- Email: [email protected]
SANSPOL at a pulsed source
ARTICLE IN PRESS
Physica B 397 (2007) 85–87 www.elsevier.com/locate/physb
SANSPOL at a pulsed source Markus Bleuela, Ed Langa, Thomas Kristb, Werner Wagnerc, Jyotsana Lala, a
Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, USA b BENSC, Hahn-Meitner Institut, D 14109 Berlin, Germany c Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
Abstract Neutron polarization has not been implemented successfully on a time-of-flight small angle neutron scattering (TOF-SANS) machine to this date anywhere in the world. Designing a suitable one for the small angle scattering instrument (SASI) at IPNS, and implementing it, is an important first on a pulsed source. To achieve this, the installation of a solid-state supermirror-based polarizer, a gradient field adiabatic spin flipper, and a new collimator package were required. A polarizing solid-state bender without adsorbing layers, designed to transmit one polarized spin state and reflect the other has been purchased from Neutron Optics Berlin (NOB). By placing this package upstream of the collimation only the transmitted spin-state passes through to the sample. The polarization achieved with this technique up to now is 80% for neutrons in a wavelength range of 3–8 A˚ and 67% for larger wavelengths. The polarizer is placed on a linear translator so it can be easily removed from the beam, when regular SANS measurements are desired. The first experimental results from a two-phase CuNiFe alloy sample are reported here. r 2007 Published by Elsevier B.V. PACS: 25.40.Dn; 61.12.Ex; 75.50.Kj Keywords: Small angle neutron scattering; Polarized neutrons; Magnetic scattering
1. Experimental set up SANS instruments at pulsed sources are well known to have one large advantage over their reactor-based counterparts, and that is the ability to collect data over a very large q range within a single measurement. Currently, there are no SANS instruments at a pulsed source which can measure with a polarized beam in transmission utilizing a wide range of incident wavelengths during one measurement. The purpose of this work is to fill this gap in capability. The first considerable challenge to this project is the polarizer. A suitable polarizer for the SASI beam line needs to be small (o.1 m) and capable of polarizing a large band of neutron wavelengths, from 3 to 14 A˚. A new polarizer design produced by NOB met these specifications, by using a solid-state bender in transmission [1]. The Corresponding author. Tel.: +1 630 252 6042; fax: +1 630 252 4163.
E-mail address: [email protected] (J. Lal). 0921-4526/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.physb.2007.02.044
polarizer has a total length of ca. 90 mm. It is built from Si wafers with the dimensions of 0.15 40 85 mm3, coated on one side with polarizing FeCo–Si supermirrors with m ¼ 2. The entrance cross section is 40 40 mm. The wafers are held and bent in a bender made from nonmagnetic materials. A system of permanent magnets surrounds the bender and produces a magnetic field above 500 G to magnetize the supermirrors. The bender reflects the spin up neutrons by 0.4–1.21 such that they are absorbed in the absorbing walls of the collimator, while the spin down neutrons are transmitted through the bender and the collimator. When tested on the polarized reflectometer at IPNS (POSY1) this component produced a very high degree of polarization (490%). When the experiment was conducted at SASI though, this value seemed closer to 80% and lower at higher wavelengths. More testing will be conducted to determine the cause of this performance degradation, though one likely possibility is that the divergence of the
ARTICLE IN PRESS 86
M. Bleuel et al. / Physica B 397 (2007) 85–87
incoming beam is larger than expected. The transmission of the polarizer ranges from 50% to 10% for the wavelength range of 3–14 A˚. The gradient field adiabatic spin flipper [2,3] has been built by M. Bleuel, and has already undergone preliminary
tests [4] showing that the device works well for neutrons in the 1.5–14 A˚ range. Collimation is essential to any small angle instrument. For this design the collimation is under additional restraints: it needs to be compatible with a polarized beam, matched to the divergence of the polarizer so it can successfully filter out the ‘wrong’ spin state, and preferably it should be small so as to fit in the limited space available at the SASI beam line. The compact solidstate collimators designed by NOB fit all of these criteria (Fig. 1). The collimators produced a well-collimated beam with a resolution that slightly exceeded our old collimators. They also had a very reasonable transmission making up for the loss through silicon by having a larger initial acceptance. The only drawback is the increase low-Q background (up to 450-fold more background for Q’s below 0.01). 2. First tests
Fig. 1. This is a layout of the components to be used for polarized-SANS. Furthest upstream is the solid-state polarizer. Following which the vacuum system begins and a glass tube passes through the center of the gradient field flipper. The beam is then transported to the collimation assembly which will not only collimates the beam, but also does the final beam filtering as the neutrons of the ‘wrong’ spin state will not have the proper divergence to pass through the collimators.
The performance of the SANSPOL option [3] at IPNS has been demonstrated on a CuNiFe alloy previously analyzed by unpolarized SANS [5]. When the magnetic moments are fully aligned along an external magnetic field the SANSPOL patterns contain an interference term FN FM which, depending on the sign of the neutron spin (+) or (), is added or subtracted to the squared amplitudes of
Fig. 2. Two-dimensional SANS pattern from the Cu-24 at% Ni-8 at% Fe alloy after annealing at 823 K for 64 h (average precipitate size 25 nm, which prefer to line up /1 0 0S direction which leads to four-fold symmetry in the scattering patterns) [5]. Furthermore b2N ¼ 0.0316 b/(sr at.) 5b2M ¼ 0.127 barn/ (sr at.) i.e. magnetic scattering dominates. I+ polarized neutrons for spins antiparallel (flipper off) and I for spins parallel (flipper on) to the external magnetic field H.
ARTICLE IN PRESS M. Bleuel et al. / Physica B 397 (2007) 85–87
nuclear and magnetic scattering F 2N and F 2M , respectively,
87
Acknowledgment
2
and I þ ðQ; aÞ ¼ F 2N þ ðF 2M 2P F N F M Þ sin a; I ðQ; aÞ ¼ F 2N þ ðF 2M þ 2Ps F N F M Þ sin2 a; a is the angle between the moment direction and the scattering vector Q. In a SANSPOL experiment very pronounced anisotropic signals are observed for the polarization states I and I+ with a dramatic change of the aspect ratios (Fig. 2). The sum signal (I++I)/2 coincides with the 2D pattern obtained with non-polarized neutrons. The difference signal (I+I) shows the expected sin2 a behavior with negligible intensity along the direction of the horizontal applied magnetic field [3]. The first test on a sample was successfully demonstrated at IPNS and efforts are underway to improve performance of all components for the neutron optics.
This work was funded by the US Department of Energy, under contract W-31-109-ENG-38.
References [1] [2] [3] [4]
T. Krist, et al., Appl. Phys. A 74 (2002) 221. A.N. Bazhenov, et al., Nuclr. Instr. and Meth. A 332 (3) (1993) 534. T. Keller, et al., J. Nucl. Instr. A 451 (2000) 474. R. Teller, J. Lal, M. Bleuel, ANL Internal LDRD Report, 2004-236NO. [5] W. Wagner, J. Kohlbrecher, Small-angle neutron scattering, in: Y. Zhu (Ed.), Modern Techniques for Characterizing Magnetic Materials, Kluwer Academic Publishers, 2005 (Chapter 2).
Physica B 397 (2007) 85–87 www.elsevier.com/locate/physb
SANSPOL at a pulsed source Markus Bleuela, Ed Langa, Thomas Kristb, Werner Wagnerc, Jyotsana Lala, a
Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, USA b BENSC, Hahn-Meitner Institut, D 14109 Berlin, Germany c Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
Abstract Neutron polarization has not been implemented successfully on a time-of-flight small angle neutron scattering (TOF-SANS) machine to this date anywhere in the world. Designing a suitable one for the small angle scattering instrument (SASI) at IPNS, and implementing it, is an important first on a pulsed source. To achieve this, the installation of a solid-state supermirror-based polarizer, a gradient field adiabatic spin flipper, and a new collimator package were required. A polarizing solid-state bender without adsorbing layers, designed to transmit one polarized spin state and reflect the other has been purchased from Neutron Optics Berlin (NOB). By placing this package upstream of the collimation only the transmitted spin-state passes through to the sample. The polarization achieved with this technique up to now is 80% for neutrons in a wavelength range of 3–8 A˚ and 67% for larger wavelengths. The polarizer is placed on a linear translator so it can be easily removed from the beam, when regular SANS measurements are desired. The first experimental results from a two-phase CuNiFe alloy sample are reported here. r 2007 Published by Elsevier B.V. PACS: 25.40.Dn; 61.12.Ex; 75.50.Kj Keywords: Small angle neutron scattering; Polarized neutrons; Magnetic scattering
1. Experimental set up SANS instruments at pulsed sources are well known to have one large advantage over their reactor-based counterparts, and that is the ability to collect data over a very large q range within a single measurement. Currently, there are no SANS instruments at a pulsed source which can measure with a polarized beam in transmission utilizing a wide range of incident wavelengths during one measurement. The purpose of this work is to fill this gap in capability. The first considerable challenge to this project is the polarizer. A suitable polarizer for the SASI beam line needs to be small (o.1 m) and capable of polarizing a large band of neutron wavelengths, from 3 to 14 A˚. A new polarizer design produced by NOB met these specifications, by using a solid-state bender in transmission [1]. The Corresponding author. Tel.: +1 630 252 6042; fax: +1 630 252 4163.
E-mail address: [email protected] (J. Lal). 0921-4526/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.physb.2007.02.044
polarizer has a total length of ca. 90 mm. It is built from Si wafers with the dimensions of 0.15 40 85 mm3, coated on one side with polarizing FeCo–Si supermirrors with m ¼ 2. The entrance cross section is 40 40 mm. The wafers are held and bent in a bender made from nonmagnetic materials. A system of permanent magnets surrounds the bender and produces a magnetic field above 500 G to magnetize the supermirrors. The bender reflects the spin up neutrons by 0.4–1.21 such that they are absorbed in the absorbing walls of the collimator, while the spin down neutrons are transmitted through the bender and the collimator. When tested on the polarized reflectometer at IPNS (POSY1) this component produced a very high degree of polarization (490%). When the experiment was conducted at SASI though, this value seemed closer to 80% and lower at higher wavelengths. More testing will be conducted to determine the cause of this performance degradation, though one likely possibility is that the divergence of the
ARTICLE IN PRESS 86
M. Bleuel et al. / Physica B 397 (2007) 85–87
incoming beam is larger than expected. The transmission of the polarizer ranges from 50% to 10% for the wavelength range of 3–14 A˚. The gradient field adiabatic spin flipper [2,3] has been built by M. Bleuel, and has already undergone preliminary
tests [4] showing that the device works well for neutrons in the 1.5–14 A˚ range. Collimation is essential to any small angle instrument. For this design the collimation is under additional restraints: it needs to be compatible with a polarized beam, matched to the divergence of the polarizer so it can successfully filter out the ‘wrong’ spin state, and preferably it should be small so as to fit in the limited space available at the SASI beam line. The compact solidstate collimators designed by NOB fit all of these criteria (Fig. 1). The collimators produced a well-collimated beam with a resolution that slightly exceeded our old collimators. They also had a very reasonable transmission making up for the loss through silicon by having a larger initial acceptance. The only drawback is the increase low-Q background (up to 450-fold more background for Q’s below 0.01). 2. First tests
Fig. 1. This is a layout of the components to be used for polarized-SANS. Furthest upstream is the solid-state polarizer. Following which the vacuum system begins and a glass tube passes through the center of the gradient field flipper. The beam is then transported to the collimation assembly which will not only collimates the beam, but also does the final beam filtering as the neutrons of the ‘wrong’ spin state will not have the proper divergence to pass through the collimators.
The performance of the SANSPOL option [3] at IPNS has been demonstrated on a CuNiFe alloy previously analyzed by unpolarized SANS [5]. When the magnetic moments are fully aligned along an external magnetic field the SANSPOL patterns contain an interference term FN FM which, depending on the sign of the neutron spin (+) or (), is added or subtracted to the squared amplitudes of
Fig. 2. Two-dimensional SANS pattern from the Cu-24 at% Ni-8 at% Fe alloy after annealing at 823 K for 64 h (average precipitate size 25 nm, which prefer to line up /1 0 0S direction which leads to four-fold symmetry in the scattering patterns) [5]. Furthermore b2N ¼ 0.0316 b/(sr at.) 5b2M ¼ 0.127 barn/ (sr at.) i.e. magnetic scattering dominates. I+ polarized neutrons for spins antiparallel (flipper off) and I for spins parallel (flipper on) to the external magnetic field H.
ARTICLE IN PRESS M. Bleuel et al. / Physica B 397 (2007) 85–87
nuclear and magnetic scattering F 2N and F 2M , respectively,
87
Acknowledgment
2
and I þ ðQ; aÞ ¼ F 2N þ ðF 2M 2P F N F M Þ sin a; I ðQ; aÞ ¼ F 2N þ ðF 2M þ 2Ps F N F M Þ sin2 a; a is the angle between the moment direction and the scattering vector Q. In a SANSPOL experiment very pronounced anisotropic signals are observed for the polarization states I and I+ with a dramatic change of the aspect ratios (Fig. 2). The sum signal (I++I)/2 coincides with the 2D pattern obtained with non-polarized neutrons. The difference signal (I+I) shows the expected sin2 a behavior with negligible intensity along the direction of the horizontal applied magnetic field [3]. The first test on a sample was successfully demonstrated at IPNS and efforts are underway to improve performance of all components for the neutron optics.
This work was funded by the US Department of Energy, under contract W-31-109-ENG-38.
References [1] [2] [3] [4]
T. Krist, et al., Appl. Phys. A 74 (2002) 221. A.N. Bazhenov, et al., Nuclr. Instr. and Meth. A 332 (3) (1993) 534. T. Keller, et al., J. Nucl. Instr. A 451 (2000) 474. R. Teller, J. Lal, M. Bleuel, ANL Internal LDRD Report, 2004-236NO. [5] W. Wagner, J. Kohlbrecher, Small-angle neutron scattering, in: Y. Zhu (Ed.), Modern Techniques for Characterizing Magnetic Materials, Kluwer Academic Publishers, 2005 (Chapter 2).