J.H. Clarke

The structure factor for liquid bromine by neutron diffraction

Thermal neutrons from steady-state (reactor) and pulsed (linac) neutron facilities have been used to study the structure factor for liquid bromine at 20°C, covering a wide range of values (0·6-35 A-1) of the momentum transfer (ħQ). The diffraction pattern at high Q-values (>10 A-1) gives information on the structural properties of the individual molecules but detailed interpretation is complicated by the vibrational motion which causes a systematic variation in the periodic oscillations of the molecular form-factor. An internuclear distance (bond length) of 2·28 ± 0·01 A is found to be suitable for data at lower Q-values and has been used in the analysis of the liquid structure. The results show that some form of orientational correlation between molecules must be present and the nuclear (atom) pair correlation function is split into two peaks for the coordination shell corresponding to nearest-neighbour molecules. The results are compared with other studies of liquid bromine and similar neutron experimen...

Structural studies of amorphous materials using a pulsed neutron source

Abstract A neutron time-of-flight spectrometer using thermal neutrons produced by the Harwell electron linear accelerator (linac) has been developed to study amorphous materials. The spectrometer uses fixed-angle detectors and is operated in the wavelength range 0.3-3.0 A. A major feature of the design is a backward-angle bank of detectors which gives a high detection efficiency for values of the scattering vector Q extending to greater than 30 A −1 . The technique is compared with conventional reactor methods and preliminary results are presented for liquid nitrogen, vitreous germania and nickel powder.

Neutron scattering by liquid nitrogen and liquid oxygen

Measurements have been made of the neutron scattering structure factors of liquid nitrogen and liquid oxygen at 77 K and 84 K respectively in the Q-value range of 3 to 36 A-1. ‘White’ incident thermal neutrons were produced in the wavelength range of 0·3 to 3·0 A by a pulsed electron linac and detected in a total-scattering time-of-flight spectrometer. Qualitative agreement has been obtained between these present data and a simple molecular form factor in the Q-value range of 12–36 A-1. Using reactor data [1], structure factors over a total Q-value range of 0·3 to 36 A-1 were obtained. Fourier transforms of the structure factor curves yield pair distribution functions which show a distinct separation of the inter and intra-molecular distances in the liquid. A comparison is made with results for the solid phase.

The structure of the heavy water molecule from neutron-diffraction measurements

The differential scattering cross section for neutron scattering from liquid heavy water (D2O) has been measured at incident neutron wavelengths of 0·694 and 0·347 A. The oscillatory pattern observed at high momentum-transfer values characterizes the structure of an individual molecule. The data are found to be in good agreement after application of a correction for molecular recoil and give a value of 0·98 ± 0·01 A for the oxygen-deuterium bond length.

Structural studies of tetrachloride liquids: I. Pulsed neutron scattering by carbon tetrachloride—molecular structure

The differential cross-sections for neutron scattering from liquid carbon tetrachloride have been measured with the TSS instrument at the Harwell Electron Linac. Data were taken at seven different scattering angles for a wavelength range of 0·2–3·5 A. The observed diffraction patterns at high momentum transfer (> 8–30 A-1) have been analysed in terms of the molecular form factor f 1(Q). It was found that the oscillation amplitudes could be satisfactorily described only by introducing an energy-dependent term into the Debye-Waller factors of the form factor. The f 1(Q) data were fitted with a four-parameter function for measurements at scattering angles of 150°, 90° and 58°. The carbon-chlorine bond length parameter was accurately defined in all cases and had a mean value of 1·766 ± 0·002 A. The inclusion of an anharmonicity constant in the form factor gave an improved χ2-fit to the data with an increased value of 1·770 ± 0·002 A for the bond length. The results are in excellent agreement with other measur...

Structural studies of tetrachloride liquids

The differential scattering cross-sections for neutrons scattering by liquid CCl4, SiCl4, TiCl4, GeCl4 and SnCl4 at 20°C have been obtained using the pulsed neutron beam from the Harwell Linac. The measured data at two scattering angles (150° and 90°) have been analysed using a molecular form factor with wavelength-dependent Debye-Waller factors. Molecular bond length parameters extracted from a x 2-fit to the observations are compared with data obtained by other techniques. Special consideration is given to the calibration of the momentum-transfer scale and the data are found to be sensitive to the variation of mean emission times for different wavelength neutrons leaving the moderator material. Correction for this effect gives an improved fit to the form factor but does not alter the fitted bond length values.

The structure factor for liquid nitrogen and liquid oxygen by neutron scattering

The structure factor S(Q) has been measured for liquid nitrogen at 77 K by neutron diffraction at four wavelengths. The effects of Placzek corrections on the self [1, 2] and interference [3] components of the observed differential scattering cross-sections at 0·35, 0·7, 0·84 and 1·06 A [4] have been investigated. Effective molecular masses and internuclear bond lengths have been obtained by fitting a modified Placzek model to the observed data. It is concluded that after correction the different structure factors agree satisfactorily within the statistical errors of the measurements and the internuclear bond length is close to that measured in the gas phase.

A cooled polyethylene moderator on a pulsed neutron source

Abstract A 40 mm thick polyethylene moderator cooled by liquid nitrogen was tested on the Harwell 5 kW electron linac. The scientific considerations underlying its design are discussed. Figures are given for its performance in terms of neutron intensity and pulse width as a function of wavelength, and compared with those from the same moderator at ambient temperature. The cooled moderator shifts the Maxwellian peak to longer wavelengths, with a factor of five gain in intensity in the 3 A region. The epithermal slowing down spectrum is extended to longer wavelengths, so that sharp pulses are preserved in the 1 A region. This is useful for diffraction experiments on a pulsed neutron source, and the loss of intensity at 1 A is compensated in terms of figure-of-merit by a 30% decrease in pulse width. The pulse shapes are also compared with those from a similar polyethylene moderator heterogeneously poisoned with a thin gadolinium foil. The choice between these two methods of pulse improvement depends on the wavelength and resolution requirements of each instrument.

The structure of liquid deuterium chloride by slow-neutron scattering

Measurements of the neutron scattering cross section for liquid deuterium chloride at 20°C are reported for incident neutron wavelengths of 0·7 A and 0·5 A. The diffraction pattern for momentum transfer/ħ, (Q), greater than about 6 A-1 is due mainly to the molecular structure. The observed cross section has weaker oscillations than expected and, with the aid of a transform to real space, it is tentatively concluded that more than half of the molecules are in unusually strong interaction with their neighbours. This is further supported by the appearance in the pair correlation function of a broad peak in the region of 2·6 A which it is suggested may be the liquified remnant of the ‘hydrogen bond’ observed in the solid phase. For longer distances the intermolecular, i.e. liquid, structure observed is weaker than predicted by recent simulation results and this may again be related to the unusually strong intermolecular interactions, or possibly to quantum mechanical effects. Our results are not dissimilar to...

Neutron diffraction studies of liquid phosphorus

Neutron diffraction measurements of liquid phosphorus at 50°C have been made using both steady-rate (reactor) and pulsed (linac) neutron facilities. The high Q-value measurements have been used to characterize the molecular form factor and to study various inelasticity corrections. The low Q-value results are used to derive an intermolecular pair distribution function which shows strong orientational correlation effects and has significant oscillations extending up to 20 A. The packing of the molecular units is different from that exhibited by the tetrachloride liquids and more closely resembles the structural arrangement observed in amorphous yellow arsenic.