Options for wide angle TOF neutron polarization analysis with polarized
^3^He

L. Passell^1^, L.D. Cooley^1^, W.C. Chen^3^, T. R. Gentile ^3^,V.
Ghosh^1^, M. Hagen ^2^, W.T. Lee^2^, W. Leonhardt^1^, S.M. Shapiro^1^,
I. Zaliznyak^1^

^1^Department of Condensed Matter Physics and Material Science,
Brookhaven National Laboratory, Upton, NY 11973-5000, USA

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

*^3^National Institute of Standards and Technology (NIST), Gaithersburg,
Maryland 20899, US*A

The large and strongly spin-dependent thermal neutron absorption cross
section of ^3^He \[σ(I-½)\>\>σ(I+½)\] has the potential to make
polarized ^3^He gas an efficient and versatile neutron polarizer and
polarization analyzer. And indeed, with the recent development of
optical pumping techniques capable of producing ^3^He gas polarizations
of 70-75 percent, ^3^He polarization analyzers are starting to appear in
neutron spectroscopic applications. Thus far, however, their use has
been limited by a number of incompletely resolved problems. Of these,
the most difficult by far is the extreme sensitivity of the gas
polarization to stray magnetic fields. To obtain a satisfactory ^3^He
gas polarization lifetime (τ~1/2~ greater than, say, 100 hours) in the
presence of a significant sample field, experience shows that the
spatial uniformity (ΔH/H) of the holding magnetic field that defines the
direction of nuclear polarization in the cell needs to be on the order
of 4-6x10^-4^/cm! There are also other impediments to the use of ^3^He.
Among these are the sensitivity of the gas polarization lifetime to the
choice of gas cell wall material, beam-related background generated by
scattering in cell entrance and exit windows, limitations imposed on
cell volumes and on gas pressures by neutron optics constraints on the
shape of the cells and the handling problems which arise because the
cells are optically pumped off-line and must be periodically
interchanged as the gas polarization decreases to maintain a reasonable
working efficiency. Here we examine some of these problems in the
context of using ^3^He polarization analyzers on neutron spectrometers
with wide angular acceptance deteector arrays such as those typically
employed in time-of-flight spectrometers.

In wide-angular-acceptance applications, uniform efficiency of
polarization analysis requires a \"wrap-around\" cell geometry (such as
that in shown schematically Fig. 1) so that all sample-to-detector
flight paths pass through the same amount of polarized ^3^He gas.
Typically such cells will be 10-15 cm long, operate with gas pressures
in the range of 1-2 atmospheres and have holding fields ranging from a
few gauss to 100 gauss or more. We consider below how such a cell might
be incorporated into three representative wide-angular-acceptance, low
temperature sample environments: (i) a low field (\<0.1T) environment
employing either a closed-cycle refrigerator or an ILL top-loading
cryostat, (ii) a medium field (1-3T) environment utilizing either a
conventional electromagnet or a cryogen-free superconducting magnet with
magnetic screening of the fringe fields done by conventional shielding
materials and (iii) a high field (10-15T) environment in which the
sample field is generated by a compensated superconducting magnet and
with both magnetic screening and the cell holding field provided by a
passive, persistent-mode superconducting shroud. As will be evident, as
the sample field increases, the cell holding field problem becomes
progressively more difficult and complex to resolve.

\(i\) Small sample field (H~s~\<0.1T)

![](media/image1.wmf)

Figure 1

When the sample field is small it is feasible to position the ^3^He cell
and sample inside a pair of Helmoltz coils as shown in Figure 1. In this
case both the cell and sample fields are produced by the Helmholtz coil
pair, the sample field being simply a guide field to maintain the
polarization of the incident and scattered neutron beams. Computer
simulations show that to obtain the necessary holding field uniformity
the cell should be located at the center of symmetry of the Helmholtz
pair and be of small volume, the latter condition also implies that a
wide-angular-acceptance cell needs to be as close to the outermost
sample radiation shield as possible. The simulations also show that
air-cooled copper coils will produce fields of the order of 0.01T of
satisfactory uniformity over a volume that can encompass a cell with an
angular acceptance of about 60 degrees. Although not shown in the
figure, an arrangement of three orthogonal Helmholtz coil pairs (the
so-called PASTIS coil geometry^(1)^ ) is also a possibility. This would
add to experimental versatility by permitting the sample and cell field
to be aligned in any direction with respect to the scattering plane.

\(ii\) Intermediate sample field (H~s~ \~1-3T)

![](media/image2.wmf)

Figure 2

Sample fields in the 1-3 T range can be supplied by either conventional
electromagnets or by cryogen-free superconducting magnets. But even with
the most careful compensation such magnets are likely to produce enough
stray field to significantly distort the ^3^He cell holding field
which - as shown schematically in Figure 2 - will in this case have to
be supplied by a separate solenoid, preferably one with aluminum
windings thin enough to be reasonably transparent to neutrons. To obtain
a workable gas polarization lifetime it will therefore almost certainly
be necessary to not only move the cell away from the sample thermal
radiation shield but screen the cell solenoid with a magnetic shield
with entrance and exit windows for the scattered neutrons. The increased
sample-to-cell distance and the need for magnetic shielding will have
several non-trivial consequences. One is that to maintain the angular
acceptance of the system the lateral and vertical dimensions of the cell
will need to be increased in proportion to the increased sample-to-cell
distance. The other is that conventional magnetic shielding is not very
transparent to thermal neutrons and, as Table I shows, is only effective
in fields less than \~ 0.1T. Assuming the maximum fringe field condition
can be satisfied at an acceptable distance from the sample magnet, it
remains an open question whether a magnetic shield with both neutron
entrance and exit windows will screen the stray field enough to make it
possible to achieve the needed holding field uniformity.

Table I

Material Field that can be shielded

(Gauss)

Annealed μ metal \~10

Partly-annealed μ metal \~100

Fe~0.97~Si~0.03~ \~1000

\(ii\) High sample field (H~s~ \~10-15T)

A typical compensated, 15T split-pair superconducting sample magnet will
produce a stray field of roughly 0.5-1T at distances of ½ meter (which
is where a. ^3^He cell of reasonable size would have to be located).
Given that conventional magnetic shielding is unlikely to be effective
in a field of this magnitude, the only realistic alternative is to think
in terms of putting the cell inside a hollow-center, ILL CRYOPOL-type
dewar^(2)^ containing a passive, persistent-mode superconducting sleeve
that would simultaneously supply a cell holding field of satisfactory
uniformity and screen the stray field from the sample magnet. In such an
application it is imperative that the superconducting sleeve, dewar
walls and internal radiation shields on either side of the sleeve be
thin enough to transmit neutrons without excessive attenuation and that
the sleeve have sufficient current carrying capacity to supply the
necessary magnetic screening. Table II lists the superconducting
materials that could, at least in principal, beemployed.

Table II

Material Field that can be shielded

with 1 mm thick foil

(Tesla)

Nb \~0.2

Al clad Nb~0.37~Ti~0.63~ \~1.0

Al clad Nb~3~Sn ?

There are established techniques for forming persistent-mode,
superconducting Nb sleeves (such as that used in the CRYOPOL instrument)
but the current carrying capacity of Nb is probably too low to screen
the fringe field from a high-field sample magnet. On paper, Al clad
Nb~0.37~Ti~0.63~ would appear to be an attractive alternative because it
has the necessary current carrying capacity and good mechanical
properties as well. But Al clad Nb~0.37~Ti~0.63~ consists of alternate
layers of Nb~0.37~Ti~0.63~ wire mesh and thin Al foil and it has yet to
be demonstrated that a monolithic, persistent-mode sleeve could be
fabricated from this material. Al clad Nb~3~Sn looks to be even better
in terms of current carrying capacity but is extremely brittle and -
given the magnetic forces that would be involved - does not appear to be
a realistic possibility. Apart from the considerable problem of finding
a suitable superconducting material, there are also major problems to be
faced in the design of the hollow-center dewar. Employing ^3^He
polarization analyzers near high field sample magnets would thus appear
to be a far-from-trivial undertaking.

\(iv\) On-line optical pumping

There are two potentially viable approaches to maintaining constant (or
near-constant) gas polarization in ^3^He cells: in-situ optical pumping
and batch gas transfer from an externally pumped cell to the working
cell.

Continuous in-situ optical pumping using the spin exchange optical
pumping (SEOP) method does not appear to be particularly difficult in
the low sample field configuration of Figure 1.

Heating the cell to about 200 C would be necessary to maintain the Rb
vapor pressure needed for efficient Rb-^3^He spin exchange, but this
does not look to be an insurmountable obstacle. For a PASTIS-type
arrangement of three orthogonal Helmholtz coil pairs, however,
continuous in-situ pumping would be more of a challenge because the
circularly polarized pumping light has to enter the cell parallel to the
holding field which, in this case, would shift from one direction to
another depending on what is regarded as the most experimentally
advantageous sample field orientation.

Batch gas transfer from an externally pumped cell connected via
capillaries to the working cell also looks to be a viable option in the
low sample field configuration of Figure 1. In this case the approach
would be a closed loop arrangement involving either metastable exchange
optical pumping (MEOP) or SEOP. MEOP is the faster of the two methods
but the gas has to be optically pumped at low pressure and then
compressed and stored under pressure for periodic exchange with gas in
the working cell. At the present time MEOP appears to be the less
attractive alternative for batch transfer in part because compressors
that can compress the gas without significant loss of polarizationare
are still in the development stage and in part because they are almost
certain to be costly and require maintenance on a regular schedule.
SEOP, on the other hand, although slower, is not constrained to low
pressures and polarized gas exchange could probably be accomplished
without any mechanical components other than the valves controlling the
gas flow and would be comparatively maintenance-free. But whatever the
choice of pumping method, issues relating to providing the necessary
holding field uniformity in the connecting capillaries would need to be
addressed.

Given the more demanding nature of the intermediate and high sample
field cases, it is likely that ^3^He polarization analyzers, if
employed, will, at least initially, be optically pumped off-line and
manually interchanged as the need arises. On-line optical pumping by
either the in-situ or batch method, although not ruled-out, would
require a significant developmental effort because of the limited access
to cells inside either solenoids or hollow-center dewars.

References:

\(1\) J.A. Stride, K.A. Andersen, A.P. Murani, H. Mutka, H. Schober and
J.R. Stewart - Physica *B356*, 146 (2005)

\(2\) J. Dreyer, L.P. Regnault, E. Bourgeat-Lami, E. Lelievre-Berna, S.
Pujol, F. Thomas,

M. Thomas and F. Tasset - Nuclear Instruemnts and Methods *A449*, 638
(2000)