[en] Full text: During the past twenty five years arrangements have been made for sharing the use of major national research facilities amongst the world community of neutron users. The administrative requirements are simple. Scientists are invited to apply for measurement time. The scientific merit of the application is assessed by a committee appointed by the host organisation. If the application is considered to have sufficient merit time is allocated. The only costs to the user are transport and living expenses. These arrangements have advantages for users and for hosts. The user can apply for time on the most suitable instrument. The host in the user country is freed from the responsibility of supplying all instruments. It can specialise in those instruments in which it has particular expertise. The host retains, through its committee, complete control over the use of instruments. The amount of time allocated to international users is dependent on the national demand. The result is efficient use of national facilities. An equally important result is the interaction between members of the international scientific community. Australian scientists routinely use overseas facilities however Australia has refused to join the international group. There is international resentment to this attitude. We have, for example powder diffraction facilities which others wish to use. We have no small-angle scattering facilities and must do our experiments at international centres. I will argue that we should join the international community now. The capacity of the replacement reactor will be far greater than the internal Australian requirements. We will become the natural host for users from countries in the Asian region. To enable us to make a smooth transition to this stage we should immediately advertise an international program for HIFAR

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[en] Abstract only. 4 refs

[en] It is shown that a recurrence relation exists such that the resolution function and luminosity of an optical system consisting of any number of alternate collimators and reflecting crystals can be calculated analytically. Certain cases of practical interest in neutron and X-ray diffraction are evaluated explicitly using a Gaussian approximation for the transmission functions of the collimators, and for the mosaic-spread distributions or reflection profiles of the crystals. It is shown that true absorption has a negligible effect on luminosity when the Darwin width of a reflection is of the order of the opening angle of the X-ray beam from a synchrotron source. (orig.)

[en] The differences between previous theoretical treatments of extinction based on the Darwin intensity equations arise because of the different functional form chosen for the coupling constant σ. When the same function is used these theories make closely similar predictions. It is shown that a limiting condition on integrated intensity as the crystal size increases puts restrictions on the functions which may be used. A Lorentzian or Fresnellian function can be used for primary extinction while secondary extinction requires a Gaussian, rectangular or triangular function. An analytical expression is given for the variation in the value of the extinction factor with scattering angle. (orig.)

[en] It is shown that the results of least squares refinement for crystallographic parameters are identical when the ordinates of the profile are refined or when integrated intensities are extracted as an intermediate step

[en] The value of the ordinate of a neutron powder is expressed as the sum of Bragg scattering, due to the existence of an average lattice, and diffuse scattering, due to local deviations from the average lattice. In this form the calculated pattern can be refined by least squares methods against the observed pattern to give both average structural parameters and disorder parameters. Significant advantages over the use of only Bragg scattering for the determination of structural parameters are: (1) a powerful limit on the value of the temperature factor is imposed by the condition that neutrons removed from Bragg peaks must be accounted for in the diffuse scattering; (2) correlation between partially occupied sites can be found. The cross sections for Bragg and disorder scattering are given

No abstract available

[en] Short communication. 2 refs