Polarized beam option for the time-of-flight spectrometer NEAT

Abstract

We present the basic design of the incoming neutron beam polarizer system recently installed at the TOF spectrometer NEAT at HZB and the first experimental test results. The recent upgrade of the instrument resulted in beam flux on the sample, which is one of the highest worldwide for this class of instruments. The substantial intensity gain obtained in the upgrade provides good conditions for polarized neutron experiments, considering the inherent cuts of neutron intensity by polarizing the beam. The neutron beam polarizer is placed inside a 3.8 m long evacuated neutron “guide changer”, which allows us to move into the beam delivery guide one of 3 alternative straight neutron guide sections mounted in parallel on a linear translation stage. The new transmission polarizer takes up the inside volume of a 1.33 m long, 60 mm x 125 mm cross section straight guide portion. The focusing of the beam after the polarizer to the 30 mm x 50 mm sample area happens over a total length of 3.5 m, of which 2.92 cm is a converging guide assembly with m=4 supermirror coating and parabolic end. This guide section leaves 58 cm space between guide exit and sample axis for sample environment equipment, in particular for high field cryomagnets. The transmission polarizer contains 8 “V cavity” channels with non-polarizing supermirror coated, opaque side walls and neutron-transparent Si cavity plates, coated by m=3 polarizing supermirror on both sides. The nominal operational wavelength band width of the polarizer is 2.5 – 8 Å, with measured polarization efficiency between 92 and 96 % for well collimated beam. The beam transmission efficiency for the preferred neutron spin state is about 65 %. The first 110 cm of the neutron spin guide field after the exit of the polarizer is provided by the stray fields of the magnet around the polarizer and of a permanent guide field magnet acting in the same direction. This section contains a wavelength tunable single coil Mezei flipper with flipping efficiency > 99 %. The rest of the guide field magnets are solenoids around the glass neutron beam guide. The supermirror coating in the polarized neutron section of the beam guide has been manufactured using a non-magnetic Ni alloy, the guide exchanger housing and its mechanical parts are made of non-magnetic alloys, primarily stainless steel. The polarized beam delivery system is by now operational and it is available for polarized neutron beam experiments on samples in a magnetic field. A 14 T vertical field cryomagnet is on hand at the HZB Sample Environment Group for use on NEAT. The NEAT sample chamber has been designed and built with non-magnetic or weakly magnetic materials in order to allow both for polarized neutron beam experiments and for polarization analysis work.