A large beam high efficiency radio frequency neutron spin flipper

Abstract

A design for a radio frequency (RF) neutron spin flipper obtained from magneto-static and neutron spin transport simulations is presented. The RF flipper constructed from this design provides a flipping probability of 0.999 or better for a beam size 6 cm wide and 15 cm high and a wavelength band between 0.4 and 0.6 nm. Three permanent magnet guide field sections with air gaps provide a linear field gradient along the beam propagation direction over a large cross-sectional area. An RF oscillator based on coupling the resonant coil of a Hartley oscillator to the excitation coil was developed, which provides a higher current and, thereby, a larger RF amplitude, as compared to a conventional RF power amplifier. Two opaque He3 neutron spin filters were employed to measure the flipping probability of the flipper with very high precision. A spatially uniform flipping probability of 0.9995(2) or higher was measured over the large cross-sectional area neutron guide. This RF neutron spin flipper will be employed in a polychromatic beam reflectometer at the National Institute of Standards and Technology Center for Neutron Research. This design can be applied to other polarized neutron instruments or applications requiring a very high continuous flipping probability of the neutron spin for a large cross-sectional area beam.

Advanced polarization analysis capability on the very small-angle neutron scattering instrument at the NIST Center for Neutron Research

“…1a, a double V-shaped m=3.5 supermirror polarizer in a transmission geometry, provided by SwissNeutronics [29,30], is used to polarize the incident neutron beam and provides a neutron polarization of 99 % over a beam cross section of 6 cm (width) × 15 cm (height) for the wavelength range from 5.5 Å to 8.0 Å that is typically used for polarized beam experiments on NG-7 SANS and VSANS at the NCNR [1]. An adiabatic RF spin flipper [31,32] is used to flip, at will, the neutron polarization of the incident beam. A schematic diagram of the polarized VSANS setup is shown in Fig.…”

“…No observable difference in polarization in the same wavelength range was seen when we changed incident beam optics from 1 neutron guide to 9 guides [1]. The RF flipper made use of a newly designed magnetostatic cavity capable of providing a more spatially uniform linear gradient field and push-pull home-made modified Hartley RF oscillators capable of providing a higher RF field than a conventional RF flipper [1,32]. The flipping probability of the RF flipper was found to be higher than 0.999.…”

“…The flipping probability of the RF flipper was found to be higher than 0.999. Both neutron polarizing efficiency and flipping efficiency were measured using the opaque 3 He analyzer approach [32]. The newly implemented supermirror polarizer and RF flipper provided an overall instrumental flipping ratio up to 290 [1] with an opaque 3 He analyzer while a typical instrumental flipping ratio of ≈100 is achieved for a higher transmission from a lower opacity 3 He analyzer.…”

A Polarization Tensor for Neutron Optics

“…This situation occurs in the frame of the neutron when some component of a magnetic field of constant magnitude rotates about a given axis. This problem is one of the corner stones of magnetic resonance [7], and is found throughout neutron optics wherever spin flipping or spin rotation is employed [8,9,10,11,12,13].…”

The very small angle neutron scattering instrument at the National Institute of Standards and Technology

“…A pair of loosely wound excitation coils of only a few turns each spread out along the primary coil inductively couples the main coil to a pair of push-pull in-house-designed modified Hartley RF oscillators, described in detail by Chen et al (2021). This circuit replaces the impedance-matched RF amplifier and waveform generator found in most RF neutron spin flipper setups.…”