A Spin-Exchange Optically Pumped Polarized 3He Target for

Low-Energy Charged Particle Scattering Experiments∗

T. Katabuchi,† S. Buscemi, J. M. Cesaratto,‡ T. B.

Clegg, T. V. Daniels, M. Fassler,‡ and R.B. Neufeld‡

Department of Physics and Astronomy, University of North Carolina,

Chapel Hill, NC 27599-3255, USA and

Triangle Universities Nuclear Laboratory (TUNL), Durham, NC 27708-0308, USA

S. Kadlecek§ and J. Nouls¶

Amersham Health, 2500 Meridian Parkway,

Suite 150, Durham, NC 27713, USA

(Dated: October 24, 2004)

Abstract

We have constructed, tested, and calibrated a new polarized 3He target system which facilitates

p-3He elastic scattering at proton energies as low as 2 MeV. This system consists of a target cell

placed in a uniform B-field inside a scattering chamber and an external optical pumping station

utilizing Rb spin-exchange. Computer-controlled valves allow polarized 3He gas to be transferred

quickly between the optical pumping station and the spherical Pyrex target cell, which has Kapton

film covering apertures for the passing beam and the scattering particles. The magnetic field

required to maintain 3He polarization in the target cell is created with a compact, shielded sine-

theta coil. Target gas polarimetry is accomplished using NMR and calibrated using the known

analyzing power of α-3He scattering.

PACS numbers: 07.55.Db, 29.25.P

∗ Work supported in part by the US Department of Energy, Office of High Energy and Nuclear Physics,

under Grant# DE-FG02-97ER41041.†Present address: Graduate School of Medicine and Faculty of Medicine, Maebashi, Gunma 371-8511, Japan;

Electronic address: katabuchi@tunl.duke.edu‡Supported in part by NSF/REU Grant# NSF PHY-02-43776.§Present address: Department of Radiology, University of Pennsylvania School of Medicine, PA 19104, USA

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¶Present address: Department of Bio-medical Engineering, Duke University, Durham, NC 27708 USA

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I. INTRODUCTION

A new polarized 3He target system has been developed[1][2] with the initial experimental

goal of measuring spin-dependent observables in the p-3He elastic scattering at incident

proton energies below 5 MeV. We are motivated by theoretical approaches which are now

capable of realistic calculations for this four-nucleon system. These microscopic calculations,

based on modern nucleon-nucleon forces determined precisely from two-nucleon experimental

data, are found to underpredict the proton analyzing power in p-3He elastic scattering at

1.20 and 1.69MeV[3]. Comparison with further accurate experimental data for the p-3He

scattering, for different spin observables, are desired to reduce theoretical ambiguities[4].

George and Knutson have performed a phase-shift analysis of p-3He scattering data from

Ep = 1.01MeV to 12.79 MeV [5]. Two phase-shift solutions were still possible when ad-

ditional low-energy measurements were included with an earlier database used in Ref. [6].

They concluded that measurements are needed of target analyzing powers or spin correlation

coefficients below 4 MeV using a polarized 3He target to define a unique set of phase-shift

parameters.

Spin polarized 3He has been used successfully as a scattering target in both nuclear and

elementary particle physics, giving important information on spin-dependent interactions

[7]. Two methods have been used to polarize the 3He: spin-exchange optical pumping

(SEOP)[8]-[11] and metastability-exchange optical pumping (MEOP)[12]-[15]. Each process

has significant advantages and disadvantages. With SEOP, typical optical pumping cell

pressures of 300 to 800 kPa are much higher than the ∼0.1 kPa typical of MEOP. Thus,

when high target pressure is needed, SEOP can provide a larger amount of polarized gas

with a simpler system, since MEOP requires subsequent gas compression during which depo-

larization is a significant concern[15]. However, the rate of polarizing 3He during MEOP is

much higher than that with SEOP. This faster polarizing rate can in some cases compensate

for undesirable sources of 3He polarization loss.

A high target density is desired in particle scattering experiments to enable high counting

rates, leading to smaller statistical uncertainties during shorter experiments. Thus, SEOP

has recently been preferred for polarizing 3He in most experiments[7]. However, a significant

concern for our application was the depolarization from wall relaxation which occurs when

the 3He interacts with interior surfaces of target cells needed at these lower bombarding

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energies. Depolarization by wall relaxation is more critical with SEOP than MEOP, because

its slower optical pumping rate makes it more difficult to overcome these losses.

Only a few aluminosilicate and borosilicate glasses are known to have sufficiently long

spin-relaxation times (∼100 h) to be good choices for polarized 3He containers[16]. However,

no technique has been found to fabricate reliably from these glasses the strong, thin (<

10µm) target cell windows required at proton bombarding energies below 5 MeV. Strong

windows are absolutely essential to hold the target gas at our desired ∼100kPa pressure.

But, the windows must also be thin enough to allow the passing beam to enter and exit the

cell. Additionally and more technically challenging, they must allow lower energy scattered

charged particles to emerge from the target cell to surrounding detectors.

A seemingly simple solution would be to cover apertures in a glass container with thin

foil of another tough material. However, no foil material has been found to provide wall-

relaxation times of comparable length to those with polarized-3He-friendly glasses. Conse-

quently, all gaseous polarized 3He target systems for low-energy charged-particle scattering

experiments have utilized MEOP[6],[17]-[21]. Those targets were operated at low 3He pres-

sures and had windows fabricated of very thin glass [17], [18] or apertures covered with thin

molybdenum foils[6], [19]-[21]. These solutions provided an acceptable wall-relaxation rate

for MEOP.

Our new polarized target system is designed to utilize SEOP with rubidium to polarize

3He, thereby allowing experiments at a high counting rate from a high target density. The

system consists of two separate parts: a unique target cell and a separate optical pump-

ing station. We report details here on both systems, which have been built and proven

satisfactory for the intended scattering experiments.

II. POLARIZED 3HE TARGET

A. System overview

Our entire polarized 3He target system is shown schematically in Fig. 1. The incident

charged particle beam passes through a sealed target cell at ∼100kPa pressure placed inside

a scattering chamber which is under vacuum. The target cell resides inside a mu-metal-

shielded, cylindrical sine-theta coil aligned coaxially with the beam axis. This produces

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a highly uniform ∼0.7 mT magnetic field whose direction can be set to any azimuthal

orientation perpendicular to its cylindrical axis. Scattered particles emerge from the cell in

the horizontal plane on both sides of the beam. They pass through small apertures in the

sine-theta coil into a movable array of Si surface barrier detectors symmetrically positioned

on the left and right sides of the beam at laboratory scattering angles between 30 and 150.

Nuclear polarized 3He gas is created externally to the chamber by spin-exchange with

optically pumped rubidium. Optical pumping occurs inside a mu-metal-shielded solenoid,

which provides a uniform magnetic field to maintain the 3He polarization. The target cell

and the optical pumping station are connected through a plastic tube. Absolute polarization

of the 3He inside the target cell is monitored by a calibrated pulsed NMR system which uses

a small coil placed next to the cell.

This system is completely different from previous closely-connected SEOP “two-chamber”

targets, which consisted of two separate glass chambers, one for the passing particle beam,

the other for optical pumping[22]. There, 3He atoms polarized in the pumping chamber in-

terchange continuously with atoms in the target chamber by diffusion through the short glass

transfer tube. By contrast, our optical pumping station allows rapid filling of the target cell

from the higher pressure optical pumping cell, and incorporates computer-controlled valve

manifolds to facilitate moving 3He gas quickly when needed between the optical pumping

cell and the target cell in the scattering chamber. Thus, we can evacuate the target cell

and introduce fresh polarized gas when the polarization of the gas becomes too small to be

useful.

Refreshing target gas frequently allows the 3He spin relaxation time there to be shorter

than in aluminosilicate glass target cells. Our experiments impose this because the spin-

relaxation time of 3He in our target cell is dominated by wall relaxation on the window

material. We thus chose easily fabricated Pyrex target cells having thin-film windows for

the primary proton beam and scattered particles.

B. Optical pumping station

The optical pumping station is shown schematically in Fig. 1. An optical pumping cell,

storage cells, a valved manifold, and a diaphragm pump are all placed inside a 29.4-cm

diameter, solenoid coil shielded externally and on both ends by 1.4-mm thick mu-metal.

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The solenoid provides a highly uniform interior magnetic field of ∼0.7 mT to establish the

3He spin quantization axis. The optical pumping cell and storage cells have a 200-cm3 inner

volume and are made of GE180 aluminosilicate glass. In this environment, our pumping

cell has a spin-relaxation time of 36 hours. The storage cells, which have no rubidium, have

spin-relaxation times of ∼7 hours. In practice, the storage cells are rarely used. Our original

plan to polarize gas in the optical pumping cell, transfer this to a storage cell from which it

could be dispensed, while a second batch of 3He was being polarized in the optical pumping

cell, has proven impractical.

A fiber-coupled, 8-diode laser array[23] is used to illuminate rubidium vapor in the optical

pumping cell. Each diode’s power level was adjusted to tune its temperature and output

wavelength to maximize light absorption by rubidium at 795 nm. The resulting width of

the composite spectral distribution was 2.2 nm FWHM. The total output of the diode array

was measured to be 80 W, of which 65 W is focused onto the pumping cell. This light enters

the solenoid through a 7.5-cm diameter central hole in its top mu-metal cover. Optical

components placed between the laser and the pumping cell focus and split the laser beam

emerging from the fiber bundle into two beams of linearly polarized light. Each beam is then

circularly polarized with a 1/4-wave plate after the splitter or reflector. Two plano-convex

lenses then converge the two light beams at the center of the pumping cell. The lens focal

lengths were chosen to magnify the fiber bundle image so the laser light covers the entire

pumping cell. The distance of the laser fiber head from the first lens was adjusted to blur the

image of individual fibers in the laser head, in order to illuminate better the entire optical

pumping cell volume.

The optical pumping cell, when placed inside a gypsum oven cavity, is heated conveniently

by the incident laser light. To maintain the∼185C needed to provide the optimum Rb vapor

density, cooling air flow to the oven cavity is regulated by a computer-controlled pneumatic

valve. The oven temperature is monitored with a platinum resistance thermometer attached

to the pumping cell and regulated with an uncertainty less than 1 degree C.

An NMR coil attached to the optical pumping cell monitors the relative 3He polarization

using a pulsed NMR technique described in Sec. IIG. At typical operating pressures of

800 kPa of 99% 3He and 1% N2 in the optical pumping cell, the time to achieve maximum

polarization of ∼30% is roughly 24 hours.

We use non-magnetic, aluminum alloy, pneumatic valves [24] for gas handling inside

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the solenoid. These are activated with 60 psi air pressure which is separately controlled by

external electronic valves. Eight such aluminum valves are connected by compression fittings

to a compact, one-piece aluminum manifold which was machined to have an 6.4 mm OD,

3.2 mm ID tube on each side. When gas transfers occur rapidly, the valves and manifold

cause only minor loss of 3He polarization.

As shown in Fig.1, the optical pumping cell, four storage cells, the target cell, the di-

aphragm pump DP1, and an external manifold are all interconnected through the valved

internal manifold. A separate manifold, external to the solenoid, has stainless-steel pneu-

matic valves for a vacuum pump, gas filling of 4He and N2, and the input and output of a

diaphragm pump DP2. Internal and external manifolds are connected through a 3.2 mm ID

stainless steel and copper tubes. The target cell in the scattering chamber and the internal

manifold are connected through a ∼1.5 m long, 6.3 mm OD, 3.2 mm ID perfluoroalkoxy

(PFA) tube.

We use the diaphragm pump DP1 inside the solenoid to compress unpolarized 3He gas

at 800 kPa into the optical pumping cell from lower pressure storage bottles. This pump

consists of two polycarbonate halves, each machined with an internal hemisphere. They are

fastened tightly together with a rubber diaphragm sandwiched between them to form two

independent interior chambers. The upper chamber is connected with the internal manifold

and the lower is connected with two outside pneumatic valves, one for a vacuum pump used

when “inhaling” gas into the upper chamber, and the other for high pressure N2 used when

“exhaling” this gas. After using polarized 3He gas in the target cell, we use the external

diaphragm pump DP2 and gas recirculation system described in Sec.II E first to transfer

much of the gas of depleted polarization into storage bottles, and then later to circulate it

thorough a purification system before transferring it again to the pumping cell.

C. Target cell

Our target cell design evolved slowly by trial and error, as we sought the best over-

all compromise between acceptably long 3He polarization lifetime, maximum 3He pressure,

minimum energy loss of the incident and scattered beam, and overall simplicity of fabri-

cation. Our cells were all made from commercial grade Pyrex glass tubing. Such Pyrex

is porous, which shortens the spin lifetime of 3He through trapping of polarized gas in the

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microvoids. Thus, the Pyrex was reflowed during fabrication to reduce its porosity[25].

For the measurements reported in Sect.III, the target cell placed in vacuum inside our

scattering chamber was roughly spherical, with a diameter of ∼5 cm (Fig.2). The cell had

10 mm wide × 6.5 mm high entrance and exit apertures for the primary beam, and a 6.5

mm high windows around each entire side for emerging scattered particles. A 20 cm long

L-shaped Pyrex capillary fill tube with an inside diameter of 0.9 mm was affixed to the

beam-exit side of the cell to restrict diffusion between the target cell and external systems

after loading the polarized gas. A tee at the end of the capillary allowed connection both

of a strain gauge pressure transducer[26] to monitor the target cell pressure, and connection

to the manifold of the 3He polarizer via a ∼2 m long, 3 mm inside diameter PFA tube with

an intermediate, manual PFA shut-off valve.

Beam entrance and exit apertures, and side windows for the emerging scattered particles,

were covered with 25 and 7.5µm thick Kapton foils, respectively. These foils were epoxied

onto the Pyrex with Varian Torr Seal[27]. Kapton and Torr Seal were chosen after testing

because they facilitated the best overall combination of high target cell pressure and long 3He

spin relaxation time. For incident proton beam energies below 3 MeV, the 7.5µm Kapton

was too thick to allow scattered protons to emerge over the full angular range. Thus, after

epoxying the films onto the target cell, the entire target cell was dipped for ∼40 sec into a

6 wt.% KOH solution in a solvent mixture (80wt.% ethanol + 20 wt.% water) held at 70C

to reduce the Kapton thickness by ∼3.7µm by etching [28]. This was found by extensive

testing to provide the thinnest windows which could still withstand the desired 100kPa

interior target cell pressure.

Two computer-controlled pneumatic valves on the internal valve manifold facilitate rapid

filling of the target cell from the optical pumping cell through a PFA tube. Before admitting

polarized 3He gas, we purge the target cell and gas transfer line in several cycles of filling

and evacuating high-purity N2. Also, to avoid depolarization of the 3He gas in large B-field

gradients, the mu-metal end cover of the polarizer solenoid is temporarily removed during

the actual polarized gas transfer.

Once all relevant lines are purged, the small volume of the internal manifold is first filled

with higher pressure polarized 3He gas by opening and closing the pneumatic valve to the

optical pumping cell. Immediately, this small amount of gas is transferred to the target

cell by opening another pneumatic valve. This process is repeated until the pressure in the

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target cell rises to 75 to 100 kPa. Three cycles are required when the optical pumping cell

is at 800kPa. In order to minimize the time polarized gas spends in the internal manifold

and transfer line, thus reduce its depolarization, the polarized gas transfer procedure is

performed as quickly as possible.

D. Materials testing

We made extensive wall relaxation measurements to select the best target cell window

materials before choosing Kapton and TorrSeal. A sample of each material with a surface

area of 77 cm2 was prepared, often by evaporating highly pure materials onto microscope

slides. We then inserted each sample into a 250-cm3 spherical Pyrex test cell filled with

polarized 3He gas. The wall relaxation time for the test cell without a sample was ∼14

hours. The wall relaxation time of 3He gas with each sample was obtained by measuring the

3He polarization as a function of time using pulsed NMR. The results are shown in Table

I. The wall relaxation times were derived from measured spin lifetimes by subtracting the

spin-relaxation time of the empty cell using the relationship 1T

= 1Te

+ 1Tw

, where T is the

measured spin-relaxation time, Te is for the empty cell and Tw is wall relaxation time for

each sample. In order to allow comparison between samples of different surface area each

table entry is the derived relaxation time constant of a 3He gas contained with a 150cm3

sphere coated with that material.

In spite of pure aluminum’s having the longest wall relaxation time of all window materials

we measured, we found that 25µm thick, pure aluminum foil had less mechanical strength

than the Kapton and could not withstand the experimentally desired 100 kPa target cell

pressure. Kapton also proved to be preferable to aluminum because it causes less energy

loss per unit thickness for low-energy protons.

We tested a 25µm Kapton foil by bombarding it with 5 MeV protons to determine if it

would withstand heating by the beam. The Kapton was glued on with Torr Seal and the

cell was pressurized to 100 kPa. The foil held the pressure for nearly 1 h while the beam

current was steadily increased, and finally held for 10 min at ∼1.5 µA. However, during later

measurements, the most frequent failure of our cell after use for several days was a slow gas

leak at the epoxy layer around the aperture for the emerging beam. We attribute this to

beam heating, which eventually caused the window’s epoxy seal to weaken.

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The measured spin relaxation times of 3He in spherical target cells described above ranged

between 2 and 3 hr. Our initial optical pumping cell pressure was 800 kPa, and the volume

ratio of the target to pumping cell allowed the target cell to be refilled from this gas twelve

times to 100 kPa. Because our typical polarized target gas refresh interval (dictated by the

depolarization rate of the target cell gas) is 1.5 to 2 hr, it is practical to conduct experiments

for 18 to 24 hours with the polarized 3He gas produced in one cycle of optical pumping.

E. 3He gas recovery and reuse

The external manifold also facilitates recovery of 3He gas of depleted polarization after use

in the target cell (Fig.1). The recovery system consists of the external diaphragm pump DP2

[29], three gas storage bottles, a liquid N2 cooled, activated charcoal trap and a purifier. The

used 3He gas is transferred to the storage bottles (1.3 ` total volume) with the diaphragm

pump which is capable of compressing gas up to ∼500kPa in the output line and evacuating

the input line to ∼10kPa. The trap and commercial purifier[30] remove oxygen, nitrogen

and water from the recovered 3He gas before reloading it into the optical pumping cell.

We measured contaminant partial pressures in the recovered 3He gas using a residual gas

analyzer before and after recirculation through the purification system. This showed that

O2 and N2 partial pressures were lowered to background levels after several recirculation

cycles.

F. Target B-field

The 0.7 mT magnetic field to maintain 3He polarization in the target cell is generated

with a shielded, compact sine-theta coil shown in Fig. 3. This consists of 24 parallel 3.2

mm diameter copper rods located at equal intervals around a hollow, 7.5 cm diameter, 30

cm long Delrin cylinder as depicted in Fig. 4. Electrically, the rods are connected such that

the current in one rod returns through the diametrically opposite rod. Each rod-pair has

its current regulated by a combination of an operational amplifier and a MOSFET which is

computer controlled via National Instruments FieldPoint I/O modules.

Figure 5 shows the diagram for one control circuit. The voltage at the positive input

of the operational amplifier is set with a FieldPoint 12-bit analog output module. The

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amplifier’s output voltage controls the current through a MOSFET which in turn is placed

in series with a common, highly regulated external power supply, a polarity reversing relay,

a 0.07 Ω resistor, and one rod-pair. The voltage across the series resistor produced by the

current provides a feedback signal to the second amplifier input. The computer monitors the

output current by measuring the voltage across the series resistor with a FieldPoint 12-bit

analog input module. This enables the computer to set and hold the 12 individual rod-pair

currents at values proportional to the sine of the angle defining their azimuthal location.

Each angle is measured clockwise from the desired spin quantization axis, as viewed when

looking in the beam direction, so the 24 rod currents collectively create a uniform B-field

in the magnitudes and directions inside the cylinder (Fig.4). The computer is programmed

in LabVIEW to set the input voltages of the amplifiers and monitor the currents running

through the rods. By successive changes in the magnitude and directions of the currents, the

computer is also capable of rotating the direction of this magnetic field while maintaining

its magnitude and overall spatial uniformity. Discrete output FieldPoint modules switch

the polarity reversing relay for each rod-pair to invert the current direction whenever its

current passes through zero. The 0.7 mT B-field can thus be reversed by 180 in less than

10 s without perceptible loss in 3He target polarization, because the 3He Larmour precession

rate around the B-field axis is much faster than the rate at which that axis orientation is

changing.

The sine-theta coil is surrounded with a 0.64 mm thick mu-metal tube which both en-

hances the magnetic field by confining it inside the cylinder and shields the enclosed target

cell region from unwanted external magnetic fields. This tube has apertures on both sides

for scattered particles to emerge to the left and right at angles from 30 to 150 in 20 steps.

The cylinder can be moved axially along the Delrin cylinder to shift the aperture locations

and allow emerging particles to be detected at other intermediate angles.

The basic design of the sine-theta coil was determined using the two-dimensional magnetic

field code POISSON/SUPERFISH[31]. Calculation showed that, when considered in a plane

perpendicular to the coil axis, our coil geometry provides a uniform magnetic field with

inhomogeneity 1B

∂B∂x

less than 1 × 10−3/cm in most of the inner region. After fabrication,

the magnetic field was mapped three-dimensionally with a computer-controlled three-axis

Hall probe. The field inhomogeneity, even with the side windows for escaping particles, was

found to be less than 2×10−3/cm, and in most of the target area was less than 1×10−3/cm.

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This satisfies the required uniformity to suppress 3He spin relaxation time in the target cell

below 20 h[32]. The shielded, compact sine-theta coil also satisfies a local requirement that

we be able to use an already-existing scattering chamber which facilitates precise angular

distribution measurements. The sine-theta coil and its enclosed target can be used in the

scattering chamber without concern from adjacent magnetic components of the chamber

and beamline support systems.

G. NMR polarimetry

The target polarization is monitored using pulsed nuclear magnetic resonance. A 2.5-cm

diameter, 0.64-cm thick coil placed against the target cell generates a 0.1-msec pulse of 24-

kHz RF magnetic field, tipping the spin of polarized 3He nuclei at a small angle (∼ 0.1).

Then an induced voltage from free precession of polarized 3He around the main magnetic

field is detected with the same coil used as a pickup. The main magnetic field produced

with the sine-theta coil is adjusted to be 7.4 Gauss, in which the 3He Lamour frequency

is 24 kHz. The amplitude of the induced NMR voltage is proportional both to the 3He

target cell polarization and pressure. The pressure is monitored using a dual port pressure

sensor [26] connected to the input gas line, with the scattering chamber vacuum serving as a

pressure reference at one sensor port. The linear calibration coefficients to convert the NMR-

voltage-to-pressure ratio into actual 3He gas polarization are determined experimentally.

This calibration procedure used α-3He elastic scattering, and results obtained are described

below in section III.

The target cell is held by a Delrin frame which fits inside the Delrin coil form (Figs. 2

and 3). The sine-theta coil is aligned carefully so that the alignment of the target cell with

the beam axis is provided from snug fit of the target frame inside the cylindrical Delrin coil

form. The target frame holds the NMR coil against the flat surface of the target cell at

the beam-exit window. The NMR coil also has a 7-mm axial hole to allow passage of the

incident beam. The entire sine-theta coil and target assembly is attached to an external

support frame which, once aligned, can easily be removed from, and reinserted into the

scattering chamber between experiments without disturbing the system’s overall alignment.

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III. TARGET POLARIZATION CALIBRATION

As mentioned in section IIG, the NMR signal provides only a relative value of 3He po-

larization. It must be calibrated experimentally to know the absolute polarization. We

determined the calibration factor by measuring the 3He asymmetry in α-3He elastic scatter-

ing interspersed with a sequence of NMR measurements and target spin reversals.

The scattering asymmetry is defined as ε = PAy(θ) = NL−NR

NL+NR, where NL and NR are

numbers of scattered 3He detected symmetrically with respect to the α-beam direction at

angles θ on the left (right) and right (left) sides of the target with target spin up (down).

Here P is the polarization of 3He target and Ay is the analyzing power, a spin-dependent ob-

servable which is a function of incident energy and scattering angle. Plattner and Bacher[33]

have shown, from analysis of phase shifts which describe α-3He elastic scattering, that an

absolute |Ay| = 1 point exists near Eα = 15.3 MeV and θ3He = 47 in the laboratory. We

have conducted a series experiments to find the absolute |Ay| = 1 point experimentally

and concluded that the point is located at 45 in laboratory, which is very close to their

prediction[34].

The actual calibration experiment was carried out using a 4He beam from a tandem elec-

trostatic accelerator focused down the axis of the sine-theta coil onto the target cell through

two sets of collimating slits located 130 and 8 cm in front of the target cell, respectively. The

first(second) slits defined horizontal and vertical apertures of 2.5 mm and 2.5 mm (1.5 mm

and 1.5 mm), respectively. Recoil 3He emerging through the Kapton windows into vacuum

were detected in a pair of 300 µm thick, silicon surface-barrier detectors located at 45 to

the left and right of the beam direction. The incident beam energy was adjusted, after losses

in the entrance Kapton foil and 3He gas, to be 15.3 MeV at the center of the target cell.

The target was bombarded continuously, such that a 4He ion current of 100 nA was

collected in a suppressed Faraday cup located 45 cm downstream. Spectra of detected

3He particles were accumulated for pairs of target spin up-down runs lasting ∼10 minutes.

Between each run pair, the target polarization was monitored by an NMR measurement,

followed by target spin reversal, followed by a second NMR measurement. Average target

polarization during the run pair was determined later after determining the overall spin-

down lifetime of the target polarization, but was usually indistinguishable from the average

of the two NMR measurements made just before and after the target spin reversal. A plot

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of the asymmetry of scattered 3He versus the NMR output voltage is shown in Fig. 6.

Assuming that Ay=-1 for 3He(α,3He)4He scattering at θlab = 45 and Eα=15.3 MeV, the

calibration factor for polarization measurements with the NMR coil was 6.31 × 10−4/mV ·atm.

As long as conditions remain the same during subsequent experiments, i.e. 1) the relative

position of the NMR coil with respect to the target cell is held constant, 2) the pressure in the

target cell remains nearly constant so the bulging Kapton foil next to the NMR coil remains

in the same location, and 3) the nitrogen partial pressure in the target cell remains small

and/or is corrected for, then this technique facilitates monitoring the absolute polarization

of 3He target nuclei using the NMR coil.

We did not observe a significant difference in the 3He spin relaxation time in the target

cell with or without beam. Depolarization of the 3He by charged-particle beam irradiation

has been considered previously[35], [36]. Depolarization is caused by atomic ions, 3He+, and

molecular ions, 3He+2 , created by beam ionization. This earlier work demonstrated that N2

mixed with the 3He suppresses the depolarization. Using Ref.[35], we calculated a depo-

larization rate of 1.5×10−5 s−1 for our experimental conditions. This value is considerably

smaller than our typical target cell depolarization rate (∼ 10−4) from wall relaxation, and

supports our observation that no beam dependent change in the depolarization rate was

ever observed.

IV. SUMMARY AND FUTURE CHALLENGES

We have constructed a unique polarized 3He target system, with physically distant Rb

spin-exchange optical pumping and target cells, which facilitates low-energy p-3He elastic

scattering experiments. To maintain 3He polarization, the Pyrex target cell with very thin

Kapton foil windows resides in the magnetic field produced by a compact sine-theta coil.

The spin relaxation time for 100 kPa of 3He in this cell is typically 2 to 3 hours. Computer

controlled gas handling facilitates transfer of polarized gas from the optical pumping cell to

the target cell, and recovery and purification of the 3He gas of depleted polarization before

reuse. An NMR coil closely coupled to the target cell, calibrated in a separate experiment

using α-3He scattering, provides continuous monitoring of the absolute polarization of the

3He target gas during experiments.

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Our largest challenge still to be faced is raising the 3He polarization. Success by oth-

ers, especially in pumping with narrowed lasers, indicates that may be possible[37]. Poor

reliability of the thinned Kapton foil windows needed at our lowest bombarding energies,

against developing small leaks, is a continuing challenge for which we have no easy solution.

The NMR calibration of our present system is reproducible only to ∼10% when the target

cell is removed from its holder for repair of leaks in Kapton windows or epoxy seals. Thus,

our present design requires a new NMR calibration experiment with 4He beam after such a

repair, a procedure which is inconvenient at best. Changes in target cell geometry will be

explored which make the NMR calibration much less sensitive to slight relative movements

of the target cell and NMR coil.

V. ACKNOWLEDGEMENTS

We benefited considerably during the early design stages of these polarized target systems

from advice of Thomas Gentile and Hans Paetz gen. Schieck. Throughout this development,

the strong encouragement and support of Bastiaan Driehuys, and the experimental assis-

tance of Hugon Karwowski, were essential. Skilled support from instrumentmakers Phillip

Thompson and Bernard Jelinek of the UNC and Duke machine shops, respectively, from

TUNL electronics technician, Bret Carlin, and from UNC glassblower Walter Boger, are

also gratefully acknowledged.

[1] T. Katabuchi et al., Proceedings of International Workshop on Polarized 3He Beams and Gas

Target and Their Applications HELION02, Oppenheim, Germany, 2002.

http://www.physik.uni-mainz.de/helion02/proceedings/Talks/Tue-06 Katabuchi.pdf

[2] M. Fassler et al., Proceedings of International Workshop on Polarized 3He Beams and Gas

Target and Their Applications HELION02, Oppenheim, Germany, 2002

http://www.physik.uni-mainz.de/helion02/proceedings/Posters/P-06 T.B. Clegg.pdf

[3] M. Viviani et al., Phys. Rev. Lett. 86, 3739 (2001)

[4] B. M. Fisher et al., Proceedings of 17th International Conference on Few-Body Problems in

Physics, Durham, USA, 2003

15

[5] E. A. George and L. D. Knutson, Phys. Rev. C67, 027001 (2003)

[6] M. T. Alley and L. D. Knutson, Phys. Rev. C 8, 1890 (1993),M. T. Alley and L. D. Knutson,

Phys. Rev. C 8, 1901 (1993)

[7] T. E. Chupp et al., Annu. Rev. Nucl. Part. Sci, 44 (1994)

[8] T. G. Walker and W. Happer, Rev. Mod. Phys. 69, 629 (1997)

[9] W. J. Cummings et al., Phys. Rev. A51, 4842(1995)

[10] M. E. Wagshul and T. E. Chupp, Phys. Rev. A49, 3854 (1994)

[11] M. A. Bouchiat et al., Phys. Rev. lett. 5, 373 (1960)

[12] E. Stoltz et al., Appl. Phys. B63, 629 (1996)

[13] T. R. Gentile and R. D. McKeown, Phys. Rev. A47, 456 (1993)

[14] F. D. Colgrove et al., Phys. Rev. 132, 2561 (1963)

[15] T. R. Gentile et al., Journal of Research of the National Institute of Standards and Technology,

106, 709 (2001)

[16] T. B. Smith et al., Nucl. Instrum. Meth. A 402, 247 (1998)

[17] Ch. Leemann et al., Helv. Phys. Acta 44, 141 (1971)

[18] U. Rohrer et al., Helv. Phys. Acta 41, 436 (1968)

[19] S. D. Baker et al., Phys. Rev. 178, 1616 (1969)

[20] D. M. Hardy et al., Nucl. Instrum. Meth. A 98, 141 (1972)

[21] D. M. Hardy et al., Nucl. Phys. A 195, 250 (1972)

[22] T. E. Chupp et al., Phys. Rev. C 45, 915 (1992)

[23] OptoPower Corporation, Model OPC-A150-795-RPPS

[24] Pneumatically actuated Swagelok toggle valves, Model A92S4-C-EP-DF-W15547, ordered

with the following modifications: SC-11 clean, clear anodize aluminum body, titanium stem,

Dow 111 grease, wetted O-ring to be FDA-EP

[25] M. J. Souza, private communication.

[26] Motorola MPX4250D silicon dual port pressure sensor,

http://www.motorola.com/semiconductors/

[27] Varian Inc., http://www.varianinc.com

[28] DuPont, http://www.dupont.com/kapton/general/caustic-etching.html

[29] KNR Neuberger Inc., Model UN035.3 TTP

[30] NuPure UltraPure PF Series, http://www.nupure.com/

16

[31] Los Alamos National Laboratory report LA-UR-96-1834

[32] R. L. Gamblin and T. R. Carver, Phys. Rev. A 4, 946 (1965)

[33] G. R. Plattner and A. D. Bacher, Phys. Lett. 36B, 211 (1971)

[34] T. Katabuchi et al., Triangle Universities Nuclear Laboratory Progress Report XLIII (2004)

[35] K. D. Bonin et al., Phys. Rev. A 37, 3270 (1988)

[36] K. P. Coulter et al., Nucl. Instrum. Meth. A 276, 29 (1989)

[37] B. Chann et al., J. Appl. Phys. 94, 6908 (2003)

Tw (h)

Kapton 1.5

Pure Aluminuma 3.7

Aluminum Alloy: AL6061 0.62

Molybdenumb 0.94

Varian Torr Seal Epoxy 8.0

Armstrong A-12 Epoxy 0.025

aImpurity: 99.999%bImpurity: 99.95%

TABLE I: Wall relaxation times Tw for several materials

17

OPC

TC

SC SC

SC SC

VP

N2

3He

VP

N2Purifier

Sine-Theta Coil

Oven

Cooling Air

CompressedAir Line

DP1

DP2

SB

SB

SB

Activated Charcoal Trap

Computer-Controlled Pneumatic Valve

Manual Valve

Regulator

4He

RGA

InOut

Solenoid Coil

Mu-metal Shield

Pressure Gauge or Vacuum Gauge

Laser Optics

FIG. 1: Schematic diagram of TUNL polarized 3He target system. TC: Target Cell, OPC: Optical

Pumping Cell, DP: Diaphragm Pump, SC: Storage Cell, RGA: Residual Gas Analyzer, VP: Vacuum

Pump, SB: Storage Bottle18

Delrin Holder

Target Cell

Beam Entrance Side Window

NMR Coil

Beam

FIG. 2: The target cell and holder. The pictures do not show Kapton foil epoxied on the cell.

19

Side Windows for Emerging Particles

Copper Rods

Glass Capillary of Target Cell

Mu-Metal Shield

Delrin Coil Form

FIG. 3: A drawing showing the sine-theta coil and target cell assembly on its mounting platform

inside the scattering chamber. Not shown are the wiring harnesses which carry currents to each

end of the copper rods, and the pressure sensor and input gas line which connect to the target cell

capillary.

20

B

θθθθInward

Outward0

Max

0

Max

Mu-Metal Tube

FIG. 4: Schematic cross sectional view of the sine-theta coil showing the 12 diagonally opposing

pairs of current carrying rods, with current directions indicated. To create the vertical magnetic

field shown, each line current is proportional to sin θ, requiring maximum (minimum) current

values at θ=90 and 270 (0 and 180).

21

FP Analog Out

-15 V

FP Analog In

FP Digital Out

High CurrentPower Supply

Sine-Theta Coil

TI TL082CP

IRFP0489310 Ohm (1%)

1 M Ohm

698 Ohm 0.07 Ohm (0.05%)

PolaritySwitching

Relay

FIG. 5: Schematic diagram of the sine-theta coil circuit. One circuit drives each rod-pair, and

the output current is computer controlled. A National Instruments FieldPoint (FP) analog output

module sets the input voltage of the operational amplifier. A FP analog input module monitors

the output current of the MOSFET by measuring voltage across 0.07 Ω(0.05 %) resistor. A FP

digital output module controls the current polarity.

22

0 100 200 300 4000

0.1

0.2

NMR Amplitude (mV)

Asy

mm

etry

FIG. 6: Plot of left-right asymmetry of recoil 3He vs NMR amplitude. The best-fit curve yields a

calibration constant of 6.31× 10−4/mV.

23