Polarized 3He Targets and Beams
Thomas B. Clegg
Department of Physics & Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA
and
Triangle Universities Nuclear Laboratory (TUNL), Durham, NC 27708-0308, USA
Abstract. This is a review of the physics underlying both why polarized targets and beams of
3
He nuclei are interesting for experiments, and how such beams and targets are realized. A
summary is given of the variety of methods used, with experimental examples to illustrate both
historical developments and current achievements. A brief look at systems proposed or being
planned for the future is also included.
Keywords: Polarized 3He beam, Polarized 3He target; Polarized 3He source; Spin-exchange
optical pumping; Metastability-exchange optical pumping; Polarized 3He imaging.
PACS: 29.25.Lg; 29.25.Ni; 29.25.Pj; 29.27.Hj; 37.20.+j; 33.80.Be
INTRODUCTION
Optical pumping methods first proposed in the early 1960s for polarizing the nuclei
of 3He atoms1,2 are in wide use today. Spurred by development in recent years of
inexpensive, powerful lasers and interest in polarized 3He for enhanced medical
imaging, research in this field has grown substantially. As a result, large increases
have been realized both in the rate at which 3He nuclei are polarized and in the
maximum value of nuclear polarization obtained. Previous workshops3,4,5 have tracked
these accomplishments, and contributions included in those and these proceedings
provide excellent descriptions of progress and lists of basic references.
Physics Motivation
As depicted in Figure 1, most of the
nuclear spin of 3He is carried by the
neutron. Because targets of polarized
neutrons are not experimentally feasible, physicists often use polarized 3He
as a substitute for a polarized neutron
target or beam. Thus, in an experiment
underway at the Jefferson Lab, the GEn
collaboration uses a polarized 3He
target in double polarized 3He(e,e'n)
scattering to measure the neutron
electric form factor at high momentum
transfer.6 Lack of alternatives for
The two protons in the 3He
nucleus S-wave ground state have their spins
predominantly paired off antiparallel, so the
unpaired neutron largely carries the spin.
Corrections for the nuclear D-state probability and for the nuclear medium make the
effective neutron polarization ~86%. (ref. 6)
FIGURE 1.
producing a high-energy polarized neutron beam is also stimulating the present feasibility studies for creating a polarized 3He beam for acceleration in RHIC at BNL.7 At
much lower beam energies, polarized 3He targets have been used for studying spindependent effects in 2-10 MeV p+3He elastic scattering8,9, with the goal of learning
how spin affects the formation of the lightest nuclei from their constituent nucleons.
Another large community of scientists uses polarized 3He targets as spin-filter
polarizers or analyzers10,11 for thermal and hot neutrons from reactors and spallation
sources. This is feasible because of strongly spin-dependent, Jπ=0+ resonant absorption
at low neutron energies in the 3He+n4He*p+t reaction (Q-value = +763 keV).
This makes transmission of neutrons through a polarized 3He target highly spin
dependent. So, if an unpolarized neutron beam is incident, the emerging beam
becomes polarized; if a polarized neutron beam is incident, comparison of its transmission when flipping the 3He target spin analyzes the incident neutron beam’s
polarization. Such systems have found wide use, for example, in studies at LANL of
the parity violating weak interaction12, measurements at NIST of the materials properties of magnetic arrays13, and measurements of a variety of condensed matter physics
properties at other international laboratories5.
Imaging Applications
Highly porous lung tissue is quite hard to image with typical X-ray, γ-ray, or MRI
techniques. By contrast, when the lungs are filled with ‘hyperpolarized’ 3He or 129Xe,
the resulting MRI signal can be larger by 104 or more than that available from the
usual room temperature spin state population difference caused by H-atom Zeeman
splitting in an external magnetic field14,15. The resulting enhanced imaging capability
has enabled sub-millimeter NMR resolution within lungs, as well as dynamic studies
of polarized gas flow during pulmonary ventilation in live subjects.
A large research community has grown up around this technique. European activity
is concentrated at the Univ. of Mainz16; a good summary of their work, with relevant
references, is available online17. Commercial development in the US of compact 3He
polarizers has been slowed by difficulties in obtaining essential licenses for use of the
process in human subjects. There is also some concern about whether 3He is widely
enough available at low enough cost to support a commercially feasible market18.
One very significant difference distinguishes the needs of the nuclear physics and
medical imaging communities. Nuclear physicists push for the highest possible 3He
polarization, because their efficiency in measuring a polarization observable with a
given statistical accuracy scales as nP2, where n signifies the density of polarized
nuclei in a target or the intensity of the 3He ion beam, and P its nuclear polarization.
By contrast, the medical imaging community enjoys such a large signal enhancement
when using these ‘hyperpolarized’ gases that P0.2 is still often very useful.
TABLE 1. Species of 3He atoms and ions available for producing polarized targets or beams.
Species
Electronic State
Critical Field, B c
Lifetime
He0
He+
1 1 S0
Grd. State
0
stable
2 3 S1
Metastable
2407 G
7860 sec
He1 1 S0
Grd. State
3087 G
stable
2S
Metastable
386 G
2 msec
1S 2S 2P
Metastable
11 and 345 μs
POLARIZATION METHODS
3
He Species
Atomic and ionic states of 3He, and properties relevant when considering methods
for polarizing its nucleus, are listed in Table 1. Atoms in both the 11S0 ground state
and in the 23S1 metastable state are routinely used for producing polarized 3He targets
via optical pumping19. Atoms in these same two states, and hydrogen-like He+ ions in
the 2S metastable state, have been used to produce polarized 3He ion beams. Also
shown for states are their lifetimes and critical magnetic field, Bc. An external B-field
much stronger than Bc is required when ionizing a polarized atom in any mixed state to
establish a known electron spin quantization axis and, via hyperfine coupling, to preserve its nuclear polarization20. Sections below provide detailed experimental methods for obtaining polarized targets or beams from these species, and results achieved.
Polarized 3He Targets
Spin-Exchange Optical Pumping (SEOP)
It was first realized1 in 1960 that rubidium atoms in a heated cell, after being
polarized by optical pumping, could transfer their nuclear polarization to 3He0 ground
state atoms included in the same cell. That early work used an rf-excited rubidium
vapor lamp and D2 filter, followed by a ¼-wave plate, to produce the circularly polarized 794.7 nm D1 light required. Modern systems instead use up to 120 W of infrared
laser power. Optical pumping cells are usually made from aluminosilicate glass, and
require extensive vacuum bakeout for cleaning before loading with Rb and 3He. A
comprehensive review21 discusses the relevant optical pumping physics, the binary
and other spin-exchange interactions between a polarized alkali-metal atom and the
nucleus of a noble gas atom which produce the polarization, and techniques used to
reduce relaxation processes which limit the ultimate 3He polarization. Technological
advances have vastly improved the method, but questions remain about the origin of
temperature-dependent, surface-relaxation limits to the 3He polarization22.
The SEOP method is used widely for experiments because it works well at 3He
pressures of 1 to 10 bar. However, the 1/e ‘spin-up’ time with Rb vapor is ~8 hrs, so
attaining maximum 3He polarization in a typical 8 bar sample requires ~30 hrs.
Research in recent years has focused both on reducing the time needed to polarize a
sample, and on trying to increase its maximum polarization.
Hybrid alkali optical pumping - ‘Spin-up’ time can be shortened by using hybridalkali-metal optical pumping23,24. For this, pure Rb is replaced in the optical pumping
cell by a mixture of Rb and K. Optically pumped Rb spin-exchanges with the K,
which then spin-exchanges with the 3He. Because of its higher vapor density at operating temperatures of 220-240 C, much less Rb than K is loaded into the optical
pumping cell at room temperature. The high Rb-K spin-exchange rate, and the lower
K-K spin-destruction rate compared with that for Rb-Rb, provides an overall optical
pumping efficiency increase of ~3 for 3He pressures of 1-10 bar. This results in a
reduced ‘spin-up’ time of ~2.5 hrs, and ~10 hrs to attain maximum 3He polarization.
The best combination of high polarizing rate and polarization24 is obtained with K-Rb
vapor density ratios between 2 and 6. With 52 W of spectrally narrowed laser light at
795 nm, one can produce 75% polarized 3He gas at a rate of 2.4 bar-l/d or 50%
polarized 3He gas at a rate of 10.5 bar-l /d.
Narrowed diode lasers - Experimenters are turning to infrared diode laser arrays, as
their cost has dropped below $50/Watt and the total power available from such arrays
has risen above 100 Watts. However, the bandwidth of light they produce is ~1 THz.
For typical cell pressures of 5 to 10 bar, this far exceeds the pressure-broadened Rb
absorption linewidth of 18 GHz/bar, so their light is inefficiently used for optical pumping.
To overcome this mismatch, the array is usually inserted into an external optical
cavity including a Littrow-mounted diffraction grating25. This directs 1st-order
diffracted light back into the array to stimulate the diodes’ emission at precisely the
desired wavelength. Emerging light from the cavity then contains ~60% of the laser
power within a linewidth of 100 GHz or less. Tuning the diode array wavelength over
a range of ~2nm can be accomplished by varying its operating temperature. With the
external optical cavity, additional precise tuning over ~1nm is also possible simply by
rotating the grating. When compared with performance of a broadband diode laser, the
optical pumping power needed to attain a given value of 3He polarization with these
narrowed laser systems has been found to be 3 times smaller26.
Neutron spin filters - Such narrowed linewidths are particularly important when
SEOP 3He cells are used as neutron spin filters, since their typical ~1 bar operating
pressure causes little Rb linewidth broadening. For long-relaxation-time spin-filter
cells with volumes up to 500 cm3, it is now routine27 with narrowed diode laser systems to
obtain 3He polarizations of 74%–77%. The cells can be polarized off-line, transported
to the experimental area in a portable solenoid, and installed there in a low-gradient
holding B-field. Their 3He polarization is typically monitored using NMR. Detailed
reviews of techniques relevant to SEOP for neutron spin filter cells are available28,29.
Nuclear scattering targets - Extensive development both of Rb and hybrid Rb-K
SEOP electron scattering targets30 supports experiments at the Thomas Jefferson Lab,
where sealed, double-chamber aluminosilicate glass cells are used. They consist of a
spherical ~180 cm3 optical pumping chamber closely connected below to a separate
cylindrical, 25 cm long chamber through which the electron beam passes. The optical
pumping chamber is housed in a temperature controlled oven to maintain the desired
K/Rb vapor densities. Very thin 0.12 mm thick glass beam entrance and exit windows
in the lower chamber withstand the internal ~8 bar pressure during experiments.
Optical pumping during electron beam bombardment has maintained 3He polarizations
of nearly 50% for more than a month.
Large Helmholtz coils provide a highly uniform guide field over the doublechamber cell to set the spin quantization axis. A second, smaller Helmholtz coil pair
provides a transverse B-field when needed to accomplish 3He spin flip via adiabatic
fast passage31. The lower chamber of the target cell rests between NMR pickup coils
whose signal during spin flip monitors the relative 3He polarization. Absolute 3He
polarizations are determined by electron paramagnetic resonance. This measures the
shift in the rubidium Zeeman resonance caused by the B-field of the polarized 3He.
This shift is directly proportional to the polarization of the gas within the cell, so measuring the rubidium EPR frequency shift determines the absolute 3He polarization32.
A similar target of larger volume has recently been developed for use with circularly
and linearly polarized gamma ray beams33.
Metastability-Exchange Optical Pumping (MEOP)
An attractive alternative for polarizing 3He was first proposed2 and demonstrated34
in the early 1960s. A weak rf discharge in a cell containing pure 3He at ~1 mbar
pressure provides the ~20 eV needed to excite 1 1S0 ground state atoms to the 2 3S1
state. These metastable atoms can then be optically pumped with circularly polarized
1083 nm light, inducing transitions to the 2 3P0 state followed by decay and hyperfine
coupling to polarize the 2 3S1 atoms’ nuclei. Then the electronic excitation of these
metastable atoms is then quickly lost to ground state 3He atoms during collisions,
without loss of their nuclear polarization. In the early experiments, 3He polarizations
of 40±5% were obtained in a cell at 1 mbar pressure.
An advantage of MEOP over SEOP is that the polarizing rate is much faster. With
several watts of incident circularly polarized light, one can produce 1019 nuclear-spinpolarized 3He atoms per second. This makes the process much less sensitive to spinrelaxation processes, and 3He gas in the optical pumping cell remains pure because no
foreign materials (e.g. Rb, K, N2) are needed. The resulting 3He polarization can easily be monitored by analysis of the circular polarization of red, 668 nm light emitted
from the discharge. The primary disadvantage of MEOP for 3He is that it works well
only at relatively low pressures between 0.1 and ~5 mbar.
Laser Development – Arc lamps were used early for MEOP but today the resulting
polarization rate and maximum polarization are enhanced with high-powered lasers.
Significant improvement came with the development of the Nd:LMA (neodymiumdoped lanthanum magnesium hexaluminate) laser35,36. Typically a Nd:LMA rod was
installed in a commercial arc-lamp-pumped Nd:YAG (neodymium-doped yttrium
aluminum garnet) laser, and etalons were added in the laser cavity to tune and narrow
the bandwidth. Several watts of laser light could be obtained with a 2 GHz bandwidth.
Today, commercially available, single-mode ytterbium fiber-coupled lasers37 provide
up to 50 W of infrared light with linewidth < 2nm.
Gas Compression – Experiments have used 3He targets polarized by MEOP at low
mbar pressures to study both analyzing powers38,8 and spin correlation observables39.
Limited improvement in target density for electron scattering was obtained40 by using
a Pyrex cell at 2 Torr for optical pumping, connected to a second copper target cell
held at 17 K. But, counting rates usually demand much higher pressure, so compression of the polarized gas is required. This adds considerable technical complexity.
Efforts beginning in 1970 to compress polarized 3He using a mercury Toepler pump
were summarized41 in 1985. Pressures of 0.2 bar were possible, but the maximum
polarization of ~2% obtained with available light sources was disappointing. Major
improvements made later by a Paris-Mainz collaboration42 led to development of a
large central production facility in Mainz where 3He is polarized, compressed, and
stored for transport to remote European locations for physics experiments and medical
imaging. That system is described in detail elsewhere43.
It consists of three sections: a 3He reservoir and gas purifiers; two 15 W ytterbium
fiber-coupled lasers illuminating five optical pumping chambers operating at pressures
of 1 mbar; and, a 2-stage, hydraulic-driven, polarization-conserving compressor. The
entire apparatus is located in a 0.8 mT field having relative field gradients (dBr/dr)/B0
of order 10–4cm–1. Such low field gradients are required, since unwanted spin flip can
occur when a polarized atom moves through a region of changing B-field. Using this
system, polarized 3He is compressed first into a 4 liter buffer cell. Once sufficient gas
is polarized, it can then be further compressed into a separate storage cell which can
be detached for transport to a remote site. Pressures of 5 to 6 bar are available.
Performance of this Mainz system is impressive. Final 3He polarizations of up to
84±2% at 0.67 mbar inside the polarizer, or 91±2% at 0.65 mbar inside a sealed
optical pumping cell, can be achieved in several minutes. At a production rate of 30
bar-l/d, P75% to 80% is readily available for fundamental physics measurements.
Higher production of 80 bar-l/d provides gas of P60%, which is acceptable for medical imaging. Polarized gas compression systems have also been developed at NIST44,
at ILL45, and other European laboratories46.
Polarized Gas Storage and Transport
If polarized 3He is stored for use in experiments at sites remote from a polarizer,
cells are needed which preserve the polarization. Research within both the SEOP and
MEOP communities has shown that aluminosilicate glass cells (GE180, Corning 1720,
or Schott Supremax) support longer 3He polarization lifetimes than do borosilicate
Pyrex cells. Pyrex, and to a lesser extent aluminosilicate, can have paramagnetic or
ferromagnetic centers which promote 3He depolarization. After an optical pumping
cell has been placed in a B-field, it can develop a magnetic memory which may be
eliminated by degaussing47. Cells coated inside with Cs, Rb, or Bi also support
increased polarization lifetimes, perhaps because these coatings fill micropores in the
glass and thereby inhibit 3He permeation48. Measurements with seemingly identical
cells reveal cell-to-cell fluctuations of polarization lifetime. Thus, surface effects
associated with cell preparation are still not understood.
As during compression described above, transport of polarized gas requires that a
highly uniform B-field be maintained over the cell. Special transport boxes containing
soft iron, mu-metal, and permanent magnets provide the controlled magnetic
environment needed17,46. Under proper conditions, the best cells enable storage and
transport of polarized gas with lifetimes of ~5 days. The very best Cs-coated quartz
cells at Mainz have been shown to have a 1/e lifetime of 11.5 days17. At NIST, large
cells of reblown GE180 glass, of volumes ranging between 790 and 940 cm3, have
been shown to have 1/e lifetimes ranging between 12.5 and 18.7 days29.
When polarized 3He gas is needed at mbar pressures, clearly MEOP is the preferred
polarization method. If pressures of several bar are needed, however, one has choices.
For large quantities of polarized gas, compression of polarized gas after MEOP can be
very successful, albeit technically rather complex. Transport of the gas after compression to remote sites for medical imaging or physics experiments is clearly feasible.
However, in many cases, the relative simplicity of a local SEOP system at 1 to 10 bar
makes its lower rate of polarizing 3He a preferable experimental inconvenience.
Polarized 3He Ion Sources
Sources Based on 2 3S1 Metastable Atoms
MEOP sources - The first polarized 3He beam was produced at Rice University49
employing methods used there earlier for polarized targets34. A weak discharge was
excited in a Pyrex cell containing ~0.1 mbar of 3He. The mixture of 23S1 metastable
and 11S0 ground state atoms produced there was polarized by the MEOP process
described above. This discharge also created He+ ions, which became polarized by ion
exchange collisions. These 3He species leaked slowly from the Pyrex cell through a
0.34 mm diameter, 2 mm long quartz canal on the axis of an accelerating column.
Emerging He+ ions were focused and pre-accelerated to 15 keV, before further
acceleration to 38 MeV by a cyclotron. After pre-acceleration and after the cyclotron,
respectively, He+ currents up to 8 μA and 100 nA were obtained. The measured
polarization in a diagnostic scattering experiment after the cyclotron was as high as
0.11, but the polarized beam was never used further for nuclear physics.
This 3He beam polarization compares poorly with the best value of ~0.4 being
obtained at that time by MEOP in sealed cells. Several causes for polarization loss
were likely: a) The ion source operating pressure of 0.1 mbar was less than optimum
for the MEOP process; b) The reported 16 s dwell time for 3He in the ion source was
likely insufficient to transfer polarization fully from metastable atoms to ground state
atoms and ions; c) Because the 2 mT B-field imposed over the entire ion source was
very small compared with the 309 mT critical field Bc for He+ (see Table 1), any
nuclear-polarized He+ ion produced in a state with mF < mF,max would have lost much
of its nuclear polarization via hyperfine coupling during acceleration.
Several possible schemes for implementing a modern MEOP 3He source are being
considered now at Brookhaven, to produce polarized 3He beams for use in the Relativistic Heavy Ion Collider (RHIC). Work is already underway there on a new, compact,
electron-beam ion source (EBIS) to produce beams of highly stripped, unpolarized
light-to-heavy ion beams for acceleration in the existing RFQ and linac. This new
EBIS source is also being considered7 as a possible ionizer for polarized 3He.
For example, 3He 23S1 and ground state atoms, polarized at ~1 mbar by MEOP,
could be fed continuously as a gas into the EBIS50 where they would be ionized to
polarized 3He++. Its 5 Tesla B-field is large enough to prevent significant depolarization via hyperfine coupling during ionization. Alternatively, in a scheme originally
proposed51 by Murnick and Mei, 3He metastable 23S1 atoms could be produced in an
external ion source by charge exchange of a 0.75 keV 3He+ beam in sodium vapor52,
and then be optically pumped in flight. With available narrowed lasers, a large fraction of the metastable atoms should be pumped into the pure |F, mF>=|3/2, 3/2> state,
which would then be insensitive to depolarization via hyperfine coupling prior to and
during stripping of the polarized atomic electrons. After being polarized, these
neutral atoms could either be injected axially into EBIS, or stripped to He+ in a 4He
gas cell before injection, before the final stripping to 3He++ occurs.53 A possible advantage of the external source would be its facilitation of interspersed beam pulses of
polarized 3He+ with pulses of different ions injected from another external ion source.
Atomic Beam Source - An alternate method for polarizing a neutral 3He 23S1 metastable atomic beam used Stern-Gerlach separation of atomic hyperfine states in
multipole magnet. This was accomplished in the mid-1980s at the Université de
Laval54 for a compact, atomic-beam-style polarized He+ ion source constructed for a
7.5 MV Van de Graaff accelerator terminal. The 3He metastable atoms created there
by electron bombardment were directed axially through a tapered multipole magnet
wired to produce a N-S-S-N-S-S pole tip B-field having both sextupole and quadrupole components. This field preferentially focused electron-spin-up atoms, which
subsequently underwent RF transitions to create atomic states having higher nuclear
polarization when ionized by electron bombardment in a B-field of 0.2 T. This
process was highly selective because the electron energy was low enough to overcome
the 4.6 eV binding energy of the polarized metastable atoms without ionizing the more
tightly bound 3He ground state atoms. The emerging He+ beam from the ionizer was
then accelerated to 0.9 MeV where it passed through a thin carbon foil to strip off the
remaining atomic electron. This produced up to ~300 nA of He++ after acceleration
with polarization measured to be as high as 60% using the 3He(d,p) reaction.
The environment for this ion source inside both the high-voltage terminal and the
pressure vessel of the Van de Graaff accelerator imposed severe technical limitations.
Insufficient power and cooling restricted the ionizer B-field to 0.2 T, a value smaller
than the ~3Bc needed to obtain the highest polarization. Also, some He+ depolarization
likely occurred via hyperfine coupling before the remaining electron was stripped to
form He++. Work with this source was discontinued before these problems could be
fully addressed.
Techniques Based on 3He+ Metastable and Ground State Ions
The most successful polarized 3He ion source to date was developed in the early
1970s for the University of Birmingham cyclotron55. It used the Lamb-shift in
hydrogen-like 3He+ metastable 2S-state ions to enable selective quenching of unwanted hyperfine states and produce a nuclear polarized beam56,20. In that source the
metastable 3He+ beam was produced by passing a 31 keV He++ beam through a
nitrogen-filled charge-exchange canal. Selective decay of 3He+ hyperfine states was
then induced to produce the polarization when the beam passed through a region of 10
GHz electric field inside a 0.25 T magnetic field. Then the polarized He+(2S) ions
passed through a second charge-exchange canal filled with 4He where they were
stripped to make a polarized He++ beam. These polarized 3He++ ions at 31 keV were
distinguished from a background of unpolarized He++ ions in the beam by decelerating
the latter to 29 keV before the final charge exchange canal. Then a velocity filter of
crossed E and B fields could select the polarized beam component. Experiments with
this polarized ion source lasted for a decade57. Typically ~90 nA at the source resulted
in target beam currents at 33 MeV of 8 nA with polarization up to 65%.
In connection with recent proposals for producing a polarized 3He beam for RHIC,
it is suggested that a similar Lamb-shift polarimetry technique in 3He+(2S) ions be
used to monitor the beam polarization at the ion source before acceleration58. Finally,
recent measurements relevant to a proposal59 to polarize low-energy 3He+ ground state
ions by spin-exchange with rubidium vapor are also discussed in these proceedings60.
Proposed Source Based on the Metastable He- Ion
The (1s2s2p) 4PJ differentially metastable 3He- ion undergoes autoionization with
different lifetimes61 for hyperfine states having different mJ (11±5μs for mJ = ±1/2 or
±3/2, and 345±90μs for mJ = ±5/2). It has been suggested that a beam of ions in the
longest lived |J,mJ> = |5/2,±5/2> states could be polarized by selectively quenching
ions of atoms with one spin projection in a resonant rf field62.
Polarized Source, Neutron Target and Polarimetry with 3He0 Atoms
Polarized 3He 11S0 ground state atoms play key roles in a highly ambitious experiment63,64 to search for a non-zero neutron electric dipole moment (nEDM). Finding a
non-zero value would be sensitive to CP-violation sources beyond the standard model.
This planned experiment seeks to reduce the present upper limit on the measured
nEDM from 6.3x10-26 to ~10-28 e-cm. The experimental concept is as follows: Fully
nuclear polarized 3He0 will be created in an atomic beam source, injected into superfluid 4He, and then mixed with polarized ultra-cold neutrons having energies < 0.13
μeV. There (since μ3He/μn = 1.1) it would serve as a co-magnetometer with the neutron
in a small, highly uniform B-field. If the nEDM ≠ 0, the neutron precession frequency
will be altered by an imposed very strong E-field. The polarized 3He then also acts as
an ultra-cold neutron target in the 3He(n,p)t reaction, which (as in a neutron spin filter)
is suppressed whenever the n and 3He spins are parallel. Photomultipliers will detect
the modulated scintillation light produced by the outgoing proton and triton as the
neutron and 3He precess with slightly different frequencies. Any change in the light
modulation frequency as the strong E-field is applied would signal a non-zero nEDM.
ACKNOWLEDGMENTS
This work was supported in part by Grants #DE-FG02-01ER41041 & -01ER41033
from the USDOE Office of High Energy and Nuclear Physics. The author acknowledges gratefully the help of T. Averett, B. Driehuys, H. Gao, and especially, T. Gentile.
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