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Journal ArticleDOI

The SLAC high-density gaseous polarized 3He target☆

TL;DR: A large-scale high-pressure gaseous 3 He polarized target has been developed for use with a high-intensity polarized electron beam at the Stanford Linear Accelerator Center as discussed by the authors, which was used successfully in an experiment to study the spin structure of the neutron.
Abstract: A large-scale high-pressure gaseous 3 He polarized target has been developed for use with a high-intensity polarized electron beam at the Stanford Linear Accelerator Center. This target was used successfully in an experiment to study the spin structure of the neutron. The target provided an areal density of about 7 × 10 21 nuclei/cm 2 and operated at 3 He polarizations between about 30% and 40% for the six-week duration of the experiment.

Summary (3 min read)

Introduction

  • A large-scale high-pressure gaseous 3He polarized target has been developed for use with a high-intensity polarized electron beam at the Stanford Linear Accelerator Center.
  • This target was used successfully in an experiment to study the spin structure of the neutron.
  • Thus the spin of the nucleus is mainly provided by the neutron, and so polarized 3He is a good approximation to a polarized neutron target, diluted by the presence of the protons but with only small corrections needed due to the polarization of the protons.
  • Second, practical 3He polarized targets have been developed [3–10] using the technique of collisional spin-exchange with optically pumped alkali-metal vapor, typically rubidium.

TARGET DESCRIPTION

  • Figure 1 shows a schematic overview of the SLAC target system.
  • Not shown is a second set of coils that produced a magnetic field (and 3He polarization) direction transverse to the beam direction for part of the experiment.
  • The laser shown in the figure represents one of five Ti:sapphire lasers, each pumped by a 20 W argon-ion laser and capable of delivering several watts of power at 795 nm, the wavelength of the D1 line of rubidium.
  • The other chamber was about 3.7 cm diameter by 8 cm long, and was enclosed in a plastic oven connected to hot air supply and return tubes (not shown).
  • The oven and the support structure for the NMR coils and target cell were constructed of plastic or other nonconducting material, to avoid disrupting the RF drive field during NMR measurements.

THE TWO-CHAMBER CELL

  • One requirement of the target arrangement was that the optical pumping of the rubidium vapor take place away from the electron beam, since the rubidium would be depolarized through ionization by the intense beam.
  • In principle, a single cylindrical chamber could be pumped in a location outside the experimental area, then installed in the beam line to replace another cell when the polarization of the latter has dropped too low.
  • In the SLAC environment, a long access time would be required to secure the experimental area, break the beam-line vacuum, exchange the target cells, then restore the vacuum, and finally retune the accelerator.
  • Also, the glass cell walls could darken from radiation damage and prevent repumping of the cell, the target polarization would be varying at all times, recovery time from an accidental loss of polarization could be very long, and the pressure (and hence stress) while pumping would be substantially higher for the same operating pressure in the cell.
  • The “double cell” solves or minimizes these problems, and in principle one good cell of this design could operate for the whole experiment.

MAXIMIZING POLARIZATION

  • The asymptotic 3He polarization is given by P3He = (γSE/γSE+Γ) 〈PRb〉, where γSE is the spin-exchange rate between the rubidium and the 3He, Γ is the 3He spin relaxation rate due to all other effects, and 〈PRb〉 is the average rubidium polarization.
  • If adequate laser power reaches all parts of the pumping chamber, 〈PRb〉 can approach 100%.
  • Since γSE is proportional to the rubidium number density, this term may be increased by increasing the oven temperature to vaporize more rubidium, but then more laser power is needed to maintain 〈PRb〉.
  • Practical limitations are then reached due to the cost and complexity of more lasers, or to the temperature limits of the oven materials, or both.
  • The remaining variable is then the spin relaxation rate Γ, which should be minimized.

RELAXATION EFFECTS

  • The total 3He spin relaxation rate is the result of several effects, here approximately Γ = Γbulk +.
  • At a pressure of about 10 atm, this limits the relaxation time to approximately 75 hours.
  • This was not a major effect during much of the experiment, but caused a noticeable (several percent) reduction in target polarization at the highest beam currents (about 4 µA).
  • The remaining terms are ones over which the target builders have some control.
  • The last two effects listed are due to collisions of 3He atoms with paramagnetic impurities in the GAS mixture, and collisions with the cell WALLS.

CELL MANUFACTURE

  • Figure 2 shows a diagram of the system used for preparing and filling target cells.
  • A small amount of rubidium is then “chased” into the pumping chamber with a torch, and the target chamber is inclosed in a vacuum-walled enclosure through which liquid 4He is blown.
  • Aluminosilicate glass has been found to highly suitable for this purpose, possibly due to its relatively low porosity to 3He.
  • For the cells used in this experiment, commercial tubing (Corning 1720) was rinsed with nitric acid to remove possible surface contaminants, then reblown to the desired dimensions on a glass-working lathe, resulting in a very clean and microscopically smooth “fire-polished” surface.
  • Cells constructed in this manner and filled as described above were measured to have net relaxation times up to 65 hours at room temperature with no incident electron beam, or nearly the bulk limit.

POLARIZATION MEASUREMENT

  • The method chosen for measuring target polarization was the NMR technique called Adiabatic Fast Passage, or AFP.
  • An adjustable circuit (“A-φ Box”) produces a signal used to cancel any direct pickup from the drive field.
  • Repeated measurements show that very little polarization (< 0.1%) is lost in this procedure.
  • The method is calibrated by replacing the 3He cell with a nearly identical cell filled with distilled water, then performing repeated measurements with the same apparatus at the same frequency.
  • Figure 4(b) shows the average of 25 proton signals from a typical water measurement.

OPERATIONAL EXPERIENCE

  • Figure 5 shows the measured 3He polarization as a function of time during the sixweek experimental run.
  • (1) The first (and best) cell installed had to be removed when the plastic oven cracked and leaked due to prolonged exposure to high temperature while surrounded by vacuum.
  • The arrangement was not capable of optical pumping in this orientation, so the polarization decayed until the longitudinal orientation was restored.
  • For the transverse running, the main holding field was provided by a second set of Helmholtz coils, as mentioned earlier.
  • This will increase the average rubidium polarization, or maintain it in the face of higher rubidium density.

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THE SLAC HIGH–DENSITY GASEOUS POLARIZED
3
He TARGET*
J. R. Johnson
Department of Physics, University of Wisconsin, Madison, WI 53706
A. K. Thompson
Department of Physics, Harvard University, Cambridge, MA 02138
T. E. Chupp and T. B. Smith
Randall Laboratory of Physics, University of Michigan, Ann Arbor, MI 48109
G. D. Cates, B. Driehuys, H. Middleton, and N. R. Newbury
Joseph Henry Laboratories of Physics, Princeton University, Princeton, NJ 08544
E. W. Hughes and W. Meyer*
Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309
ABSTRACT
A large-scale high-pressure gaseous
3
He polarized target has been developed for use with
a high-intensity polarized electron beam at the Stanford Linear Accelerator Center.
This target was used successfully in an experiment to study the spin structure of
the neutron. The target provided an areal density of about 7×10
21
nuclei/cm
2
and operated
at
3
He polarizations between about 30% and 40% for the six-week duration of the
experiment.
*Work supported in part by Department of Energy contracts DE–AC03–76SF00515
(SLAC), DE–FG02–90ER40557 (Princeton), and DE–AC02–76ER00881 (Wisconsin);
by National Science Foundation grants 8914353 and 9200621 (Michigan); and by the
Bundesministerium für Forschung und Technologie (W. Meyer).
*Permanent address: Universität Bonn, Bonn, Germany.

INTRODUCTION
Experiment E-142 at SLAC [1] was proposed to measure the spin-dependent structure
function of the neutron by studying deep-inelastic scattering of high-energy polarized
electrons from polarized neutrons. Polarized
3
He was chosen as the target material to
provide the polarized neutrons for two main reasons. First,
3
He nuclear wave-function
calculations [2] indicate that the nucleus is primarily in a spatially symmetric S-state where
the spins of the two protons must be anti-aligned. Thus the spin of the nucleus is mainly
provided by the neutron, and so polarized
3
He is a good approximation to a polarized
neutron target, diluted by the presence of the protons but with only small corrections
needed due to the polarization of the protons. Second, practical
3
He polarized targets have
been developed [3–10] using the technique of collisional spin-exchange with optically
pumped alkali-metal vapor, typically rubidium. Using modern high-power Ti:sapphire
lasers for the depopulation optical pumping of rubidium, such targets have been shown to
be capable of operating effectively at high
3
He pressures (more than 10 atm [8]) and in the
presence of intense electron beams [10], as required by the SLAC experiment.
A disadvantage of this technique is the weakness of the hyperfine interaction between the
polarized rubidium valence electron and the
3
He nucleus, leading to typical time constants
for the build-up of
3
He polarization of the order of 10 hours. Thus, long
3
He spin
relaxation times are required to achieve high polarization.
TARGET DESCRIPTION
Figure 1 shows a schematic overview of the SLAC target system. The large main
Helmholtz coils have a diameter of about 1.5 m and provide a uniform magnetic field of
about 20 G in the vicinity of the target cell during operation. The coils shown produced
a field direction parallel (or antiparallel) to the electron beam direction, and this direction
(and thus the direction of the
3
He polarization) was reversed every few hours during

the experiment. Not shown is a second set of coils that produced a magnetic field (and
3
He
polarization) direction transverse to the beam direction for part of the experiment. The RF
drive coils are about 50 cm in diameter and, together with the small pickup coils shown,
were used for NMR measurements of the
3
He polarization, as described later in this paper.
The laser shown in the figure represents one of five Ti:sapphire lasers, each pumped by
a 20 W argon-ion laser and capable of delivering several watts of power at 795 nm, the
wavelength of the D
1
line of rubidium. Together, the five laser systems typically produced
nearly 20 W of infrared photons for optical pumping. The quarter-wave plate in each laser
beam line was adjusted to produce circularly polarized photons, and the mirrors shown
were oriented so as to preserve the circular polarization.
As indicated in fig. 1, the
3
He gas was contained in a sealed glass vessel constructed as
two chambers connected by a transfer tube, for reasons discussed below. The chamber in
the electron beam was a cylinder about 2.1 cm in diameter and 30 cm long, with end
windows about 125 µm thick. The other chamber was about 3.7 cm diameter by 8 cm
long, and was enclosed in a plastic oven connected to hot air supply and return tubes (not
shown). The oven had an optically coated laser-beam entrance window on one end, and
transparent plastic windows on two adjacent sides to allow observation of the rubidium
fluorescence during optimization of laser steering and wavelength adjustment. The oven
was operated typically at about 175˚C, giving a typical rubidium vapor density of a few
times 10
14
atoms/cm
3
and a
3
He pressure in the sealed vessel of about 11 atm.
The target cell, oven, and NMR coils were required to operate in a completely evacuated
beam line, to minimize backgrounds and unpolarized material intercepting the electron
beam. The oven and the support structure for the NMR coils and target cell were
constructed of plastic or other nonconducting material, to avoid disrupting the RF drive

field during NMR measurements. The entire target structure was constructed of
nonmagnetic materials so as not to degrade the uniformity of the main holding field.
TARGET DESIGN CONSIDERATIONS
THE TWO-CHAMBER CELL
One requirement of the target arrangement was that the optical pumping of the
rubidium vapor take place away from the electron beam, since the rubidium would be
depolarized through ionization by the intense beam. In principle, a single cylindrical
chamber could be pumped in a location outside the experimental area, then installed in the
beam line to replace another cell when the polarization of the latter has dropped too low.
However, in the SLAC environment, a long access time would be required to secure the
experimental area, break the beam-line vacuum, exchange the target cells, then restore the
vacuum, and finally retune the accelerator. Also, the glass cell walls could darken from
radiation damage and prevent repumping of the cell, the target polarization would be
varying at all times, recovery time from an accidental loss of polarization could be very
long, and the pressure (and hence stress) while pumping would be substantially higher for
the same operating pressure in the cell. The “double cell” solves or minimizes these
problems, and in principle one good cell of this design could operate for the whole
experiment. In this design, the
3
He is polarized in the upper (pumping) chamber and
diffuses through the transfer tube to the lower (target) chamber with a time constant of
about 10 min—small compared with the characteristic spin exchange and relaxation times.
Due to the low thermal conductivity of glass, the target chamber remains at a low enough
temperature (about 60˚C) that the rubidium vapor density there is negligible. The
disadvantage of this design is that the effective spin-exchange time is increased, and the cell
is more difficult to construct.

MAXIMIZING POLARIZATION
The asymptotic
3
He polarization is given by P
3
He
= (γ
SE
/γ
SE
+Γ) P
Rb
, where γ
SE
is the spin-exchange rate between the rubidium and the
3
He, Γ is the
3
He spin relaxation
rate due to all other effects, and P
Rb
is the average rubidium polarization. If adequate
laser power reaches all parts of the pumping chamber, P
Rb
can approach 100%. Since
γ
SE
is proportional to the rubidium number density, this term may be increased by
increasing the oven temperature to vaporize more rubidium, but then more laser power is
needed to maintain
P
Rb
. Practical limitations are then reached due to the cost and
complexity of more lasers, or to the temperature limits of the oven materials, or both. The
remaining variable is then the spin relaxation rate Γ, which should be minimized.
RELAXATION EFFECTS
The total
3
He spin relaxation rate is the result of several effects, here approximately
Γ = Γ
bulk
+ Γ
beam
+ Γ
field
+ Γ
gas
+ Γ
wall
, as identified below. The first two of these are
unavoidable.
BULK relaxation results from
3
He–
3
He collisions, where a dipolar interaction
couples the nuclear spin to the orbital angular momentum of the colliding atoms. At a
pressure of about 10 atm, this limits the relaxation time to approximately 75 hours. BEAM
depolarization results from ionization and recombination of
3
He exposed to an electron
beam. This was not a major effect during much of the experiment, but caused a noticeable
(several percent) reduction in target polarization at the highest beam currents (about 4 µA).
The remaining terms are ones over which the target builders have some control.
Magnetic FIELD inhomogeneities induce depolarization as the
3
He atoms diffuse
throughout the cell volume. The solution for this target was to use large Helmholtz coils to
produce the holding field, as such a design was required in any case to provide access to
the target vacuum chamber. The last two effects listed are due to collisions of
3
He atoms

Citations
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Journal ArticleDOI
TL;DR: Both SEOP and MEOP are now widely applied for spin-polarized targets, neutron spin filters, magnetic resonance imaging, and precision measurements, and have benefitted from development of storage methods that allow forspin-relaxation times of hundreds of hours, and specialized precision methods for polarimetry.
Abstract: This article reviews the physics and technology of producing large quantities of highly spin-polarized 3He nuclei using spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping. Both technical developments and deeper understanding of the physical processes involved have led to substantial improvements in the capabilities of both methods. For SEOP, the use of spectrally narrowed lasers and K-Rb mixtures has substantially increased the achievable polarization and polarizing rate. For MEOP nearly lossless compression allows for rapid production of polarized 3He and operation in high magnetic fields has likewise significantly increased the pressure at which this method can be performed, and revealed new phenomena. Both methods have benefitted from development of storage methods that allow for spin-relaxation times of hundreds of hours, and specialized precision methods for polarimetry. SEOP and MEOP are now widely applied for spin-polarized targets, neutron spin filters, magnetic resonance imaging, and precision measurements.

102 citations


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References
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TL;DR: The theory of spin exchange between optically pumped alkali-inetal atoms and noble-gas nuclei is presented in this article, where the main spin interactions are assumed to be the spin-rotation interactions yN S between the rotational angular momentum N of the alkali ion and the electron spin S of the noble ion.
Abstract: The theory of spin exchange between optically pumped alkali-Inetal atoms and noble-gas nuclei is presented. Spin exchange with heavy noble gases is dominated by interactions in long-lived van der Waals molecules. The main spin interactions are assumed to be the spin-rotation interactions yN S between the rotational angular momentum N of the alkali-metal — noble-gas pair and the electron spin S of the alkali-metal atom, and the contact hyperfine interaction aK S between the nuclear spin K of the noble-gas atom and the electron spin S. Arbitrary values for EC and for the nuclear spin I of the alkali-metal atom are assumed. Precise formal expressions for spin transfer coefficients are given along with convenient approximations based on a perturbation expansion in powers of (o.'/yX), a quantity which has been shown to be small by experiment.

404 citations

Journal ArticleDOI
TL;DR: In this article, the Overhauser nuclear polarization effect involving dipolar interactions between an optically polarized atom and the nucleus of a suitable buffer gas was observed in He/sup 3/ gas used as the buffer for the optical pumping of rubidium vapor.
Abstract: The Overhauser nuclear polarization effect involving dipolar interactions between an optically polarized atom and the nucleus of a suitable buffer gas was observed in He/sup 3/ gas used as the buffer for the optical pumping of rubidium vapor. The rubidium was polarized and the degree of nuclear polarization of He/sup 3/ was determined. The polarization was reduced by relaxation processes. The relaxation time was proportional to the density and doubled in going from 300 to 77 deg K. (M.C.G.)

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TL;DR: The authors report the results of the experiment E142 which measured the spin dependent structure function of the neutron, which confirms the fundamental Bjorken sum rule with O({alpha}{sub s}{sup 3}) corrections to within one standard deviation, a major success for perturbative Quantum Chromodynamics.
Abstract: The authors report the results of the experiment E142 which measured the spin dependent structure function of the neutron, g{sub 1}{sup n}(x, Q{sup 2}). The experiment was carried out at the Stanford Linear Accelerator Center by measuring an asymmetry in the deep inelastic scattering of polarized electrons from a polarized {sup 3}He target, at electron energies from 19 to 26 GeV. The structure function was determined over the kinematic range 0.03 < BJorken x < 0.6 and 1.0 < Q{sup 2} < 5.5 (GeV/c){sup 2}. An evaluation of the integral {integral}{sub 0}{sup 1} g{sub 1}{sup n}(x,Q{sup 2})dx at fixed Q{sup 2} = 2 (GeV/c){sup 2} yields the final result {Lambda}{sub 1}{sup n} = -0.032 {+-} 0.006 (stat.) {+-} 0.009 (syst.). This result, when combined with the integral of the proton spin structure function measured in other experiments, confirms the fundamental Bjorken sum rule with O({alpha}{sub s}{sup 3}) corrections to within one standard deviation. This is a major success for perturbative Quantum Chromodynamics. Some ancillary results include the findings that the Ellis-Jaffe sum rule for the neutron is violated at the 2 {sigma} level, and that the total contribution of the quarks to the helicity of the nucleon is 0.36 {+-}more » 0.10. The strange sea polarization is estimated to be small and negative, {Delta}s = -0.07 {+-} 0.04.« less

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X. Zeng1, Z. Wu1, T. Call1, E. Miron1, D. Schreiber1, William Happer1 
TL;DR: The three-body formation rates and the spin-transfer probabilities for alkali- metal-atom vapors and noble-gas van der Waals molecules are determined.
Abstract: By analyzing the measured spin-relaxation transients of $^{129}\mathrm{Xe}$ nuclear spins and alkali-metal-atomic spins in mixtures of alkali-metal-atom vapors, Xe gas, and larger amounts of ${\mathrm{N}}_{2}$ gas, we have determined the three-body formation rates and the spin-transfer probabilities for alkali- metal--noble-gas van der Waals molecules. Three parameters, in addition to the spin quantum numbers of the alkali-metal and noble-gas nuclei, are needed to predict the spin-transfer rates. These parameters, which we have determined from experimental measurements, are x=\ensuremath{\gamma}N/\ensuremath{\alpha}, the ratio of the spin-rotation interaction \ensuremath{\gamma}N to the spin-exchange interaction \ensuremath{\alpha}; ${p}_{0}$, the third-body pressure for which the molecular breakup rate ${\ensuremath{\tau}}^{\mathrm{\ensuremath{-}}1}$ is equal to the spin-rotation frequency \ensuremath{\gamma}N/h; and Z, the three-body rate constant for forming van der Waals molecules.

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TL;DR: In this paper, the authors measured the spin exchange between Rb spins and rubidium nuclei and deduced the binary spin exchange cross-section from their measurements, which was shown to be approximately 7.3.
Abstract: By directly observing the nuclear polarization of $^{129}\mathrm{Xe}$, the efficiency $\ensuremath{\eta}$ of spin exchange between optically pumped Rb spins and $^{129}\mathrm{Xe}$ nuclei has been measured. It is found that $\frac{1}{\ensuremath{\eta}}=23\ifmmode\pm\else\textpm\fi{}4$ rubidium ${D}_{1}$ resonance-line photons are required to polarize a $^{129}\mathrm{Xe}$ nucleus when long-lived van der Waals molecules are unimportant. The binary spin-exchange cross section deduced from our measurements is ${\ensuremath{\sigma}}_{\mathrm{ex}}=(7.3\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ ${\mathrm{cm}}^{2}$.

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Frequently Asked Questions (17)
Q1. What are the contributions in "The slac high–density gaseous polarized 3he target*" ?

This target was used successfully in an experiment to study the spin structure of the neutron. The target provided an areal density of about 7×1021 nuclei/cm2 and operated at 3He polarizations between about 30 % and 40 % for the six-week duration of the experiment. Polarized 3He was chosen as the target material to provide the polarized neutrons for two main reasons. Thus the spin of the nucleus is mainly provided by the neutron, and so polarized 3He is a good approximation to a polarized neutron target, diluted by the presence of the protons but with only small corrections needed due to the polarization of the protons. 

Due to the low thermal conductivity of glass, the target chamber remains at a low enough temperature (about 60˚C) that the rubidium vapor density there is negligible. 

The laser shown in the figure represents one of five Ti:sapphire lasers, each pumped by a 20 W argon-ion laser and capable of delivering several watts of power at 795 nm, the wavelength of the D1 line of rubidium. 

The largest contribution for this experiment was from the extraction of the proton signal from water measurements, with an estimated uncertainty of ±5.6%. 

The quarter-wave plate in each laser beam line was adjusted to produce circularly polarized photons, and the mirrors shown were oriented so as to preserve the circular polarization. 

(5) Near the end of the experiment the beam intensity was increased, leading to additionaldepolarization due to ionization by the electrons. 

The disadvantage of this design is that the effective spin-exchange time is increased, and the cell is more difficult to construct. 

In principle, a single cylindrical chamber could be pumped in a location outside the experimental area, then installed in the beam line to replace another cell when the polarization of the latter has dropped too low. 

The oven and the support structure for the NMR coils and target cell were constructed of plastic or other nonconducting material, to avoid disrupting the RF drivefield during NMR measurements. 

The evidence is that the electron scattering rates dropped linearly with the 3He NMR signals during this episode, implying a decrease in 3He density rather than polarization. 

The asymptotic 3He polarization is given by P3He = (γSE/γSE+Γ) 〈PRb〉, where γSE is the spin-exchange rate between the rubidium and the 3He, Γ is the 3He spin relaxation rate due to all other effects, and 〈PRb〉 is the average rubidium polarization. 

The large main Helmholtz coils have a diameter of about 1.5 m and provide a uniform magnetic field of about 20 G in the vicinity of the target cell during operation. 

Cells constructed in this manner and filled as described above were measured to have net relaxation times up to 65 hours at room temperature with no incident electron beam, or nearly the bulk limit. 

In this design, the 3He is polarized in the upper (pumping) chamber and diffuses through the transfer tube to the lower (target) chamber with a time constant of about 10 min—small compared with the characteristic spin exchange and relaxation times. 

The 3He density in the target chamber was determined from measurements made during cell filling, and from temperature measurements of the two chambers of the cell during operation, and contributes an estimated uncertainty of ±2.5%. 

The RF drive coils are about 50 cm in diameter and, together with the small pickup coils shown, were used for NMR measurements of the 3He polarization, as described later in this paper. 

This was not a major effect during much of the experiment, but caused a noticeable (several percent) reduction in target polarization at the highest beam currents (about 4 µA).