The forward and backward scattering parts (phase II) of the new neutron spectrometer, TOSCA, replacing the old TFXA and TOSCA (phase I), have been successfully installed at the ISIS pulsed neutron source. The results show a significant enhancement in the counting rate due to the larger detector area. The improved resolution (to 1.5–3% of the energy transfer) as compared with the previous instruments (TXFA: 2.5–3.5%; TOSCA-I: 2–3.5%) has been achieved by increasing the primary flight path from 12 m to 17 m. A chopper has been added in order to avoid neutron frame overlap and to reduce the fast neutron background. Additional diffraction capability will be installed in the near future. An example of the high resolution that is routinely available is provided by the spectrum of potassium hydrogen phtalate at 20 K.

Inelastic neutron scattering experiments were performed at intermediate and high momentum transfer, up to 88.2 Å−1, to study the temperature dependence of single hydrogen mean kinetic energy in polycrystalline and liquid hydrogen sulphide (H2S), in the temperature range 16–206 K. Values of the hydrogen mean kinetic energy were extracted, within the impulse approximation, by fitting to the high momentum transfer data a model response function, obtained from a momentum distribution which is the orientational average of a multivariate Gaussian function. The extracted kinetic energies are compared with a harmonic model for the vibrational and roto-translational dynamics. The model makes use of the hydrogen-projected density of states worked out from intermediate momentum transfer data, as well as of optical frequencies determined from Raman and infrared (IR) spectroscopy. A fairly good agreement is obtained in the whole temperature range, while noticeably lower values for the kinetic energy are found if a single atom momentum distribution of isotropic Gaussian shape is assumed in the model response function.

We report good agreement between neutron and X-ray measurements of the static structure factor S(Q) of liquid mercury, as well as the first determination of its isothermal density derivative from neutron measurements.

The study of the density dependence of neutron diffraction spectra in dilute gases, already applied in recent papers to the determination of two-and three-body interaction properties in monatomic insulating fluids, is extended here to the case of a molecular fluid. Deuterated methane has been chosen for its good neutron scattering properties and its small molecular anisotropy. The centre-of-mass static structure factor S(Q) has been measured at four densities between 0.55 nm(-3) and 1.84 nm(-3) and at a temperature just below critical. Density analysis of the Fourier transform c(Q) of the direct correlation function makes it possible to show that valuable information on the intermolecular interactions is contained in the neutron data.

Neutron diffraction on low-density systems offers the possibility of a direct probing of the intermolecular forces. This method, used in the last few years for monatomic systems like rare gases, is applied here to the case of gaseous CD4 at low densities on the 180 K isotherm. The intermolecular scattering has been analysed in order to extract C(q), i.e. the Fourier transform of the direct correlation function. The zero-density limit of C(q), which depends on the (spherical average of the) pair potential only, is compared to the same quantity obtained from effective intermolecular pair potentials found in the literature.

The crystallization kinetics of poly(ethylene oxide) doped with various alkaline perchlorate salts were measured at room temperature by means of the new energy dispersive x-ray diffraction method for the phase transition. The experimental points of the transformation coordinate were fitted using the phenomenological Liquori-Tripiciano law, the parameters of which were evaluated for each case. The influence of the concentration and of the cation dimensions on the crystallization rate is discussed. Further details about the application of this nonconventional diffractometric technique to polymers are also reported, as is an intuitive model for describing the method.

A new energy dispersive x-ray diffraction method was used to observe isothermal phase transitions of a polyethylene oxide sample contained in a variable temperature cell. A series of scattering measurements were performed to follow the time evolution of the system at each temperature. The collected spectra were suitably processed and the transition coordinate x was obtained as a function of time. A detailed treatment of the errors was accomplished to evaluate the reliability of the method. The experimental points (dx/dt)(max) - T were plotted both for the amorphous-crystalline (below the melting temperature T-m) and for the crystalline-amorphous (over T-m) transitions. They reproduce the usual behavior for T < T-m and show a characteristic unexpected feature for T > T-m which is partially justified.

A new application of energy dispersive x-ray diffraction to the kinetics of phase transitions is reported. The aim of this work is to provide a new investigation tool to follow the evolution of polymeric systems from the initial to the final phase. The theoretical treatment is developed both for constant density transitions and for the variable density ones. The experimental route is illustrated by an example.

The structure in aqueous solutions of the hydrated selenate and chromated ions SeO42- and CrO42- have been derived from X-ray and neutron diffraction measurements using CrO42- as an isomorphous substitute for SeO42-. The Se(Cr)-O bond lengths are 1.63 +/- 0.02 angstrom and do not differ from those found in crystal structure.A well defined hydration shell of about 12 water molecules surrounds the XO42- ions at a Se(Cr)-H2O distance of 3.96 angstrom. The diffraction results indicate a geometrical orientation of the water molecule around the tetrahedral anions very similar to that one found in the case of simple anions such as Cl-, with Se(Cr)-D distances of 3.25 and 4.4 angstrom and with a Se(Cr)-O distance of 3.96 angstrom.

Inelastic neutron scattering experiments on single crystal specimens at a pulsed neutron source require new instrumentation and measuring techniques. In this paper we describe the development and performance of the PRISMA spectrometer which has recently been installed at the ISIS spallation neutron source, Rutherford Appleton Laboratory, UK for the purpose of carrying out such measurements. This spectrometer is particularly suited to the performance of quick overview measurements of the dispersion curves in single crystals over a wide neutron energy transfer range. It is also very efficient at measuring changes in the dynamical scattering function introduced by external parameters such as temperature, pressure or magnetic field. Experimental results from the commissioning phase which demonstrate the potential of the PRISMA spectrometer are presented.

A data acquisition system designed to acquire time of flight spectra in neutron scattering experiments is described. The system is completely hardware, so that even employing a relatively slow clock (1 MHz) it is adequate to work at 300 kHz with no loss of information.

We report on neutron-diffraction measurements of liquid benzene under standard thermodynamic conditions. The measurements are taken at two different incident neutron wavelengths to cover an extended Q range. The diffraction data are normalized and inelastic effects corrected, considering only the total sample scattering cross-section. To determine the molecular structure from the diffraction data, we have to introduce direction-dependent meansquare displacements for the carbon and deuterium atoms. An analysis is made of the pair distribution function in terms of a first-shell solid-like structure, and good agreement is found with previous x-ray measurements.

We report on a method to normalize neutron scattering data (accuracy of the order of 1%) from liquids based on an accurate knowledge of the total scattering cross section. This procedure can be used when it is not possible or it is unlikely to perform a normalization run. The method has been developed for continuous sources but it can be in principle applied to the pulsed neutron sources case. Some experimental results, obtained using the presented normalization procedure, will be presented and discussed.

In the present paper we describe a low cost pulsed neutron scattering facility based on a circular electron accelerator being built at our laboratory and mainly intended as a routine work and personnel training facility. A solution like the one we propose will make neutron scattering available to small institutions with relatively low cost but still useful performances.

The measurement of nuclear and magnetic excitation spectra from single crystal samples remains central to condensed matter physics. The requirements in terms of the range and resolution of the scattering vector Q and energy transfer h̵ ω are reviewed and typical experiments with a well defined cross-section are chosen. The performance and limitations of existing instruments are reviewed. A design for a new spectrometer, PRISMA, to be installed on the UK spallation neutron source, ISIS, is presented. Its performance for chosen experiments is given in terms of the Q and h̵ ω range covered in a single scan, the resolution and the count rate.

Measurements of the neutron scattering static structure factor S(Q) are reported for orthobaric liquid fluorine at 77K for an incident wavelength of 1·2Å. The observed S(Q) and the atom-atom correlation function are discussed and compared with those of other halogens and oxygen. From the d(r) pair distribution function it is shown that liquid fluorine has a coordination number of first neighbours more similar to liquid oxygen than halogens. The number of atoms in the first and second coordination shell is in good agreement with a close-packed arrangement of atoms.

We report neutron diffraction measurements of liquid bromine at T = 298 K and T = 473 K and orthobaric densities. The structure factor, S(Q), in the range 0·8–15Å-1 and the non-bonded bromine atom-atom correlation function, g(r), are discussed and compared with previous results on Br2 and Cl2. Monte Carlo (MC) computer simulations using the two-centre Lennard-Jones interaction model with idealized point quadrupole were performed at the higher temperature. This effective pair potential turns out to be unable to reproduce the Br2 structure.

We report neutron diffraction measurements of liquid chlorine at 200 K and 290 K and orthobaric densities. The scattering functions, S(K), and the radial distribution functions, g(r), are discussed by comparison with those of other homonuclear diatomic liquids like N2, O2, Br2 and I2. Strong orientational correlations, found in both thermodynamic states, are suggested to be related mainly to the quadrupolar interaction. The shapes of the g(r) functions and the number of first neighbours strongly indicate that, at short distance in the liquid, we have correlations among first neighbouring molecules very similar to that we find in solid chlorine. Some implications for the σ value used in the 2LJC intermolecular potential are discussed, and consequences are also indicated for the MD experiments already done to simulate the liquid halogens.