Lithium Fluoride Crystals Research Articles

Isotopically Enriched Lithium Fluoride Crystals for Detection of Neutrons with the Fluorescent Track Technique.

In this work, the properties of LiF crystals grown using Li of different isotopic compositions are described from the standpoint of their application as fluorescent nuclear track detectors used in measurements in the neutron radiation fields. The crystals were grown using two techniques: the Czochralski method and the micro-pulling-down method. Three isotopic compositions of Li were studied: natural, highly enriched in 6Li, and highly enriched in 7Li. It was found that 6LiF detectors are about six times more sensitive to thermal (low energy) neutrons than natural LiF, which significantly decreases the lower detection limit. 7LiF detectors are insensitive to thermal neutrons, which makes it easier to detect tracks due to other radiation modalities, such as energetic ions or nuclei recoiled in collisions with high-energy neutrons. Besides the response to neutron radiation, no other significant differences in the crystal properties were identified, irrespective of the isotopic composition and crystal growth method employed.

Nonlinear Excitation of Color Centers in a LiF Crystal by Femtosecond Laser Radiation

Experiments were carried out in which the luminescence of color centers in lithium fluoride crystals was excited by two different femtosecond lasers with significantly different energies, durations and pulse repetition rates. It was established that in all experiments the main center, the luminescence of which was excited nonlinearly, was the F3+ color center. Unusual experimental data were obtained; a laser with low pulse energy (4 nJ) excited triplet luminescence of these centers (570 nm) but did not excite singlet luminescence (540 nm). Another laser with a higher pulse energy (0.3 mJ), on the other hand, excited singlet luminescence and did not excite triplet luminescence. A proposed diagram of energy levels and quantum transitions is presented, explaining the possible mechanisms of nonlinear excitation of luminescence in these experiments.

Thermal annealing of radiation damages produced by swift 14N and 16O ions in LiF crystals

Annealing of color centers was studied in lithium fluoride crystals, irradiated with 23-MeV nitrogen and 28-MeV oxygen ions. Basing on the optical absorption spectroscopy and reaction-rate modelling, a new interpretation of the annealing kinetics at the practically important temperatures below 500 K is suggested. Proposed model explains simultaneous decrease of the F and F2/F3 + peaks as a result of migration of the F centers and formation of larger aggregates, and does not include additional assumptions about impurities and cation vacancies. It specifies the migration energy of the F centers in the ground state to be about 1.3 eV, that corresponds to earlier studies.

Detection of fluorescent low-energy proton tracks in lithium fluoride crystals

The exploitation of visible radiophotoluminescence in lithium fluoride (LiF) crystals, due to local aggregate point defects produced by ionizing radiation in the crystal lattice, is demonstrated for fluorescent imaging of single tracks of protons at low energies. LiF crystals were irradiated perpendicularly with respect to nearly monochromatic, collimated proton beams at the energies of about 1 MeV in a fluence range from ∼5.8 × 105 p/cm2 up to ∼3.4 × 107 p/cm2 and of about 2.6 MeV in a fluence range from ∼9.6 × 105 p/cm2 up to ∼4.7 × 106 p/cm2. The fluence values were estimated by detecting and automatically counting the tracks on the images acquired with a fluorescence microscope at high magnification; they were found to be in agreement with those obtained using CR39 plastic detectors irradiated under the same experimental conditions. Despite the very short range in matter of these charged particles, by focusing the excitation blue light at discrete depths in the irradiated LiF crystals, an estimate of the proton energies was also obtained. These first results are encouraging for the utilization of LiF crystals as fluorescent nuclear track detectors under typical conditions employed in ion radiobiology.

Advanced spectroscopic investigation of colour centres in LiF crystals irradiated with monochromatic hard x-rays

Nominally-pure lithium fluoride (LiF) crystals were irradiated with monochromatic hard x-rays of energy 5, 7, 9 and 12 keV at the METROLOGIE beamline of the SOLEIL synchrotron facility, in order to understand the role of the selected x-ray energy on their visible photoluminescence (PL) response, which is used for high spatial resolution 2D x-ray imaging detectors characterized by a wide dynamic range. At the energies of 7 and 12 keV the irradiations were performed at five different doses corresponding to five uniformly irradiated areas, while at 5 and 9 keV only two irradiations at two different doses were carried out. The doses were planned in a range between 4 and 1.4 × 103 Gy (10.5 mJ cm−3 to 3.7 J cm−3), depending on the x-ray energy. After irradiation at the energies of 7 and 12 keV, the spectrally-integrated visible PL intensity of the F2 and F3 + colour centres (CCs) generated in the LiF crystals, carefully measured by fluorescence microscopy under blue excitation, exhibits a linear dependence on the irradiation dose in the investigated dose range. This linear behaviour was confirmed by the optical absorption spectra of the irradiated spots, which shows a similar linear behaviour for both the F2 and F3 + CCs, as derived from their overlapping absorption band at around 450 nm. At the highest x-ray energy, the average concentrations of the radiation-induced F, F2 and F3 + CCs were also estimated. The volume distributions of F2 defects in the crystals irradiated with 5 and 9 keV x-rays were reconstructed in 3D by measuring their PL signal using a confocal laser scanning microscope operating in fluorescence mode. On-going investigations are focusing on the results obtained through this z-scanning technique to explore the potential impact of absorption effects at the excitation laser wavelength.

Enhanced thermoluminescence, radioluminescence, and optically stimulated luminescence from lithium fluoride and silver nanoparticles composites

Plasmonic properties of metals can increase the sensitivity of ionizing radiation dosimeters. In this work, we study the plasmon resonance of silver nanoparticles (AgNp) to increase the optically stimulated luminescence (OSL) intensity of lithium fluoride (LiF) polycrystals. We produced LiF/Ag nanocomposites with different volumes of a silver nanoparticle dispersion and investigated their OSL response. We also investigated their thermoluminescence (TL) and radioluminescence/scintillation (RL) responses. X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–vis spectroscopy characterized the synthesized materials. XRD results clearly show the characteristic peaks of LiF and AgNps. TEM images revealed silver nanoparticles with diameters between 80 and 120 nm and LiF crystals with sizes between 1 and 5 μm. Reflectance spectroscopy showed a band around 420 nm, attributed to the plasmon resonance of silver nanoparticles. The responses of the OSL curves to X-ray irradiation were analyzed, showing higher OSL sensitivity with 14 mL AgNp, attributed to the AgNp plasmon resonance. A significant increase in TL and RL intensity could be observed for the LiF/Ag nanocomposites. TL, RL, and OSL results support that addition of AgNps do not lead to the creation of more and new traps or luminescent centers. The increase of TL and RL intensities is therefore related to the coupling of the luminescent centers with the AgNPs plasmons, in an energy-transfer mechanism (plasmon-coupled emission), whereas the enhanced OSL intensity directly correlates with the increased photoionization cross-section of the electronic traps under plasmon resonance conditions (plasmon-coupled excitation). Therefore, these results indicate that AgNp are potential candidates to increase the OSL, TL, and RL intensity from LiF dosimeters.

Proton-beam energy diagnostics by color-center photoluminescence imaging in LiF crystals: Implementation of multiple Coulomb scattering into an analytical Bragg-curve model

Lithium fluoride crystals are used to assess proton-beam energy spectra. Proton irradiation induces laser-active color centers in the crystal, whose density correlates with the absorbed dose. The spatial distribution of photoluminescence emitted by these color centers is exploited to estimate the proton-beam energy spectrum using an analytical Bragg-curve model. This study integrates the effects of multiple Coulomb scattering (MCS) into the model. At high enough energies, MCS leads to proton leakage through the crystal faces with a reduction in absorbed dose along the crystal length. The model incorporates MCS using an empirical approach based on Monte Carlo simulations.

2D Helium Atom Diffraction from a Microscopic Spot.

A method for measuring helium atom diffraction with micron-scale spatial resolution is demonstrated in a scanning helium microscope (SHeM) and applied to study a micron-scale spot on the (100) plane of a lithium fluoride (LiF) crystal. The positions of the observed diffraction peaks provide an accurate measurement of the local lattice spacing, while a combination of close-coupled scattering calculations and MonteCarlo ray-tracing simulations reproduce the main variations in diffracted intensity. Subsequently, the diffraction results are used to enhance image contrast by measuring at different points in reciprocal space. The results open up the possibility for using helium microdiffraction to characterize the morphology of delicate or electron-sensitive materials on small scales. These include many fundamentally and technologically important samples which cannot be studied in conventional atom scattering instruments, such as small grain size exfoliated 2D materials, polycrystalline samples, and other surfaces that do not exhibit long-range order.

Dynamics of laser-induced formation of color centers in a lithium fluoride crystal

The dynamics of the laser-induced formation of aggregate color centers under the action of the first (950 nm) and second (475 nm) harmonics of a femtosecond Ti:sapphire laser has been studied. During the irradiation of the samples with laser radiation, the photoluminescence spectra of the induced centers were repeatedly measured. It was found that intensity of the spectral luminescence band of F3+ centers exceeds intensity of the band of F2 centers in samples with color centers induced by the first harmonic. At the same time, the opposite ratio of the intensities of these bands was observed in the luminescence spectra of color centers produced by second harmonic radiation for the same energy of laser pulses. It has been established that the observed effect is caused by different degree of heating of the crystalline medium in the regions of passage of laser filaments. Heating accelerates the migration of kinetic particles that carry out the aggregation process. The ratio of the concentrations of F2 and F3+ centers depends on the ratio of the duration of irradiation and the lifetime of kinetic particles, which significantly depends on temperature. A multiphoton-tunneling mode of interband ionization of a crystalline substance was implemented in the experiment. It is shown that the identity of the absorption spectra of color centers induced by the first and second harmonics of femtosecond laser radiation is a new distinguishing characteristic of experimental implementation of the tunneling ionization mechanism in experiment.

Bragg Curve Detection of Low-Energy Protons by Radiophotoluminescence Imaging in Lithium Fluoride Thin Films.

Lithium fluoride (LiF) crystals and thin films are utilized as radiation detectors for energy diagnostics of proton beams. This is achieved by analyzing the Bragg curves in LiF obtained by imaging the radiophotoluminescence of color centers created by protons. In LiF crystals, the Bragg peak depth increases superlinearly with the particle energy. A previous study has shown that, when 35 MeV protons impinge at grazing incidence onto LiF films deposited on Si(100) substrates, the Bragg peak in the films is located at the depth where it would be found in Si rather than in LiF due to multiple Coulomb scattering. In this paper, Monte Carlo simulations of proton irradiations in the 1-8 MeV energy range are performed and compared to experimental Bragg curves in optically transparent LiF films on Si(100) substrates. Our study focuses on this energy range because, as energy increases, the Bragg peak gradually shifts from the depth in LiF to that in Si. The impact of grazing incidence angle, LiF packing density, and film thickness on shaping the Bragg curve in the film is examined. At energies higher than 8 MeV, all these quantities must be considered, although the effect of packing density plays a minor role.

Low-energy protons shallow spread-out Bragg peak imaging with a lithium fluoride crystal

The 35 MeV proton beam produced by a modular linear accelerator was used to irradiate at grazing incidence a LiF crystal through a spinning Mylar multi-sector range modulation wheel and a Pyrex range shifter. Starting from a measured pristine depth dose curve, these energy-modulation devices were designed to plan a test shallow spread-out Bragg peak in water. The irradiation of the LiF crystal produced in its lattice a volume distribution of color centers, which could be visualized in a fluorescence microscope as a fluorescent image under blue-light illumination. Since the intensity of this fluorescent image was proportional point-by-point to the absorbed energy, it was elaborated to obtain an experimental depth dose curve in the LiF crystal, from which the proton beam energy distribution was estimated. This latter was finally used to evaluate the corresponding spread-out Bragg peak that would be obtained in water and compare it to the designed one.

Optical characterization of lithium fluoride thin-film imaging detectors for monochromatic hard X-rays

Lithium fluoride (LiF) crystals and thin films have been successfully investigated as X-ray imaging detectors based on optical reading of visible photoluminescence emitted by stable radiation-induced F2 and F+ 3 colour centres. In this work, the visible photoluminescence response of optically-transparent LiF film detectors of three different thicknesses, grown by thermal evaporation on Si(100) substrates and irradiated with monochromatic 7 keV X-rays at several doses in the range between 13 and 4.5 × 103 Gy, was carefully investigated by fluorescence optical microscopy. For all the film thicknesses, the photoluminescence response linearly depends on the irradiation dose in the investigated dose range. The lowest detected dose, delivered to the thinnest LiF film, only 0.5 μm thick, is estimated 13 Gy. Edge-enhancement imaging experiments, conducted by irradiating LiF film detectors at the same energy placing an Au mesh in front of them at a distance of 15 mm, allowed estimating a spatial resolution of (0.38 ± 0.05) μm, which is comparable to the microscope one. This very high spatial resolution in LiF film radiation detectors based on colour centres photoluminescence is combined with the availability of a wide field of view on large areas.

Dynamic range and dose linearity of the radiophotoluminescence intensity in lithium fluoride crystals irradiated with 2.3 and 26 MeV protons

Irradiation of lithium fluoride with protons causes the formation of electronic defects, known as colour centres, in the crystal lattice. Two stable defects among them, the F2 and F3+ aggregate ones, under blue light simultaneous excitation show broad photoluminescence in the red and green visible spectral ranges, respectively, their peculiar characteristics being well known for applications in optically-pumped tuneable lasers, broad-band miniaturized light-emitting photonic devices and passive radiation imaging detectors. Recently, exploitation of their radiophotoluminescence has been successfully proposed for proton-beam advanced diagnostics and dosimetry, even at low dose values typical of radiotherapy. In the present study, lithium fluoride crystals were irradiated with 2.3 MeV protons in the dose range from ∼104 to ∼107 Gy and their dynamic range, that is the capability of detecting the widest possible range of radiophotoluminescence intensity emitted by the colour centres, was found to be 114 dB at the F2 peak emission wavelength of 675 nm under continuous-wave laser excitation at 445 nm. Moreover, after including in the analysis a set of lithium fluoride crystals irradiated with 26.1 MeV protons in the dose range from 0.5 to 48 Gy at a much lower dose-rate, it was found that the radiophotoluminescence intensity of the F2 colour centres shows a linear dependence with dose from 0.5 to ∼2 × 105 Gy.

Proton Bragg peak imaging by colour centre radiophotoluminescence in lithium fluoride thin film radiation detectors on silicon

Optically transparent lithium fluoride (LiF) thin films, thermally evaporated on Si(100) substrates, are under investigation as novel radiation detectors based on radiophotoluminescence for imaging of the full Bragg curves of proton beams produced by a linear accelerator for proton therapy under development at ENEA C.R. Frascati. Proton irradiation induces the formation of stable colour centres in LiF, amongst which the broadband light-emitting F2 and F3+ aggregate defects, whose concentrations are locally proportional to the energy deposited in the material. Their spatial distributions in the irradiated LiF thin films and crystals are carefully measured by acquiring the latent two-dimensional visible fluorescence images with an optical microscope under blue lamp excitation. Several LiF films grown on silicon substrate were irradiated in air at increasing proton energies up to 35 MeV with their surface parallel to the particle beam and a cleaved edge perpendicularly facing it; for each sample, the fluorescence image acquired from the top surface side of the film allows to obtain the depth profile of the energy released by protons. Differences in colour centre distributions detected in LiF films with respect to LiF crystals are presented and discussed. Accurate Monte Carlo simulations allow to fully explain their experimental behaviours, paving the way towards using LiF film radiation detectors on silicon for the advanced diagnostics of proton beams at typical particle energies used for proton therapy.

Микроспектроскопия единичных центров окраски в щелочно-галоидных кристаллах

Single color centers in the LiF crystal were studied by confocal fluorescence microscopy with analysis and statistical processing of the time dependences of their fluorescence intensity. The possibility of identifying quantum systems whose selection by luminescence excitation (absorption) spectra, luminescence spectra, and luminescence decay time is difficult due to their overlap or due to the coincidence of the corresponding characteristics of different centers is shown. An additional identification feature in this case is the flicker frequency and other characteristics of the time dependences of the luminescence intensity. The efficiency of the method is demonstrated by the example of two color centers of different nature (F2 and F3, stabilized by an impurity–vacancy dipole) in a lithium fluoride crystal.

Design concepts of a new imaging system for a high-intensity XUV source beam by colour centres excitation in lithium fluoride crystals

Stable colour centre production in lithium fluoride (LiF) crystals can employ as a high-spatial-resolution imaging tool for extreme ultraviolet (XUV) irradiation, as well as the possibility for images of the unfocused beam and the beam focused by a multi-layer mirror.The LiF crystal sensitivity has sufficient to impress high-contrast photo-luminescent patterns with XUV single-pulse irradiation on an area up to 40mm2. The suggested imaging technique, using LiF as a detector, can contribute to reducing the lack of sufficient knowledge for XUV beam characterization and profile featurization which can open a very wide range of XUV metrology and tomography applications.The experimental results explain the concepts of detection of high-intensity source at13.5nm using a YAG:Ce scintillator crystal embedded with a CMOS camera, additionally using LiF as a 2D high-resolution detector, and the work shows investigations outcomes and improvement procedure and analysis.The results demonstrate the potential of LiF crystals as a sub-micrometre resolution two-dimensional imaging tool for XUV irradiation applications. Moreover, the research study explains the optimization sequences of the new imaging technique that will play an important role to predict the achievable spot size, geometry, beam profile and intensity distribution, as well as the characterization complexity of XUV source features.

Infrared LiF ridge waveguide fabricated by carbon ion irradiation

We report on the fabrication of an optical ridge waveguide in lithium fluoride crystal using the method of C5+ ion irradiation with precise diamond blade dicing. The guiding properties of the waveguide at the infrared wavelengths of 1064 nm, 2200 nm and 4000 nm are investigated in detail. All the near-field modal profiles exhibit single-mode guidance and the minimum propagation losses of the ridge waveguide after annealing are reduced to 0.7 dB cm−1 (λ = 1064 nm), 0.4 dB cm−1 (λ = 2200 nm) and 0.2 dB cm−1 (λ = 4000 nm), respectively. The Raman spectroscopy of the waveguide shows that the ion irradiation does not obviously change the lattice structure of the crystal.

(Digital Presentation) Color Centers Photoluminescence in Lithium Fluoride Thin-Film-on-Silicon Detectors for Proton Bragg Curves Imaging

The photoluminescence (PL) properties of radiation-induced color centers (CCs) in lithium fluoride (LiF) crystals and thin films find applications in optically-pumped solid-state lasers and photonic light-emitting microdevices operating at room temperature (RT) in the visible and near-infrared spectral range [1]. Among their peculiarities, the broad tunability and high emission quantum efficiency, combined with the wide optical transparency of the hosting LiF matrix.On the other hand, LiF dosimeters based on thermoluminescence (TL) of point defects in LiF crystals and pellets have been the most widely used family of phosphors in TL dosimetry, mainly for their high radiation sensitivity at low doses and the LiF tissue-equivalence, which is essential for meaningful medical applications.The excellent thermal and optical stabilities of the laser-active F2 and F3 + electronic defects, consisting of two electrons bound to two and three adjacent anion vacancies, respectively, whose efficient visible photoluminescence is located in the green-red spectral range under simultaneous excitation with blue-light pumping, make radiation detectors based on LiF crystals and thin films attractive for X-ray imaging [2] at nanoscale, related to the atomic-scale dimensions of such point defects.These passive radiation imaging sensors are based on the optical reading of visible radiophotoluminescence (RPL) of aggregate CCs locally created and stored in LiF, by using conventional and advanced fluorescence microscopy techniques for latent images acquisition.In the last years they were successfully tested for proton beam advanced diagnostics and dosimetry at increasing energies from 1.5 to 35 MeV, showing a linear RPL response as a function of absorbed doses in a wide dynamic range [4], long-term stability against fading and non-destructive reading capability. With suitable irradiation geometries of LiF crystals, it is possible to record the energy that protons deposit in the material (Bragg curve) as a bi-dimensional spatial distribution of luminescent CCs, even at dose values below 50 Gy, typically utilized in protontheraphy [5].We have been investigating the feasibility to extend this approach to optically transparent LiF thin films thermally evaporated on Si(100) substrates. Despite of their low thickness, we take advantage of the enhanced PL response of F2 and F3 + CCs, related to the presence of the flat and smooth Si substrate [6], which is optically reflective at the excitation and emission wavelengths utilized in the fluorescence microscope. The irradiations were performed with proton beams produced by the linear accelerator TOP-IMPLART (Oncological Therapy with Protons - Intensity Modulated Proton Linear Accelerator for RadioTherapy), under development at ENEA C.R. Frascati, with the cut edge perpendicular to the proton beam direction. The latent two-dimensional fluorescence images of the CC distributions generated in the polycrystalline LiF layers show a systematic increase in depth of the Bragg peak with respect to LiF crystals.The results obtained in LiF films at increasing proton energies are presented and discussed, also in comparison with those in LiF crystals, in order to explain the observed behaviors and highlight advantages and limits of versatile LiF film radiation detectors.[1] R. M. Montereali, in Handbook of Thin Film Materials, H. S. Nalwa, Editor, Vol.3: Ferroelectric and Dielectric Thin Films, Ch.7, p. 399, Academic Press, S. Diego (2002).[2] G. Baldacchini, F. Bonfigli, A. Faenov, F. Flora, R. M. Montereali, A. Pace, T. Pikuz, L. Reale, J. Nanosci. Nanotechno., 3, 483 (2003).[3] A. Ustione, A. Cricenti, F. Bonfigli, F. Flora, A. Lai, T. Marolo, R. M. Montereali, G. Baldacchini, A. Faenov, T. Pikuz and L. Reale, Jpn. J. Appl. Phys. 45, 2116 (2006).[4] M. Piccinini, F. Ambrosini, A. Ampollini, L. Picardi, C. Ronsivalle, F. Bonfigli, S. Libera, E. Nichelatti, M. A. Vincenti and R. M. Montereali, Appl. Phys. Lett., 106, 261108 (2015).[5] R.M. Montereali, E. Nichelatti, V. Nigro, M. Piccinini, M.A. Vincenti, ECS J. Solid State Sci. and Technol. 10, 116001 (2021).[6] M. A. Vincenti, M. Leoncini,S. Libera, A. Ampollini, A. Mancini, E. Nichelatti, V. Nigro, L. Picardi, M. Piccinini, C. Ronsivalle, A. Rufoloni, R. M. Montereali, Opt. Mater., 119, 111376 (2021).

Thermal enhancement of the intensity of fluorescent nuclear tracks in lithium fluoride crystals

Lithium fluoride fluorescent nuclear track detectors (FNTD) irradiated with alpha-particles were subjected to annealing at temperatures ranging up to 400 °C. It was found that for the annealing temperatures between 200 °C and 300 °C the intensity of fluorescent tracks increases. The maximum enhancement factor of 2.5 is reached for a temperature of 290 °C. For temperatures exceeding 300 °C the intensity decreases and above 340 °C the tracks are not visible. The effect of track enhancement is permanent - no differences were observed for several weeks after the thermal treatment. An interesting phenomenon is the influence of the particle fluence: with increasing fluence the enhancement factor decreases and for the fluence around 2.9*107 cm−2, the intensity of light registered after heat treatment is lower than the initial one. It is still unclear what is the reason behind the observed effect. It however seems probable that the mechanism involves transforming part of F centers into F2 centers. Nevertheless, the effect is promising, as it may lead to signal-to-noise ratio improvement and improved measuring capabilities of LiF FNTDs.

Visible radiophotoluminescence of colour centres in lithium fluoride: from lasers to versatile radiation sensors

The peculiar photoluminescence characteristics of radiation-induced colour centres in lithium fluoride (LiF), well known for applications in optically-pumped tuneable lasers and broad-band miniaturised light-emitting photonic devices operating at room-temperature, are under exploitation in passive imaging detectors and dosimeters based on visible radiophotoluminescence in LiF crystals and polycrystalline thin films. Their high intrinsic spatial resolution, wide dynamic range and large field of view, combined with easy handling, ambient-light operation and no development need, allow to successfully extend their use from X-ray imaging to proton-beam advanced diagnostics and dosimetry, even at those low dose values that are typical of hadrontherapy. After exposure, the latent images stored in LiF as local formations of F2 and F3 + aggregate defects are read with an optical fluorescence microscope under illumination in the blue spectral range. Their visible emission intensity was found to be linearly proportional to the dose over at least three orders of magnitude, so that bi-dimensional LiF solid-state dosimeters based on spectrally-integrated radiophotoluminescence reading can be envisaged. Taking advantage of the low thickness of LiF thin films, transversal proton beam dose mapping was demonstrated at low proton energies, even at high doses. Recent results and advances concerning LiF crystals and polycrystalline thin film characterisation in the linearity range are presented and discussed with the aim of highlighting challenges related to increasing the LiF film detector radiation sensitivity to both particles (protons) and photons (X-rays), although therapeutic dose values typical of clinical radiotherapy are still a big challenge.