- Email: [email protected]
Polarized primary spectrometer on the LET instrument at ISIS
Physica B: Physics of Condensed Matter xxx (2017) 1–4
Contents lists available at ScienceDirect
Physica B: Physics of Condensed Matter journal homepage: www.elsevier.com/locate/physb
Polarized primary spectrometer on the LET instrument at ISIS J. Košata, G.J. Nilsen * , M. Devonport, R.I. Bewley, D.J. Voneshen, P.J. Galsworthy, D. Raspino, J.R. Stewart ISIS Neutron and Muon Facility, Rutherford Appleton Laboratory, Science and Technology Facilities Council, Didcot OX11 0QX, UK
A R T I C L E
I N F O
Keywords: Neutron spectroscopy Instrumentation Polarized neutrons
A B S T R A C T
The combination of neutron time-of-flight spectroscopy and longitudinal polarization analysis opens up the possibility of simultaneously separating cross-section components over vast regions in (Q, ΔE) space. In this paper, we describe initial results of a project to install longitudinal polarization analysis on the LET cold neutron spectrometer at the ISIS Neutron and Muon Facility; namely, the design and commissioning of the polarizing primary spectrometer. The polarized neutron beam is generated by a supermirror-based transmission device, for which ray tracing simulations indicate a high neutron polarization P ∼ 96% and transmission T ∼ 40%. Despite the short distance between this device and the precession coil flipper, and the consequent large stray magnetic fields on the latter, careful optimization of the field environment yields predictions of excellent flipping performance ∼99%. Both simulation results are confirmed via measurements using a polarizing Cu2 MnAl Heusler crystal as an analyzer.
Over the last two decades, neutron time-of-flight spectroscopy has become the default technique for wide reciprocal-space mapping of excitations in condensed matter systems. In many materials, however, situations arise where the nature of an excitation cannot easily be distinguished, or where excitations arising from different scattering crosssection components overlap. For example, in quasi-elastic neutron scattering (QENS) experiments, scattering from incoherent (single particle) excitations is often partly or fully obscured by that due to coherent (many body) processes. Alternatively, in magnetic materials, scattering from weak longitudinal fluctuations of the magnetic moment can be hidden among much more intense scattering from transverse excitations. Neutron scattering with longitudinal (uniaxial) polarization analysis (LPA), where the scattered neutron polarization is analyzed with respect to the incident polarization direction, provides a solution to both of these problems by permitting the separation of the components of the total cross-section – nuclear and isotope incoherent, nuclear spin-incoherent, and magnetic – as well giving access to the directional components of the magnetism [1,2]. LPA has a long history on single detector instruments, such as triple-axis spectrometers, where it has been applied to systems ranging from quantum magnets like Cu(CDOO)2·4D2 O [3] to unconventional superconductors such as YBa2 Cu3 O7 [4] and BaFe2-x Nix As2 [5]. However, it is often desirable to probe excitations over a broader range in (Q, ΔE) space than can be
conveniently covered on a triple-axis instrument. In these cases, the use of time-of-flight (TOF) spectrometers with large solid angle multidetectors gives simultaneous access to a wide (Q, ΔE) range. After some years of effort dedicated towards the goal of equipping polarization analysis on TOF spectrometers [6–8], experiments are now being successfully undertaken, as demonstrated by two recent studies. Burankova et al. [9] were able to isolate the dynamics due to a protonated functional group from the remainder of the quasi-elastic scattering in an ionic liquid sample, exploiting the large spin-incoherent cross section of hydrogen. More recently, Hong et al. [10] measured magnetic excitations in a quantum-spin-ladder compound and were able to distinguish an amplitude (Higgs) mode from transverse Goldstone modes.1 Despite the great promise shown by these studies, LPA on TOF spectrometers as a technique is far from mature, owing to the technical difficulties inherent in polarizing and analyzing a broad-band beam, as well as the requirement of large solid angle and broad bandwidth for the analyzer. In this paper, we will present our implementation of broad-band incident beam polarization, which is part of a wider project to add polarization analysis to the LET time-of-flight spectrometer at the ISIS Neutron and Muon Facility in Oxfordshire, UK [11,12]. On LET, the polarized beam is obtained using a two-channel supermirror transmission device, for which simulations predict excellent polarization and transmission. Inconveniently, the limited space between the polarizer
* Corresponding author. E-mail address: [email protected] (G.J. Nilsen). 1 A multi-analyzer triple-axis spectrometer was used for this study – most of the challenges in implementing polarization analysis on such an instrument are identical to those faced on a TOF spectrometer. https://doi.org/10.1016/j.physb.2017.11.072 Received 31 August 2017; Received in revised form 22 November 2017; Accepted 24 November 2017 Available online XXX 0921-4526/Crown Copyright © 2017 Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Košata, et al., Polarized primary spectrometer on the LET instrument at ISIS, Physica B: Physics of Condensed Matter (2017), https://doi.org/10.1016/j.physb.2017.11.072
J. Košata et al.
Physica B: Physics of Condensed Matter xxx (2017) 1–4
and the flipper means that the strong magnetic field required to saturate the polarizing supermirrors causes severe stray fields at the flipper position. To compensate these fields, the placement of permanent magnets around the precession coil spin flipper is optimized using the simulated annealing algorithm, resulting in a predicted efficiency close to 100%. Both sets of simulations are verified using an experimental setup consisting of the polarizer, two flippers, and a magnetized single crystal Cu2 MnAl (Heusler) analyzer. We begin by summarizing the design and construction of the polarizer. Both this and the flipper are situated ∼20 m from the moderator, and ∼2.5 m from the sample position. The upstream guide consists of a straight section of dimensions 64 × 40 mm2 (height, width) with supermirror coatings m = 10 𝜃c ∕𝜆 = 3, 2 (top/bottom, sides). In this section, the beam traverses four disc choppers, which select multiple incident wavelengths from the cold neutron spectrum provided by the coupled H2 moderator. In unpolarized mode, the beam then enters a linearly converging section of length 2 m with m = 4, 2 (top/bottom, sides) coatings, beyond which the guide is straight and coated with m = 4, 4 (top/bottom, sides). This section includes the final chopper, defining the energy resolution. The first 0.88 m of the converging section is removable, with the guide mounted on a set of rails to facilitate rapid changeover. In polarized mode, it is here that the polarizer and flipper are installed. In view of the short distance available (0.88 m) to polarize and flip the beam, as well as the requirement of no beam deviation, the options considered for the polarizer were so-called “V”-cavity and “S”-bender supermirror-based devices. The latter option was judged to yield a considerably lower transmission than the former, offset by only a modest improvement in polarization, and was therefore not considered further. The “V”-cavity was designed with two channels and a high m = 5 coating to cover a wavelength range of between 3 and 9 Å and with a length of 780 mm, leaving a space of ∼100 mm for the flipper. A section of (m = 2) supermirror was included between the channels to suppress spurious reflections from the back surfaces of the double-coated mirror. The top/bottom and sides of the guide body were coated in the same supermirror m values as the unpolarized section. Ray-tracing simulations using the McStas package indicated that the expected performance of the device was excellent, with a flat P ∼ 96% and T ∼ 40% (figure-ofmerit P2 T ∼ 0.37) across the operating wavelength range. The polarizer was constructed by Swiss Neutronics, using Ni/Ti and Fe/Si supermirrors as the non-polarizing and polarizing supermirror coatings, respectively, and yoked Nd2 Fe14 B magnets to generate the saturating field for the polarizing coatings. The device is shown in position on LET in Fig. 1. Measurements to verify the polarizer transmission were made using a time-of-flight position-sensitive detector at the sample position. The counts integrated in the wavelength range 5 Å to 9 Å for a collection time of 2 s are shown in Fig. 2(a). The beam footprint is similar to that measured using unpolarized neutrons, and the transmission, obtained by dividing the two measurements, agrees well with that predicted by the McStas simulation [Fig. 2(b)]. Using this result and the flux in unpolarized mode, we estimate a polarized flux Φ = 4.4 × 104 ncm−2 s−1 at 𝜆 = 5 Å (Ei = 3.3 meV) and 2% energy resolution. Although the transmission of the polarizer is close to simulations, the large stray field produced by its magnetic field environment presents a considerable challenge for the flipper, situated <100 mm from the polarizer exit. Very few designs both fit in the allotted space and are capable of functioning in this stray field. Among these, the technically most straightforward to implement is the precession coil or “Mezei” flipper [13]. This consists of two flat solenoids, both wound perpendicular to the beam and to each other. The role of the vertical coil is to compensate the major (vertical) component of the guide field, while the horizontal coil creates an orthogonal field, about which the neutron precesses by an angle 𝜋 on passing through the device. While this design is relatively sensitive to stray magnetic fields, it was possible to eliminate the effect of these by careful placement of permanent
Fig. 1. (a) A top view of the area around the polarizer and flipper on LET. The grey lines indicate the rails on which the polarizer is rolled into position. (b) The assembled LET polarizer and flipper, showing all components mentioned above and in the text.
Fig. 2. (a) PSD counts at the sample position integrated between 5 Å and 10 Å. (b) The polarizer transmission T integrated over the entire beam spot compared with McStas simulation. The maximum deviation between the two above 3 Å is around 10%.
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magnets around the flipper. The magnet positions were found using a simulated annealing algorithm written for the Radia software within Mathematica [14]. A figure of merit was calculated by considering the flipping efficiency effects of stray field components parallel and perpendicular to the guide field and the field variation between the opposing flipper faces. Before optimization, this gave a predicted flipping efficiency of 87% at 9 Å, increasing to 98% at 3 Å, with the loss mainly attributable to flipping field inhomogeneity. The optimized configuration consists of a permeable soft-iron box around the flipper and a permeable soft-iron screen with an aperture on the face of the polarizer. An arrangement of permanent magnets situated near the face of the flipper cancels the flipping field inhomogeneities while providing a strong (∼70 G) vertical guide field. This solution, shown in Fig. 3 gives a predicted flipping efficiency of 99.5% at 9 Å and 99.9% at 3 Å. Importantly, opting for very thin coils (3 mm) also helped to mitigate the efficiency loss - a thinner flipper uses higher flipping fields, which makes it less susceptible to external inhomogeneities. In the present case, a thicker precession coil gives a predicted polarization loss of ∼1% per extra millimeter at 9 Å. The flipper was wound using Cu wire on an Al frame, and the magnetic field environment was constructed from soft-iron and Nd2 Fe14 B permanent magnets. The field profile at the flipper was verified by measurements with an xyz Hall probe and Gaussmeter (Lakeshore 460 3-axis). The probe was mounted on an XYZ translation stage, and scans were made with the flipper in situ. The resulting maps of the critical field component (parallel to the precession field) taken at ∼40 mm from the flipper face and ∼40 mm from the polarizer exit are shown in Fig. 3. The agreement between measurement and simulations is excellent.
To transport the polarization adiabatically from the flipper to the boundary of the sample chamber, guide fields are installed around the final 2 m of guide. The guide fields consist of soft iron boxes with Nd2 Fe14 B permanent magnets placed at regular intervals. There is a small gap where the beam passes through the resolution chopper. The adiabaticity parameter 𝛼 = 𝜔L ∕𝜔B , where 𝜔L is the Larmor frequency and 𝜔B is the angular rotation frequency of the magnetic field, is shown over the path between the exit of the flipper and the outer edge of the sample chamber in Fig. 4(a). Values in excess of 30 are found throughout for 3 Å neutrons, indicating good transport of polarization for the whole 𝜆 range. This is confirmed by a numerical simulation of spin evolution through the setup, which showed a <0.02% polarization loss across the beam area [Fig. 4(b)]. In order to verify the polarizing performance of the polarizer and flipper, a temporary polarizing secondary spectrometer was constructed. This consisted of a set of electromagnetic coils, a second precession coil flipper (with accompanying magnetic field environment) just beyond the entrance to the sample area, and a magnetized Cu2 MnAl Heusler alloy crystal analyzer. From the four experimental intensities accessible in this configuration, it is possible to unambiguously extract the flipper efficiencies, as well as the product of the polarizer and analyzer efficiencies, pa [15,16]. To isolate the polarizer efficiency p, it is therefore also necessary to independently determine the analyzer efficiency. The Cu2 MnAl crystal was thus measured on the Larmor instrument, also at ISIS, using a polarizer and flipper of known efficiency. The spin flip and non-spin-flip intensities were measured for a broad range of 𝜆 using a white beam. Integrating over the entire polarizing (111) Bragg peak, the analyzer efficiency was found to be broadly indepen-
Fig. 3. The optimized flipper setup (top). The lower panels show the simulated and measured magnetic field component along the precession field direction at ∼40 mm from the flipper face, as indicated by the shaded rectangle in the upper panel. The flipper was off during the measurement.
Fig. 4. Simulated 𝛼∕𝜆[Å] for a neutron trajectory in the beam cross-section corner - dips correspond to gaps between the soft iron guides, with the largest dip appearing due to the gap at the resolution chopper. (b) Simulated polarization loss due to the guide field at 3 Å. 3
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Physica B: Physics of Condensed Matter xxx (2017) 1–4
points are available) and ∼96%, again close to simulations [Fig. 5(b)]. This quantity should be assumed to have a slightly larger error bar than plotted, due to the systematic error inherent to measuring the analyzer efficiency on a different instrument. The combination of the high flipper and polarizer efficiencies means that flipping ratios in excess of 20 should be accessible for a broad wavelength range on the completed instrument. To conclude, we have shown the design and realization of a polarized primary spectrometer on the LET instrument at ISIS. This consists of a transmission-based supermirror polarizer, a precession coil flipper, and magnetic guide fields leading up to the sample tank. The simulated performance of all components was confirmed by measurements using a PSD at the sample position, as well as a two-flipper setup with a Cu2 MnAl crystal as the analyzer. The next part of the project will deal with the polarized secondary spectrometer, which will be based on a wide-angle 3 He analyzer [12]. References [1] R.M. Moon, T. Riste, W.C. Koehler, Polarization analysis of thermal-neutron scattering, Phys. Rev. 181 (2) (1969) 920–931, https://doi.org/10.1103/PhysRev. 181.920. [2] O. Schärpf, H. Capellmann, The XYZ-difference method with polarized neutrons and the separation of coherent, spin incoherent and magnetic scattering cross sections in a multidetector, Phys. Stat. Sol. A135 (1993) 359–379, https://doi.org/ 10.1002/pssa.2211350204. [3] B. Dalla Piazza, et al., Fractional excitations in the square-lattice quantum antiferromagnet, Nat. Phys. 11 (1) (2015) 62–68, https://doi.org/10.1038/ nphys3172. [4] H.A. Mook, M. Yethiraj, G. Aeppli, T.E. Mason, T. Armstrong, Polarized neutron determination of the magnetic excitations in YBa2 Cu3 O7 , Phys. Rev. Lett. 70 (1993) 3490–3493, https://doi.org/10.1103/PhysRevLett.70.3490. [5] H. Luo, M. Wang, C. Zhang, X. Lu, L.-P. Regnault, R. Zhang, S. Li, J. Hu, P. Dai, Spin excitation anisotropy as a probe of orbital ordering in the paramagnetic tetragonal phase of superconducting BaFe1.904 Ni0.096 As2 , Phys. Rev. Lett. 111 (2013) 107006, https://doi.org/10.1103/PhysRevLett.111.107006. [6] J.R. Stewart, K.H. Andersen, E. Babcock, C.D. Frost, A. Hiess, D. Jullien, J.A. Stride, PASTIS : an insert for polarization analysis studies on a thermal inelastic spectrometer, 386 (2006) 1142–1145, https://doi.org/10.1016/j.physb.2006.05. 393. [7] T. Yokoo, K. Ohoyama, S. Itoh, J. Suzuki, M. Nanbu, N. Kaneko, K. Iwasa, T.J. Sato, H. Kimura, M. Ohkawara, Construction of polarized inelastic neutron spectrometer in J-PARC, J. Phys. Conf. Ser. 502 (1) (2014), https://doi.org/10. 1088/1742-6596/502/1/012046. [8] J. Voigt, H. Soltner, E. Babcock, R.J. Aldus, Z. Salhi, R.R. Gainov, T. Bruckel, Polarization analysis for the thermal chopper spectrometer TOPAS, in: EPJ Web of Conferences, vol. 83, 2015, https://doi.org/10.1051/epjconf/20158303016. [9] T. Burankova, R. Hempelmann, A.R. Wildes, Collective ion diffusion and localized single particle dynamics in pyridinium-based ionic liquids, J. Phys. Chem. B 118 (2014) 14452–14460, https://doi.org/10.1021/jp5092416. [10] T. Hong, M. Matsumoto, Y. Qiu, W. Chen, T.R. Gentile, S. Watson, F.F. Awwadi, M.M. Turnbull, S.E. Dissanayake, H. Agrawal, R. Toft-Petersen, B. Klemke, K. Coester, K.P. Schmidt, D.A. Tennant, Higgs amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point, Nat. Phys. 13 (7) (2017) 638–642, https://doi.org/10.1038/nphys4182. [11] R.I. Bewley, J.W. Taylor, S.M. Bennington, LET, a cold neutron multi-disk chopper spectrometer at ISIS, Nucl. Instrum. Methods Phys. Res. A 637 (1) (2011) 128–134, https://doi.org/10.1016/j.nima.2011.01.173. [12] G.J. Nilsen, J. Kosata, M. Devonport, P. Galsworthy, R.I. Bewley, D.J. Voneshen, R. Dalgliesh, J.R. Stewart, Polarisation analysis on the let time-of-flight spectrometer, J. Phys. Conf. Ser. 862 (2017), https://doi.org/10.1088/1742-6596/862/1/ 012019. [13] F. Mezei, Neutron spin echo: a new concept in polarized thermal neutron techniques, Z. Phys. 255 (1972) 146–160, https://doi.org/10.1007/BF01394523. [14] O. Chubar, P. Elleaume, J. Chavanne, A three-dimensional magnetostatics computer code for insertion devices, J. Synchrotron Radiat. 5 (Pt 3) (1998) 481–484, https://doi.org/10.1107/S0909049597013502. [15] A.R. Wildes, The polarizer-analyzer correction problem in neutron polarization analysis experiments, Rev. Sci. Instrum. 70 (1999) 4241–4245, https://doi.org/10. 1063/1.1150060. [16] A. Wildes, Scientific reviews : neutron polarization analysis corrections made easy, Neutron News. 17 (July 2006) (2016) 17, https://doi.org/10.1080/ 10448630600668738.
Fig. 5. (a) Analyzing efficiency of the Cu2 MnAl Heusler crystal, extracted from a measurement using a polarizer and flipper of known efficiency. The sharp dips are due to multiple Bragg scattering. (b) The polarizer (blue) and flipper (orange) efficiencies calculated according to the method described in the text. Simulations of both quantities are shown by dashed lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
dent of wavelength and ∼94%, although it does show several dips, most notably at 3.8 Å and 4.1 Å [Fig. 5]. These were ascribed to multiple scattering from the (200) and (220) reflections, which predominantly scatter the opposite spin state to the (111) reflection. For the polarized test measurements, LET was operated in white beam mode, with all the choppers stationary. To prevent issues with detector saturation, the first chopper was closed so as to only admit ∼5% of the total flux, and a 3 mm pinhole was placed approximately 30 cm from the Cu2 MnAl crystal. Since this was approximately 2 m from the flipper, the measurements represent its average performance over the beam area. The Cu2 MnAl (111) Bragg peak was then measured at several angles for the four flipper states mentioned above. The resulting efficiency of the flipper adjacent to the polarizer is shown in Fig. 5(b). The performance is again excellent, and agrees well with simulations for the optimal permanent magnet placement. Dividing pa by the analyzer efficiency extracted from the Larmor measurement, the polarizer efficiency was furthermore found to be flat (though few experimental
4
Contents lists available at ScienceDirect
Physica B: Physics of Condensed Matter journal homepage: www.elsevier.com/locate/physb
Polarized primary spectrometer on the LET instrument at ISIS J. Košata, G.J. Nilsen * , M. Devonport, R.I. Bewley, D.J. Voneshen, P.J. Galsworthy, D. Raspino, J.R. Stewart ISIS Neutron and Muon Facility, Rutherford Appleton Laboratory, Science and Technology Facilities Council, Didcot OX11 0QX, UK
A R T I C L E
I N F O
Keywords: Neutron spectroscopy Instrumentation Polarized neutrons
A B S T R A C T
The combination of neutron time-of-flight spectroscopy and longitudinal polarization analysis opens up the possibility of simultaneously separating cross-section components over vast regions in (Q, ΔE) space. In this paper, we describe initial results of a project to install longitudinal polarization analysis on the LET cold neutron spectrometer at the ISIS Neutron and Muon Facility; namely, the design and commissioning of the polarizing primary spectrometer. The polarized neutron beam is generated by a supermirror-based transmission device, for which ray tracing simulations indicate a high neutron polarization P ∼ 96% and transmission T ∼ 40%. Despite the short distance between this device and the precession coil flipper, and the consequent large stray magnetic fields on the latter, careful optimization of the field environment yields predictions of excellent flipping performance ∼99%. Both simulation results are confirmed via measurements using a polarizing Cu2 MnAl Heusler crystal as an analyzer.
Over the last two decades, neutron time-of-flight spectroscopy has become the default technique for wide reciprocal-space mapping of excitations in condensed matter systems. In many materials, however, situations arise where the nature of an excitation cannot easily be distinguished, or where excitations arising from different scattering crosssection components overlap. For example, in quasi-elastic neutron scattering (QENS) experiments, scattering from incoherent (single particle) excitations is often partly or fully obscured by that due to coherent (many body) processes. Alternatively, in magnetic materials, scattering from weak longitudinal fluctuations of the magnetic moment can be hidden among much more intense scattering from transverse excitations. Neutron scattering with longitudinal (uniaxial) polarization analysis (LPA), where the scattered neutron polarization is analyzed with respect to the incident polarization direction, provides a solution to both of these problems by permitting the separation of the components of the total cross-section – nuclear and isotope incoherent, nuclear spin-incoherent, and magnetic – as well giving access to the directional components of the magnetism [1,2]. LPA has a long history on single detector instruments, such as triple-axis spectrometers, where it has been applied to systems ranging from quantum magnets like Cu(CDOO)2·4D2 O [3] to unconventional superconductors such as YBa2 Cu3 O7 [4] and BaFe2-x Nix As2 [5]. However, it is often desirable to probe excitations over a broader range in (Q, ΔE) space than can be
conveniently covered on a triple-axis instrument. In these cases, the use of time-of-flight (TOF) spectrometers with large solid angle multidetectors gives simultaneous access to a wide (Q, ΔE) range. After some years of effort dedicated towards the goal of equipping polarization analysis on TOF spectrometers [6–8], experiments are now being successfully undertaken, as demonstrated by two recent studies. Burankova et al. [9] were able to isolate the dynamics due to a protonated functional group from the remainder of the quasi-elastic scattering in an ionic liquid sample, exploiting the large spin-incoherent cross section of hydrogen. More recently, Hong et al. [10] measured magnetic excitations in a quantum-spin-ladder compound and were able to distinguish an amplitude (Higgs) mode from transverse Goldstone modes.1 Despite the great promise shown by these studies, LPA on TOF spectrometers as a technique is far from mature, owing to the technical difficulties inherent in polarizing and analyzing a broad-band beam, as well as the requirement of large solid angle and broad bandwidth for the analyzer. In this paper, we will present our implementation of broad-band incident beam polarization, which is part of a wider project to add polarization analysis to the LET time-of-flight spectrometer at the ISIS Neutron and Muon Facility in Oxfordshire, UK [11,12]. On LET, the polarized beam is obtained using a two-channel supermirror transmission device, for which simulations predict excellent polarization and transmission. Inconveniently, the limited space between the polarizer
* Corresponding author. E-mail address: [email protected] (G.J. Nilsen). 1 A multi-analyzer triple-axis spectrometer was used for this study – most of the challenges in implementing polarization analysis on such an instrument are identical to those faced on a TOF spectrometer. https://doi.org/10.1016/j.physb.2017.11.072 Received 31 August 2017; Received in revised form 22 November 2017; Accepted 24 November 2017 Available online XXX 0921-4526/Crown Copyright © 2017 Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Košata, et al., Polarized primary spectrometer on the LET instrument at ISIS, Physica B: Physics of Condensed Matter (2017), https://doi.org/10.1016/j.physb.2017.11.072
J. Košata et al.
Physica B: Physics of Condensed Matter xxx (2017) 1–4
and the flipper means that the strong magnetic field required to saturate the polarizing supermirrors causes severe stray fields at the flipper position. To compensate these fields, the placement of permanent magnets around the precession coil spin flipper is optimized using the simulated annealing algorithm, resulting in a predicted efficiency close to 100%. Both sets of simulations are verified using an experimental setup consisting of the polarizer, two flippers, and a magnetized single crystal Cu2 MnAl (Heusler) analyzer. We begin by summarizing the design and construction of the polarizer. Both this and the flipper are situated ∼20 m from the moderator, and ∼2.5 m from the sample position. The upstream guide consists of a straight section of dimensions 64 × 40 mm2 (height, width) with supermirror coatings m = 10 𝜃c ∕𝜆 = 3, 2 (top/bottom, sides). In this section, the beam traverses four disc choppers, which select multiple incident wavelengths from the cold neutron spectrum provided by the coupled H2 moderator. In unpolarized mode, the beam then enters a linearly converging section of length 2 m with m = 4, 2 (top/bottom, sides) coatings, beyond which the guide is straight and coated with m = 4, 4 (top/bottom, sides). This section includes the final chopper, defining the energy resolution. The first 0.88 m of the converging section is removable, with the guide mounted on a set of rails to facilitate rapid changeover. In polarized mode, it is here that the polarizer and flipper are installed. In view of the short distance available (0.88 m) to polarize and flip the beam, as well as the requirement of no beam deviation, the options considered for the polarizer were so-called “V”-cavity and “S”-bender supermirror-based devices. The latter option was judged to yield a considerably lower transmission than the former, offset by only a modest improvement in polarization, and was therefore not considered further. The “V”-cavity was designed with two channels and a high m = 5 coating to cover a wavelength range of between 3 and 9 Å and with a length of 780 mm, leaving a space of ∼100 mm for the flipper. A section of (m = 2) supermirror was included between the channels to suppress spurious reflections from the back surfaces of the double-coated mirror. The top/bottom and sides of the guide body were coated in the same supermirror m values as the unpolarized section. Ray-tracing simulations using the McStas package indicated that the expected performance of the device was excellent, with a flat P ∼ 96% and T ∼ 40% (figure-ofmerit P2 T ∼ 0.37) across the operating wavelength range. The polarizer was constructed by Swiss Neutronics, using Ni/Ti and Fe/Si supermirrors as the non-polarizing and polarizing supermirror coatings, respectively, and yoked Nd2 Fe14 B magnets to generate the saturating field for the polarizing coatings. The device is shown in position on LET in Fig. 1. Measurements to verify the polarizer transmission were made using a time-of-flight position-sensitive detector at the sample position. The counts integrated in the wavelength range 5 Å to 9 Å for a collection time of 2 s are shown in Fig. 2(a). The beam footprint is similar to that measured using unpolarized neutrons, and the transmission, obtained by dividing the two measurements, agrees well with that predicted by the McStas simulation [Fig. 2(b)]. Using this result and the flux in unpolarized mode, we estimate a polarized flux Φ = 4.4 × 104 ncm−2 s−1 at 𝜆 = 5 Å (Ei = 3.3 meV) and 2% energy resolution. Although the transmission of the polarizer is close to simulations, the large stray field produced by its magnetic field environment presents a considerable challenge for the flipper, situated <100 mm from the polarizer exit. Very few designs both fit in the allotted space and are capable of functioning in this stray field. Among these, the technically most straightforward to implement is the precession coil or “Mezei” flipper [13]. This consists of two flat solenoids, both wound perpendicular to the beam and to each other. The role of the vertical coil is to compensate the major (vertical) component of the guide field, while the horizontal coil creates an orthogonal field, about which the neutron precesses by an angle 𝜋 on passing through the device. While this design is relatively sensitive to stray magnetic fields, it was possible to eliminate the effect of these by careful placement of permanent
Fig. 1. (a) A top view of the area around the polarizer and flipper on LET. The grey lines indicate the rails on which the polarizer is rolled into position. (b) The assembled LET polarizer and flipper, showing all components mentioned above and in the text.
Fig. 2. (a) PSD counts at the sample position integrated between 5 Å and 10 Å. (b) The polarizer transmission T integrated over the entire beam spot compared with McStas simulation. The maximum deviation between the two above 3 Å is around 10%.
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magnets around the flipper. The magnet positions were found using a simulated annealing algorithm written for the Radia software within Mathematica [14]. A figure of merit was calculated by considering the flipping efficiency effects of stray field components parallel and perpendicular to the guide field and the field variation between the opposing flipper faces. Before optimization, this gave a predicted flipping efficiency of 87% at 9 Å, increasing to 98% at 3 Å, with the loss mainly attributable to flipping field inhomogeneity. The optimized configuration consists of a permeable soft-iron box around the flipper and a permeable soft-iron screen with an aperture on the face of the polarizer. An arrangement of permanent magnets situated near the face of the flipper cancels the flipping field inhomogeneities while providing a strong (∼70 G) vertical guide field. This solution, shown in Fig. 3 gives a predicted flipping efficiency of 99.5% at 9 Å and 99.9% at 3 Å. Importantly, opting for very thin coils (3 mm) also helped to mitigate the efficiency loss - a thinner flipper uses higher flipping fields, which makes it less susceptible to external inhomogeneities. In the present case, a thicker precession coil gives a predicted polarization loss of ∼1% per extra millimeter at 9 Å. The flipper was wound using Cu wire on an Al frame, and the magnetic field environment was constructed from soft-iron and Nd2 Fe14 B permanent magnets. The field profile at the flipper was verified by measurements with an xyz Hall probe and Gaussmeter (Lakeshore 460 3-axis). The probe was mounted on an XYZ translation stage, and scans were made with the flipper in situ. The resulting maps of the critical field component (parallel to the precession field) taken at ∼40 mm from the flipper face and ∼40 mm from the polarizer exit are shown in Fig. 3. The agreement between measurement and simulations is excellent.
To transport the polarization adiabatically from the flipper to the boundary of the sample chamber, guide fields are installed around the final 2 m of guide. The guide fields consist of soft iron boxes with Nd2 Fe14 B permanent magnets placed at regular intervals. There is a small gap where the beam passes through the resolution chopper. The adiabaticity parameter 𝛼 = 𝜔L ∕𝜔B , where 𝜔L is the Larmor frequency and 𝜔B is the angular rotation frequency of the magnetic field, is shown over the path between the exit of the flipper and the outer edge of the sample chamber in Fig. 4(a). Values in excess of 30 are found throughout for 3 Å neutrons, indicating good transport of polarization for the whole 𝜆 range. This is confirmed by a numerical simulation of spin evolution through the setup, which showed a <0.02% polarization loss across the beam area [Fig. 4(b)]. In order to verify the polarizing performance of the polarizer and flipper, a temporary polarizing secondary spectrometer was constructed. This consisted of a set of electromagnetic coils, a second precession coil flipper (with accompanying magnetic field environment) just beyond the entrance to the sample area, and a magnetized Cu2 MnAl Heusler alloy crystal analyzer. From the four experimental intensities accessible in this configuration, it is possible to unambiguously extract the flipper efficiencies, as well as the product of the polarizer and analyzer efficiencies, pa [15,16]. To isolate the polarizer efficiency p, it is therefore also necessary to independently determine the analyzer efficiency. The Cu2 MnAl crystal was thus measured on the Larmor instrument, also at ISIS, using a polarizer and flipper of known efficiency. The spin flip and non-spin-flip intensities were measured for a broad range of 𝜆 using a white beam. Integrating over the entire polarizing (111) Bragg peak, the analyzer efficiency was found to be broadly indepen-
Fig. 3. The optimized flipper setup (top). The lower panels show the simulated and measured magnetic field component along the precession field direction at ∼40 mm from the flipper face, as indicated by the shaded rectangle in the upper panel. The flipper was off during the measurement.
Fig. 4. Simulated 𝛼∕𝜆[Å] for a neutron trajectory in the beam cross-section corner - dips correspond to gaps between the soft iron guides, with the largest dip appearing due to the gap at the resolution chopper. (b) Simulated polarization loss due to the guide field at 3 Å. 3
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points are available) and ∼96%, again close to simulations [Fig. 5(b)]. This quantity should be assumed to have a slightly larger error bar than plotted, due to the systematic error inherent to measuring the analyzer efficiency on a different instrument. The combination of the high flipper and polarizer efficiencies means that flipping ratios in excess of 20 should be accessible for a broad wavelength range on the completed instrument. To conclude, we have shown the design and realization of a polarized primary spectrometer on the LET instrument at ISIS. This consists of a transmission-based supermirror polarizer, a precession coil flipper, and magnetic guide fields leading up to the sample tank. The simulated performance of all components was confirmed by measurements using a PSD at the sample position, as well as a two-flipper setup with a Cu2 MnAl crystal as the analyzer. The next part of the project will deal with the polarized secondary spectrometer, which will be based on a wide-angle 3 He analyzer [12]. References [1] R.M. Moon, T. Riste, W.C. Koehler, Polarization analysis of thermal-neutron scattering, Phys. Rev. 181 (2) (1969) 920–931, https://doi.org/10.1103/PhysRev. 181.920. [2] O. Schärpf, H. Capellmann, The XYZ-difference method with polarized neutrons and the separation of coherent, spin incoherent and magnetic scattering cross sections in a multidetector, Phys. Stat. Sol. A135 (1993) 359–379, https://doi.org/ 10.1002/pssa.2211350204. [3] B. Dalla Piazza, et al., Fractional excitations in the square-lattice quantum antiferromagnet, Nat. Phys. 11 (1) (2015) 62–68, https://doi.org/10.1038/ nphys3172. [4] H.A. Mook, M. Yethiraj, G. Aeppli, T.E. Mason, T. Armstrong, Polarized neutron determination of the magnetic excitations in YBa2 Cu3 O7 , Phys. Rev. Lett. 70 (1993) 3490–3493, https://doi.org/10.1103/PhysRevLett.70.3490. [5] H. Luo, M. Wang, C. Zhang, X. Lu, L.-P. Regnault, R. Zhang, S. Li, J. Hu, P. Dai, Spin excitation anisotropy as a probe of orbital ordering in the paramagnetic tetragonal phase of superconducting BaFe1.904 Ni0.096 As2 , Phys. Rev. Lett. 111 (2013) 107006, https://doi.org/10.1103/PhysRevLett.111.107006. [6] J.R. Stewart, K.H. Andersen, E. Babcock, C.D. Frost, A. Hiess, D. Jullien, J.A. Stride, PASTIS : an insert for polarization analysis studies on a thermal inelastic spectrometer, 386 (2006) 1142–1145, https://doi.org/10.1016/j.physb.2006.05. 393. [7] T. Yokoo, K. Ohoyama, S. Itoh, J. Suzuki, M. Nanbu, N. Kaneko, K. Iwasa, T.J. Sato, H. Kimura, M. Ohkawara, Construction of polarized inelastic neutron spectrometer in J-PARC, J. Phys. Conf. Ser. 502 (1) (2014), https://doi.org/10. 1088/1742-6596/502/1/012046. [8] J. Voigt, H. Soltner, E. Babcock, R.J. Aldus, Z. Salhi, R.R. Gainov, T. Bruckel, Polarization analysis for the thermal chopper spectrometer TOPAS, in: EPJ Web of Conferences, vol. 83, 2015, https://doi.org/10.1051/epjconf/20158303016. [9] T. Burankova, R. Hempelmann, A.R. Wildes, Collective ion diffusion and localized single particle dynamics in pyridinium-based ionic liquids, J. Phys. Chem. B 118 (2014) 14452–14460, https://doi.org/10.1021/jp5092416. [10] T. Hong, M. Matsumoto, Y. Qiu, W. Chen, T.R. Gentile, S. Watson, F.F. Awwadi, M.M. Turnbull, S.E. Dissanayake, H. Agrawal, R. Toft-Petersen, B. Klemke, K. Coester, K.P. Schmidt, D.A. Tennant, Higgs amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point, Nat. Phys. 13 (7) (2017) 638–642, https://doi.org/10.1038/nphys4182. [11] R.I. Bewley, J.W. Taylor, S.M. Bennington, LET, a cold neutron multi-disk chopper spectrometer at ISIS, Nucl. Instrum. Methods Phys. Res. A 637 (1) (2011) 128–134, https://doi.org/10.1016/j.nima.2011.01.173. [12] G.J. Nilsen, J. Kosata, M. Devonport, P. Galsworthy, R.I. Bewley, D.J. Voneshen, R. Dalgliesh, J.R. Stewart, Polarisation analysis on the let time-of-flight spectrometer, J. Phys. Conf. Ser. 862 (2017), https://doi.org/10.1088/1742-6596/862/1/ 012019. [13] F. Mezei, Neutron spin echo: a new concept in polarized thermal neutron techniques, Z. Phys. 255 (1972) 146–160, https://doi.org/10.1007/BF01394523. [14] O. Chubar, P. Elleaume, J. Chavanne, A three-dimensional magnetostatics computer code for insertion devices, J. Synchrotron Radiat. 5 (Pt 3) (1998) 481–484, https://doi.org/10.1107/S0909049597013502. [15] A.R. Wildes, The polarizer-analyzer correction problem in neutron polarization analysis experiments, Rev. Sci. Instrum. 70 (1999) 4241–4245, https://doi.org/10. 1063/1.1150060. [16] A. Wildes, Scientific reviews : neutron polarization analysis corrections made easy, Neutron News. 17 (July 2006) (2016) 17, https://doi.org/10.1080/ 10448630600668738.
Fig. 5. (a) Analyzing efficiency of the Cu2 MnAl Heusler crystal, extracted from a measurement using a polarizer and flipper of known efficiency. The sharp dips are due to multiple Bragg scattering. (b) The polarizer (blue) and flipper (orange) efficiencies calculated according to the method described in the text. Simulations of both quantities are shown by dashed lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
dent of wavelength and ∼94%, although it does show several dips, most notably at 3.8 Å and 4.1 Å [Fig. 5]. These were ascribed to multiple scattering from the (200) and (220) reflections, which predominantly scatter the opposite spin state to the (111) reflection. For the polarized test measurements, LET was operated in white beam mode, with all the choppers stationary. To prevent issues with detector saturation, the first chopper was closed so as to only admit ∼5% of the total flux, and a 3 mm pinhole was placed approximately 30 cm from the Cu2 MnAl crystal. Since this was approximately 2 m from the flipper, the measurements represent its average performance over the beam area. The Cu2 MnAl (111) Bragg peak was then measured at several angles for the four flipper states mentioned above. The resulting efficiency of the flipper adjacent to the polarizer is shown in Fig. 5(b). The performance is again excellent, and agrees well with simulations for the optimal permanent magnet placement. Dividing pa by the analyzer efficiency extracted from the Larmor measurement, the polarizer efficiency was furthermore found to be flat (though few experimental
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