Fig. 4-13 Drabkin energy filter A zigzag-folded Al sheet is placed in the center of a solenoid coil. The Al sheet generates a periodic alternating field and the solenoid coil generates a uniform field orthogonal to the alternating field.

Fig. 4-14 Spatial neutron spin resonance phenomena Only neutrons having a wavelength near a resonance wavelength are spin-flipped due to the interaction with the periodic magnetic field in a Drabkin energy filter. Period numbers "0" and "100" indicate the entrance and exit of the field region, respectively.

Fig. 4-15 Simulation of pulse shaping by a Drabkin energy filter for the J-PARC coupled moderator The red dots represent an incident pulse of 0.36 nm and the blue dots show a sharpened pulse formed by two Drabkin filters installed in sequence (a double Drabkin filter). The results show that a double Drabkin filter will eliminate 99.975% of tail neutrons. Pulse shaping shown in this figure could be performed over a wide range of wavelengths simultaneously.

In neutron experiments to be performed at the J-PARC spallation neutron source, pulsed neutrons are emitted from a moderator. The initial energy (or wavelength) of a neutron is determined by measuring the time-of-flight between the moderator surface and a reference position. Hence, the uncertainty of the emission-time (pulse width), which is from tens to hundreds of microseconds, degrades the experimental resolution. A method is being developed to reduce pulse widths (pulse shaping) by means of spatial neutron spin resonance. By reducing the widths of very high intensity neutron pulses from the J-PARC coupled moderator, experiments can be performed with high resolution and high intensity neutron pulses. In terms of spatial neutron spin resonance phenomena, neutrons with a wavelength very close to a particular wavelength (the resonance wavelength) are spin-flipped due to interactions with a spatially periodic magnetic field. Hence, neutrons in the neutron beam are filtered into a monochromatic beam by extracting the spin-flipped neutrons. A monochromator employing this principle is called a Drabkin energy filter. A Drabkin energy filter is composed of a zigzag-folded aluminum (Al) current-sheet that is placed in the center of a solenoid coil. The current-sheet (Fig. 4-13) produces a spatially alternating field while the solenoid coil produces uniform field orthogonal to the alternating field. The synthetic field brings about the spatial resonance spin-flip (Fig. 4-14). The resonance wavelength and resonance peak width are electronically variable. If the resonance wavelength is varied in synchronization with the time-of-flight from the moderator surface, neutrons that are emitted late can be eliminated and the pulse widths can be reduced. Fig. 4-15 shows a pulse shaping simulation for neutron pulses from the J-PARC coupled moderator. The Drabkin filter can raise the experimental resolution dramatically and provide coupled-moderator instruments with high resolution competitive with decoupled-moderator instruments.

Reference D. Yamazaki et al., Pulse Shaping by Means of Spatial Neutron Spin Resonance, Nucl. Instrum. Methods Phys. Res., Sect. A, 529, 204 (2004).

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