CONCEPTS AND PROSPECTS FOR NEUTRON SOURCES FOR BEAMHOLE RESEARCH

The two classical methods of energy selection in neutron scattering, time of flight and reflection from crystals, are used on the traditional steady state neutron sources (fission reactors) with more or less comparable figures of merit. The advent of accelerator-based spallation neutron sources with their option for pulsed operation provided a new boost to the development of time-of-flight methods with the source pulse used as the primary phase space selection element. This lead to extensive efforts to provide short source pulses even at the expense of overall intensity, based on the criterion that the figure of merit was inversely proportional to the square-of the pulse duration. More recent thinking, however, tends to overcome this trend. Conceptual design work for new instruments suggests that on a high average intensity source, a time structure can be exploited in various ways to gain an order of magnitude or more in overall performance over a cw-source of similar average flux. Another clue for the optimum design parameters of an advanced neutron source with high overall performance comes from the finding that the important feature for good resolution is the width of the Fourier spectrum of the resolution function rather than its actual FWHM. Results of work carried out at various locations can be combined to assess the technical aspects of such a project.