1,721,067 research outputs found
Influence of depth-dependent excitation-light absorption on photoluminescence emission from a slab
Full text linkIn questo Rapporto Tecnico viene analizzato come l’assorbimento della luce di eccitazione (pompa ottica) influenzi le misure di intensità di fotoluminescenza (FL) in solidi a facce piane parallele contenenti una distribuzione di centri emettitori variabile lungo lo spessore. Viene proposta una metodologia di correzione basata sulla misura della densità ottica, per tenere conto dell’attenuazione progressiva della luce di pompa attraverso il materiale. Nel caso specifico di centri di colore generati da irraggiamento ionizzante in cristalli di LiF, lo studio evidenzia i possibili errori sistematici introdotti dall’assorbimento della pompa nella stima della dose di saturazione, assumendo che quest’ultima venga ottenuta tramite best fit degli andamenti di intensità di FL in funzione della dose assorbita. Vengono infine presentati alcuni esempi che mostrano come tali andamenti siano influenzati non solo dall’assorbimento della pompa ottica, ma anche dalla distribuzione spaziale della dose assorbita all’interno del cristallo. I risultati sottolineano l’importanza di correggere l’effetto di assorbimento della pompa nella valutazione del comportamento di saturazione della FL in cristalli di LiF irradiati.This Technical Report investigates how the absorption of excitation light (optical pump) affects photoluminescence (PL) intensity measurements in solids with parallel planar faces containing a depth-dependent distribution of emitting centers. A correction method based on optical density measurements is proposed to account for the gradual attenuation of the pump light across the material thickness. In the specific case of color centers generated by ionizing radiation in LiF crystals, the study highlights the possible systematic errors introduced by pump absorption in estimating the saturation dose, assuming it is derived from best fits of PL intensity as a function of absorbed dose. Several examples are presented to illustrate how PL trends are influenced not only by pump absorption, but also by the spatial distribution of the absorbed dose within the crystal. The findings emphasize the importance of correcting for pump absorption when evaluating PL saturation behavior in irradiated LiF crystal slabs
Diagnostica energetica di fasci di protoni da fotoluminescenza di centri di colore in cristalli di fluoruro di litio: correzione del disallineamento angolare tra fascio e cristallo
Full text linkLa fotoluminescenza da centri di colore nei cristalli di fluoruro di litio (LiF) è sfruttata per caratterizzare lo spettro energetico di fasci di protoni accelerati nell’intervallo di energie da qualche MeV a diverse decine di MeV. L’approccio, applicato negli ultimi anni con successo all’acceleratore lineare TOP-IMPLART, è stato sviluppato dal nostro laboratorio e consiste di diverse fasi: (a) irraggiamento a incidenza radente del cristallo di LiF con il fascio di protoni; (b) visualizzazione in un microscopio a fluorescenza della map-pa di fotoluminescenza emessa dai centri di colore formatisi nel cristallo per l’interazione ionizzante con i protoni; (c) elaborazione di tale mappa per ricavare una stima dello spettro energetico del fascio di pro-toni. In quest’ultima fase, il ricorso a un modello analitico per il calcolo di curve di Bragg di protoni acce-lerati in cristalli di LiF, opportunamente corretto per includere la fuoriuscita di protoni dal cristallo dovuta alla diffusione multipla di Coulomb, permette l’esecuzione di un algoritmo per determinare lo spettro energetico del fascio. In questo Rapporto Tecnico si propone un aggiornamento di tale algoritmo, me-diante elaborazione esterna con il software Monte Carlo FLUKA, per correggere l’effetto di un eventuale disallineamento angolare, nella prima fase, tra fascio di protoni e cristallo di LiF.Photoluminescence emitted by color centers within lithium fluoride (LiF) crystals is utilized to characterize the energy spectrum of accelerated proton beams ranging from a few MeV to several tens of MeV. This method, successfully employed at the TOP-IMPLART linear accelerator, was developed by our laborato-ry and involves several stages: (a) exposing the LiF crystal to the proton beam at grazing incidence; (b) visualizing the photoluminescence map emitted by the color centers formed in the crystal due to ionizing interactions with protons using a fluorescence microscope; (c) processing this map to estimate the ener-gy spectrum of the proton beam. In this final phase, the use of an analytical model for the calculation of Bragg curves of protons accelerated in LiF crystals, appropriately corrected to include the exit of protons from the crystal due to multiple Coulomb scattering, allows the execution of an algorithm to determine the energy spectrum of the beam. This Technical Report proposes an update of this algorithm, through external processing with the Monte Carlo FLUKA software, to correct the effect of a possible angular misalignment, in the first phase, between the proton beam and the LiF crystal
Energy spectrum and Bragg curve of a protron beam in low-density lithium fluoride
No full textL’irraggiamento del fluoruro di litio (LiF) con un fascio di protoni provoca la formazione di centri di colore aggregati F2 ed F3+ che emettono luminescenza nel visibile se eccitati otticamente con luce blu. La distribuzione spaziale della radiofotoluminescenza emessa è proporzionale alla curva di Bragg dei protoni nel LiF, purché la dose ricevuta dal materiale non sia troppo elevata (minore di circa 105–106 Gy) da provocare saturazione della densità dei centri. Dall’analisi della curva di Bragg luminescente, è possibile stimare lo spettro energetico del fascio di protoni. Se il metodo di analisi si basa sulla sovrapposizione di curve di Bragg teoriche calcolate nel LiF bulk, ma il LiF irraggiato è quello di un film sottile con densità d’impacchettamento minore del 100%, lo spettro che si ottiene non è quello reale. Infatti, i protoni in un materiale con densità inferiore a quella di bulk penetrano più in profondità rispetto a quanto farebbero nel bulk. In questo Rapporto Tecnico, derivo una formula correttiva approssimata che permette di ricavare lo spettro reale nel LiF a densità minore.Irradiation of lithium fluoride (LiF) with a proton beam induces the formation of F2 and F3+ aggregate color centers that luminesce in the visible spectrum when optically excited by blue light. The spatial distribution of the emitted radiophotoluminescence is proportional to the proton Bragg curve in LiF, provided that the dose received by the material is not too high (less than approximately 105–106 Gy) to cause saturation of the center density. From the analysis of the luminescent Bragg curve, it is possible to estimate the energy spectrum of the proton beam. If the analysis method is based on the superposition of theoretical Bragg curves calculated in bulk LiF, but the irradiated LiF is a thin film with a packing density lower than 100%, the resulting spectrum will not reflect the actual one. In fact, protons in a material with lower density than bulk penetrate deeper than they would in bulk. In this Technical Report, I derive an approximate correction formula that allows obtaining the actual spectrum in lower-density LiF
Modello semianalitico per il calcolo della dose tridimensionale depositata in acqua da un fascio di protoni a sezione gaussiana
Full text linkNell’ambito del progetto TOP-IMPLART, è in fase di sviluppo e collaudo un prototipo di acceleratore lineare di protoni a radiofrequenza e modulazione attiva destinato a un utilizzo per terapia adronica. In previsione dell’elaborazione di piani di trattamento ideati ad hoc per testare le prestazioni del linac, è desiderabile disporre di uno strumento snello per il design di picchi di Bragg allargati nelle tre dimensioni spaziali. La possibilità di sfruttare una rappresentazione matematica analitica del volume di dose depositato in acqua dal fascio di protoni emesso dal linac permette, in linea di principio, una realizzazione più agevole di tale strumento. A tale scopo, in questa relazione si illustra la derivazione di una formula analitica per il calcolo approssimato della distribuzione spaziale di dose nelle tre dimensioni utilizzando i modelli analitici di Bortfeld e di Preston-Koehler. I risultati ottenuti con tale formula vengono confrontati con simulazioni Monte Carlo condotte in FLUKA.As part of the TOP-IMPLART project, a prototype of a radiofrequency proton linear accelerator with active modulation is currently being developed and tested for use in hadron therapy. In anticipation of designing tailored treatment plans to test the linac's performance, it is desirable to have a streamlined tool for designing expanded Bragg peaks in three spatial dimensions. The ability to utilize an analytical mathematical representation of the dose volume deposited in water by the proton beam emitted from the linac facilitates, in principle, the creation of such a tool. To this end, this report presents the derivation of an analytical formula for the approximate calculation of the three-dimensional spatial dose distribution using the analytical models of Bortfeld and Preston-Koehler. The results obtained with this formula are compared with Monte Carlo simulations conducted in FLUKA
Thermoluminescence in LiF crystals and the role of impurities
No full textEight 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
Tetraphenyl-butadiene films: VUV-Vis optical characterization from room to liquid argon temperature
No full textA 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
VUV-Vis optical characterization of Tetraphenyl-butadiene films on glass and specular reflector substrates from room to liquid Argon temperature
No full textThe 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
Color-center waveguides in low-energy electron-bombarded lithium fluoride
No full textWe employ a differential version of m-line spectroscopy through grating coupling in order to measure the refractive index of low-energy electron irradiated lithium fluoride crystal channel waveguides for broadband emission. Using photoresist films and a holographic setup for grating fabrication, we perform an accurate characterization of bidimensional structures and a direct quantitative comparison between treated and blank materials
Optimization of the theoretical dose distribution in the “Spread out Bragg Peak” (SOBP) region in proton therapy by means of semi-analytical techniques
No full textProton therapy uses proton beams to destroy cancer cells. Since the energy deposition of protons peaks at the end of the trajectory (Bragg peak), healthy tissues close to the target are partially spared. In clinical practice, beams of different energies are composed to obtain a broadened peak (SOBP, Spread Out Bragg Peak) to treat the entire lesion region in a uniform manner. In the framework of the TOP-IMPLART project (linear accelerator for proton therapy under construction at ENEA-Frascati), we present a semi-analytical method that allows to optimize SOBP uniformity for two different energy-modulation techniques, passive and active
In-line X-ray lensless imaging with lithium fluoride film detectors
No full textIn 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. © 2013 Elsevier B.V. All rights reserved
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