Neutron Scattering Facility Requirements for the Study of Complex Matter: Very High Power Spallation Source
F. Mezei
Los Alamos National Laboratory
LANSCE-DO, MS H805
Los Alamos, NM 87545
(505) 667-7633
(505) 665-2676
Neutron scattering study of condensed matter is currently dominated by two complementary European facilities: the high flux reactor ILL at Grenoble, France and the short pulse spallation source ISIS near Oxford, UK. These two facilities are complementary, with ILL covering about 70 % of the research needs with vastly superior capabilities, and ISIS providing best for the remaining 30 % of neutron work. There is some limited overlap in the potentials of the two sources. The ratio 70 - 30 also roughly corresponds to the scientific out-put of the two facilities. The strength of ILL (and the reactor based source approach) is essentially all applications of thermal and cold neutrons - diffraction, diffuse scattering, (high resolution) inelastic spectroscopy, small angle scattering - and single crystal work with hot neutrons. ISIS advantageously complements these opportunities in hot neutron spectroscopy and powder diffraction involving hot neutrons. The impact of neutron scattering work in the exploration of frontiers in matter has been reviewed at a Los Alamos workshop, December 12-14, 1997. Over 85 % of the high-lights discussed at that meeting happened to be results of neutron scattering experiments performed at continuous reactor sources, primarily ILL.
Collective and adaptive phenomena in complex matter in general involve a length scale intermediate between the nearest neighbor atomic distances and macroscopic dimensions. The characteristic time scale of processes on such a length scale tends to be much longer than that of local atomic vibrations, such as typical Debye cut-off frequencies in crystalline matter. Neutron scattering is the method of choice for the direct investigations of what happens on such intermediate time and length scales, namely by methods which allow us to explore relatively low wavenumbers (typically 0.001 - 3 Å-1) and energies (typically 1 neV - 5 meV). In this domain of parameters existing reactor sources perform incomparably better than existing pulsed spallation sources. The systematic exploration of complex matter will set more and more challenging requirements. A vital experimental ingredient for CAM research is to advance neutron scattering capability in exploring intermediate and large scale structures and slow processes, i.e. to build up a "next generation ILL".
This requirement exactly correspond to the recommendation of the Seitz panel for building the ANS super-reactor. Since that time it became clear, that a reactor source substantially more powerful than ILL is not feasible at reasonable costs, but neutron scattering performance more than an order of magnitude superior to ILL, (i.e. well beyond those planned with ANS) can be achieved by spallation sources with proton beam energy per pulse beyond 200 kJ and a repetition rate of about 20 Hz. This very high power can readily be envisaged in long (2-3 ms) pulses delivered by linear accelerators developed for transmutation applications.
The short pulse spallation source SNS to be built at Oak Ridge will provide the US with the "Next Generation ISIS". The short pulse technique (SNS pulses will carry 17 kJ only) is not adequate for achieving the energy per pulse required for equaling and exceeding core research capabilities of ILL, which are crucial for probing complex matter. For this purpose the US needs to restore a competitive edge in the full breadth of neutron research, which requires, in addition to SNS, a "Very High Power Spallation Source", the "Next Generation ILL", based on novel neutron scattering instrumentation and linear accelerator techniques developed or being developed at Los Alamos.