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Characterisation of the polarised neutron beam at the small angle
scattering instrument SANS-I with a polarised proton target
V.K. Aswala, B. van den Brandtb, P. Hautleb,, J. Kohlbrecherc, J.A. Konterb, A. Michelsd,
F.M. Piegsab, J. Stahnc, S. Van Petegemb, O. Zimmere
a
Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai, India
b
Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
c
Laboratory for Neutron Scattering, ETH Zurich & Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
d
Technische Physik, Universität des Saarlandes, DE-66041 Saarbrücken, Germany
e
Technische Universität München, James-Franck Strasse, DE-85748, Germany
Abstract
A transmission neutron polariser (Fe/Si supermirror) has been successfully implemented in the small angle neutron scattering
instrument SANS-I at the SINQ neutron source. The polariser is needed for investigations of magnetic nanostructures as well as for spin
contrast variation techniques relying on the spin-dependent neutron scattering length of polarised nuclei. The V-shaped polariser is
installed in the first section of the collimator system of the SANS instrument and its performance is optimised for neutrons with a
wavelength between 0.5 and 1.0 nm. For a precise polarisation analysis of a beam with selectable incident divergence, such as in SANS
experiments, an opaque spin filter is ideal. We used a solid polarised proton target exploiting the strong spin-dependent neutron
scattering cross-section of hydrogen and determined the neutron beam polarisation to a precision of dp=p0:5% for different
collimations in a broad wavelength band.
r 2008 Elsevier B.V. All rights reserved.
PACS: 61.12.Ex; 29.25.Pj; 76.70.Fz
Keywords: Neutron polarisation analysis; Spin filters; Dynamic nuclear polarisation
1. Introduction
Small angle scattering of a polarised neutron beam is a
unique tool to investigate samples with combined magnetisation, density or compositional fluctuations which are
not accessible to conventional SANS. The polarisation
degree of freedom allows one to discern contributions of
magnetic and nuclear scattering. It has been successfully
applied, e.g. to study interfaces and surfaces of magnetic
nanocrystalline materials [1]. Other fields of application are
in biology and polymer chemistry where the strong spin
dependent nuclear scattering of protons is exploited and a
scattering contrast in hydrogenous samples is created by
dynamic nuclear polarisation (DNP) [2–4].
Corresponding author.
E-mail address: [email protected] (P. Hautle).
A polarisation option has already been implemented at
many SANS instruments. Our set up is similar to the one
installed at HMI/Berlin [5], as both SANS instruments
have a similar design (Fig. 1). In the following we briefly
describe its components and then discuss in some detail the
advantages of using a spin filter to analyse the neutron
beam polarisation. The results of this characterisation
show the excellent performance of the polarising device.
The measurements also demonstrate the high accuracy that
can be readily achieved employing a polarised proton spin
filter as an analysing device.
2. Layout of the polarisation option at SANS-I
The following requirements had to be met by the beam
polariser: (1) the supermirror has to polarise neutrons with
l40:5 nm in transmission, i.e. without changing the
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V.K. Aswal et al. / Nuclear Instruments and Methods in Physics Research A ] (]]]]) ]]]–]]]
Fig. 1. Sketch of the polarisation option at SANS-I. The polarising cavity is located inside a neutron guide (not shown) within the first exchangeable
segment of the collimator.
direction of the beam incident on the SANS instrument, (2)
it should be removable for experiments requiring an
unpolarised beam and (3) placed far away from the sample
position to suppress parasitic scattering from the polariser
itself. We therefore decided to install a polarising cavity in
the first exchangeable collimation section, following a
design of Mezei [5,6]. This reduces to 15 m the longest
collimation distance possible for polarised neutrons. Since
magnets could not be installed within the collimation tube,
a supermirror coating requiring a rather weak magnetization field was favoured. Due to its low absorption and high
magnetic remanence an Fe/Si supermirror (SM) coating
[7,8] on Si substrate was selected. The polariser has a
V-shape to reduce its overall length. Nevertheless, due to
the size of the neutron guide of 50 50 mm2 into which the
polariser had to be fitted, and even using a coating with
m ¼ 2:6, a length of 2 m was still needed to cover the
wavelength range between 0.5 and 1.0 nm. On the 15 m
flight path between polariser and sample a small magnetic
guide field of about 1–2 mT has to be applied to preserve
the neutron polarisation. Such a small field is easily
produced by iron plates magnetized by FeNdB permanent
magnets. The layout of the beamline is sketched in Fig. 1.
The wavelength range of the polarising cavity is limited
by the fixed mean angle of incidence of the beam upon the
supermirror. Since the Fe/Si coating (and the Si wafer
substrate) reflects both spin states below the critical
momentum transfer on pure Si, the transmittance of the
device goes down for long wavelengths. The measured data
shown in Fig. 2 illustrate this effect. On the other hand
both spin states will be transmitted above the critical
momentum transfer of the supermirror coating. This takes
effect first for the neutrons with the largest angle of
incidence. If these are filtered afterwards by a strong
collimation, the polarisation can be kept high. Thus, a
wavelength- and collimation dependence of the efficiency
of the device might be expected and was actually measured
(see Fig. 4).
In principle the supermirror coatings chosen would allow
to switch the polarisation just by inverting the magnetization direction with respect to the applied magnetic field.
However, this results in slightly different transmitted
Fig. 2. Transmittance of the Fe/Si supermirror neutron polariser in
comparison to the piece of neutron guide installed for unpolarised beam
measurements. The measurement error is smaller than 2%.
Fig. 3. Schematic drawing of the spin flipper setup with (1) the gradient
field, (2) the radio-frequency solenoid, (3) the magnetic shielding box and
(4) the magnetic guiding field.
intensities, due to increased small angle scattering in one
state. For this reason an adiabatic radio frequency spin
flipper (see, e.g. Ref. [9]) was installed within a magnetic
shielding at the end of the collimation section, about 1.5 m
in front of the sample position. A schematic drawing is
shown in Fig. 3. A static field transverse to the beam is
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produced by a wedge shaped iron yoke, with magnitude
9 mT in the center of the spin flipper, and with a gradient of
0:3 mT cm1 along the beam. The oscillating field amplitude of 1.8 mT is produced by a solenoid. Its frequency
(260 kHz) is chosen to meet the resonance condition for the
neutron magnetic moment in the center of the gradient
field. The theoretically expected spin-flip probability is
better than 0.999 for neutrons with wavelength above
0.5 nm [10]. In order to shield the spin flipper from external
fields, e.g. stray fields from magnets of the sample
environment, it is placed in a double walled iron box.
Tests performed in combination with a spin analysing
bender indicate that the flipper has indeed an efficiency
very close to 1.
in the present application, a smooth transition to a lower
value that stays practically constant from 200 meV up to
the sub-MeV region. A detailed study comparing the three
methods (supermirrors, 3He, protons) for cold neutrons
has shown the principle advantages of spin filters for high
precision neutron polarimetry [14,15].
3. Neutron spin filters
where the þ and the stand for the two eigenstates of the
neutron spin with respect to the target’s polarisation axis.
The part depending on the neutron spin is the product of
the ‘‘polarisation cross-section’’ sp and the nuclear
polarisation P. s0 denotes the neutron spin-independent
cross-section. The exponential attenuation factors
expðs NdÞ for the respective beam polarisation components lead to a filter analysing power A (defined here as
positive to avoid confusion with signs),
Supermirror type polarisers, which rely on spin-dependent neutron optical reflection, are very popular for
neutron beam polarisation and analysis of scattered
neutrons. They are easy to set up and use, have a high
polarising efficiency for cold and thermal neutrons and
need no maintenance. However, if high accuracy is
requested for polarisation analysis, the spin- and momentum-transfer-dependent analysing efficiency and transmission of such instruments leads to systematic limitations.
Neutron spin filters on the other hand rely on spindependent nuclear scattering, e.g. on polarised protons
[11], or absorption on polarised 3He [12]. They can polarise
the whole range of cold, thermal and hot neutrons. With
the advent of strong pulsed neutron spallation sources this
property becomes crucial since time-of-flight information
can be employed to analyse all wavelengths in a single
experiment. More important for the present purpose, the
analysing power and transmission of a homogeneous spin
filter with parallel entrance and exit windows perpendicular
to the beam are practically independent on the beam
divergence. For a typical value of some 10 mrad the relative
difference of neutron flight paths through the spin filter has
negligible influence on the neutron polarisation analysis.
The spin filter of optically polarised 3He has become very
popular at several neutron research centers. Filter cells
suited to polarise wide beams are available. A comprehensive overview of the current state-of-the-art can be found in
Ref. [13]. The strong energy dependence of the absorption
cross-section makes it however difficult to optimise the
filter thickness if a large neutron energy range is to be used
in the experiment. On the other hand, hydrogenous
materials with dynamically polarised protons offer an
attractive alternative for a broadband spin filter, as realised
and demonstrated already long ago [11]. The singlet crosssection for slow neutrons is more than twenty times higher
than the triplet cross-section; neutrons polarised antiparallel to the protons will thus be much stronger scattered
in 4p solid angle than those with polarisation parallel. The
large scattering cross-section has very little energy dependence below 20 meV and shows, however not of relevance
4. Opaque spin filter polarisation analysis of the SANS-I
neutron beam
A spin filter may be characterised by its thickness d, the
number density N of the polarised nuclei and by a spindependent effective cross-section
s ¼ s0 sp P
A ¼ j tanhðsp PNdÞj.
(1)
(2)
A simple way to determine an unknown neutron beam
polarisation p employs a single analyser with known
analysing power A. One measures a ‘‘flipping ratio’’
Rðp; A; f Þ ¼ N 0 =N 1 of two count rates with an inactive,
respectively, active spin flipper with efficiency f, situated
between polariser and analyser. The accuracy of the
polarisation analysis is limited by the knowledge of A
and f. Whereas it is difficult to characterise the analysing
power A of a single supermirror analyser, for a spin filter
one may choose the value of the opacity x ¼ sp PNd
sufficiently large and thereby approximate an ideal
analyser with A ! 1 (opaque spin filter). As demonstrated
in Ref. [14], already a modest knowledge of the actual filter
parameters results in a precise knowledge of A, the
corresponding uncertainties being related by
dA
2x
dx
¼
.
A
sinhð2xÞ x
(3)
On the other hand, the closer A is to unity, the smaller the
transmission becomes. Hence, using the freedom in the
choice of x, there is a tradeoff between the systematic
uncertainty of A and the time required to reach the
corresponding counting statistical uncertainty.
As pointed out in Ref. [16], the free choice of the
polarisation direction of a dynamically polarised nuclear
target can be employed to measure two different flipping
ratios with the opaque spin filter either polarised parallel or
antiparallel to the polarising direction of the beam. One
ratio is almost insensitive to the flipper efficiency. As a
result one can easily determine f from only four neutron
count rates. A first experimental demonstration of this
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14.5 m was chosen to achieve a precision for the neutron
polarisation measurement of dp=p5 103 within reasonable time. In about 2 h the necessary statistics was
accumulated for one polarisation orientation and for four
different wavelengths. In order to determine the second
count rate appearing in the ratio (4), the direction of the
proton polarisation was reversed by DNP within a few
minutes. The whole procedure was performed for several
collimation lengths and the results are shown in Fig. 4.
5. Conclusion
Fig. 4. Polarisation of the neutron beam at SANS-I at SINQ for different
collimation lengths. For clarity the measurement error is shown for one
data set only. The polariser was designed to reach a polarisation 497%
for l40:5 nm for all collimations. The decay for smaller l is due to the
transmission of both spin states above the critical momentum transfer,
starting with the neutrons with a large angle of incidence. This explains the
collimation dependence.
method was published in Ref. [17]. In the present study, the
spin flipper was not yet available, and therefore we decided
to determine the beam polarisation simply from the ratio of
count rates with an opaque spin filter polarised parallel and
antiparallel to the neutron beam, respectively. This ratio is
given by
R¼
N þ T þ 1 þ pAþ coshðsp Pþ NdÞ 1 þ p
¼
N T 1 pA coshðsp P NdÞ 1 p
(4)
where Aþ and A are the analysing powers for the ‘‘bright’’
and the ‘‘dark’’ state of the opaque analyser, and T þ and
T are corresponding transmission factors. Since the
nuclear polarisation in the two states, Pþ and P , may
be different, Aþ and A may possess different numerical
values. However, they are close to unity in the opaque spin
filter limit. Note that the ratio of transmission factors
appears here as a consequence of not using a neutron spin
flipper. However, choosing Pþ P , which requires only
simple relative NMR measurements during DNP, their
uncertainty does not dominate the final accuracy attained
in the present study.
For the polarisation analysis of the neutron beam of
SANS-I we employed a polarised target system previously
used for a series of spin contrast variation experiments. It is
described in Ref. [18]. The protons contained in frozen 1,2propanediol (with N ¼ 6:7 1022 cm3 ) doped with CrV
complexes (1020 cm3 ) were polarised by dynamic nuclear
polarisation [19] at a temperature of 1 K in a magnetic field
of 3.5 T to about 42%. The polarisation cross-section
of this material was determined previously [17] and is
sp ðlÞX51 1024 cm2 in the wavelength range l40:2 nm.
A beam 1 mm wide and 10 mm high was defined by a
cadmium aperture close to the target, which was a frozen
platelet with cross-section 4:5 16 mm2 . Its length of
The transmission neutron polariser implemented in the
SANS-I instrument at the SINQ neutron source has met
the design goal to polarise neutrons to 497% in a
wavelength range of 0:5olo1 nm for all collimations. Its
performance has been charactertised with a polarised
proton spin filter, which is very suitable for a reliable and
accurate polarisation analysis of a neutron beam with
different divergences as present at such an instrument.
A precision of dp=p0:5% was achieved within short time.
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