Superconducting magnetic Wollaston prism for neutron spin encoding

Abstract

A magnetic Wollaston prism can spatially split a polarized neutron beam into two beams with different neutron spin states, in a manner analogous to an optical Wollaston prism. Such a Wollaston prism can be used to encode the trajectory of neutrons into the Larmor phase associated with their spin degree of freedom. This encoding can be used for neutron phase-contrast radiography and in spin echo scattering angle measurement (SESAME). In this paper, we show that magnetic Wollaston prisms with highly uniform magnetic fields and low Larmor phase aberration can be constructed to preserve neutron polarization using high temperature superconducting (HTS) materials. The Meissner effect of HTS films is used to confine magnetic fields produced electromagnetically by current-carrying HTS tape wound on suitably shaped soft iron pole pieces. The device is cooled to ~30 K by a closed cycle refrigerator, eliminating the need to replenish liquid cryogens and greatly simplifying operation and maintenance. A HTS film ensures that the magnetic field transition within the prism is sharp, well-defined, and planar due to the Meissner effect. The spin transport efficiency across the device was measured to be ~98.5% independent of neutron wavelength and energizing current. The position-dependent Larmor phase of neutron spins was measured at the NIST Center for Neutron Research facility and found to agree well with detailed simulations. The phase varies linearly with horizontal position, as required, and the neutron beam shows little depolarization. Consequently, the device has advantages over existing devices with similar functionality and provides the capability for a large neutron beam (20 mm × 30 mm) and an increase in length scales accessible to SESAME to beyond 10 μm. With further improvements of the external coupling guide field in the prototype device, a larger neutron beam could be employed.

High resolution neutron Larmor diffraction using superconducting magnetic Wollaston prisms

“…As part of a project to develop neutron spin manipulation devices 11–15 , we have built a Wollaston prism that consists of two triangular shaped regions of opposing magnetic field separated by high-temperature superconducting films. As described by Li et al .…”

“…As described by Li et al . 16–18 , using four such prisms 11 , we proposed a new technique for measuring the linewidths of dispersive phonons, which traditionally also requires tilting of the magnetic field boundaries in a similar way to the LD method described above. However, instead of physical tilting of the field boundaries, we proposed that the tuning process can be achieved by varying the magnetic field configuration in the Wollaston prisms.…”

“…With this electromagnetic tuning, no physical movement of the coils is necessary and a large effective tilting angle can be achieved (up to 85°), higher than currently available. Also, since the regions of homogeneous magnetic field are defined by sharp boundaries provided by the Meissner effect of thin films of YBCO deposited on extremely flat, 0.5 mm-thick, single-crystal sapphire plates, both the transmission and polarization efficiency have been found to be high 11, 15 in these devices.…”

Spin echo small angle neutron scattering using a continuously pumped 3He neutron polarisation analyser

“…Moreover this suggests that improvements in the flipping efficiency would allow a significant increase in echo polarisation. Such improvements may be possible using a new superconducting Wollaston prism that is under development [20], although this would still not recover the total instrumental polarisation based upon the calculated values. …”

“…The instrument is used for SESANS and also the development of polarised neutron instrumentation [17][18][19][20]. The neutrons are produced at a repetition rate of 20 Hz, are thermalized within a 15 cm diameter water reflector, and cooled with a 10 mm thick methane moderator at T≈6 K. The neutrons are transported from the moderator via supermirror guides.…”