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

Abstract

Accurately measuring the neutron beam polarization of a high flux, large area neutron beam is necessary for many neutron physics experiments. The Fundamental Neutron Physics Beamline (FnPB) at the Spallation Neutron Source (SNS) is a pulsed neutron beam that was polarized with a supermirror polarizer for the NPDGamma experiment. The polarized neutron beam had a flux of ∼ 10 9 neutrons per second per cm 2 and a cross sectional area of 10×12 cm 2 . The polarization of this neutron beam and the efficiency of a RF neutron spin rotator installed downstream on this beam were measured by neutron transmission through a polarized 3 He neutron spin-filter. The pulsed nature of the SNS enabled us to employ an absolute measurement technique for both quantities which does not depend on accurate knowledge of the phase space of the neutron beam or the 3 He polarization in the spin filter and is therefore of interest for any experiments on slow neutron beams from pulsed neutron sources which require knowledge of the absolute value of the neutron polarization. The polarization and spin-reversal efficiency measured in this work were done for the NPDGamma experiment, which measures the parity violating γ-ray angular distribution asymmetry with respect to the neutron spin direction in the capture of polarized neutrons on protons. The experimental technique, results, systematic effects, and applications to neutron capture targets are discussed.

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

“…where A PC and A PV are the parity-conserving and parity-violating physics asymmetries, A i app is a term encompassing contributions that dilute the asymmetry, P n is the measured neutron beam polarization [7], ∆ dep is the neutron depolarization correction, and ∆ sf is the spin flip efficiency. Using the super ratio method is superior to using the difference between detector signals because it perfectly accounts for the differences in the gains of the detectors in each pair and is less sensitive to differential detector pedestal values.…”

Neutron Polarimetry Using a Polarized 3He Cell for the aCORN Experiment

“…The experiment performed in June used two 3 He spin filter cells of different length; Olaf (N He L = 5.5 bar-cm) and Syrah (N He L = 13.3 bar-cm). [17] The small variation in GE180 glass transmission with wavelength was neglected [8,18] and T e = 0.88 was assumed for both cells. [10] Two cells were used as opposed to only one because we believed this would allow us to observe any effect due to a polychromatic neutron wavelength spectrum.…”

“…Such spin filters have also been used for polarization analysis on a range of neutron-based instruments such as triple-axis spectrometers, small-angle neutron scattering instruments, and reflectometers. [5] Spin filters have been used for highly accurate neutron beam polarization measurements on polarized neutron beams, [6,7,8,9,10] but aCORN is in a class of experiments that is sensitive to a slight polarization on a nominally unpolarized beam. [1,11,12] Polarized 3 He has a large absorption cross section for neutrons of antiparallel spin and a negligible cross section for neutrons of parallel spin.…”

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

“…and which should be compared to equation 15 below. The spin flip terms for a neutron with spin along theẑ factor in equation 10. The probability that any given particle interaction leads to a spin flip is given by…”

“…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. The average neutron beam polarization is measured with an empty liquid hydrogen target vessel, and a 3 He spin filter [10] followed by a 3 He transmission monitor [11] downstream of the vessel are used to measure the beam polarization. A uniform 9.5 gauss magnetic field aligned within 3 mrad to the vertical axis (ẑ) transports polarized neutrons to the target vessel [1].…”