C.J. Carlile

The design of the IRIS inelastic neutron spectrometer and improvements to its analysers

The design principles of the IRIS high-resolution inverted-geometry spectrometer on the ISIS pulsed source are described and the performance in terms of energy transfer and resolution derived. Recent improvements to the crystal analysers are described, notably the effect of cooling the graphite to 25 K. Future plans are indicated.

Molecular dynamics of water in oriented DPPC multilayers studied by quasielastic neutron scattering and deuterium‐nuclear magnetic resonance relaxation

The dynamics of water between highly oriented multilayers of 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC) has been studied in two time domains at different hydration levels. Incoherent quasielastic neutron scattering (QENS) and deuterium‐nuclear magnetic resonance (NMR) longitudinal (T1) relaxation were employed to investigate both the high‐frequency motions of water (10−9–10−11 s time scale) and their anisotropy, while 2H‐NMR transverse (T2) relaxation was used for obtaining information on low frequency dynamical processes (microsecond time scale). Our results show that high frequency dynamics (picosecond‐time scale) at low hydration (three to four water molecules per lipid) can be understood solely as a uniaxial rotation of the water molecules tightly bound to DPPC head groups with a correlation time τrot≊62 ps at 55 °C and a rotational radius of 1±0.1 A, but with no detectable translational degrees of freedom. The 2H‐NMR T1 data (nanosecond‐time scale) can be explained satisfactorily on the basis...

Molecular spectroscopy and Bayesian spectral analysis : how many lines are there ?

We demonstrate the Bayesian spectral analysis approach for analyzing neutron scattering molecular tunneling data. It is a generalized form of model fitting, which is appropriate when the number of parameters to be optimized is not known. Specifically, it addresses the question of how many excitation lines there is evidence for in the data. We review the theory of Bayesian spectral analysis relevant to our particular application, describe an efficient algorithm for its implementation, and illustrate its use with both simulated and real data. We believe that this powerful method of analysis will be a very useful tool in experimental molecular spectroscopy.

Bayesian analysis of quasielastic neutron scattering data

We consider the analysis of quasielastic neutron scattering data from a Bayesian point-of-view. This enables us to use probability theory to assess how many quasielastic components there is most evidence for in the data, as well as providing an optimal estimate of their parameters. We review the theory briefly, describe an efficient algorithm for its implementation and illustrate its use with both simulated and real data.

Rotational motions of CH4 molecules in weak orientational potentials of cubic symmetry

Nearly free rotational motions of CH4 molecules as substitutional impurities in argon, krypton, and xenon have been observed at low temperatures with inelastic neutron scattering. Besides energy transfer, the dependence of the scattered intensities on momentum transfer Q is used for the assignment of the experimentally observed lines to the various transitions of a spherical quantumrotor in an orientational potential of cubic symmetry. The measured intensities are in good agreement with theoretical predictions based on the extended James–Keenan model. Measurements with high energy resolution on solid CH4 in its antiferrorotational phase II were devoted to the determination of the Q dependence of the tunneling lines and the nearly free rotor lines. The results give the first direct experimental evidence for the value 3 for the ratio of the orientationally ordered molecules to those which are orientationally disordered in CH4‐II.

Test of a perfect crystal neutron storage device

Abstract A system of two perfect silicon crystal plates (1.07 m apart) both placed in back reflection position ((1, 1, 1)-planes) was used to capture highly monochromatic (6.27 A) neutrons. We have been able to show that a pulsed magnetic field is a proper means to fill the system with neutrons and to release the neutrons after a period of up to 30 ms. Here we present our first results obtained from measurements at the ISIS pulsed neutron source at Rutherford Appleton Laboratory.

A quasielastic neutron scattering study of protonic transport in hydrogen-bonded alkali metal hydrogen sulphates and selenates

The diffusion of protons in the superprotonic phases of CsHSO4, Cs3H(SeO4)2, Rb3H(SeO4)2 and Cs5H3(SO4)4-H2O have been studied by quasielastic neutron scattering on the IRIS spectrometer at the ISIS pulsed neutron spallation source. The data were analysed on the basis of the jump diffusion model via unfilled proton positions in the superprotonic phases. Quantitative characteristics of the diffusion process (mean residence time, jump length, activation energy) were found to be consistent with the structural features of the superionic phases and with the experimental data obtained by other methods. Some common characteristic features of the mechanism of protonic transport in these compounds are discussed.

Rotational spectra and the phase diagram of CH4/Ar mixtures

The rotational dynamics, i.e., tunneling and almost free quantum rotation at low temperature and rotational diffusion at higher temperature, have been studied in methane–argon mixtures in the whole concentration range and at temperatures up to T=40 K. Changes of the rotational spectra allow parts of the phase diagram to be established in detail. At T=5 K, a transition from a perturbed partially ordered phase II to be a disordered glassy phase has been observed at an argon concentration of 10%. There is a small regime of coexistence of these two phases. The miscibility gap at larger argon concentrations is wider than determined by x‐ray diffraction with methane being almost insoluble in argon.

TOSCA: a world class inelastic neutron spectrometer

Abstract Present applications are reaching the limitations of the inelastic neutron scattering spectrometer, TFXA at ISIS. The new instrument TOSCA, currently under construction, builds on the strengths of TFXA (a wide energy range, good resolution and ease of operation), but will have detector banks in forward and backward directions to give a much larger detector area. A longer primary flight path of 17 m will result in a significant improvement in resolution. The open geometry in forward scattering allows a larger range of momentum transfers to be accessed. TOSCA will have a diffraction capability with detectors at a range of angles to provide a good coverage of Q -space and d -plane spacing.

Rotational Excitations in Mixed Ammonium Potassium Halides: a Neutron Scattering Study

The rotational dynamics of NH4+ ions in dilute solutions in potassium halides (I, Br, Cl) have been investigated at temperatures below 20 K by inelastic neutron scattering. Tunnel splitting in the (NH4)xK1-xCl mixed crystal is reported for the first time. An almost free uniaxial quantum rotation of the NH4 is observed in all the halides. An increase in NH4 concentration in all mixed salts produces a collapse of the free-rotor line into a broad intense feature to lower energies. The onset of this collapse occurs at different NH4 concentrations x in the different halides, with x(Cl) < x(Br) < x(I). The tunnel splittings of the different halides exhibit a systematic relationship to the nearest-neighbour distance of the respective matrices. The remarkable similarity to the methane-rare-gas systems is discussed.