed study on bulk LiF has been performed at different electronic excitations and ion fluences for understanding the ion matter interaction under these dense electronic excitations.

Tl+-doped lithium fluoride crystals exhibit very interesting properties concerning the formation of colour centres after X-rays irradiation. The presence of Tl+ ions, even in small concentrations, increases the number of traps which stabilize the H-centre aggregates. These H-centre aggregates become smaller and their number increases, inducing two sets of trapping levels. Photoluminescence measurements after subsequent thermal bleaching of the thermoluminescence peaks show a faster decreasing of the F3+ centres, in three steps, compared with the F2 ones. The F-H recombination processes may induce the excitation-emission transitions of Tl+-ions.

LiF films and crystals containing lithium colloids have been studied by scanning near-field optical microscopy (SNOM). Scattering phenomena and possible wave-guided modes have been observed studying the physical mechanism of the interaction between the LiF samples and an external laser source.

Electron-beam lithography has been used to define color center stripes of about one hundred micrometers length and variable width (100 nm to 5 μm) in lithium fluoride. These structures have for the first time been illustrated and spectrally characterized in near field optical microscopy (SNOM) operating in local illumination mode with an optical fiber probe and far field fluorescence detection.

Primary and aggregate color centers in lithium fluoride (LiF) crystals and polycrystalline LiF films were produced by an innovative irradiation technique using extreme ultraviolet radiation and soft X-rays generated by a laser-plasma source. This irradiation facility allowed the efficient formation of active color centers on luminescent patterns with submicron spatial resolution on large areas and short exposure times. The method looks promising for the realization of low-dimensionality photonic devices. The optical characterization of the colored structures was performed by means of absorption and photoluminescence measurements on LiF samples colored under different irradiation conditions.

Research activities concerned with color centers in alkali halide films started recently. The use of versatile, well-assessed, and low-cost fabrication techniques consisting of physical vapor deposition of Lithium Fluoride (LiF) films combined with direct writing lithographic processes allows the realization of miniaturized structures, like broad-band emitters, channel waveguides, optical microcavities and point-light sources emitting in the visible spectral range. Promising results have been obtained in the generation, amplification and waveguiding of visible light in LiF treated by low energy electron beams, where the efficient formation of stable primary and aggregate color centers also induces a local modification of the refractive index. A brief overview of the investigated optical properties is presented together with a short discussion about their perspectives of applications in optoelectronics.

Optically transparent LiF thin films thermally evaporated on glass and Si(100) substrates were used for advanced diagnostics of proton beams of energies from 1.4 to 7 MeV produced by a linear accelerator for protontheraphy under development at ENEA C.R. Frascati. The proton irradiation induces the formation of stable colour centres, among them the aggregate F2 and F3 + optically active defects. After exposure of LiF films grown on glass perpendicularly to the proton beams, their accumulated transversal spatial distributions were carefully measured by reading the latent two-dimensional (2-D) fluorescence images stored in the LiF thin layers by local formation of these broad-band visible light-emitting defects with an optical microscope under blue lamp excitation. Taking advantage from the low thickness of LiF thin films and from the linear behaviour of the integrated F2 and F3 + photoluminescence intensities up to the irradiation fluence of ∼5x1015 p/cm2, placing a cleaved LiF film grown on Si substrate with the cutted edge perpendicular to the proton beam, the 2-D fluorescence image of the film surface could allow to obtain the depth profile of the energy released by protons, which mainly lose their energy at the end of the path. © Published under licence by IOP Publishing Ltd.

Lithium fluoride (LiF) is a well-known dosimeter material in pure (McLaughlin et al. Nucl Instrum Methods 175:17–18, 1980) and doped (Lakshmanan et al. Phys Status Solidi (a) 153:265–273, 1996) form. Radiation detectors based on microcrystalline dispersion of LiF in a polymeric matrix have been introduced for gamma and electron high-dose dosimetry (Kovacs et al. Radiat Phys Chem 57:691–695, 2000). In recent years the area of growth and characterisation of LiF thin films has seen a considerable expansion. Polycrystalline LiF films grown by thermal evaporation were proposed and tested as nuclear sensors for neutrons (Cosset et al. Thin Solid Films 303:191–195, 1997) and for gamma dosimetry (Montecchi et al. Point defects in lithium fluoride films induced by gamma irradiation, ch. 116. In: Proceedings of the 7th international conference on advanced technology and particle physics, Como, pp 819–825, 2002). In the last years many efforts have been devoted to the development of novel imaging detectors for extreme-ultraviolet radiation and soft X-rays (Baldacchini et al. J Nanosci Nanotechnol 3:483–486, 2003), hard X-rays (Almaviva et al. Appl Phys Lett 89(5):054102, 2006), as well as low (Baldacchini et al. J Phys Condens Matter 10:857–867, 1998) and high energy electrons. Such solid-state detectors are based on the optical reading of photoluminescence (PL) from stable, visible-emitting colour centres (CCs), produced by irradiation with ionising radiations. These aggregate electronic defects are F2 and F3 + CCs (two electrons bound to two and three close anion vacancies, respectively), which possess almost overlapping absorption bands, peaked at about 450 nm, called M band (Nahum, Phys Rev 157:817–830, 1967). By optical pumping in this spectral region, F2 and F3 + CCs emit broad PL bands peaked at 678 and 541 nm, respectively. LiF films of different thickness were grown by thermal evaporation on different substrates, such as glass and silica, as well as plastic ones (Di Lazzaro et al. Extreme ultraviolet marking system for anti-counterfeiting tags with adjustable security level.

Waveguiding light-emitting strips have been written in a blank LiF crystal with a femtosecond laser, by translating the sample under the pulsed focused beam. Light guiding through these buried micro-structures was observed at several wavelengths between 458 and 1550 nm. Visible photoluminescence spectra of aggregate F2 and F3 + laser active color centers were measured. The refractive index increase, estimated of the order of 10-4 in the visible and near-infrared spectral intervals, is consistent with the stable formation of electronic point defects in LiF. These results are promising for the realization of miniaturized optical waveguiding solid-state amplifiers and lasers based on color centers. © 2014 AEIT.

Proton beams of energy 3 and 7 MeV, produced by a linear accelerator for protontherapy under development at ENEA Frascati, were used to irradiate lithium fluoride (LiF) crystals and 1 μm thick LiF films grown by thermal evaporation on glass substrates, in a fluence range from 10 11 to 10 15 protons/cm 2 . The irradiation of LiF induced the stable formation of aggregate visible-emitting F 2 and F 3 + color centers, which possess almost overlapping absorption bands around 450 nm and, under optical pumping in this spectral region, they simultaneously emit broad photoluminescence bands, peaked at 678 nm and 541 nm, respectively. Their laser excited emission spectra were acquired and the integrated photoluminescence signal was measured using a fluorescence optical microscope equipped with a cooled s-CMOS camera. The photoluminescence of proton irradiated LiF crystals and films shows an interesting linear response as a function of the irradiation fluence. By optical fluorescence microscopy, it was possible to record the transversal proton beam intensity profile by acquiring the photoluminescence image of irradiated LiF. Moreover, using cleaved LiF films grown on silicon substrates irradiated in a particular geometry allowed one to measure the colour centres photoluminescence distribution with proton penetration depth and provided a direct imaging of the Bragg peak.