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Using Fourier Transform Infrared (FTIR) spectroscopy, we investigate the temperature dependence of the bending vibrations of water confined in the pores of a silica hydrogel in the temperature interval of 270–180 K. We also investigate the presence of thermal hysteresis by cooling and reheating temperature scans. The results clearly show the presence, at about 230 K, of a crossover in the temperature dependence of the IR spectra; moreover, the presence of hysteresis is clearly demonstrated. By comparing FTIR data with neutron diffraction data and previous calorimetric data on the same samples, we conclude that the crossover and the hysteretical behavior do not involve a water glass transition or crystallization but are related to a first-order-like liquid-liquid transition. [ABSTRACT FROM AUTHOR]

The transient radiation response from the visible to the infrared (IR) domains (400–2200 nm) of an ultralow loss pure‐silica‐core and fluorine‐doped single‐mode optical fiber (ULL‐SMF) exposed to a short (tens of ns) 1 MeV X‐ray pulse is investigated. Online (hundredths of millisecond time resolution) radiation‐induced attenuation (RIA) spectra measurements are performed at both room and liquid nitrogen temperatures (RT and LNT). The main goals are to assess the vulnerability of this class of fibers as well as to characterize the metastable defects recovering just after the shot and responsible for the RIA decay kinetics. Their identification relies on a Gaussian spectral decomposition exploiting the optical absorption bands characteristics of point defects already known in literature. By comparing the ULL‐SMF behaviors at the two temperatures, the RIA unstable contributions are highlighted from the visible to IR domains, in particular the ones that are bleached in a timescale faster than the time resolution of a few milliseconds at RT. The origin of the induced losses is discussed revealing the fundamental role of the self‐trapped holes as intrinsic metastable defects in the response of this class of pure‐silica‐core OF. [ABSTRACT FROM AUTHOR]

An unexpected high sensitivity to radiation has recently been discovered in a new generation of ultralow loss pure‐silica core optical fiber, the Corning Vascade EX1000. It is investigated how the injected power level of the probing signal at 1310 and 1550 nm affects the radiation‐induced attenuation (RIA) during and after steady‐state X‐ray exposure up to a total ionizing dose (TID) of 250 kGy (SiO2) at room temperature. The experiments confirm this fiber's high sensitivity to radiations, mainly due to the generation of metastable defects absorbing in the infrared domain. RIA kinetics are characterized by a non‐monotonic behavior, reaching a maximum of the radiation‐induced losses at 3.3 kGy (SiO2). Through successive radiation tests, the RIA dependence on the signal power level at the dose corresponding to the maximum of the RIA is studied, finding out that, for the two investigated wavelengths, the RIA follows a power law dependence in the 100 nW–1 mW injected power range, whose exponent seems independent on the dose rate (between 1 and 6 Gy s−1). The results highlight a clear photobleaching phenomenon, with the RIA associated to these metastable defects strongly reduced at higher injected powers of mW levels. [ABSTRACT FROM AUTHOR]

- Oxygen isotope and chemical measurements were carried out on 25 samples of Libyan Desert Glass (LDG), 21 samples of sandstone, and 3 of sand from the same area. The δ18O of LDG samples range from 9.0‰ to 11.9‰ (Vienna Standard Mean Ocean Water [VSMOW]); some correlations between isotope data and typological features of the LDG samples are pointed out. The initial δ18O of a bulk parent material may be slightly increased by fusion due to the loss of isotopically light pore water with no isotope exchange with oxygen containing minerals. Accordingly, the δ18O of the bulk parent material of LDG may have been about 9.0 ± 1‰ (VSMOW). The measured bulk sandstone and sand samples have δ18O values ranging from 12.6‰ to 19.5‰ and are consequently ruled out as parent materials, matching the results of previous studies. However, separated quartz fractions have δ18O values compatible with the LDG values suggesting that the modern surface sand inherited quartz from the target material. This hypothesis fits previous findings of lechatelierite and baddeleyite in these materials. As the age of the parent material reported in previous studies is Pan-African, we measured the δ18O values of bulk rock and quartz from intrusives of Pan-African age and the results obtained were compatible with the LDG values. The main element abundances (Fe, Mg, Ca, K, Na) in our LDG samples conform to previous estimates; Fe, Mg, and K tend to be higher in heterogeneous samples with dark layers. The hypothesis of a low-altitude airburst involving silica-rich surface materials deriving from weathered intrusives of Pan-African age, partially melted and blown over a huge surface by supersonic winds matches the results obtained. [ABSTRACT FROM AUTHOR]

The transient radiation‐induced attenuation (RIA) of two different versions of pure‐silica‐core (PSC) multimode optical fibers (so‐called "solarization‐resistant" fibers) exposed to nanosecond 1 MeV X‐ray pulses are investigated. On‐line RIA spectra measurements at both room temperature (RT) and liquid nitrogen temperatures (LNT) in the range 1–3.5 eV are performed. Following the RIA kinetics, the properties of the metastable defects that are bleached just after the pulse are discussed. The spectral decomposition of the RIA is performed using known Gaussian bands associated to point defects absorbing in this spectral range. For both fiber types, the generation and the post‐irradiation kinetics of the self‐trapped holes (STHs) that are the main contributor to the transient RIA are investigated particularly. Moreover, thanks to the measurements performed at low temperatures the complex temperature dependence of the kinetics of self‐trapped holes and some chlorine‐related species is characterized. The transient radiation‐induced attenuation (range 1–3.5 eV) of solarization‐resistant optical fibers exposed to an ultra‐short X‐ray pulse are studied at room temperature and liquid nitrogen temperature to investigate the recovery kinetics of metastable defects and their temperature dependences. The decomposition of the RIA spectra highlights the contribution from different STHs and chlorine‐related defects. [ABSTRACT FROM AUTHOR]

The radiation vulnerability of various classes of multimode silica‐based optical fibers is investigated for space applications operating from the ultraviolet up to near‐infrared spectral domains. For this, radiation‐induced attenuation (RIA) levels and kinetics in the 300 ÷ 1100 nm wavelength range are monitored during and after steady state 105 MeV proton exposure at room temperature (RT, equivalent dose of ∼250 Gy(SiO2)). The responses of three types of "radiation hardened" optical fibers with either a pure‐silica core (PSC) or fluorine‐doped cores are compared to the one of a Telecom‐grade germanosilicate optical fiber. RIA growth during irradiation and decay after irradiation (recovery phase) reveal that the highly fluorine (2 wt.%)‐doped optical fiber, manufactured by axial vapor deposition process, presents higher RIA levels above 600 nm than other tested optical fibers. Indeed, if the high F‐doping level reduces the UV‐RIA related to SiE', NBOHC or chlorine‐related centers this composition appears as associated with a strong RIA increase in the visible‐near‐IR through the more efficient generation of RT unstable strain‐assisted or inherent self‐trapped holes. The response of fluorine‐doped optical fibers to space constraints is investigated. The results show that depending on their manufacturing conditions, these fibers can present higher radiation induced attenuation in the visible and near infrared spectral domains than telecom‐grade fibers. These induced losses are mainly caused by the self‐trapped holes. [ABSTRACT FROM AUTHOR]