Antonino Alessi

Interstitial O2 distribution in amorphous SiO2 nanoparticles determined by Raman and photoluminescence spectroscopy

The O2 content and emission properties in silica nanoparticles after thermal treatments in oxygen rich atmosphere have been investigated by Raman and photoluminescence measurements. The nanoparticles have different sizes with average diameter ranging from 7 up to 40u2009nm. It is found that O2 concentration in nanoparticles monotonically increases with nanoparticles size. This finding is independent on the measurement technique and evidences that oxygen molecules are not present in all the nanoparticles volume. This dependence is interpreted on the basis of a structural model for nanoparticles consisting of a core region able to host the oxygen molecules and a surface shell of fixed size and free from O2.

O 2 -Loading Treatment of Ge-Doped Silica Fibers: A Radiation Hardening Process

The effects of a high-pressure O2-loading treatment on the radiation response of Ge-doped optical fibers (OFs) were investigated. We found that the incorporation of high concentration of interstitial molecular oxygen remarkably enhances the resistance to ionizing radiation of Ge-doped OFs in the UV-Visible domain and, at the same time, improves the transmission of UV light in the unirradiated OF sample. By comparison with previously reported results, the O2-loading treatment turned out to increase the radiation resistance of Ge-doped OFs more efficiently than F or Ce codoping. The understanding of such amelioration relies in basic radiation-induced mechanisms that were characterized with three complementary experimental techniques: Confocal microluminescence (CML), online radiation-induced attenuation (RIA), and electron paramagnetic resonance (EPR). We have shown that the almost intrinsic oxygen-deficient character of germanosilicate fibers can be overturned by forcing O2 diffusion in the glass matrix. The Germanium lone pair centers, which are precursor defects invariantly present in the as-drawn Ge-doped OFs, are converted to some other yet-undetermined species. Consequently, the usual chain of radiation-activated processes leading to the creation of Ge(1) and Ge(2) is substantially suppressed. The experiments have also highlighted an increased production of oxygen-excess related defects under irradiation. Although in terms of RIA, the tradeoff between the oxygen-excess and oxygen-deficient defects is already a positive one, it is conceivable that the radiation resistance of Ge-doped OFs can be further improved by optimizing the O2-loading treatment.

Irradiation Temperature Influence on the In Situ Measured Radiation Induced Attenuation of Ge-Doped Fibers

We report an experimental investigation on the radiation-induced attenuation (RIA) in the ultraviolet-visible domain for Ge-doped optical fibers, during X-ray (10 keV) exposure at different temperatures. The objective is to characterize the impact of the irradiation temperature on the RIA levels and kinetics. Our data highlight that for dose exceeding 1 kGy(SiO2) the RIA spectrum changes with the irradiation temperature. In particular, for wavelengths below 470 nm the RIA depends both on the dose and on the irradiation temperature, whereas at higher wavelengths the RIA depends only on the dose. From the microscopic point of view the origin of this behavior is explained by a larger impact of the irradiation temperature on the Ge(1) defects generation mechanism with respect to the one of GeX defects, which appears as poorly temperature sensitive in the tested range. This finding prevents us from easily establishing a conclusive relation between the generation mechanisms of these two types of defects. The lower content of radiation induced Ge(1), in fiber irradiated at higher temperature, is supported by the electron paramagnetic resonance (EPR) results acquired after the irradiation. In situ RIA and postmortem EPR data show a significant correspondence of the Ge(1) growth as a function of the dose. Confocal microscopy luminescence experiments indicate that the non-bridging oxygen hole center concentration is higher at 473 K in comparison with those observed at 300 and 373 K.

submitter : Distributed Optical Fiber Radiation Sensing in the Proton Synchrotron Booster at CERN

In this paper, we report on the development and first deployment of a distributed optical fiber radiation sensor (DOFRS) for the online monitoring of radiation levels in the high-energy accelerator facilities of CERN. The DOFRS is composed of two basic parts: a set of suitably chosen radiation sensitive optical fibers (OFs), to be installed in the machine tunnel, and an optical time domain reflectometer (OTDR), to be installed in a radiation-free area. We carried out a calibration of the radiation response of a P-doped multimode OF under 60Co

Cathodoluminescence Characterization of Point Defects in Optical Fibers

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Radiation Induced Attenuation in Single-Mode Phosphosilicate Optical Fibers for Dosimetry

-rays and in the mixed-field radiation environment of the CHARM facility at CERN. By performing OTDR measurements, we are able to probe the radiation-induced attenuation in the OF along its length and calculate the deposited radiation dose with a spatial resolution of 1 m. In this paper, we describe the main features associated with DOFRS implementation in the proton synchrotron booster (PSB) at CERN. We also report the first results obtained from the monitoring of PSB radiation levels since its recommissioning in April 2017. The performances and advantages of the DOFRS system are discussed.

Optical properties of peroxy link defects from first principles

We evaluate the potential of the cathodoluminescence (CL) spectroscopy to characterize the nature and the spatial distribution of point defects in the main classes of optical fibers (OFs): Telecom-grade, radiation-hardened, and radiation sensitive. Canonical samples, that are differently doped in their cores (Ge, N, P, Ce) or their claddings (F), have been investigated through CL technique using a 10-keV electron beam. Obtained results are compared with those obtained by photoluminescence spectroscopy. CL benefits and limits are discussed on the basis of the obtained experimental data. CL is shown to be efficient to investigate the kinetics of defect generation and bleaching under the electron exposure, being the unique technique allowing us to determine in situ the spatial distribution of emitting defects in the fiber transverse cross sections. New insights are given for some of the defects related to the Ge, P, Ce and N dopants.