Small-angle neutron polarimetry apparatus (SANPA): Development at the NIST Center for Neutron Research

Krycka

Watson

et al. 2023

J. Phys.: Conf. Ser.

The Very Small Angle Neutron Scattering (VSANS) diffractometer at the National Institute of Standards and Technology has been commissioned and is in the user program. A large available space of nearly 2 m along the beam in the sample area not only enhances the existing SANS polarization analysis capability, but also makes it possible for implementation of other polarization analysis capabilities which would not be easily available on existing SANS instruments, including grazing-incidence small-angle neutron scattering with polarization analysis and spherical neutron polarimetry. We present two polarized setups, one for high magnetic sample fields and the other for low magnetic sample fields, together with a versatile and flexible operational platform for polarized beam experiments. The design of a magnetostatic cavity device that provides better field homogeneity and thus longer 3He polarization relaxation time is discussed. It consists of an end-compensated magnetic shielded solenoid with non-identical holes to accommodate the divergent scattered beam in a constrained distance. Significant improvement in polarized neutronic performance, 3He polarization relaxation time, and an extended momentum transfer range for polarization analysis are presented. Improved neutron polarizing devices, double V-shaped supermirror polarizer, adiabatic radio-frequency spin flipper, and a 3He spin analyzer have yielded an initial instrumental flipping ratio of 100, allowing for higher sensitivity to detection of weak magnetic features in the sample.

Section: Future Developmentsmentioning

confidence: 99%

Advanced polarization analysis capability on the very small-angle neutron scattering instrument at the NIST Center for Neutron Research

Krycka

Watson

et al. 2023

J. Phys.: Conf. Ser.

Sensitive neutron transverse polarization analysis using a 3He spin filter

Jau

Review of Scientific Instruments

Gentile

et al. 2020

We report an experimental implementation for neutron transverse polarization analysis that is capable of detecting a small angular change (≪10−3 rad) in neutron spin orientation. This approach is demonstrated for monochromatic beams, and we show that it could be extended to polychromatic neutron beams. Our approach employs a 3He spin filter inside a solenoid with an analyzing direction perpendicular to the incident neutron polarization direction. The method was tested with polarized neutron beams and a spin rotator placed inside a μ-metal shield just upstream of the analyzer. No cryogenic superconducting shields or additional neutron spin manipulations are needed. With a counting detector, we experimentally show that the angular resolution δθ=1/(PnAN) rad is only determined by the counting statistics for the total counts N and the product of the neutron polarization Pn and the analyzing power A. With a high-flux neutron beam, 10−6 rad angular sensitivity is feasible within a day. This simple, classical-quantum-limited transverse polarization analysis scheme may reduce the overall complexity of experimental implementation for applications requiring sensitive neutron polarimetry and improve the precision in fundamental science studies and polarized neutron imaging.

Optimizing magnetically shielded solenoids

Review of Scientific Instruments

Hassan

Erwin

et al. 2020

An important consideration when designing a magnetostatic cavity for various applications is to maximize the ratio of the volume of field homogeneity to the overall size of the cavity. We report a design of a magnetically shielded solenoid that significantly improves the transverse field gradient averaged over a volume of 1000 cm3 by placing compensation coils around the holes in the mu-metal end caps rather than the conventional design in which the compensation coils are placed on the main solenoid. Our application is polarized 3He-based neutron spin filters, and our goal was to minimize the volume-averaged transverse field gradient, thereby the gradient induced relaxation time, over a 3He cell. For solenoids with end cap holes of different sizes, additional improvements in the field gradient were accomplished by introducing non-identical compensation coils centered around the non-identical holes in the end caps. The improved designs have yielded an overall factor of 7 decrease in the gradient in the solenoid, hence a factor of 50 increase in the gradient induced relaxation time of the 3He polarization. The results from both simulation and experiments for the development of several such solenoids are presented. Whereas our focus is on the development of magnetically shielded solenoids for 3He neutron spin filters, the approach can be applied for other applications demanding a high level of field homogeneity over a large volume.