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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.

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.

Point defects in the host lattice of a scintillator material can trap carriers, slowing down their migration or even preventing their transfer to luminescent centers. Such competition schemes between defects and luminescent centers may explain also the hysteresis effect, which consists of a progressive enhancement of scintillation efficiency with accumulated dose. We propose a comparison between the scintillation hysteresis effect of Yb-doped sol-gel silica glasses in bulk and fiber forms, and we correlate them with traps monitored by wavelength-resolved thermally stimulated luminescence in both materials. The results demonstrate that the fiber-drawing process is responsible for modifications of the defectiveness of the glass network, with a change of the local distribution of the traps surrounding the luminescent center. The consequence of such modifications is the removal, in the fiber samples, of the thermally stimulated luminescence peak ascribed to traps closer to Yb ions and unstable at room temperature. We highlight that suitable postdensification thermal treatments can significantly modify the concentration and spatial distribution of defects around a luminescent center and can therefore be used as a tool for the engineering of scintillating glasses.

Radio-luminescence (RL) optical fiber sensors enable a remote, punctual, and real time detection of ionizing radiation. However, the employment of such systems for monitoring extended radiation fields with energies above the Cerenkov threshold is still challenging, since a spurious luminescence, namely, the "stem effect," is also generated in the passive fiber portion exposed to radiation. Here, we present experimental measurements on Yb-doped silica optical fibers irradiated with photon fields of different energies and sizes. The results demonstrate that the RL of Yb3+, displaying a sharp emission line at about 975 nm, 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. © 2014 AIP Publishing LLC.

Significant improvements have continuously been made in radiation therapy technologies, and therefore innovative detectors must also be developed to ensure the beam quality of these new irradiation systems and to allow in vivo dosimetry measurements. Optical fiber based radioluminescence (RL) dosimeters are a promising option for these purposes. They can enable real time dose measurement and their small size can be exploited in small radiation field dosimetry. A major difficulty involved in marketing these systems is the spurious luminescence, generally known as stem effect, that is intrinsically present in this type of detector. The possible mechanisms causing the stem effect during irradiation are fluorescence phenomena and especially Cerenkov light, produced in the undoped fiber portion. In this study we have faced the problem by making use of a spectral discrimination method exploiting the sharp 5D0-7F2 line emission of Eu3+ [1]. First, the incorporation of Eu3+ ions in sol-gel silica was investigated as a function of dopant concentration and synthesis parameters. Preliminary structural (TEM), vibrational (Raman, FTIR) and optical absorption studies [2, 3] have allowed to find the most suitable rare-earth (RE) concentrations and synthesis conditions for optimizing both RL efficiency and RE dispersion, avoiding the formation of aggregates. Indeed, clusters of few tens of nanometers were observed already for SiO2:3 mol% Eu3+. The 10 mol% doped sample contains also aggregates of much larger dimensions, whose diffraction patterns are in agreement with the presence of rare earth crystalline oxide silicates. Eu3+-doped fibers were realized by drawing cluster-free silica with 600 ppm doping level. The spectral emission of composite fibers made of a small portion (1 cm) of doped silica fiber coupled to a long (15 m) undoped fiber for remote signal transport has then been investigated under irradiation with photons and electrons of different energies, field sizes and ori

Significant improvements have continuously been made in radiation therapy technologies, and therefore innovative detectors must also be developed to ensure the beam quality of these new irradiation systems and to allow in vivo dosimetry measurements. Optical fiber based radioluminescence (RL) dosimeters are a promising option for these purposes. They can enable real-time dose measurement and their small size can be exploited in small radiation field dosimetry. A major difficulty involved in marketing these systems is the spurious luminescence, generally known as stem effect, that is intrinsically present in this type of detector. The possible mechanisms causing the stem effect during irradiation are fluorescence phenomena and especially Cerenkov light, produced in the undoped fiber portion. In this study we have faced the problem by making use of a spectral discrimination method exploiting the sharp 5D0–7F2 line emission of Eu3+ [1]. First, the incorporation of Eu3+ ions in sol-gel silica has been investigated as a function of dopant concentration and synthesis parameters. Structural (Raman, TEM) and optical (absorption and RL) studies have allowed to find the most suitable rare-earth (RE) concentrations and synthesis conditions for optimizing both RL efficiency and RE dispersion, avoiding the formation of aggregates. The spectral emission of composite fibers made of a small portion (1 cm) of Eu3+-doped silica fiber with 600 ppm doping level coupled to a long (15 m) undoped fiber for remote signal transport has then been investigated under irradiation with photons and electrons of different energies, field sizes and orientations, in order to discover the origin of the stem effect and evaluate its influence on the RL spectral shape. A comparison with previously investigated Ce3+-doped silica fibers [2] has also been made. The possibility of an efficient discrimination between the RL dosimetric signal and the spurious one is proved and discussed.References 1. I. Veron

The depth-dose-profile and the transversal beam profile of a 6 MV photon beam are measured by means of Eu-doped silica optical fibres. In both cases, a non-negligible influence of the stem effect was observed as consequence of the change in the relative contribution of the Cerenkov light while varying the vertical and lateral position of the fibre. The approach for removing the spurious luminescence signal based on the radioluminescence (RL) spectral analysis proved to be effective and independent of the irradiation configurations.

In this study, the spectral emission of Eu3+-doped silica optical fibres is investigated under irradiation with photons and electrons of different energies, field sizes and orientations, in order to discover the origin of stem effect and evaluate its influence on the radioluminescence (RL) spectral shape under experimental conditions. A comparison with previously investigated Ce-doped silica fibres is also made. Below the Cerenkov radiation energy threshold, a slight contribution to the total RL signal is observed. This is due to fluorescence effects occurring in the passive fibre. Above the energy threshold, the stem effect is mainly caused by Cerenkov radiation. A method for removing the stem effect, based on RL spectral analysis, is proposed and validated by measuring the output factors of extended photon fields and by studying the angular dependence of the dosimeter

LiCaAlF6 (LiCAF) crystals doped with two different ions (europium and lead) have been investigated as potential new dosimetric materials. The stability of thermally stimulated luminescence (TSL) glow peaks in LiCAF:Eu was evaluated by means of the initial rise technique. The decay times at room temperature of the traps related to the dosimetric glow peaks were found to range between 40 and 2 × 104 years confirming the good dosimetric characteristics of this crystal. The glow curve of LiCAF:Pb is dominated by a peak at approximately 300 °C emitting in the UV region ( 3P0,1-1S0 transition of Pb 2+) superimposed to a very broad structure at lower temperature (20-200 °C) featuring recombination at an intrinsic defect centre. The anomalous behavior of the low temperature structure during thermal cleaning procedures prevented any reliable numerical analysis of the TSL glow peak at 300 °C. © 2009 Elsevier Ltd. All rights reserved.

A radioluminescent (RL) dosimeter based on cerium-doped silica optical fibre was recently developed by means of the sol-gel and powder-in-tube techniques. The detector, already tested with both soft X-rays and more energetic photon beams used in conventional radiation therapy and in stereotactic radiosurgery, showed interesting and promising properties. A feasibility study for the use of the doped fibre in proton therapy is here presented. The measurements were performed at the proton therapy facility of the Paul Scherrer Institute (PSI) using a 138 MeV scanned proton pencil beam. The preliminary results showed a good sensitivity of the RL dosimeter to proton beams and a satisfactory reproducibility of the response (better than 0.5%) following repeated irradiations under the same experimental conditions. A linear dose response of the signal was observed in the investigated range (0.1-10 Gy) and, moreover, the stem effect proved to be negligible. The characteristic of the system to enable a real-time measurement of the dose rate was exploited for time-resolving different spot scanning sequences. Finally, the detector was used for assessing the depth dose curve in water: by comparing the curve obtained with the doped fibre with the corresponding one measured with a standard ionization chamber, a quenching effect in the order of 12% in the region of the Bragg peak was observed, as consequence of the decrease of the scintillation efficiency with increasing the Linear Energy Transfer (LET) of protons. © 2009 Elsevier Ltd. All rights reserved.

Interest is increasing in the development of new methodologies for accidental dose assessment, exploiting the luminescence and dosimetric properties of objects and materials which can be usually found directly on exposed subjects and/or in the contaminated area. In this work, several types of ceramics employed for dental prosthetics restoration, including both innovative materials used as sub-frames for the construction of the inner part of dental crowns (core), and conventional porcelains used for the fabrication of the external layer (veneer), were investigated with regard to their thermally and optically stimulated luminescence (TSL and OSL respectively) properties, in view of their potential application in accidental dosimetry. The sensitivity to ionizing radiation proved to strongly depend on the type and brand of ceramic, with minimum detectable dose ranging from few mGy up to several tens of mGy. A linear dose-response was observed for most of the samples. However, the luminescence signals were characterised by a significant fading, which has to be taken into account for a reliable accidental dose assessment after a radiation exposure event.