Günther, G.; Mezei, F.; Füzi, J.; Gainov, R.; Krist, T.; Sánta, Z.; Russina, M.: Polarized beam option for the time-of-flight spectrometer NEAT. Journal of Physics: Conference Series 1316 (2019), p. 012008/1-7
10.1088/1742-6596/1316/1/012008
Open Access Version
Abstract:
We present the basic design of the incoming neutron beam polarizer system recently installed at the TOF spectrometer NEAT at HZB and 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 very 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 3.3 m long 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 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 section 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 m=3 polarizing supermirror coated neutron-transparent Si cavity plates, coated on both sides [1]. The nominal operational wavelength band width of the polarizer is 2.5 – 8 Å, with measured polarization efficiency between 92 and 96 %. The beam transmission efficiency for the preferred neutron spin state is about 65 %. The polarization guide field between the exit of the polarizer and the end of the converging guide in front of the sample contains a wavelength tunable single coil Mezei flipper coil with guide field with flipping efficiency > 99 %. The rest of the guide field is provided by solenoids around the guide. The supermirror coating in the polarized neutron section of the guide has been manufactured using a non-magnetic Ni alloy, the guide exchanger housing and mechanical parts inside 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 and a 14 T maximal vertical field cryomagnet is available 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 to allow both for polarized neutron beam experiments and polarization analysis work.