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Radiation-induced color centers locally produced in lithium fluoride (LiF) are successfully used for radiation detectors. LiF detectors for extreme ultraviolet radiation, soft and hard X-rays, based on photoluminescence from aggregate electronic defects, are currently under development for imaging applications with laboratory radiation sources, as well as large-scale facilities. Among the peculiarities of LiF-based detectors, noteworthy ones are their very high intrinsic spatial resolution across a large field of view, wide dynamic range, and versatility. LiF crystals irradiated with a monochromatic 8 keV X-ray beam at KIT synchrotron light source (Karlsruhe, Germany) and with the broadband white beam spectrum of the synchrotron bending magnet have been investigated by optical spectroscopy, laser scanning confocal microscopy in fluorescence mode, and confocal Raman micro-spectroscopy. The 3D reconstruction of the distributions of the color centers induced by the X-rays has been performed with both confocal techniques. The combination of the LiF crystal capability to register volumetric X-ray mapping with the optical sectioning operations of the confocal techniques has allowed performing 3D reconstructions of the X-ray colored volumes and it could provide advanced tools for 3D X-ray detection.

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.

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 F2 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 F2 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.

In recent years, lithium fluoride (LiF) detectors based on the optical reading of photoluminescence emitted by radiation-induced F2 and F3+ colour centres have been successfully used for advanced diagnostics of proton beams. In this work, the visible photoluminescence response of LiF film-based radiation detectors of three different thicknesses, grown by thermal evaporation on glass and Si(100) substrates and irradiated with 27 MeV proton beams at doses in the range between 4.2 × 103 and 1.7 × 105 Gy, was carefully investigated by fluorescence optical microscopy. Before irradiation, the structural and morphological properties of LiF films were investigated by atomic force microscopy and X-ray diffraction techniques. Substrate-enhanced photoluminescence response up to 48% was observed in LiF film-based detectors grown on Si(100) substrates with respect to those deposited on glass in the same deposition run and irradiated in the same conditions. This photoluminescence enhancement can be mainly ascribed to the reflective properties of the Si substrate in the visible spectral range, where the emission bands of F2 and F3+ colour centres are located, combined with the proper choice of the film thickness. Other complex effects related to the polycrystalline nature of the LiF films cannot be excluded.

The peculiar photoluminescence properties of radiation-induced electronic defects in lithium fluoride (LiF), known as color centers, are well known for applications in tunable lasers and photonic light-emitting microdevices operating at room temperature. In the last two decades they have been under exploitation in radiation imaging detectors and dosimeters based on the optical reading of the visible photoluminescence in LiF crystals and polycrystalline thin films. They were proposed for soft X-ray imaging at high spatial resolution and, more recently, their use has been successfully applied to advanced diagnostics of proton beams. After proton irradiation of LiF, the emission intensity of the radiation-induced point defects is proportional to the absorbed dose over several orders of magnitude, thus novel solid-state dosimeters with imaging capabilities can be envisaged, although dose values that are typical of clinical radiotherapy are still a challenge. In this brief review, the main experimental results and recent advances concerning radiophotoluminescence of color centers in optically transparent LiF crystals and polycrystalline thin films are presented and discussed in order to highlight the advantages, challenges and limits related to the feasibility of detectors for protontherapy applications.

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.

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.

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.

In the last few years, the peculiar photoluminescence properties of radiation-induced color centers in lithium fluoride (LiF) films have been successfully used for advanced diagnostics of low-energy proton beams produced by the TOP-IMPLART linear accelerator for protontheraphy under development at ENEA C.R. Frascati. The two-dimensional spatial dose map of the transversal section of proton beams was fully reconstructed in a wide interval of doses. In this work the optical emission properties of LiF thin films, grown by thermal evaporation on glass and Si(100) substrates and subsequently irradiated by proton beams of nominal energy 3 MeV at doses higher than 105 Gy, were carefully investigated. Their structural and morphological analyses were performed by X-ray diffraction and atomic force microscopy. A careful comparison of the photoluminescence and photoluminescence-excitation spectra of F2 and F3+ electronic defects was performed. Substrate-enhanced photoluminescence intensity increase up to 100% was observed in colored LiF films grown on Si substrates with respect to glass ones. This behavior can be substantially ascribed to the reflective properties of the Si substrate at the emission wavelengths of F2 and F3+ CCs, although other complex effects due to the polycrystalline nature of the films cannot be excluded.

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.

Double-nanotextured black diamond films with different geometries were fabricated by double-step femtosecond laser treatments at different split ratios of accumulated laser fluence. A “2D-like” pseudo-periodic nanostructure was obtained for the first time when the split ratio was slightly unbalanced in favour of the first step of the treatment, as inferred by scanning electron microscopy. Raman analysis showed that a residual biaxial stress, composed by a superposition of a tensile and a compressive component, is always present after the laser writing process, and that the two components tend to balance each other in the 2D pseudo-periodic case. Spectrophotometric measurements in the 200–2000 nm wavelength range returned outstanding solar absorptance values for all the fabricated films (reaching the unprecedented value of 99.1% in the “2D-like” structure), launching double-nanotextured black diamond as a possible alternative to black silicon as absorbing layer for high-efficiency solar cells. © 2018 Elsevier Ltd

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.

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.

In the last few years, the visible photoluminescence (PL) of radiation-induced laser-active F2 and F3+ aggregate color centers (CCs) in lithium fluoride, LiF, has been successfully used for advanced diagnostics of low-energy proton beams. A systematic study of the optical properties of LiF crystals and thin films exposed in different irradiation geometries to a proton beam of nominal energy 7 MeV is under way. The ionization induces the stable formation of primary and aggregate CCs in the crystalline lattice, among which the F2 and F3+ defects possess broad emission bands in the red and green spectral ranges, respectively, when optically pumped at wavelengths close to 450 nm. Their PL intensity has been found to be linearly proportional to the dose absorbed by the host material over at least three orders of magnitude, so that LiF solid-state dosimeters based on spectrally-integrated PL reading can be envisaged. A simple defect formation model that takes into account the energy released in the material, crystals and films, together with the saturation of CC concentration at high doses, allows obtaining both the full Bragg curve reconstruction with its dose distribution with depth and the accumulated bi-dimensional transversal dose-mapping from the visible fluorescence images latently stored in LiF crystals and thin films, respectively. Results obtained by using a fluorescence microscope are presented and discussed with the aim of highlighting some quantitative aspects of the PL behavior of CCs both in LiF crystals and films, which are relevant for low-energy proton-beam Bragg curve imaging and dose mapping in a wide interval of doses up to the ones at which strong CC saturation occurs.

Lithium fluoride crystals are irradiated at various doses by gamma rays at 77 K. The time evolution of photoluminescence signals from aggregated F 2 +, F2, F3 + and F 3 color centers, and of the absorption intensity of primary F centers are measured at various annealing temperatures. The lifetimes of anionic vacancies υa and F2 + centers, the characteristic times of concentration growth of F2, F 3 + and F3 centers, and also the activation energies of diffusion of vacancies and F2 + centers together with various processes of aggregation are determined. It is found that lifetime decreases for vacancies while increases for F2 + centers by increasing the irradiation dose. It is also shown that, after irradiation during annealing, vacancies are formed as a result of the reaction F2 ++H→υa+Fl-, where Fl- is a fluorine ion in a lattice site and H is a fluorine interstitial atom. Then these vacancies participate in color centers aggregation kinetics. The presence of F- centers in the irradiated crystal is established, and the processes which lead to the formation of F2, F3 + and F3 centers after irradiation, are unveiled. © 2013 Elsevier B.V.

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. (c) 2013 Elsevier Ltd. All rights reserved.

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.

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.

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

An Aluminum coated FBG was heated to 650 °C in order to study the regeneration process at low temperature. When compared to the standard regeneration process (at 800 °C -1000 °C) the regeneration occurred at longer times, the onset of the process took around 150 days to appear. The reflected signal after regeneration also is weaker than that obtained in the usual process, although it presents a good stability after the recovery phase - measured for a time span of 200 days. © 2015 SPIE.

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.

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.

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.

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.

A recursive procedure is applied to the measured near-field profiles of buried optical waveguides recorded in a lithium fluoride (LiF) crystal by femtosecond laser pulses in order to estimate the core dimensions and the refractive index increase. Albeit the waveguides transversal section geometry is quite complex it is possible to obtain the horizontal and vertical widths and the average refractive index maximum increase assuming a simplified rectangular transversal section in the simulation. The procedure is validated by comparing the simulated results with the experimental near-field profiles and the maximum refractive index values of two commercial optical fibers. Typical dimensions of ∼(8x10)μm2 and refractive index changes of ∼(2-10)x10 -4 were obtained for the LiF waveguides at several wavelengths. © 2014 SBMO/SBMag.

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.

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 3 MeV 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 F 3 + defects, which are stable at room temperature. By optical pumping in the blue spectral region, the F2 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. 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.

Proton beams of 3 MeV energy, produced by the injector of a linear accelerator for proton therapy, were used to irradiate at room temperature lithium fluoride crystals and polycrystalline thin films grown by thermal evaporation. The irradiation fluence range was 1011-1015 protons/cm2. The proton irradiation induced the stable formation of primary and aggregate color centers. Their formation was investigated by optical absorption and photoluminescence spectroscopy. The F2 and F3 + photoluminescence intensities, carefully measured in LiF crystals and thin films, show linear behaviours up to different maximum values of the irradiation fluence, after which a quenching is observed, depending on the nature of the samples (crystals and films). The Principal Component Analysis, applied to the absorption spectra of colored crystals, allowed to clearly identify the formation of more complex aggregate defects in samples irradiated at highest fluences. © Published under licence by IOP Publishing Ltd.

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.

In this work we tested use of thin film aluminium coating to enhance the temperature sensing features of FBG sensors. Thin film coating was evaporated on the Bragg grating after removal of the original acrylate recoating, using a modified vacuum evaporation system that allows uniform radial deposition around the fibre. Results show that recoating thickness up to 0.5 μm do not produce appreciable effects, whereas thickness of 2.3 μm appreciably increases temperature sensitivity without increasing time response constant. © 2013 SPIE.

The doping process of LiF with Pb has been described by using structural ab initio modeling and experimental results. The values of formation energy of several complexes involving Pb ions show that the presence of charge-balancing vacancies ease the metal incorporation in the crystal. The metal successively captures the vacancies to form a final stable complex Pb-Li(center dot) + V-Li('). Experimental data from X-ray absorption spectroscopy confirm the presence of Pb in LiF crystals in this site revealing both Pb-F bonds and collinear Pb-F-Li atomic configurations. A maximum solubility of Pb in LiF of the order of 3 x 10(20)/cm(3) can be estimated from these calculations. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793751]

The optical properties of Ni2+ -ion impurities in a new crystal, the inverted fluoroperovskite BaLiF3, have been studied. In particular we have investigated the fine structure of the transition by means of absorption and emission spectra. By analogy to previous results we have assigned the structured absorption band around 1200 nm to the 3A2 ↠ 3Ti transition. Four zero-phonon lines are clearly visible in the absorption spectrum. The emission in the region 1250–1600 nm, due to the same 3A23T1 transition, consists of two sharp lines with their vibrational sidebands. © 1995, Taylor & Francis Group, LLC. 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.

Broad-band light-emitting radiation-induced F2 and F3+ electronic point defects, which are stable and laser-active at room temperature in lithium fluoride crystals and films, are used in dosimeters, tuneable color-center lasers, broad-band miniaturized light sources and novel radiation imaging detectors. A brief review of their photoemission properties is presented, and their behavior at liquid nitrogen temperatures is discussed. Some experimental data from optical spectroscopy and fluorescence microscopy of these radiation-induced point defects in LiF crystals and thin films are used to obtain information about the coloration curves, the efficiency of point defect formation, the effects of photo-bleaching processes, etc. Control of the local formation, stabilization, and transformation of radiation-induced light-emitting defect centers is crucial for the development of optically active micro-components and nanostructures. Some of the advantages of low temperature measurements for novel confocal laser scanning fluorescence microscopy techniques, widely used for spatial mapping of these point defects through the optical reading of their visible photoluminescence, are highlighted. © 2012 American Institute of Physics.

LiF:Pb doped crystals were successfully grown by Kyropoulos method, starting with drying powders. The presence of Pb2+ ions in the LiF crystals were evidenced by the absorption band at 278 nm and by 375 nm photoluminescence. The presence of some other Pb structures with oxygen compounds in the as made samples was evidenced, decreasing after some annealing procedures. The local environment and valence state of Pb in LiF were studied by X-ray Absorption Spectroscopy at the Pb L-III and L-I edges. XANES data reveal that Pb is present as Pb2+ whereas EXAFS data show that it is incorporated in the crystal and not forming PbF2 precipitates. Identical spectra are obtained for samples as prepared and after thermal annealing up to 650 degrees C demonstrating the stability of the incorporation site. Also the concentration of Pb in the crystal has no effect on the location site of the metal as the same spectrum is obtained for specimens with different dopant concentrations.

Thin films of KCl, KBr, RbCl have been obtained by thermal evaporation on amorphous substrates with different deposition parameters. The crystalline structure and orientation have been determined, and the films resulted to be policrystals with high uniformity of orientation. Production of colour centres, achieved by irradiation with low-energy electrons, leads to F center concentrations barely observed in large crystals. The colouration kinetics is similar to that in the bulk, and shows after a maximum an exponential decay at high doses because of centre aggregation coupled to thermal effects. The films exhibit a bleaching process of the colour centres at room temperature, whose kinetics depends on the irradiation damage. © 1992, Taylor & Francis Group, LLC. All rights reserved.

Systematic irradiation of thermally evaporated 0.8 μm thick polycrystalline lithium fluoride films on glass was performed by proton beams of 3 and 7 MeV energies, produced by a linear accelerator, in a fluence range from 1011to1015protonscm2. The visible photoluminescence spectra of radiation-induced F2 and F3+laser active color centers, which possess almost overlapping absorption bands at about 450 nm, were measured under laser pumping at 458 nm. On the basis of simulations of the linear energy transfer with proton penetration depth in LiF, it was possible to obtain the behavior of the measured integrated photoluminescence intensity of proton irradiated LiF films as a function of the deposited dose. The photoluminescence signal is linearly dependent on the deposited dose in the interval from 103 to about 106Gy, independently from the used proton energies. This behavior is very encouraging for the development of advanced solid state radiation detectors based on optically transparent LiF thin films for proton beam diagnostics and two-dimensional dose mapping. © 2015 AIP Publishing LLC.

Local fluorescent probes based on low-energy electron beam irradiated lithium fluoride (LiF) thin films were preparated and tested for their applications in Scanning Near-Field Optical Microscopy. © 2004 American Institute of Physics.

The use of versatile, well-Assessed and low-cost fabrication techniques consisting in physical vapor deposition of LiF films combined with an electron-beam direct writing lithographic process allows the realization of optically confined active structures, like broad-band emitters, channel waveguides and optical microcavities operating in the visible. © 2000 American Institute of Physics.

Broadband waveguiding through light-emitting strips directly written in a blank lithium fluoride crystal with a femtosecond laser is reported. Light guiding was observed at several optical wavelengths, from blue, 458 nm, to near-infrared, at 1550 nm. Visible photoluminescence spectra of the optically active F2 and F3+ color centers produced by the fs laser writing process were measured. The wavelength-dependent refractive index increase was estimated to be in the order of 10-3-10 -4 in the visible and near-infrared spectral intervals, which is consistent with the stable formation of point defects in LiF. © 2014 Chinese Physical Society and IOP Publishing Ltd.

We measured the optical absorption spectra of thermally treated, gamma irradiated LiF crystals, as a function of temperature in the range 16–300 K. The temperature dependence of intensity, peak position and bandwidth of F and M absorption bands were obtained. Keywords: Lithium fluoride, Optical absorption, Low temperature, Color centers