Neutron scattering is an indispensable experimental technique in a wide range of fields including physics, chemistry, and engineering. In 1949, Nobel Laureate C.G. Shull demonstrated its usefulness by elucidating the magnetic structure of the antiferromagnet MnO using neutron scattering [1]. Particularly, inelastic neutron scattering (INS) has proven to be a powerful tool for observing collective excitations of atoms and spins in condensed matter. These collective modes are characterized by a wavevector $\bm{Q}$ and energy $E$, thus measuring them allows for the determination of the system’s Hamiltonian. The dynamics of crystals [2], magnetic materials [3], and other systems have been actively studied using this technique.

The neutron triple-axis spectrometer (TAS) has been widely used in both inelastic and elastic scattering experiments since its development in the 1950s, establishing its position as a versatile and important spectrometer [4]. By using a single analyzer and detector along with a focused neutron beam, TAS allows for high signal-to-noise (S/N) ratio measurements at specific $\bm{Q}$-$E$ points. On the other hand, chopper spectrometers employ an array of detectors surrounding the sample, combined with time-of-flight energy analysis, enabling efficient measurements across a wide $\bm{Q}$-$E$ space. Recently, there has been a global trend in the design, construction, and operation of multiplex-type spectrometers, which combine high S/N measurements with efficient $\bm{Q}$-$E$ space coverage [5]. In this study, we constructed a multiplex spectrometer called HODACA (HOrizontally Defocusing Analyzer Concurrent data Acquisition) based on the inverse Rowland inelastic spectrometer (IRIS) concept proposed by Harriger and Zaliznyak [6]. The instrument was installed at the C11 beam port of the research reactor JRR-3 [7].

As shown in Fig. 1, the HODACA spectrometer employs an array of analyzers arranged on a Rowland circle to refocus scattered neutrons. Due to the inscribed angle theorem, the reflection angles of all analyzers remain constant, and the trajectories of the neutrons form a pattern as if they are diffused from a sample image. Neutrons are then detected by an array of detectors positioned on a circle centered on the sample image. The use of radial collimators before and after the analyzers is expected to effectively reduce background noise. This spectrometer enables efficient and high-S/N measurements across a wide $\bm{Q}$-space at constant energy. As a result, HODACA became a spectrometer capable of measuring spectra from −1 meV to 7 meV by fixing the scattered neutron energy Ef at 3.635 meV. It consists of 24 analyzers and 24 detectors spaced at 2◦ intervals, covering a scattering angle A2 of 46◦. Each analyzer is composed of 3 to 7 PG crystals mounted in a vertically focusing configuration. The vertical size of each analyzer (number of PG crystals) is determined to ensure that the solid angles spanned by the analyzer viewed from the sample position are the same. Radial collimators with divergence angle of 2° are installed between the sample-analyzer and analyzer-detector to minimize cross-talk of scattered neutrons from neighboring analyzers.

For the standard sample to measure INS spectra, we selected the frustrated magnetic compound CsFeCl₃. Its dispersion relation at ambient pressure is well described by the Extended Spin Wave Theory (ESWT) [8]. The INS spectrum measured by the HODACA spectrometer is shown in Fig. 1(b), where $\bm{Q} = (1/3, 1/3, l)$ direction. The white lines in the figure represent the dispersion curves calculated using ESWT parameters from previous studies. The experimental results are well reproduced by the calculations using the previously reported parameters. An ideal spectrometer for measuring dynamics in the energy range of cold neutrons is now ready for users.

References

• [1] C. G. Shull and J. S. Smart, Phys. Rev. 76, 1256 (1949).
• [2] R. Pynn and G. L. Squires, Proc. R. Soc. A 326, 347 (1972).
• [3] M. F. Collins et al., Phys. Rev. 179, 417 (1969).
• [4] G. Shirane, S. M. Shapiro, and J. M. Tranquada, Neutron Scattering with a Triple-Axis Spectrometer (Cambridge University Press, Cambridge, U.K., 2008).
• [5] F. Groitl et al., Rev. Sci. Instrum. 87, 035109 (2016).
• [6] L. Harriger and I. Zaliznyak, 2015 NCNR Annual Report (2015) p. 46
• [7] H. Kikuchi et al., J. Phys. Soc. Jpn. 93, 091004 (2024).
• [8] S. Hayashida, M. Matsumoto, M. Hagihala, N. Kurita, H. Tanaka et al., Sci. Adv. 5, eaaw5639 (2019).

Authors

• H. Kikuchi, S. Asai, T. J. Sato^a, T. Nakajima, L. Harriger^b, I. Zaliznyak^c and  T. Masuda
• ^aTohoku University
• ^bNational Institute of Standards and Technology
• ^cBrookhaven National Laboratory