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Since the discovery of the neutron in 1934 neutron beams have been used in a very broad range of applications, As an aging fleet of nuclear reactor sources is retired the use of compact accelerator-driven neutron sources (CANS) are becoming more prevalent. CANS are playing a significant and expanding role in research and development in science and engineering, as well as in education and training. In the realm of multidisciplinary applications, CANS offer opportunities over a wide range of technical utilization, from interrogation of civil structures to medical therapy to cultural heritage study. This paper aims to provide the first comprehensive overview of the history, current status of operation, and ongoing development of CANS worldwide. The basic physics and engineering regarding neutron production by accelerators, target-moderator systems, and beam line instrumentation are introduced, followed by an extensive discussion of various evolving applications currently exploited at CANS.

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We have studied by neutron spectroscopy the concentration (1.5xlO~4^x^l.lxlO~2) and temperature dependence (0.5 K^ F*S 10 K) of H tunneling in Nb(OH) x . For T < 5 K a narrow inelastic line is observed for x = 1.5 x 10~4 which changes into a broad peak at higher concentrations. This shows that interactions between the defects lead to a distribution of both the asymmetry of the potentials and the matrix element. Between 5 and 10 K the data reveal a renormalization of the matrix element and an increased damping due to a Korringa-type coupling of the tunnel system with conduction electrons.PACS numbers: 61.12. Ex, 61.40.4b, 63.20.Kr, 63.20.Mt We report on the first spectroscopic measurements of both the concentration and temperature dependence of the tunnel splitting of the H vibrational ground state in Nb(OH) x . The inelastic scattering at low temperatures shows the progressive influence of distortions with increasing defect concentration and a detailed analysis of the line shape allows us to distinguish between distortions of the tunnel system which lead to a purely energetic shift of the pocket states and distortions which cause a change of the matrix element. The evolution of the inelastic scattering with temperature is strongly indicative for the interaction of the tunnel systems with conduction electrons. This causes both different matrix elements for the superconducting and normal conducting states and a dynamical damping of the tunnel state, from which the electrondefect coupling constant can be determined.Tunneling in Nb(OH) x has been observed by specific heat, 1 " 3 ultrasonic or inelastic studies, 4 " 9 and neutron-scattering 10 ' 11 measurements. It is likely the best-known example for H tunneling in the solid state and it represents a unique model system with atomic transfer studies relevant for quantum diffusion theory. 12 " 14 A two-site model as commonly used to account for low-temperature anomalies in amorphous substances 15,16 has been proposed to describe the phenomenon. The energy difference E between the two lowest eigenstates of the H is then given bywhere JQ is the matrix element and € is an energetic shift between the two sites due to (elastic) defectdefect interaction. In view of the statistical spatial arrangement of the defects a distribution Z(e) of Lorentzian shape 17 needs to be taken into account:where Ae is the average value for the energy shift. Similarly, a distribution Z(J) withcan be assumed for the matrix element. The widths of both distributions will increase with increasing defect concentration when interactions become more important. 3 Equation (4) is a generalization of the cross section for the inelastic scattering from a tunnel system [see Eq. (4) in Ref. 10], which includes the distribution functions for both the energy shift and the matrix element: d 2