A novel γ−sensitive detector apparatus for eV neutron spectroscopy applied to hydrogen-bonded systems

The work presented in this thesis collects the activities I addressed during the three-year PhD project in Materials for Health, Environment and Energy. The objective of my PhD project is the development of a novel detector apparatus, named γDA, and methodology for eV neutron spectroscopy applied to the characterization of hydrogen-bonded systems. Neutrons, being uncharged particles, are the probe of choice to study the nuclear dynamics in condensed-matter systems. The nuclear dynamics is an increasingly requested feature in material characterization as it has a central role in many open question in Physics and Chemistry. The nuclear dynamics is studied with neutrons of energy in the eV range, in a technique called Deep Inelastic Neutron Scattering. I used this technique at high momentum, Q, and energy transfers, hω¯ , to study the single-particle dynamics in hydrogen-bonded systems with the above mentioned device that will broaden the capability of Deep Inelastic Neutron Scattering. The work is a prosecution of the e.Verdi project where the cornerstones of the detection technology development of neutrons at the eV were established. The γDA is thought to operate on the VESUVIO spectrometer at the ISIS pulsed neutron and muon source. In the first part of the thesis I described the basic theory of neutron scattering and the facility were neutrons are produced, with a particular attention on VESUVIO and the detection strategy used at present. The second part of the thesis is devoted at the configurations tested for the apparatus, with a detailed description of the preliminary tests performed on a polyethylene sample, used as a neutron spectroscopy standard, together with the calibration and the response simulation, performed within the FLUKA environment, of the detector. Once the optimal configuration was found I investigated the nuclear dynamics in biphenyl that is generally used as a model compound to emulate the lignin decomposition in monocyclic hydrocarbons in the biofuel-production research. The measurements demonstrated a detection count-rate increase of five times and a signal-background ratio improvement of 50%. This means faster and cleaner measurements. The biphenyl dynamics was also simulated with ab-initio calculations performed within the Quantum Espresso and CASTEP environments and compared with the experimental results accessed by neutron scattering techniques. The simulations allowed us to determine the reason why structures with a considerable number of internal vibrations show an isotropic dynamics, i.e., same kinetic energy in all the Cartesian directions. In fact, if the number of nuclei is big enough the Central Limit Theorem can be applied as I have shown with the simulation of saturated hydrocarbons from methane to decane where a decreasing trend in the anisotropy was observed from the simulation as a function of the nuclei present in the molecule. In the last part of the thesis, after a brief introduction to the physics of ice, I described the neutron scattering experiments, in collaboration with the University College London, performed at ISIS, used to demonstrate the unusual stability of ice II. In fact, ice II is the only hydro- vi gen ordered phase that does not have any disordered countrepart and it is stable up to high temperatures. The stability of the phase is motivated by the very low kinetic energy of hydrogens, probed by eV neutron scattering, and by the high displacement, probed by quasi elastic neutron scattering. In particular, as the stability is given by the Gibbs free energy, at lower energy and higher entropy it corresponds a more stable phase. The beauty and the reason why this study was so fascinating is that the hydrogen displacement brings an additional therm to the entropy, a dynamic disorder that makes, together with the low energy, the ice II very stable. Hence ice II is ordered by the disorder, a wonderful joke of Nature.