First Demonstration of Novel Method for Inelastic Neutron Scattering Measurement Utilizing Multiple Incident Energies

Abstract

We succeeded in experimentally demonstrating that a series of two-dimensional maps of a dynamical structure factor in momentum-energy space with multiple incident energies can be simultaneously obtained by one measurement. This method reduces the dead time of time-of-flight measurement, and thus, it markedly increases the measurement efficiency. Our achievement realized using the Fermi chopper spectrometer 4SEASONS in J-PARC is expected to open up new possibilities of inelastic neutron scattering measurements.KEYWORDS: pulsed neutron source, Fermi chopper spectrometer, inelastic neutron scattering, multi-E i measurement, J-PARC DOI: 10.1143/JPSJ.78.093002A ''Fermi chopper'' spectrometer is a typical instrument for inelastic neutron scattering (INS) measurement at a pulsed neutron source. The Fermi chopper is used for monochromating the incident neutron beam. Inelastic scattering from a sample is measured by a time-of-flight (TOF) technique over a neutron source period. The accumulated TOF data can be converted to a dynamical structure factor. In fact, the INS measurements conducted using Fermi chopper spectrometers have revealed various mysteries of materials science, and thus, a Fermi chopper spectrometer should remain to be a flagship instrument in new-generation pulsed neutron sources such as the Japan Proton Accelerator Research Complex (J-PARC), which is one of the world's highest intensity pulsed neutron sources.A conventional Fermi chopper has a set of curved slits. The curvature is adjusted to the passage of neutrons with a selected velocity, which means that neutron transmission is optimized for a specific single incident energy (E i ). From another point of view, the curved slits would bring about the inefficiency of an INS measurement. Chopper rotation frequency (100 -600 Hz) is typically much higher than the frequency of the pulsed source (25 Hz in J-PARC), and the utilized TOF region of INS measurement with single E i is approximately less than 10% of the source period (40 ms in J-PARC). To reduce the dead time of TOF measurement, we have proposed the method of utilizing multiple E i beams. 1,2)We apply a set of straight and wide slits to a Fermi chopper, and consequently, a high transmission of neutrons can be achieved over a wide E i range, though the energy resolution is relaxed. Thus, this type of Fermi chopper can select multiple E i beams within the source period, and the simultaneous INS measurements with multiple E i 's can be realized. We refer to this method as Multi-E i measurement.Another technique, the so-called ''repetition rate multiplication (RRM) method'', for reducing the dead time of TOF measurement has also been developed, 3,4) and recently its first implementation has been reported. 5) The RRM method is mainly applied to measurements with cold neutrons, such as those involving quasi-elastic neutron scattering. It requires several disk choppers to suppress the contamination of slow neutrons and produce the symmetric pulse shape of incident neutrons. On the other hand, Multi-E i m...

Neutron Scattering Study of Spin Fluctuations in La2−xSrxCuO4 by Four-Dimensional Mapping of Neutron Cross-Sections Using the 4SEASONS Chopper Spectrometer

“…We used spectrometer conditions with the incident neutron energy E i = 48 meV and the chopper frequency of 200 Hz. 4SEASONS allows simultaneous measurements with multiple incident neutron energies [10]. For the above conditions, simultaneous measurements with E i = 48 and 120 meV are possible.…”

Materials and Life Science Experimental Facility at the Japan Proton Accelerator Research Complex III: Neutron Devices and Computational and Sample Environments

“…On 4SEASONS, however, we decided to adopt a Fermi chopper with straight slits that sacrifices the transmitted intensity but passes through a wide energy range of Ei. This decision brought us success in the experimental demonstration of the usefulness of Multi-Ei measurement, even in the Fermi chopper spectrometer [39].…”

“…These development products were utilized in actual user measurements at the chopper spectrometers at MLF, while we were proceeding with development of effective analysis for event recording data. In particular, we successfully contributed to the realization of the Multi-E i method for inelastic scattering for the first time in the world [39]. As the other beamlines also required event-recording analysis, we made the data reduction and visualization parts in Utsusemi independent of the original to give it greater transferability as universal analysis software at MLF.…”

Present Status of the Materials & Life Science Experimental Facility of J-PARC

“…Therefore, spectrum observed with several different incident energies are automatically obtained. The method is very effective to make a survey in the different energy scales to find unknown phenomena [13].…”