Absolute measurements of the stretching mode density of states in polycrystalline ice Ih

Hydrogen Dynamics in Supercritical Water Probed by Neutron Scattering and Computer Simulations

In this work, an investigation of supercritical water is presented combining inelastic and deep inelastic neutron scattering experiments and molecular dynamics simulations based on a machine-learned potential of ab initio quality. The local hydrogen dynamics is investigated at 250 bar and in the temperature range of 553-823 K, covering the evolution from subcritical liquid to supercritical gas-like water. The evolution of libration, bending, and stretching motions in the vibrational density of states is studied, analyzing the spectral features by a mode decomposition. Moreover, the hydrogen nuclear momentum distribution is measured, and its anisotropy is probed experimentally. It is shown that hydrogen bonds survive up to the higher temperatures investigated, and we discuss our results in the framework of the coupling between intramolecular modes and intermolecular librations. Results show that the local potential affecting hydrogen becomes less anisotropic within the molecular plane in the supercritical phase, and we attribute this result to the presence of more distorted hydrogen bonds.

Hydrogen nuclear mean kinetic energy in water down the Mariana Trench: Competition of pressure and salinity

The Mariana Trench is one of the most famous and extreme environments on our planet. We report experimental values of the hydrogen nuclear mean kinetic energy in water samples at the same physical and chemical conditions than in the Challenger Deep within the Mariana Trench: a pressure of 1092 bars, a temperature of 1 °C, and a salinity of 35 g of salt per kg of water. Results were obtained by deep inelastic neutron scattering at the VESUVIO spectrometer at ISIS. We find that the effect of pressure is to increase the hydrogen nuclear mean kinetic energy with respect to ambient conditions, while ions in the solution have the opposite effect. These results confirm the recent state-of-the-art simulations of the nuclear hydrogen dynamics in water. The changes in the nuclear mean kinetic energy likely correspond to different isotopic fractionation values in the Challenger Deep compared to standard sea water.

Radiative neutron capture as a counting technique at pulsed spallation neutron sources: a review of current progress

Neutron scattering techniques are attracting an increasing interest from scientists in various research fields, ranging from physics and chemistry to biology and archaeometry. The success of these neutron scattering applications is stimulated by the development of higher performance instrumentation. The development of new techniques and concepts, including radiative capture based neutron detection, is therefore a key issue to be addressed. Radiative capture based neutron detectors utilize the emission of prompt gamma rays after neutron absorption in a suitable isotope and the detection of those gammas by a photon counter. They can be used as simple counters in the thermal region and (simultaneously) as energy selector and counters for neutrons in the eV energy region. Several years of extensive development have made eV neutron spectrometers operating in the so-called resonance detector spectrometer (RDS) configuration outperform their conventional counterparts. In fact, the VESUVIO spectrometer, a flagship instrument at ISIS serving a continuous user programme for eV inelastic neutron spectroscopy measurements, is operating in the RDS configuration since 2007. In this review, we discuss the physical mechanism underlying the RDS configuration and the development of associated instrumentation. A few successful neutron scattering experiments that utilize the radiative capture counting techniques will be presented together with the potential of this technique for thermal neutron diffraction measurements. We also outline possible improvements and future perspectives for radiative capture based neutron detectors in neutron scattering application at pulsed neutron sources.