Abstract
Among polarizers based on the neutron reflection from Super-Mirrors (SMs), solid-state neutron-optical devices have many advantages. The most relevant is the 5-10 times smaller size along the neutron beam direction compared to more traditional air-gap devices. An important condition for a good SM polarizer is the matching of the substrate SLD (Scattering Length Density) with the SM coating SLD for the spin-down component. For traditional Fe/Si SM on the Si substrate, this SLD step is positive when a neutron goes from the substrate to the SM, which leads to a significant degradation of the polarizer performance in the small Q region. This can be solved by replacing single-crystal Si substrates by single-crystal sapphire or quartz substrates. The latter shows a negative SLD step for the spin-down neutron polarization component at the interface with Fe and, therefore, avoid the total reflection regime in the small Q region. In order to optimize the polarizer performance, we formulate the concept of sapphire V-bender. We 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, as well the strength and quality of magnetizing field have dramatic effect on the polarizer performance. 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 (300 mT) provides an excellent agreement between the simulated and measured polarization values. For the double-reflection 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 using only a double polarizer in the "crossed" (X-SM) geometry. These results are applied in a full-scale polarizer for the PF1B instrument.
Highlights
PF1B is a user facility at the Institut Laue-Langevin (ILL) in Grenoble, France, for experiments in elementary particle and nuclear physics using polarized or unpolarized cold neutrons.[1]
In our previous paper,[24] we showed that the replacement of single-crystal Si substrates by single-crystal quartz or sapphire wafers results in a negative Scattering Length Densities (SLDs) step for spin-down neutrons that eliminates the total reflection regime, see Fig. 1, and expands the polarizer bandwidth into the low Q region
An important condition for a good solid-state polarizer is the matching of the substrate SLD with the SM coating SLD (ρ−Fe for FeSi SM on Si) for the spin-down component of neutron polarization
Summary
PF1B is a user facility at the Institut Laue-Langevin (ILL) in Grenoble, France, for experiments in elementary particle and nuclear physics using polarized or unpolarized cold neutrons.[1]. When ultrahigh polarization was required, we installed a second similar polarizer in the “crossed geometry,” obtaining the mean polarization of Pn = 99.7% and the transmission ∼25%17 for the “good” polarization component This type of double polarizer, where reflecting planes in the second half are orthogonal to reflecting planes of the first half, is known as the X-bender. During 15 years of successful exploitation, this polarizer was irradiated with a very high neutron fluence which resulted in significant radiation damage to the mirrors’ glass substrates [mainly by charged particles products from the reaction 10B(n, α) in the glass substrate] It is strongly activated, mainly due to the presence of Co in the SM coatings, which makes its handling more complicated. We present a project of a new advanced polarizer for PF1B with an improved polarization and transmission performance and free from radiation damage and activation issues
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