Time of Flight (TOF) Spectroscopy and Powder Diffraction at the European Spallation Source
Time of Flight (TOF) Spectroscopy
Time of flight spectroscopy is an extremely versatile method to investigate the dynamics in condensed matter in an energy range from µeV to hundreds of meV. This energy range covers the dynamics in biological samples, magnetic excitations in high Tc superconductors and other correlated electron systems, or the motion of hydrogen or other ions important for energy research.
The long pulse spallation source combines advantages of a high peak brightness realized otherwise at a short pulse spallation source with the flexibility of a reactor based spectrometer, where resolution and intensity can be traded according to the needs of the experiment. In direct geometry they can fully exploit the high intensity in the peak flux, which is a factor of 30 higher comparing the ESS and the most powerful reactor today, the ILL. Therefore, such an instrument at the ESS opens new perspectives for the investigation of the structure and dynamics in complex materials (see figure 1).
An instrument at a long pulse spallation source will feature a very long flight path to enable measurements ranging from very high to modest energy resolution. Modern neutron transport systems provide an intense incoming neutron beam over a wide range of energy. Novel chopper systems using the concept of "Repetition Rate Multiplication" optimize the usage of the produced neutrons and adjust the resolution according to the scientific needs.
At the moment in the Forschungszentrum Jülich the TOF spectrometer "TOPAS" for thermal neutrons is being developed and will be built at the FRMII reactor in Garching. Characteristics of TOPAS are a 50 m double elliptic neutron guide providing a high flux at the sample and the polarization setup, which has not yet been realized today at any thermal time-of-flight spectrometer.
With that respect this instruments is very similar to a TOF instrument at a long pulse spallation source. However, first calculations for the envisaged ESS pulse characteristics show that a distance of about 100 m or more is necessary to exploit the full pulse of the source at a simultaneous good energy resolution. The "Repetition Rate Multiplication“ allows to choose a TOF frame rate that is determined by the necessary time to reasonably separate spectra with minimal overlap. This rate is typically significantly higher (several 100 Hz) than the ESS pulse frequency. Thus the repetition rate concept increases the data collection efficiency by the ratio of timing chopper frequency and pulse rate in order to obtain the full peak flux gain factor of 30.
The detailed elaboration of concepts optimized with respect to the ESS parameters is currently done in collaboration with the Helmholtz Centre Berlin and the Technical University of Munich.
Powder diffraction is an important tool to determine the structure of new compounds. The outstanding role of neutrons originates from the abilities to localize exactly the distribution of light elements and to determine magnetic structures. Further typical applications are parametric and kinetic studies of phase transitions and chemical reactions. Important current challenges are, for example, to follow rapid reactions or to determine the structures of new materials, which could be synthesized only in little quantity.
Looking at today’s pulsed spallation sources, instruments for powder diffraction rely on time-of-flight methods using a broad bandwidth of neutron wave-lengths. Hence, these instruments conceptually differ from the traditional monochromatic angle-dispersive instrument type at continuous sources and may exploit, in particular, high resolution for backward scattering. Typical requests for resolution in powder diffraction are relatively high and fit better to the short pulses at the existing new spallation sources. The ESS is an intense long-pulse neutron source. Using pulse shaping choppers the neutron pulse can be tailored to the requested resolution. Due to the high intensity of the ESS obtained by the full neutron moderation, even for the very competitive case of short pulses the performance the ESS is expected to be comparable to the best existing instruments. An instrument flexible in resolution, utilizing more of the long ESS pulse, will maximize the full potential of this new source (see figure 2).
Within the German contribution to the Redesign-Update Phase of the ESS two diffraction instruments will be developed. There will be one instrument dedicated for engeneering applications and developed by the Helmholtz Zentrum Gheesthacht, a second one will be designed for a broad user community and for general purposes in powder diffraction under a common development by the Research Centre Jülich and the Helmholtz Centre Berlin.
Flexibility in flux and resolution is a major issue for the general purpose powder diffractometer, which for the design phase we shall briefly name POWHOW. Counter rotating disc choppers provide a large variety of options to define the requested time structure and pulse length; hence, parameters determining the resolution are easily changeable by the relative phase and frequency of the choppers. This pulse shaping chopper system will be the first element close to the moderator in a focus of the subsequent elliptic neutron guide, which will minimize the background near the sample in the second focus. A most homogeneous divergence profile can be realized by elliptic guides of octagonal rather than squared cross section according to previous simulations for the POWTEX instrument under construction at the FRM2.(link) An optimization for small samples favors a point instead of slit-like geometry and to make use of very high detector coverage of the solid angle. The figure illustrates the result for a time-of-flight diffraction pattern for a cubic structure as calculated by Monte Carlo simulation. A typical feature of the Bragg lines shown, here calculated for the resolution at POWTEX, is the high intensity at medium scattering angles and the high resolution near back scattering. The POWHOW instrument will be tunable to significant better resolution and larger bandwidth. It is worthwhile to note that the instrument concept has potential also beyond powder diffraction, which we currently explore further, one valuable additional option could be single crystal Laue diffraction.
The work is done in collaboration with the Helmholtz Centre Geesthacht and the Helmholtz Centre Berlin.