A Spin-Exchange Optically Pumped Polarized 3 He 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 3 He target system which facilitates p-3 He 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 3 He 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 3 He polarization in the target cell is created with a compact, shielded sinetheta coil. Target gas polarimetry is accomplished using NMR and calibrated using the known analyzing power of α-3 He 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:
[email protected] ‡
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
1
¶
Present address: Department of Bio-medical Engineering, Duke University, Durham, NC 27708 USA
2
I.
INTRODUCTION
A new polarized 3 He target system has been developed[1][2] with the initial experimental goal of measuring spin-dependent observables in the p-3 He 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-3 He elastic scattering at 1.20 and 1.69MeV[3]. Comparison with further accurate experimental data for the p-3 He scattering, for different spin observables, are desired to reduce theoretical ambiguities[4]. George and Knutson have performed a phase-shift analysis of p-3 He scattering data from Ep = 1.01MeV to 12.79 MeV [5]. Two phase-shift solutions were still possible when additional 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 3 He target to define a unique set of phase-shift parameters. Spin polarized 3 He 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 3 He: 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 depolarization is a significant concern[15]. However, the rate of polarizing 3 He during MEOP is much higher than that with SEOP. This faster polarizing rate can in some cases compensate for undesirable sources of 3 He 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 3 He in most experiments[7]. However, a significant concern for our application was the depolarization from wall relaxation which occurs when the 3 He interacts with interior surfaces of target cells needed at these lower bombarding 3
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 3 He 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 wallrelaxation times of comparable length to those with polarized-3 He-friendly glasses. Consequently, all gaseous polarized 3 He target systems for low-energy charged-particle scattering experiments have utilized MEOP[6],[17]-[21]. Those targets were operated at low 3 He pressures 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 3
He, 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 pumping station. We report details here on both systems, which have been built and proven satisfactory for the intended scattering experiments.
II.
POLARIZED 3 HE TARGET A.
System overview
Our entire polarized 3 He 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-metalshielded, cylindrical sine-theta coil aligned coaxially with the beam axis. This produces
4
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 3 He 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 3 He polarization. The target cell and the optical pumping station are connected through a plastic tube. Absolute polarization of the 3 He 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, 3 He atoms polarized in the pumping chamber interchange 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 3 He 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 3 He spin relaxation time there to be shorter than in aluminosilicate glass target cells. Our experiments impose this because the spinrelaxation time of 3 He 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. 5
The solenoid provides a highly uniform interior magnetic field of ∼0.7 mT to establish the 3
He 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 3 He 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 3 He polarization using a pulsed NMR technique described in Sec. II G. At typical operating pressures of 800 kPa of 99% 3 He 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 6
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 3 He polarization. As shown in Fig.1, the optical pumping cell, four storage cells, the target cell, the diaphragm pump DP1, and an external manifold are all interconnected through the valved internal manifold. A separate manifold, external to the solenoid, has stainless-steel pneumatic valves for a vacuum pump, gas filling of 4 He 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 3 He 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 3 He 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 overall compromise between acceptably long 3 He polarization lifetime, maximum 3 He pressure, minimum energy loss of the incident and scattered beam, and overall simplicity of fabrication. Our cells were all made from commercial grade Pyrex glass tubing. Such Pyrex is porous, which shortens the spin lifetime of 3 He through trapping of polarized gas in the 7
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 3 He 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 3 He 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 3 He 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 3 He 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 3 He 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 8
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 3 He gas. The wall relaxation time for the test cell without a sample was ∼14 hours. The wall relaxation time of 3 He gas with each sample was obtained by measuring the 3
He 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
1 T
=
1 Te
+
1 , Tw
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 3 He 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. 9
The measured spin relaxation times of 3 He 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 3 He gas produced in one cycle of optical pumping.
E.
3 He
gas recovery and reuse
The external manifold also facilitates recovery of 3 He 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 3 He 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 3 He gas before reloading it into the optical pumping cell. We measured contaminant partial pressures in the recovered 3 He 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 3 He 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
10
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 3 He target polarization, because the 3 He 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 enhances 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
1 ∂B 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. 11
This satisfies the required uniformity to suppress 3 He 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 24kHz RF magnetic field, tipping the spin of polarized 3 He nuclei at a small angle (∼ 0.1◦ ). Then an induced voltage from free precession of polarized 3 He 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 3 He Lamour frequency is 24 kHz. The amplitude of the induced NMR voltage is proportional both to the 3 He 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 NMRvoltage-to-pressure ratio into actual 3 He gas polarization are determined experimentally. This calibration procedure used α-3 He 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.
12
III.
TARGET POLARIZATION CALIBRATION
As mentioned in section II G, the NMR signal provides only a relative value of 3 He polarization. It must be calibrated experimentally to know the absolute polarization. We determined the calibration factor by measuring the 3 He asymmetry in α-3 He elastic scattering interspersed with a sequence of NMR measurements and target spin reversals. The scattering asymmetry is defined as ² = P Ay (θ) =
NL −NR , NL +NR
where NL and NR are
numbers of scattered 3 He 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 3 He target and Ay is the analyzing power, a spin-dependent observable which is a function of incident energy and scattering angle. Plattner and Bacher[33] have shown, from analysis of phase shifts which describe α-3 He elastic scattering, that an absolute |Ay | = 1 point exists near Eα = 15.3 MeV and θ3 He = 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 4 He beam from a tandem electrostatic 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 3 He 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 3 He gas, to be 15.3 MeV at the center of the target cell. The target was bombarded continuously, such that a 4 He ion current of 100 nA was collected in a suppressed Faraday cup located 45 cm downstream. Spectra of detected 3
He 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 spindown 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 13
of the asymmetry of scattered 3 He versus the NMR output voltage is shown in Fig. 6. Assuming that Ay =-1 for 3 He(α,3 He)4 He 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 3 He target nuclei using the NMR coil. We did not observe a significant difference in the 3 He spin relaxation time in the target cell with or without beam. Depolarization of the 3 He by charged-particle beam irradiation has been considered previously[35], [36]. Depolarization is caused by atomic ions, 3 He+ , and molecular ions, 3 He+ 2 , created by beam ionization. This earlier work demonstrated that N2 mixed with the 3 He suppresses the depolarization. Using Ref.[35], we calculated a depolarization 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 3 He target system, with physically distant Rb spin-exchange optical pumping and target cells, which facilitates low-energy p-3 He elastic scattering experiments. To maintain 3 He 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 3 He 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 3 He gas of depleted polarization before reuse. An NMR coil closely coupled to the target cell, calibrated in a separate experiment using α-3 He scattering, provides continuous monitoring of the absolute polarization of the 3
He target gas during experiments. 14
Our largest challenge still to be faced is raising the 3 He polarization. Success by others, 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 4 He 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 assistance 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 3 He 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 3 He 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)
a
Impurity: 99.999%
b
Impurity: 99.95%
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
TABLE I: Wall relaxation times Tw for several materials
17
Optics
Laser
Sine-Theta Coil
Solenoid Coil
TC
Mu-metal Shield
Oven OPC
Cooling Air
DP1
Compressed Air Line
4
He
3
He
SC
SC
SC
SC
VP
N2
Purifier
N2
RGA
VP
Activated Charcoal Trap Computer-Controlled Pneumatic Valve Out
DP2
Manual Valve
In
Regulator
SB
Pressure Gauge or Vacuum Gauge
SB SB
FIG. 1: Schematic diagram of TUNL polarized 3 He target system. TC: Target Cell, OPC: Optical Pumping Cell, DP: Diaphragm Pump, SC: Storage Cell, RGA: Residual Gas Analyzer, VP: Vacuum Pump, SB: Storage Bottle
18
Delrin Holder Target Cell NMR Coil
Beam Entrance
Side Window
Beam
FIG. 2: The target cell and holder. The pictures do not show Kapton foil epoxied on the cell.
19
Mu-Metal Shield
Copper Rods
Side Windows for Emerging Particles
Delrin Coil Form
Glass Capillary of Target Cell 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
0
Outward Inward
θ Max
Max
B
0
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
High Current Power Supply Polarity Switching Relay
Sine-Theta Coil
FP Analog Out 9310 Ohm (1%)
IRFP048 FP Digital Out
TI TL082CP
0.07 Ohm (0.05%)
698 Ohm 1 M Ohm
FP Analog In
-15 V
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
Asymmetry
0.2
0.1
0 0
100 200 300 NMR Amplitude (mV)
400
FIG. 6: Plot of left-right asymmetry of recoil 3 He vs NMR amplitude. The best-fit curve yields a calibration constant of 6.31 × 10−4 /mV.
23
