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After a brief general and historical discussion, the main design features of spallation sources are described. At the present time, Europe not only has the world-leading pulsed neutron spallation source, the SNS-ISIS at RAL, UK, but it is on the point of commissioning a world-leading continuous cyclotron-driven source, the SINQ at PSI, Switzerland. Looking to the future, yet more powerful pulsed sources are actively under study and the difficult problem of high-power target design (>250 kW) is leading to a new technology for liquid targets. The accelerator designs, although basically classical, require custom-built solutions that are often at the limit of presentday accelerator technology.

The use of synchrotrons for pulsed neutron spallation sources is an example of scientific and technological spin-off from the accelerator development for particle physics. Accelerator-driven sources provide an alternative to the continuous-flux, nuclear reactors that currently furnish the majority of neutrons for research and development. Although the present demand for neutrons can be adequately met by the existing reactors, this situation is unlikely to continue due to the increasing severity of safety regulations and the declared policies of many countries to close down their reactors within the next decade or so. Since the demand for neutrons as a research tool is, in any case,expected to grow, there has been a corresponding interest in sources that are synchrotron-driven or linac-driven with a pulse compression ring and currently several design studies are being made. These accelerator-driven sources also have the advantage of a time structure with a high peak neutron flux. The basic requirement is for a fast-cycling, medium-energy, high-intensity accelerator complex. In the synchrotron-driven sources, the synchrotrons typically work at the limit of the technology used in their design and require sophisticated injection and "painting" systems, RF cages in ceramic vacuum chambers, large resonant power converters and powerful RF systems. Radiation management is one of the main concerns. The activation of the exhaust air and ground water must be kept below limits agreed with licensing authorities and where possible the activation of machine elements should be kept within the limits for "hands-on-maintenance". The degradation of materials such as the coil insulation needs to be estimated and radiation-hard elements used in critical places. These requirements imply tight tolerances and high efficiencies for the collimator systems, the ventilation system, the shielding and the parameters of the incoming linac beam. Presently operating neutron sources and newly-proposed sources are listed with their main parameters after the discussion of the above points.

The broadening produced by 2,5-dichloropyridine and 3,6-dichloropyridazine in p-dichlorobenzene has been measured. Theoretical estimates of the broadening are obtained using both a simple model and a Monte Carlo approach. Because of the small size difference between host and impurity molecules, it is concluded that the electric dipole moment can have a very significant effect on the broadening of the NQR line.

Variations of T2★ of the n.q.r. signals of 35Cl, 81Br and 127I in their p-dihalobenzenes were observed to depend on the previous history of the sample. Zonerefined samples were cooled from the melt to room temperature, XXX20°C, in two ways (a) slowly over several hours and (b) quickly over a few minutes. Samples (a) gave higher T2★ values because the fewer crystal defects produced in the slowcooling process gave a more homogeneous electric field gradient. The results gave new measurements of T2★ , emphasized the need for describing past history when quoting linewidth measurements and showed the importance of T2★ measurements in the study of phase transitions.

One of the major broadening mechanisms in Nuclear Quadrupole Resonance is the spread in the electric field gradient produced by defects in the crystal. If the defects are randomly distributed, the electric field gradient follows a normal distribution and the resulting lineshape is symmetrical. This paper shows that asymmetric lineshapes may be produced when the asymmetry parameter of the field gradient is equal to or very close to its extreme values of zero or one even though the field gradient distribution is random. Diagrams of some possible lineshapes are given.

The major broadening mechanism in Nuclear Quadrupole Resonance is the spread in the electric field gradient produced by crystal defects. In multilevel spin systems, the frequencies of the various possible resonances are different functions of the components of the electric field gradient. This paper examines the role of both the principal component of the field gradient and the asymmetry component in the relationship between the linewidths of the two resonances of I = 5 2 nuclei. Comparison with experimental data shows various ways in which defects interact with quadrupolar nuclei.

A basic research program to determine fundamental phenomena involved in lubrication by graphite and other lamellar solids is described. Lamellar cleavage experiments in known environments of air, ultra-high vacuum, water vapor, and oxygen are reported. A theoretical calculation of the interlamellar binding energy of the ideal graphite lattice is described. A stress-etch process is reported for graphite, in dry oxygen, water vapor, and air environments, which significantly lowers the cleavage energy and provides a basis for understanding lubrication phenomena. A general hypothesis of lamellar solid lubrication is thereby proposed on the basis of interlamellar binding forces. A correlation of this general hypothesis is made with experimental results for the lamellar materials: graphite, molybdenite, pyrophyllite, muscovite, margarite, talc, and phlogopite.

Extensive computer calculations of the variation of output phase with pump strength, structure length and input power level have shown that the a.m.-p.m. conversion, in a d.c.-pumped cyclotron-synchronous-wave amplifier, is minimised at a frequency below the degenerate cyclotron frequency. Two forms of the optimised amplifier are suggested.