Development of Drabkin energy filters for J-PARC project

In the J-PARC project, the high intensity spallation neutron source has been developed. Very intensive pulsed neutron beam will be available from a coupled moderator installed at the spallation source. Wavelengths of neutrons is generally determined by its time-of-flight (TOF) from the source to the detector, but the available precision is limited by the non-zero emission time-width of the moderator system. It follows that high precision experiments cannot be performed with the intensive pulsed neutrons from the coupled moderator. We have been developing Drabkin energy filters, which effectively reduces the emission time-width by the spatial neutron spin resonance. In this paper, firstly, we describe the physics in the Drabkin spin flipper, which is the main part of the Drabkin energy filter, and derive the spin-flip probability by the flipper in the quantum-mechanical manner. Secondly, the properties of the resonance spin flipping are described. Thirdly, sweep mode for the application to pulsed neutrons are described and the effects of the field sweeping to the spin-flip probabilities were numerically demonstrated. Suppressions of sub-peaks in the resonance spin flipping by modulated magnetic fields are also demonstrated numerically. We have developed Drabkin flippers with 10 periods of field and carried out performance test of the flipper at JRR-3M. Firstly, field dependency of the spin-flip probability was measured with monochromatic beam (lambda=8.8 A, FWHM 2.5%). Secondly, wavelength dependency of the spin-flip probability was measured with chopped beam (lambda=8.3 A, FWHM 14.9%). The results were consistent with the expected values by numerical calculations.