High-efficiency resonant rf spin rotator with broad phase space acceptance for pulsed polarized cold neutron beams

Abstract

High precision fundamental neutron physics experiments have been proposed for the intense pulsed spallation neutron beams at JSNS, LANSCE, and SNS to test the standard model and search for new physics. Certain systematic effects in some of these experiments have to be controlled at the few ppb level. The NPDGamma experiment, a search for the small parity-violating -ray asymmetry A in polarized cold neutron capture on parahydrogen, is one example. For the NPDGamma experiment we developed a radio-frequency resonant spin rotator to reverse the neutron polarization in a 9:5 cm 9:5 cm pulsed cold neutron beam with high efficiency over a broad cold neutron energy range. The effect of the spin reversal by the rotator on the neutron beam phase space is compared qualitatively to rf neutron spin flippers based on adiabatic fast passage. We discuss the design of the spin rotator and describe two types of transmission-based neutron spin-flip efficiency measurements where the neutron beam was both polarized and analyzed by optically polarized 3 He neutron spin filters. The efficiency of the spin rotator was measured at LANSCE to be 98:8 0:5% for neutron energies from 3 to 20 meV over the full phase space of the beam. Systematic effects that the rf spin rotator introduces to the NPDGamma experiment are considered.

Measurement of the absolute neutron beam polarization from a supermirror polarizer and the absolute efficiency of a neutron spin rotator for the NPDGamma experiment using a polarized 3He neutron spin-filter

“…The NPDGamma experiment uses a resonant RF spin rotator (RFSR), also known as a Rabi coil, to rotate the neutron spins π radians, thereby isolating the small parityviolating signal and canceling several systematic uncertainties in the γ-ray asymmetry measurement [17]. The RFSR creates a magnetic field B RF = B RF cos(ω RF t)ẑ parallel to the neutron beam and on resonance such that the frequency of the RF magnetic field ω RF is equal to the Larmor frequency of the neutrons in the guide field ω 0 = γ n B 0 .…”

Monte Carlo calculation and verification of the geometrical factors for the NPDGamma experiment

“…The neutron beam is 10 cm x 12 cm at the exit of the Fundamental Neutron Physics Beamline (FNPB) [1] at the Spallation Neutron Source at Oak Ridge National Laboratory. The beam passes through a supermirror polarizer [2] before entering the resonant frequency spin rotator (RFSR) [3]. The RFSR alternates the neutron spin over a sequence of 8 beam pulses in a ↑↓↓↑↓↑↑↓ pattern in order to cancel first and second order fluctuations in the beam intensity.…”

Monte Carlo calculation of the average neutron depolarization for the NPDGamma experiment

“…Neutrons exit the 10 × 12 cm neutron guide on the Fundamental Neutron Physics Beamline (FNPB) [3] and enter a supermirror polarizer [4], after which is the NPDGamma apparatus [5,6]. Polarized neutrons then enter the resonant frequency spin rotator (RFSR) [7] and are rotated according to a pattern (↑↓↓↑↓↑↑↓) that cancels beam power fluctuations to second order and the signals from opposite spin states are used to isolate the asymmetry signal. The neutrons are incident on the 16-liter liquid hydrogen target [8] that is surrounded by 48 CsI(Tl) detectors arranged in 4 rings of 12 detectors each [9] that detect the γ-rays from neutron capture.…”