3He spin filters for a thermal neutron triple axis spectrometer

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Abstract

We have tested two 3He neutron spin filters (NSF), one for the polarizer and one for the analyzer, in conjunction with a doubly focusing pyrolytic graphite (PG) monochromator on the state-of-the-art BT-7 thermal triple axis spectrometer (TAS) at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). This system will provide significantly better neutronic performance for polarization analysis over a conventional TAS with Heusler crystals. We discuss the scheme for employing NSFs on the TAS instrument, including the 3He cell design, spin-exchange optical pumping (SEOP) of these large 3He cells, and the holding fields on the spectrometer. Using Rb/K hybrid SEOP, we have produced 75% 3He polarization for the 11 cm diameter cells for TAS in less than two days.

Introduction

Polarized neutron scattering (PNS) is a powerful tool that probes magnetic structures and spin dynamics in a wide variety of magnetic materials [1]. It has been successful for many applications with polarizers such as supermirrors and Heusler crystals. However, the severe intensity loss or the inability to cover divergent beams with these polarizers precludes many PNS experiments. Polarized 3He gas, produced by optical pumping, can be used to polarize or analyze neutron beams because of the strong spin dependence of the neutron absorption cross section for 3He gas. Polarized 3He neutron spin fitters (NSF) are of growing interest in the PNS community due to recent significant improvements in their performance [2], [3], [4]. Compared to supermirrors and Heusler crystals, 3He NSFs have two main advantages: (1) they are broadband and can polarize thermal or hot neutrons effectively and (2) they can polarize both large area and large divergence neutron beams.

Heusler crystals used in today's neutron spectrometers typically have thermal neutron reflectivity of 25–30% for the reflected spin state and routinely provide a neutron polarization of 95% [5]. Modernizing thermal triple axis spectrometers (TASs) require higher flux at the sample, higher useful detected count rates, lower background, and faster data acquisition. A large double-focusing pyrolytic graphite (PG) monochromator can provide a dramatic increase in flux at the sample compared to a traditional thermal TAS [6]. Heusler crystals are difficult to assemble and magnetize in the large focusing arrays required for these new methods. Additionally, 3He spin filters have greater versatility in the selection of energy and polarization. In order to utilize these instrumental advances for polarized thermal neutron beams, it is advantageous to use 3He polarizers instead of Heusler crystals.

On the ILL IN20 thermal TAS, a PG monochromator in conjunction with a 3He polarizer outperformed a Heusler crystal monochromator for wavelengths below 0.18 nm, despite a relatively low 3He polarization of 48% [7]. During the last several years, the achievable 3He polarization has been significantly improved for both the spin-exchange optical pumping (SEOP) and metastability-exchange optical pumping (MEOP) methods. For SEOP, using 52 W of spectrally narrowed 795 nm diode laser light, we now routinely polarize 1 l, long relaxation time cells to 75% 3He polarization with a polarizing rate up to 2.4 bar-L/day [8]. Recently, we obtained a 3He polarization of 76% on the Center for Neutron Research NG-1 polarized neutron reflectometer. For the MEOP method, 3He polarization up to 72% in the storage NSF cell has been achieved with typical relaxation times of 150 h [9].

In this paper, we discuss applications of 3He NSFs in conjunction with a doubly focusing PG monochromator for the BT-7 thermal TAS at the NCNR. Using a doubly focusing PG monochromator has allowed us to reach a neutron flux at the sample on BT-7 of 1.8×108cm-2s-1 at 40 meV (0.143 nm) [10]. At a 3He polarization of 75%, the total neutron transmission through a 3He polarizer with a polarizing efficiency of 95% is about 0.23. Hence the polarized neutron flux at the sample will be 4.1×107cm-2s-1, which is about a factor of 18 higher than the decommissioned BT-2 thermal TAS at the NCNR. Combined with another factor of 1.8 gained from a 3He analyzer over a Heusler crystal, the intensity gain in the detector will be a factor of 35.

Section snippets

BT-7 TAS beam geometry

The BT-7 thermal TAS at the NCNR [10] features the choice of either a Cu(220) or PG(002) doubly focusing monochromator, providing a useful continuous incident neutron energy range from 5 to 200 meV. Each doubly focusing monochromator has an active area 20 cm high and 20 cm wide. The distances from the monochromator to the exit of its drum and to the sample are 117 and 208 cm, respectively. Due to the doubly focusing geometry, it would be ideal to locate the 3He NSF close to the sample in order to

3He neutron spin filter design

Employing 3He NSFs requires suitable design of the 3He cells, optical pumping, and holding field. Due to the large size of these cells, we employed mixtures of potassium and rubidium to increase the spin-exchange optical pumping efficiency [8], [11].

3He NSF test on the BT-7 triple axis spectrometer

We performed a test on the BT-7 thermal TAS at a wavelength of 0.236 nm using a 3He polarizer and a 3He analyzer. Besides standard TAS elements [10], there was a 3He polarizer immediately outside the monochromator drum, a 3He analyzer between the sample enclosure and the detector enclosure, a PG filter, and a spin flipper between the sample and the 3He polarizer. A longitudinal guide field (10–15 G) was used to adiabatically rotate the neutron spin from the longitudinal direction to the vertical

Conclusions

We have employed a polarized 3He spin filter to polarize a 10 cm tall thermal neutron beam at 14.7 meV (0.236 nm) and a second polarized 3He spin filter to spin-analyze the scattered beam on the BT-7 thermal TAS at the NCNR. We have constructed a MSS 20 cm in diameter and 25 cm long to fit into available space on the instrument. The solenoid has a reasonably uniform field despite the limited space, yielding a 3He polarization relaxation time of 130 h for the large BT-7 cells. Both 3He polarizer and

Acknowledgments

The authors thank J. Anderson and J. Fuller of the NIST Optical Shop for assistance with cell fabrication. The work at NIST was partially supported by Department of Energy Interagency Agreement no. DE-AI02-00ER45814.

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