Progress in High Resolution Chopper Spectrometer HRC by improving collimator and Fermi chopper
Introduction
The High Resolution Chopper Spectrometer (HRC) installed at the Materials and Life Science Experimental Facility (MLF) at the Japan Proton Accelerator Research Complex (J-PARC), provides the opportunity for dynamical studies of materials over a wide energy-momentum space, with high resolution using relatively high energy neutrons [[1], [2], [3], [4], [5]]. In particular, the HRC can be used to perform three types of inelastic neutron scattering (INS) experiments: high-resolution experiments in a conventional energy-momentum space, eV neutron spectroscopy, and neutron Brillouin scattering (NBS).
The HRC is a chopper spectrometer: a polychromatic pulsed neutron beam is monochromatized using a Fermi chopper, where the incident neutron energy (Ei) is selected, and the neutrons scattered by the experimental sample are detected with a detector array that covers wide scattering angles. The scattering angle (ϕ) and the time-of-flight (TOF) of the detected neutron are then analyzed, and the detected neutron counts are converted to the dynamical structure factor as a function of the momentum transfer Q and the energy transfer E. The layout of the HRC is illustrated in Fig. 1. This instrument is positioned to face a decoupled moderator to utilize sharp neutron pulses. The moderator has an area of 100 × 100 mm2, and the maximum sample size of 50 × 50 mm2 is acceptable. In the primary flight path, a T0 chopper reduces the background noise which originates from the high-energy neutrons, and a supermirror guide tube is used to increase the neutron flux at the sample position. The Fermi chopper and the sample are located at 14 m and 15 m (L1 = 15 m) from the neutron source, respectively. The HRC has a detector array of 3He position sensitive detectors (PSDs) with a length of 2.8 m and a diameter of 19 mm (3/4 inch). These detectors are located at 4 m from the sample position (L2 = 4 m) and cover scattering angles up to ϕ = 62° for conventional experiments. In addition, another detector array of 3He PSDs with a length of 0.8 m and a diameter of 12.7 mm (1/2 inch) are located at L2 = 5.2 m and cover scattering angles down to ϕ = 0.6° for NBS. Each PSD is mounted with its length direction oriented vertically. A collimator system composed of slits of vertical sheets of Cd is installed at the up stream of the sample [2,5]. One of the two collimators with collimation angles of 1.5° and 0.3° can be selected, which reduce the background noise at low angles down to ϕ = 3° for conventional experiments, and ϕ = 0.6° for NBS, respectively [5]. Experiments with the HRC are performed by optimizing the experimental conditions. Typical experimental conditions are as follows: Ei = 5–500 meV, ϕ = 3–62°, ΔE∕Ei ≥ 2.5% for high-resolution experiments in a conventional (Q, E) space, Ei = 300–2000 meV, ϕ = 0.6–62°, ΔE∕Ei ≥ 5% for eV neutron spectroscopy, and Ei = 100–500 meV, ϕ = 0.6–5.1°, ΔE∕Ei ≥ 2% for NBS, where ΔE is the energy resolution.
NBS is INS near the forward direction. This technique is effective for observing coherent excitations in non-single-crystal samples such as ferromagnetic spin waves in powder samples. Owing to the kinematic constraints of neutron spectroscopy, Ei in the sub-eV region with a high resolution is necessary, and the scattered neutrons need to be detected at very low scattering angles, for measuring NBS. The NBS option differentiates the HRC from other INS spectrometers at pulsed neutron sources, because the HRC utilizes low angle detectors and high energy neutrons with high resolutions. Because the solid angle of the detection area for NBS experiments is limited to very low scattering angles, improvement for gaining the neutron flux is essential. Since the recent success of NBS on the HRC [6], neutron intensities for NBS experiments have been improved by using a double-layered array of low angle PSDs, a less deformed slit package of the Fermi chopper and a longer collimator for 0.3° collimation [7]. We here report further improvements.
Section snippets
Optimization of high resolution Fermi chopper
For the HRC, one of two Fermi choppers, which include a high-intensity chopper (Fermi A) and a high-resolution chopper (Fermi B), can be selected to optimize the experiments. Each Fermi chopper is composed of a slit package, which is a layered assembly of components that consist of shielding plates and spacers (spacer width w) sandwiched by curved walls (wall curvature R). The slit package of a vertical slit, with a cross-section of 75 mm (width) × 64 mm (height) and length of D = 100 mm, is
Improvement of collimator and NBS
The collimator system is effective for reducing background noise at low angles. Initially, we fabricated two collimators with 1.5° and 0.3° collimation angles composed of Cd sheets with a thickness of tc = 0.1 mm and a length of Lc = 150 mm, as listed in Table 2. Since the transmission of the 0.3° collimator (0.3° short) was very low, we fabricated a new 0.3° long collimator with a length of Lc = 295 mm and a slit width of wc = 1.55 mm to increase the transmission, because it is difficult to
Summary
Since the start of the operation of the HRC with the high-intensity Fermi chopper and PSDs mounted at ϕ = 3–42° at L2 = 4 m and −0.5 – −2.8° at L2 = 5.2 m [1], we have made successive efforts to improve its performance. Currently, PSDs are mounted at ϕ = 3–62° and −13 – −31° at L2 = 4 m, and at ϕ = 0.8–0.6° and −0.6 – −5.1° at L2 = 5.2 m, as illustrated in Fig. 1 [5]. Initially, we installed the collimator system using collimators with 2.3° and 0.6° collimation angles [2,3]. The 2.3° collimator
Acknowledgments
This study was approved by the Neutron Scattering Program Advisory Committee of the Institute of Materials Structure Science, High Energy Accelerator Research Organization (No. 2017S01).
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