Search

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 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.

Optical photoluminescence of thermally evaporated Alq3 thin films has been studied for over six years in a few samples annealed and non-annealed and afterwards exposed to the laboratory atmosphere. It was found that the measured emission intensity decays with four different time-spectral behaviors, which imply the existence of four molecular aggregations, or components. Insights have unveiled their interactions with environment agents, which are useful both for optoelectronic utilizations of this ubiquitous organo-metallic molecule possessing a long emission lifetime and for basic research and application of similar organic molecules.

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.

Intensity Modulated Proton Therapy (IMPT) can take great advantage of fast tools based on analytical or semianalytical approaches providing dose evaluation in a target. Here, a semianalytical method is described that has been developed for the TOP-IMPLART (Intensity Modulated Proton Linear Accelerator for RadioTherapy) accelerator, consisting of a pulsed full linear accelerator for proton therapy being developed at ENEA Frascati, where preliminary tests of active beam energy and intensity modulation are undergoing. The method relies on a two-step procedure that provides the main accelerator parameters and optimizes the number and the weights of the energetic components needed to obtain a uniform Spread-Out Bragg Peak (SOBP) in the target region. In the present paper, the theoretical basis of the method is described in detail together with two practical example applications.

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.

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.

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.

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.

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.

Large and transparent LiF:Pb and LiF:Tl crystals were successfully grown by using Kyropoulos method. They were characterized using spectroscopic measurements combined with differential thermal analysis and thermogravimetry (TG) analysis, in order to investigate the effect of dopants upon the optical properties of these crystals. The optical absorption peaks at 278 nm (for Pb) and 273 nm (for Tl) have been assigned to the A-type transition of these ions. Photoluminescence spectra have shown bands at 350 nm (for Tl) and 375 nm (for Pb), which have been associated to the dopant ions in different configurations in the lattice.

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.

The influence of Pb 2+ doping on the optical properties of LiF crystals before and after irradiation with X-ray was studied by using optical absorption and photoluminescence measurements. After irradiation it was observed an increase of the absorption due to primary and aggregate electronic defects together with a higher photoluminescence signals associated with the F 2 and F 3 + colour centres in doped samples compared with the undoped ones.

A theoretical model to evaluate normal-incidence specular, direct, hemispherical, and diffuse reflectance and transmittance of a single layer affected by refractive-index inhomogeneity along the growth axis, roughness, and thickness wedge was introduced in recent papers. In the present work, the model is extended to the case of a multilayer optical coating. To that purpose, suitable transfer matrices that account for layer-index inhomogeneity and random deviations of the interfaces from smooth planes are introduced, and statistical averages are applied to the resulting amplitude and intensity coefficients. Eventually, thickness wedge is included as a deterministic average on the intensity coefficients.

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.

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.

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.

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.

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.

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.

The F 2 centers in LiF display an efficient broad emission in the red spectral range when excited in their absorption band located around 450 nm and promising results have been obtained in transferring the optical properties to low-dimensional emitting structures, where their density reaches values higher than 10 17 F 2 /cm 3 . The emission decay times measured in the entirely colored LiF crystals irradiated by 3 MeV electrons with increasing doses do not show any variation up to 10 17 F 2 /cm 3 and decrease by 20% of the value approaching 10 18 F 2 /cm 3 . The values at low concentrations practically coincide with the lifetime of the excited level. The comparison among samples irradiated at different temperatures seems to exclude relevant interactions with the F 3 + centers, while it supports an energy transfer mechanism with the F 2 + ones.

Optical absorption and emission spectra have been measured in LiF crystals colored with γ-rays. The optical bands of various samples, suitably treated in order to create different relative concentrations of F2 and F 3 + defects in the crystal lattice, have been studied in detail at 77 K and at room temperature, and resolved into their components. An accurate determination of the spectroscopic parameters of the absorption and emission bands of the aggregated defects in LiF has been achieved, and a critical comparison with previous data has been performed.

Lithium fluoride films, about 2.6 μm of thickness, were thermally evaporated onto a 10 μm thick SiO2 film grown on silicon 〈1 0 0〉 substrate. Coloured stripes in the LiF films were created by low energy electron (12 keV) beam lithography. Intense photoluminescence (PL), extending from green to red in the visible spectral range, was measured at room temperature due to the presence of the laser active F2 and F3+ colour centres. Atomic force microscopy (AFM) analysis was used to study the film morphology inside and outside the irradiated areas. A confocal microscope was used to observe the surface layer of the film.

Note: Sp. Iss. SI Reference LPMV-ARTICLE-2007-004doi:10.1016/j.jlumin.2006.01.190View record in Web of Science Record created on 2007-06-20, modified on 2017-12-03

Optical properties of alkali halide crystals with anisotropic color centers are considered in terms of non-linear optics. It is shown that, taking into account the peculiarities of the reorientation of optically excited centers, typical non-linear phenomena should be expected at modest excitation level. The possibility of polarization optical bistability in cubic crystals with F A centers is clarified and a new luminescence behavior is predicted.

Up to now more than ten color center systems in various host lattices of alkali halides have been successfully developed in tunable lasers; among them are the oxygen stabilized F2+ centers. In this work we have studied the optical properties of these laser-active centers in mixed KCl : KBr crystals doped with oxygen, having in mind an extended range of tunability with only one laser crystal. We have observed the emission band shifting from 1.7 to 2.0 μm depending on the mixing ratio of the two halides, and also other luminescences due to more complex centers related to Na impurity.