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Register a new userAdvanced broad-band solid-state supermirror polarizers for cold neutrons
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Added by
Valery Nesvizhevsky
Publication date
2016
fields
Physics
and research's language is
English
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An ideal solid-state supermirror (SM) neutron polarizer assumes total reflection of neutrons from the SM coating for one spin-component and total absorption for the other, thus providing a perfectly polarized neutron beam at the exit. However, in practice, the substrates neutron-nucleai optical potential does not match perfectly that for spin-down neutrons in the SM. For a positive step in the optical potential (as in a Fe/SiN(x) SM on Si substrate), this mismatch results in spin-independent total reflection for neutrons with small momentum transfer Q, limiting the useful neutron bandwidth in the low-Q region. To overcome this limitation, we propose to replace Si single-crystal substrates by media with higher optical potential than that for spin-down neutrons in the SM ferromagnetic layers. We found single-crystal sapphire and single-crystal quartz as good candidates for solid-state Fe/SiN(x) SM polarizers. To verify this idea, we coated a thick plate of single-crystal sapphire with a m=2.4 Fe/SiN(x) SM. At the T3 instrument at the ILL, we measured the spin-up and spin-down reflectivity curves with 7.5 A neutrons incident from the substrate to the interface between the substrate and the SM coating. The results of this experimental test were in excellent agreement with our expectations: the bandwidth of high polarizing power extended significantly into the low-Q region. This finding, together with the possibility to apply a strong magnetizing field, opens a new road to produce high-efficient solid-state SM polarizers with an extended neutron wavelength bandwidth and near-to-perfect polarizing power.
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Among Super-Mirror (SM) polarizers, solid-state devices have many advantages. The most relevant is 5-10 times smaller length compared to air-gap polarizers allowing to apply stronger magnetic fields. An important condition for a good SM polarizer is the matching of the substrate SLD (Scattering Length Density) with the SM coating SLD for spin-down neutrons. For traditional Fe/Si SM on Si substrate, this SLD step is positive when a neutron goes from the substrate to the SM, which leads to a significant loss of the polarizer performance at small Q. Instead, we use single-crystal Sapphire/Quartz substrates. The latter show a negative SLD step for spin-down neutrons at the interface with Fe and, therefore, avoid the total reflection regime at small Q. To optimize the polarizer performance, we formulate the concept of Sapphire V-bender, perform ray-tracing simulations of Sapphire V-bender, compare results with those for traditional C-bender on Si, and study experimentally V-bender prototypes with different substrates. Our results show that the choice of substrate material, polarizer geometry and the strength and quality of magnetizing field have dramatic effect. In particular, we compare the performance of polarizer for the applied magnetic field strength of $50 mT$ and $300 mT$. Only the large field strength provides an excellent agreement between the simulated and measured polarization values. For the double-collision configuration, a record polarization $>0.999$ was obtained in the neutron wavelength band of $0.3-1.2 nm$ with only $1%$ decrease at $2 nm$. Without any collimation, the polarization averaged over the full outgoing capture spectrum, $0.997$, was found to be equal to the value obtained previously only using a double polarizer in the crossed (X-SM) geometry. These results are applied in a full-scale polarizer for the PF1B instrument.
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