The near-infrared radioluminescence and dosimetric properties of Yb-doped silica optical fibers, coupled with an optical detector prototype based on an avalanche photo-diode, were studied by irradiating the fibers with clinical beams generated by a Varian Trilogy accelerator. The performances of the system in standard and small field sizes have been also investigated comparing the output factor, percent depth dose and off axis ratio measurements of the prototypal dosimetric system with other commercial sensors.
The results demonstrated that the drawback due to the stem effect in Yb-doped silica optical fibers can be managed in a simple but effective way by optical filtering. These features, together with the accuracy and precision achieved by Ybdoped fibers in relative dose assessments make the device promising for in-vivo dosimetry studies in radiation therapy.
Cristina De Mattia, Ivan Veronese, Mauro Fasoli, Norberto Chiodini, Eleonora Mones, Marie Claire Cantone, Simone Cialdi, Marco Gargano, Nicola Ludwig, Letizia Bonizzoni, Anna Vedda
Rare earths-doped silica optical fibers have shown promising results for ionizing radiation monitoring, thanks to their radio-luminescence (RL) properties. However, the use of these systems for accurate and precise dosimetric measurements in radiation fields above the Cerenkov energy threshold, like those employed in radiation therapy, is still challenging, since a spurious luminescence, namely the “stem effect,” is also generated in the passive fiber portion exposed to radiation. The spurious signal mainly occurs in the UV-VIS region, therefore a dopant emitting in the near infrared may be suitable for an optical discrimination of the stem effect.
In this work, the RL and dosimetric properties of Yb-doped silica optical fibers, produced by sol-gel technique, are studied, together with the methods and instruments to achieve an efficient optical detection of the Yb3+ emission, characterized by a sharp line at about 975 nm.
The results demonstrate that the RL of Yb3+ is free from any spectral superposition with the spurious luminescence. This aspect, in addition with the suitable linearity, reproducibility, and sensitivity properties of the Yb-doped fibers, paves the way to their use in applications where an efficient stem effect removal is required.
The recent progresses in the development and characterization of doped silica fiber optics for dosimetry applications in the modern radiation therapy, and for high energy physics experiments, are presented and discussed. In particular, the main purpose was the production of scintillating fiber optics with an emission spectrum which can be easily and efficiently distinguished from that of other spurious luminescent signals originated in the fiber optic material as consequence of the exposition to ionizing radiations (e.g. Cerenkov light and intrinsic fluorescence phenomena). In addition to the previously investigated dopant (Ce), other rare earth elements (Eu and Yb) were considered for the scintillating fiber optic development. The study of the luminescent and dosimetric properties of these new systems was carried out by using X and gamma rays of different energies and field sizes.
In the last decade, the interest in scintillating fiber optics for ionizing radiation monitoring is constantly increasing.
Among the fields of possible applications of these sensors, radiation therapy represents a driving force for the research
and development of new devices. In fact, the small dimensions of fiber optics based detectors, together with their realtime
response, make these systems extremely promising both in quality assurance measurements of intensity modulated
radiotherapy beams, and in in-vivo dosimetry. On the other hand, two specific aspects might represent limiting factors:
(i) the “stem effect”, that is the spurious luminescence originating as a consequence of the irradiation of the light guide,
and (ii) the “memory effect”, that is the radioluminescence sensitivity increase during prolonged exposition to ionizing
radiation, typical of many scintillating materials.
These two issues, representing the main challenges to face for the effective use of scintillating fiber as dosimeters in
radiotherapy, were studied considering amorphous silica matrices prepared by sol-gel method and doped with europium.
The origin of the stem effect was investigated by means of spectral measurements of the doped fibers irradiated with Xrays
and electrons of different energies, field sizes and orientations. New approaches for removing the stem effect on the
basis of the radioluminescent spectral analysis are presented and discussed. Furthermore, the causes and phenomenology
of the memory effect are described, considering also the effect of dose accumulation with different dose rates and
energies of ionizing radiation.
Norberto Chiodini, Anna Vedda, Mauro Fasoli, Federico Moretti, Alessandro Lauria, Marie Claire Cantone, Ivan Veronese, Giampiero Tosi, Marco Brambilla, Barbara Cannillo, Eleonora Mones, Gilberto Brambilla, Marco Petrovich
Scintillating materials, able to convert energy of ionizing radiation into light in the visible-UV interval, are presently
used in a wide class of applications such as medical imaging, industrial inspection, security controls and high energy
physics detectors.
In the last few years we studied and developed a new radiation sensor based on silica-glass fiber-optic technology. In its
simplest configuration such device is composed by a short portion (about 10 mm) of scintillating fiber coupled to a
photomultiplier through a suitably long passive silica fiber.
In this work, we present new results concerning the characterization of silica based Ce and Eu doped fibers glasses
obtained by a modified sol-gel method and drawn by a conventional drawing tower for optical fibers. The radio-luminescence
of Eu doped fibers is rather weak; moreover it displays a marked sensitivity increase during subsequent
irradiations, preventing the use of such fibers in dosimetry. On the other hand Ce-doped fibers show very high radiation
hardness, signal stability and reproducibility, and high sensitivity to radiations with energies from 10 keV to several
tens of MeV. Numerous tests with photons (X and gamma rays), electrons, and protons have already been successfully
performed.
At the early stage of its market introduction it is the smallest radiation sensor, also compared to MOSFET and diode
technology and it appears to be the ideal choice for in vivo measurements in medical field or remote sensing.
Silica xerogels doped with Ge(IV), substituting for Si(IV) in the oxide network, are prepared from tetraethylorthosilicate
and germanium-tetraethoxide. The sintering process is carried out in reducing atmosphere at 700 - 900° C by reaction
with H2. Raman spectroscopy and high resolution transmission electron microscopy (TEM) show that reactions with H2 give rise, in the porous silica network, to uncontrolled islands of crystallites of elemental cubic germanium with average size of 50 nm. Sintering process in reducing H2 atmosphere at temperatures just below the phase separation, at about 610°C, gives materials where Ge atoms are dispersed in the matrix in conditions of incipient clustering. Evidences of segregation of germanium nanocrystals are observed with electron irradiation during TEM analysis. Furthermore, the
electron beam induced precipitation leads to the formation of isolated quantum dots-like nanocrystals (5-6 nm in
diameter) and with narrower size dispersion. The ranges of suitable temperature and germanium concentration are analysed, as well as the size dispersion of the resulting Ge nanophases.
Optically transparent nanostructured SiO2 glassceramics containing a high density of monodispersed nanometer-sized
clusters of semiconducting SnO2 have been obtained by phase separation from a sol-gel derived synthesis. Complex
impedance spectroscopy analysis has been performed to get information about the conduction mechanisms to
understand the electrical behavior of the material. Measurements have been taken with applied bias ranging from +40 V
to -40 V and with an alternated voltage signal in the range 20 Hz - 1 MHz and amplitude from 10 mV to 300 mV. An
equivalent circuit, based on a metal-oxide-semiconductor model, comprising nanostructuring contributions, allows
measurements fitting. Capacity-voltage and conductance-voltage curves have been obtained for each component.
We present the spectroscopic study of the mechanisms of excitation transfer between rare earth ions excited by energy transfer from SnO2 nanocrystals in silica. Bulk samples of pure and Er-doped silica with SnO2 nanoparticles were prepared by a sol gel technique and further thermal sintering process. Transmission electron microscopy (TEM) reveals the formation of spherical nanoclusters with a size distribution strongly determined by erbium doping. Small angle neutron scattering (SANS) experiments confirm and detail the TEM data evidencing the existence of a interphase region at the cluster boundaries where a SnOlike phase compensates the structural mismatch between the crystalline lattice in SnO2 nanoparticles and the amorphous silica network. The analysis of the SANS patterns show what kind of modification of the interphase morphology of SnO2 nanoparticles in silica brings to the passivation of interfacial defects. Surface states, which may preclude the exploitation of UV excitonic emission, are reduced after doping by rare earth ions. We demonstrate, by means of transmission-electron-microscopy and small-angle-neutron-scattering data, that a smooth interphase with a non negligible thickness takes the place of the fractal and discontinuous boundary observed in undoped material. The time resolved photoluminescence spectra of erbium in the infrared region show the spectral profile ascribable to ions in a ordered environment. Moreover, the absence of the broad contribution of the radiative decay of erbium ions dispersed in the silica amorphous matrix indicates that the excitation transfer follows paths enveloped in the interphase region. The spectroscopic analysis allows us to conclude that the excitation is transferred from ion to ion within a quasi-crystalline region where each site is surrounded by a different distribution of PL quenching sites which are responsible for the multi-exponential decay kinetics.
Silica glass with SnO2 nanocrystals, obtained from sol-gel synthesis and thermal densification at 1100 °C, was
poled by means of a two-step process consisting of infrared 1064 nm laser irradiation followed by 532 nm laser
exposure in high-voltage static electric field. Maker fringe experiments were then carried out at 1064 nm.
The results show the formation of second-order nonlinearity with macroscopic nonlinear thickness (about 1
mm) and nonlinear susceptibility comparable with thermally poled silica (about 0.1 pm/V). Photoluminescence
measurements suggest that mechanisms for this process should involve the activation and anisotropic ionization of defects at the interface between nanocrystals and glass.
In this paper different sol gel syntheses strategies are adopted in order to produce Ce-doped silica-glasses scintillators with improved properties.
Variations on the synthesis procedure are adopted to reduce the hydroxyl content of the sol gel glass; to this purpose, a fluorine derivative of silicon alkoxide was added in the sol gel reaction.
Since a drawback of Ce-doped sol gel glasses is the partial oxidation of Ce3+ to Ce4+ in the matrix probably occurring during the sintering procedure, both Ce-doped xerogel and fluorinated xerogel were treated under reducing atmosphere (H2:Ar) instead of the usual oxidizing atmosphere.
Glasses obtained in such ways are compared with commercial synthetic glasses and with previously produced sol gel samples by spectroscopic IR absorption measurements, and by radio-luminescence experiments.
Optically transparent nanostructured SiO2 glassceramics containing a high density of monodispersed, nanometer sized clusters of SnO2 were obtained by phase separation from sol-gel synthesized xerogel. This material was produced either as bulk samples or as thin films on silicon and it can be easily doped with rare earth ions during the sol-gel synthesis. Spectroscopic measurements were carried out on bulk samples, demonstrating an effective energy-transfer between nanoclusters and rare earth ions. A particular sol-gel derived synthesis was employed, producing silica-based films with relatively low electrical resistivity and thickness ranging between 300 and 1000 nm. Suitable sol features allowed thick single-step deposition by spin-coating techniques, assuring the homogeneous nanosized clustering of the semiconducting SnO2 phase. Refractive index and thickness were analyzed in films produced in different conditions of synthesis and thermal treatments of densification. The current-voltage response and the electro-induced optical emission in this material were investigated, suggesting potential technological applications in photonics and UV-emitting devices.
New Ce3+ - doped high-grade silica glasses are synthesized by a modified sol-gel method, to be used as scintillators for
the detection of X-rays and low energy particles. These glasses have efficiency a factor of 2 higher than state-of-the-art Bi4Ge3O12 crystals, high radiation hardness and high compatibility with the silica-based photonic technology. Powder-in-tube and rod-in-tube techniques are used to fabricate low-loss optical fibres with 135, 220 and 660 microns diameter. Device prototypes are obtained by fusion-splicing these fibres to commercial high numerical aperture optical fibres.
Wide-band-gap semiconductor-doped-glasses were obtained by synthesizing SnO2:SiO2 nanostructured glassceramics. In this binary system, comprising two chemically compatible oxides, crystalline SnO2 nanoclusters were embedded in a pure silica matrix in a controlled way, by setting appropriate thermochemical parameters, up to 10% of volume fraction of the semiconductor crystalline phase. Measurements of third order non-linearity were carried out by means of z-scan technique at 1064 nm finding a non linear refractive index comparable with that of glasses doped with Cd chalcogenides. Optical spectroscopy, micro-Raman scattering and electron microscopy indicated good optical and nano-structural features, suitable for stable optical applications, both in bulk and film samples.
Tin(IV) doped SiO2 xerogel and SnO2/SiO2 glass-ceramics were synthetised by sol-gel route. Synthesis and physical properties of those materials will be disclosed in this paper. Particularly, the solubility behaviour of tin(IV) in SiO2 and the oversaturation condition in glass and xerogel intermediate were investigated. Both materials (glass and glass ceramics) are useful in photonics, and different application will be proposed in this work.
Tin-doped silica glass has been recently investigated as photosensitive optical material for optoelectronic device applications. The mechanisms responsible for the material photosensitivity and the optical activity induced by Sn doping are presented. Studies performed on perform slides and on sol-gel bulk samples show that the refractive index change can be ascribed to structural rearrangements induced by photochemical reactions. Photoluminescence measurements indicate that tin atoms are embedded in Sn-substituted Si sites of the silica network. The modified structure shows extremely high stability, and gratings written in fibers exhibit a negligible erasure in 30 minutes below 600 °C. At high UV radiation fluences the refractive index modulation saturates and does not exhibit any decrease. Optical measurements and electron paramagnetic resonance data show that different processes contribute to the refractive index change. The comparison between samples with and without optical absorption at the UV laser wavelength shows that the presence and the consequent laser-induced bleaching of the 5 eV absorption band due to oxygen deficient centers does not appear crucial for photosensitivity. In fact a refractive index change is also observed in samples without detectable absorption at this energy.
There is considerable interest in Er-doped silica-based materials with photosensitive properties for lasing and photorefractive applications. Tin doping has been recently found to induce strong and stable photosensitivity in silica glass. In this work, Er co-doped Sn-doped silica samples are obtained by sol-gel method by hydrolysis and condensation of TEOS and tin dibutyl diacetate, Er(NO3)3, or Er(CH3COO)3 as dopant precursors. Samples with Er content ranging between 0.1 and 1 mol% are investigated. Sn doping (0.4 mol%) is employed to induce photorefractivity properties in the glassy host. Time resolved photoluminescence, optical absorption and refractive index measurements are carried out and analyzed as a function of the Er content. The specific effects of Er content and Sn doping on the rare earth emission efficiency and photosensitivity of the glass are discussed. In particular it is shown that the Er emission shows the typical features of Er doped sol gel silica and the strong photosensitivity induced by tin doping is not affected by rare earth ions.
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