ICANS-XVII

17^th^ Meeting of the International Collaboration on Advanced Neutron
Sources

April 25-29, 2005

Santa Fe, New Mexico

The Methods of Producing and Analyzing Polarized Neutron Beams for
HYSPEC at the SNS

S. M.Shapiro ^1^, L. Passel1^l^, I. A. Zaliznyak^1^, V. J. Ghosh^1^, W.
L. Leonhardt^1^, M. E. Hagen ^1,2^

^1^Department of Physics, Brookhaven National Laboratory, PO Box 5000,
Upton, NY 11973, USA

^2^ Oak Ridge National Laboratory, Spallation Neutron Source, PO Box
2008, Oak Ridge, TN 37830, USA

Abstract

The [Hy]{.underline}brid [Spec]{.underline}trometer (HYSPEC), under
construction at the SNS on beam line 14B, is the only inelastic
scattering instrument designed to enable polarization of the incident
and the scattered neutron beams. A Heusler monochromator will replace
the graphite crystal for producing polarized neutrons. In the scattered
beam it is planned to us a collimator - multi-channel supermirror bender
array to analyze the polarization of the scattered beam over the final
energy range from 5 -- 20 meV. Other methods of polarization analysis
under consideration such as transmission filters using He^3^, Sm, and
polarized protons are considered. Their performance is estimated and a
comparison of the various methods of polarization is made.

1\. Introduction.

HYSPEC is an abbreviation for [Hy]{.underline}brid
[Spect]{.underline}rometer, a unique direct geometry inelastic
scattering instrument under construction at the Spallation Neutron
Source at Oak Ridge National Laboratory \[1\]. The hybrid nature stems
from the use of a focusing crystal, normally used in crystal
spectrometers at continuous sources, and the time-of-flight (TOF)
analysis of the energy of the scattered neutrons. A Fermi chopper will
be used to monochromate the beam before it strikes the focusing crystal.
This combination of monchromating the beam and using a crystal is a more
efficient way of focusing a large beam to a small size (2x2 cm^2^) than
using convergent guides. It will be situated at beam line 14B, which
looks at a liquid hydrogen coupled moderator. The instrument is designed
to operate in the incident energy range of 3.6 to 90 meV with a variable
energy resolution 2%\<∆E/E\<10%. Importantly, HYSPEC will be capable of
polarizing the incident beam and analyzing the polarization of the
scattered beam -- a unique capability for inelastic instruments at a
pulsed source \[2\].

2\. Instrument Description

An engineering drawing of HYSPEC is shown in Fig.1 \[3\]. The beam
emerging from the liquid hydrogen coupled moderator at beam line 14B
will pass through a T0 and frame overlap chopper, which will stop the
very fast neutrons and neutrons from other frames, respectively. The
neutrons will traverse a long curved supermirror (m=3) guide to an
external building where it will pass through Fermi and order suppressor
choppers. The monochromatic beam will then strike a focusing crystal
placed inside a well-shielded drum. The beam emerging from the drum will
strike the sample, which is external to the detector bank. The sample
area should be accessible to the experimenter and can accommodate
standard or exotic sample environments. The scattered beam first passes
through a radial collimator, then a coarse 3° collimator and finally to
the detector bank located 4.5m from the sample. The detector bank covers
an angle of 60° horizontal and 15° vertical and is moveable inside a
shield to cover a two-theta scattered angle range of 0-120°. There will
be 160, 1.2m high position sensitive He^3^ detectors arranged in 20
banks of 8 counters. There will be a choice of several radial
collimations available to the experimenter and these will be manually
interchangeable.

![](media/image1.pct){width="5.993055555555555in"
height="3.1055555555555556in"}

Figure 1: Engineering drawing showing the components of the HYSPEC
instrument. The sample placed 35 m from the moderator. The focusing
crystal, sample area and detector bank will be located in an external
building.

2\. Polarized beam operations.

A vertical and horizontal focusing pyrolytic graphite(PG) crystal will
be used in non-polarized beam operation. For polarized beam work, a
vertical focusing Heusler (Cu~2~MnAl) crystal purchased from the
Institute Laue Langevin (currently the only supplier of these crystals)
will replace the PG crystal. The (111) reflection of this crystal will
be used and a polarization of 95% is achievable when the Mn moments are
fully aligned. For polarization analysis of the scattered beam, HYSPEC
is designed with an array of

> ![](media/image2.pct){width="2.2284722222222224in" height="1.9in"}
> ![](media/image3.pct){width="1.4569444444444444in"
> height="2.2916666666666665in"}

Figure 2: (a) Schematic of a single collimator-bender array showing the
separation of the spin down(no-deflected) and the spin up (deflected)
beams. (b) Drawing a single collimator bender array. The tilt angle in
this figure is very exaggerated

supermirror bender transmission polarizers \[4\] that can be positioned
where the radial collimator is located in the detector shielding. Figure
2a shows the schematic of one bender array. A 20 min. collimator will be
placed before the supermirror bender to limit the divergence of the beam
so the neutrons will strike the bender at angles less than the critical
angle of the supermirror coating. Figure 2b shows a drawing of one
collimator-bender array. There is a small tilt angle between the two
that can be varied. The bender will consist of a stack of bent Si
crystals with a supermirror coating of Fe-Si placed in a magnetic field
to saturate the iron moments. For a magnetized film the spin state
parallel to the polarized bender will be deflected and the spin state
anti-parallel will be transmitted. It is thus possible to detect both
spin states simultaneously on the detector. This concept has been tested
and is in routine use at the NIST cold beam neutron reflectometer \[5\].
To assess the performance we performed Monte Carlo simulations using the
NISP package from Los Alamos \[6\]. Figure 3a shows the result of the
calculated intensity distribution on the detector bank for a 5 cm long
bender with a radius of curvature of 5m, a tilt angle of 0.3° and a
channel width of 0.025cm. (If a solid state Si collimator is used this
would be the thickness of the individual plates.) The critical angles
chosen are: θ (spin up) =3.0 θ~c~^(Ni)^ and θ (spin down) =0.6
θ~c~^(Ni)^, where θ~c~^(Ni)^ is the critical angle for natural nickel.
The sample-to-detector distance is 4.5m and the detector bank is 3.9 m
from the polarizer's rear face. The two spin states are clearly
separable. The spacing increases with increasing wavelength and become
less resolvable for shorter wavelengths. The spacing also varies with
tilt angle. It is envisioned to have two bender arrays optimized for 10
and 20 meV.

![](media/image4.pct){width="5.894444444444445in"
height="2.527083333333333in"}

Figure 3: (a) Profile of neutron intensity at the detector determined by
Monte-Carlo simulation using the NISP package. The red (green) curve is
the spin up (down)polarization corresponding to the deflected
(transmitted) beam. (b) The polarization efficiencies (upper two curves)
and transmission (lower two curves) for the deflected and non-deflected
beams

For this direct geometry instrument it would be possible to perform
polarization analysis of scattered neutrons for energies from \~5 -- 20
meV. Figure 3 b shows the polarization efficiency and transmission of a
bender for 10 meV neutrons as a function of tilt angle between the
bender and the collimator. It is seen that the polarization efficiency
is about 90% and an acceptable transmission of near 50% for a given tilt
angle of 0.3°.

3\. Transmission Polarizers.

HYSPEC will be built with the bender polarizer array discussed above.
However, there are other options to produce and analyze polarized beams
that have been considered and are currently under development. These are
transmission polarizers, which have the advantage that they are suitable
over a relatively broad energy range compared to crystals or
multilayers. For the latter, diffraction plays the key role and the
performance is very wavelength dependent. The transmission polarizers
work on the principle that one spin state is preferentially absorbed
when passing through a medium of polarized nuclei \[7\]. We shall
calculate the polarization and the transmission of three types of
transmission polarizers: polarized protons, Sm and He^3^. Only the
latter is under intense development and currently in use at several
facilities, mainly reactor based. The compound nuclear state of a
neutron when it is resonantly captured by a nucleus has either a spin
*I+1/2* or *I-1/2*, where *I* is the spin of the nucleus before neutron
capture. The spin dependent neutron cross section, σ, for the combined
system was worked out by Rose \[8\] many years ago:

> $s = (\frac{1}{2I + 1})(1 - f_{n}f_{N})s_{I - \frac{1}{2}} + \frac{I + 1}{2I + 1}(1 + \frac{I}{I + 1})f_{n}f_{N})s_{I + \frac{1}{2}}$
> (3.1)

*f~n~* and *f~N~* are the neutron and nucleus polarization,
respectively, and σ~*I*-1/2~ and σ~*I*+1/2~ are the cross sections of
the different spin states of the compound nucleus. From this the neutron
polarization, P~n~, and the combined transmission, T~n~, of the two spin
states are:

> $\begin{array}{r}
> P_{n}(f_{N}\text{,N}t) = \text{tanh}(N\text{ts}_{p}f_{N}) \\
> T_{n}(f_{N},Nt) = \text{exp}( - {Ns}_{0}t)\text{cosh}(N\text{ts}_{p}f_{N})
> \end{array}$ (3.2)

N = the number of atoms/volume in the neutron beam

τ = the length of the filter

σ~p~= (I/(2I+1)(σ~*I*+1/2~-σ~*I*-1/2~)

σ~0~= (I/(2I+1)σ~*I*-1/2~ + (I+1)/(2I+1)σ~*I*+1/2~

Equations 3.2 were used to calculate the polarization and transmission
of the three types of transmission polarizers.^.^

3.1 Polarized protons:

Polarized protons have been used for many years as targets in high
energy physics experiments so the technology is well established \[9\].
A high degree of polarization is obtained by using the technique of
dynamic nuclear polarization (DNP). In this method the sample is cooled
to low temperatures (\<1 K) in a homogeneous magnetic field to polarize
the paramagnetic spins. A microwave field is applied to equalize the
popoulation of the two spin states and to transfer the polarization to
the nuclear spins via dipole---dipole coupling. Several different proton
targets have been used such as ammonia, lanthanum-magnesium nitrate
doped with neodymium (LMN), and butanol. We have chosen to study butanol
(CH~3~(CH~2~)~3~OH), doped with paramagnetic centers, as the source of
polarized protons. Figure 4a gives the results of the calculations with
a 90% proton polarization and a density length corresponding to
N~h~τ=4.2 x10^22^ atoms/cm^2^. This shows the broad-band nature of the
filter. The polarization is greater the 75% over the energy range of
3.6-90 meV. The transmission is also very high for neutron spins
polarized parallel to the proton polarization (spin-up). It is low for
spins anti-parallel to the proton polarization so one would have to do
two measurements in order to get a complete picture of the polarization
of the scattered neutrons from the sample.

3.2 Nuclear polarized Sm:

^149^Samarium is particularly useful as a polarizer. It has a broad 98
meV resonance and a highly spin-dependent cross section, which enables a
high degree of polarization. The nuclei have to be cooled to very low
temperature to get them to order and a modest field is needed to orient
the atomic moments. Because of the very high hyperfine field of Sm (335
T), a high degree of nuclear polarization can be achieved. There were
several studies of Sm compounds to test its feasibility as a neutron
polarizer. One study \[10\] used a filter of Sm-doped cerous magnesium
nitrate. Because this material is an insulator it was difficult to
maintain the low temperatures necessary for maintaining the nuclear
polarization. Another study used fine particles of SmCo~5~, a high
remnance ferromagnet \[11\]. The difficulty with this material for
neutron polarization was the magnetic alignment of all the small
particles. This misalignment leads to a depolarization of the neutron
beam. We chose a foil of ^149^Sm~.04~Y~0.96~Al~2~, to study since this
is either a paramagnet or weak ferromagnet at millikelvin temperatures
and the problem associated with domains in the other Sm magnets does not
exist. The amount of nuclear polarization is determined by the (low)
temperature and the size of the holding field. The results calculated
from equations 3.2 are shown in Fig.4b for an achievable nuclear
polarization of 90% and a thickness of the foil of 0.7mm. Over the range
of energy (3.6 -- 90 meV) the neutron polarization is very high (\>80%).
However, the transmission of the spin-up state varies due to the rapid
variation of the spin-dependent cross section and averages around 40%.
The transmission for the other spin state is very low so two
measurements would be required to study both spin states of the
scattered neutrons.

![](media/image5.pct){width="5.854861111111111in"
height="3.3291666666666666in"}

Figure 4:Calculated polarization and transmission of (a) Butanol and (b)
^149^Sm transmission polarizers

3.3 He^3^ Transmission Polarizer:

The transmission polarizer that shows the greatest promise for
applications in pulsed neutron beams is He^3^. It is under intense
development at ILL \[12\] and NIST \[13\] and is currently being used on
the diffractometers D1B andD3 at ILL \[14\]. For the neutron
polarization parallel to the He^3^ spin direction the absorption is
nearly zero, whereas for the neutron spin anti-parallel the absorption
cross section is very large. The cross section is directly proportional
to the wavelength or 1/$\sqrt{E}$. Polarized He^3^ is produced by two
optical pumping methods: spin-exchange (SEOP) \[15\] and
metastability-exchange (MEOP) \[16\]. Figure 5 shows the calculated
polarization and transmissions for a He^3^ nuclear polarization of 75%
and 90%. In Fig. 5a, the nuclear polarization is 75%, which is the
current state of the art for He^3^ polarization. The neuron polarization
varies from 100% for low energies (\~3.6meV) to a low of 60% for 90 meV.
The transmission of the spin-up state is quite low at low energies and
increase to 80% for 90 meV. As with all transmission filters the
down-spin state is low over the entire energy range. If the He^3^
polarization can be increased to 90% (Fig. 5b), there is a marked
improvement in performance. The polarization is greater than 65% over
the entire energy range and the transmission is greater than 60%, a
truly suitable performance. A central filling station would be required
and He^3^ has the attractive feature that it would be relatively easy to
change the gas pressure of the cell and thus change the transmission and
the polarization depending upon the energy range of the experiment.

4\. Comparison of different polarization analysis methods

Table 1 summarizes the advantages and disadvantages of the four types of
polarization methods discussed above. For the benders the clear
advantage is that the technique is well established and maintenance
free. You can also measure both spin states simultaneously. Importantly
the device is

![](media/image6.pct){width="6.352083333333334in"
height="3.4347222222222222in"}

Figure 5: Calculated polarization and transmission for He^3^ spin
filters with nuclear polarization (a) f~N~=75% and (b) f~N~=90%.

insensitive to stray magnetic fields. It's disadvantages include a
limited energy range of applicability and also that the tilt angle for
optimum performance depends on the neutron energy. Also, all counters
will not be used in a single measurement. Moreover, there is
considerable transmission loss in the 20 minute collimator needed to
limit the divergence of the beam striking the bender.

For the He^3^ transmission polarizer the distinct advantage is its broad
band nature with a high polarization efficiency, which can be varied
with pressure to suit the energy range of the experiment. One can also
cover a wide angular range with a single cell or a number of different
cells. However, the He^3^ polarization cells are very sensitive to
magnetic field gradients and its use may be limited to studies not
involving high sample magnetic fields where stray fields will affect the
performance. To cover the angular range of HYSPEC, a large He^3^ volume
is required. Also, a central facility is required of fill and polarize
the He^3^ nuclei in the cells and this adds greatly to the cost of the
technique and requires a significant facility infrastructure.

There are also tradeoffs for the less well know methods of polarizing
neutrons. For the Sm filters, a high degree of polarization can be
achieved and this is magnetic field insensitive. However, very low (mK)
temperatures and a holding magnetic field are needed to maintain the Sm
nuclear polarization. Also, the polarization efficiency, once fixed,
cannot be varied. For the polarized proton transmission filter a high
degree of polarization can be achieved and it performs well over a broad
energy range. The means of producing polarized protons has been studied
over many years by the high energy community. Its disadvantages are the
complicated nature of the technique in that mK temperatures and a
microwave pumping field are needed to maintain the proton polarization.
Also, it, too, is sensitive to magnetic field gradients around the
device and the polarizing efficiency, once fixed, cannot be varied.

5\. Summary and conclusions

HYSPEC is unique instrument at a pulsed source and will be the only
inelastic instrument at SNS with a designed-in capability to do full
neutron polarization. Other methods using transmission polarizers, whose
transmission and polarization depend upon the absorption of one spin
state and transmission of the other, are more broad-band in nature.
There are clear trade-offs to be considered in each of these systems.
The collimator-bender polarizer is the chosen method to be used in
HSYPEC because it has been demonstrated to work and is maintenance free.
He^3^ transmission polarizers are under intense investigation and would
be desirable if a number of hurdles can be overcome. The feasibility of
using Sm and polarized protons has been demonstrated, but more R&D would
have to be undertaken to establish this method as competing techniques
for polarization analysis.

Table 1: Advantages and disadvantages of four polarization techniques
discussed in this paper

  ----------------- ----------------- ----------------- -----------------
  **BENDERS**       **He^3^**         **Sm**            **Polarized
                                                        Protons**

  ----------------- ----------------- ----------------- -----------------

ADVANTAGES

+-----------------+-----------------+-----------------+-----------------+
| Established     | High            | High            | High            |
|                 | polarization    | polarization    | polarization    |
| technique       |                 |                 |                 |
|                 | efficiency      | efficiency      | efficiency      |
| Maintenance     |                 |                 |                 |
| free            | Vary            | Magnetic field  | Broad-band      |
|                 | polarization    |                 |                 |
| Measure both    |                 | insensitive     | Established     |
| spins           | efficiency with |                 |                 |
|                 |                 |                 | technology      |
| Magnetic field  | pressure        |                 |                 |
|                 |                 |                 |                 |
| insensitive     | Wide angular    |                 |                 |
|                 |                 |                 |                 |
|                 | acceptance      |                 |                 |
|                 |                 |                 |                 |
|                 | Broad-band      |                 |                 |
+-----------------+-----------------+-----------------+-----------------+

DISADVANTAGES

+-----------------+-----------------+-----------------+-----------------+
| Polarization    | Magnetic field  | Very low        | Low             |
|                 | sensitive       | temperatures    | temperatures    |
| depends upon    |                 |                 |                 |
|                 | Large He^3^     | required        | required        |
| energy          | volumes         |                 |                 |
|                 |                 | Polarization    | Microwave       |
| Limited energy  | required        | efficieny is    | pumping         |
|                 |                 |                 |                 |
| range           | Need central    | fixed           | needed          |
|                 | facility        |                 |                 |
| Not all         |                 |                 | Magnetic field  |
| detectors       |                 |                 | sensitive       |
|                 |                 |                 |                 |
| used            |                 |                 |                 |
|                 |                 |                 |                 |
| Transmission    |                 |                 |                 |
| loss in         |                 |                 |                 |
|                 |                 |                 |                 |
| collimators     |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+

Acknowledgements are the same as the subhead style.

Work at BNL is supported by the US DOE under contract No.
DE-AC02-98CH10886

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