RECENT TECHNICAL ADVANCES FOR POLARIZATION STUDIES

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RECENT TECHNICAL ADVANCES FOR POLARIZATION STUDIES

T. Clegg

To cite this version:

T. Clegg. RECENT TECHNICAL ADVANCES FOR POLARIZATION STUDIES. Journal de

Physique Colloques, 1990, 51 (C6), pp.C6-533-C6-539. �10.1051/jphyscol:1990667�. �jpa-00230937�

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C o l l o q u e C 6 , s u p p l 6 m e n t au n022, Tome 51, 15 n o v e m b r e 1990

RECENT TECHNICAL ADVANCES FOR POLARIZATION STUDIES T . B . CLEGG

D e p a r t m e n t of P h y s i c s and A s t r o n o m U n i v e r s i t y o f N o r t h C a r o l i n a C h a p e l H i l l , NC 2 7 5 9 9 - 3 2 5 5 , U . S. A. and T r i a n g l e U n i v e r s i t i e s N u c l e a r L a b o r a t o r y , D u r h a m , NC 2 7 7 0 6 , U . S . A . 1 2 )

Resume

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Un resume des contributions techniques ZL ce congrks et du progrks technique rkcent et significatif en physique de polarisation est present6

Abstract - A summary of technical contributions to this conference and recent technical progress in fields related to nuclear polarization studies is presented.

1

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INTRODUCTION

The contributions to this meeting and recent advances in technical fields can largely be grouped into four major areas: polarized targets, polarized ion beams, methods for polarizing short-lived nuclear reaction products, and polarimeters and spin handling systems. In each area, significant advances have been made since the Osaka conference. Many have been described at this conference while others, equally important, have eluded discussion here. In the following article, I will attempt to summarize the major technical advances of the last five years in the field of polarization physics and will close with comments about what we should expect in the coming years. Those desiring further experimental details about the many works discussed may consult the original contributions to this meeting, published reports, and proceedings for recent conferences and workshops. /1,2/

2 - POLARIZED TARGETS

A

-

For hvdro~en and deuterium - Major projects are underway to produce polarized gas targets of H and D for use in storage rings. A minimum target thickness of 1014 polarized atoms/cm2 is sought to provide adequate luminosity for experiments. This must be achieved without disturbing the stored beam in the ring.

At Heidelberg the FILTEX target /3/ is being assembled. It is based on the traditional Stern-Gerlach-type atomic-beam sources 141. The differences are technical: massive pumping systems allow the atomic flux to be maximized; multiple sextupole magnets from very high flux Sm-CO permanent magnets optimize the transport of the desired atomic beam to the target cell 151; and-cooled, coated target cells /6/ are designed to accept and "store" the polarized jet beam. The cell chosen must minimize the loss of polarization via wall collisions while allowing injection, storage and acceleration of the main beam in the ring. The system at Heidelberg is in the final stages of construction. Initial measurements to optimize the atomic beam flux and sextupole system design calculations indicate that 8x1016 polarized atomsls can be expected into the target cell. It is also now lcnown that if the the internal cell wall is coated with a fluorocarbon film, polarizations as high as 70-80% of that in the free beam can be maintained while cooling the target cell to -60K. This is encouraging because it results in a -90% increase in the density of polarized gas in the target over that available at room temperature. The permanent magnet sextupoles will soon be installed and the first tests of the target in the storage ring at Heide1berg.m expected before mid-1991.

(l) Work supported in part by the US Department of Energy, Office of High Energy and Nuclear Physics under Grant No. DE- FG005-88ER40442.

(2) Work supported in part by the US Department of Energy, Office of High Energy and Nuclear Physics under Contract No.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990667

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C6-534 COLLOQUE DE PHYSIQUE

A second type of polarized atomic hydrogen jet target is under development by a group based at Brookhaven /7/. This ultra-cold jet relies on the principle that at 300 mK hydrogen atoms of only one electron-spin polarization are trapped inside a strong (-5T) solenoidal magnetic field. If these trapped atoms are then subjected to p-wave radiation at the characteristic electron-spin-flip frequency for the confining B-field, the atoms' spins will be flipped and they will be ejected as a jet along the solenoidal field axis. The method has been shown to work in principle. In a prototype experiment 1.2x1016 spin-polarized atomsls were ejected from the solenoid, and via sextupole focusing 3x1010 atomslcm2 were extracted into a useful target volume. Improvements are being incorporated into a new system. Better component alignment, improved gas feed, higher frequency p-wave extraction, and sextupole focusing are planned for subsequent tests with the ambitious goal of achieving a target thickness of -1014 atomdcm2 in the "free" extracted jet without the need for a storage cell.

In a third, entirely different approach, polarized jet targets for hydrogen or deuterium are being provided already for experiments in the VEPP-3 electron storage ring at Novosibirsk 181. These targets polarize the H or D via spin exchange with an optically pumped K vapor. This must occur in a pyrex cell with its interior walls coated to minimize depolarization during wall collisions. At present the cell is maintained in a -10 G rnagne- tic field and the polarized atoms emerge through a small spout toward the interaction region. To date for an emerging flux of 2.5~1017 atomsls the vector polarizations achieved have attained 30%, still substantially below the -75% value expected from the atomic beam targets described above. In an effort to improve the target's polarization the effective optical pumping laser power will be increased by a factor of 4 using a Ti- sapphire laser and the pyrex cell will be placed at higher magnetic field to raise the spin-exchange efficiency.

All such experiments using a polarized gas target rely on accurate knowledge of the nuclear polarization of the target during the experiment. Depolarizing mechanisms are a concern once the target is actuallv vlaced in an operating storage ring. ~ossibilides consiaered include 191: spin-flip transitions of ;he target at;& caused by the intense rf magnetic fields associated with the circulating beam in the storage ring, an effect which has actually been observed /10/; depolarizing collisions of the atoms with walls if a storage cell is used, and ionization and excitation of the polarized atoms by the passage of the circulating beam through the target.

Several methods have been conceived during the development of these targets to measure their polarization:

monitoring the transmission through a diagnostic sextupole system which samples a small flux emerging from the target D/; monitoring the circular polarization of the Balmer-a light following electron beam excitation of the polarized atoms in the cell 161; or measuring the nuclear polarization ~roduced when fast (-50 keV) D+ ionstraverse the target volume and pick up a pofarized electroh to emerge &i polarized D atoms / l l/.

It will be verv important to refine such techniaues to become reliable indicators of the actual in-beam target polarization kthdut interfering with the operatibn of the target or the storage ring where the target is install&.

B

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For 3He

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In the absence of a target of isolated spin-polarized neutrons, targets of polarized 3He are considered an experimentally important and tractable alternative. According to simple models, the polarized 3He nuclear ground state contains an unpaired, polarized S-state neutron which carries the angular momentum. The idea of using dense targets of 3He as a polarized neutron target has driven extensive development 112,131. Two basic methods are used. Both depend on optical pumping to introduce angular momentum to an intermediate species whose preferential spin orientation is then transferred to the 3He ground state via spin exchange. Both methods have been quite successful 1141.

In the earliest olarized 3He targets this intermediate species was atoms in the 23S1 metastable excited state

!

created in the He target cell by a weak rf discharge 1151. This technique has recently been advanced sig- nificantly by the development of a high power YAG-ty e laser at 1.083 pm to optically pum the3He

P 4'

metastables 116,171. An isotopically pure, 72% polarized He sample of density n=1017 atomslcm has been achieved after only -5 seconds of optical pumping 1141. For an operating polarized target the density could be increased, for example by cooling to -15K. It is estimated that a steady-state polarization of -30% can be achieved with n=6x1017. Since 3He atoms have no net electronic spin, when used in storage rings this target is expected to be insensitive to the main depolarizing mechanisms mentioned above for H or D targets.

The alternative polarization scheme uses optical pumping light from a Ti-sapphire laser to polarize Rb vapor as the intermediate species. Spin exchange with the Rb then allows 11413He targets of density -1020 atomslcm3 and polarizations which can reach 65-70%. Since the Rb polarization can be monitored, the associated 3He polarization can be calibrated and monitored during an actual experiment. This target is not mono- isotopic since the addition of 60 Torr of N2 is required to eliminate radiation trapping which would otherwise reduce the efficiency of Rb optical pumping 1131.

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3

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POLARIZED ION SOURCES

A- For H and D beams

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A large number of contributions to this conference have reported on new or improved, traditional atomic-beam polarized sources for hydrogen or deuterium. Such systems as those at Seattle 1181, Saclay 1191, and the new, commercially constructed source reported from Karlsruhe I201 have been heavily used in many accelerator laboratories for many years. The new feature for these sources since the Osaka conference is the replacement of the traditional electron-bombardment ionizer. At the Institute for Nuclear Research in Moscow their new ionizer contains a neutral plasma of fast D+ ions and electrons 1211.

An alternative approach is taken by several contributors to this conference who describe the performance of their systems equipped with a plasma ionizer heated by electron cyclotron resonance /22,23,241.

The record for polarized ion current is obtained from the Moscow device in which the entering polarized H undergoes the H

+

D+ -> H+

+

D charge-exchange reaction. Up to 6 mA of 76% polarized H+ beam is available in 100 ps pulses at 1 Hz with a normalized emittance of 5 mm-mrad. When this ionizer is pushed to give more frequent or longer pulses, the background pressure in the source rises and the beam intensity and polarization are reduced.

Continuous rather t h a ~ pulsed beams are available from the new electron-cyclotron-resonance (ECR) ionizers /25/. In these devices the electron beam of the traditional ionizer is replaced by the fast electrons in the neutral plasma. The ECR source has been known for many years as an efficient means to produce high-charge-state, heavy-ion beams /26/. Although its advantages of simplicity, reliability, and high ionization efficiency were attractive to polarized source builders, using the ECR plasma was initially believed risky for beams of spin- polarized atoms. The concern was based on the fact that the electron-cyclotron-resonance frequency is essen- tially equal (to within the equality of the electron's g-factor to 2) to the atomic electron spin-flip frequency.

Careful analysis showed that the power density needed to produce the ECR plasma was substantially lower than that ne&ed to flip the atom's s$n in the plasma volume 1271. -This gave enough impetus that an initial successful test of the concept could be mounted by a Karlsruhe-PSI collaboration 1281.

The ECR ionizer consists of solenoidal mirror coils to confine the plasma along the beam axis and a coaxial permanent magnet sextupole to provide radial plasma confinement. With -50 W of injected d power, a density of -1011 electrons/cm3 is achieved. More importantly, the ionizing plasma in these devices is space- charge neutral, so the internal energy spread and the resulting longitudinal emittance of the beam extracted are substantially lower than for traditional electron-bombardment ionizers. This makes it easier to bunch the emerging beam efficiently for injection into cyclic accelerators.

At present eleven such ECR ionizers are either operating, under construction, or planned for laboratories in Euro e, the United States, and Japan. Typically extracted and usable polarized beams up to 150

P

p4 (for H+

or D ) and 10 p4 (for H- or D-) are reported. The remaining concern about these new ionizers is that they seem to provide beams whose polarizations (-80% for H, -85% for D) are still -10% below those obtained from the traditionaI eIectron-bombardment and cesium-charge-exchange ionizers. For users who prefer the absolute maximum beam polarization for experiments over the gain in beam intensity these sources provide, there is still development work to be done. In fact, comparatively little is known yet about detailed ECR plasma parameters and how they influence the output beam's intensity and polarization.

There are also hints that the new ECR ionizer is very efficient. If this is indeed true, then the most intense output ion beams may.be obtained in the future by maximizing t h e m of polarized atoms entering the ionizer rather than the density in the entering atomic beam as is now thought best. The beam polarization may improve if the radial extent of the ionizing plasma can better be limited to cover only the axial volume occupied by the beam, thereby reducing the probability of ionizing unpolarized background particles at larger radius.

Another type of polarized source which has been the subject of considerable effort since the Osaka meeting has not been discussed in contributions to this conference. In these sources polarization of the spin-112 hydrogen nucleus is produced by pickup of a polarized electron from an optically pumped, alkali vapor.

These sources are not competitive with traditional atomic beam sources for polarizing the spin-l deuterium nucleus since its higher angular momentum cannot be fully oriented by attaching a single polarized electron 1291. Such sources have been developed for use on accelerators at Moscow 1301, KEK 1311, TRIUMF 1321, and at LANL 1331. In Moscow output H+ (H-) ion currents have reached 4.0 (0.40) mA with a polarization of 65%. Such sources are proving to be more successful when the beams required are pulsed, since the high optical pumping laser power needed is more readily available from pulsed lasers.

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B

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For heavier ions

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Polarized sources for heavier ions have also received some attention since the Osaka meeting. An entirely new 3He polarized source has been constructed at Osaka I341 and is now being commis- sioned. In this source a 3He-H beam is extracted from an ECR source and deflected along the axis of a cell containing optically pumped Na vapor. After pickup of a polarized electron, nuclear polarization is produced in the emerging 3He+ beam via hyperfine coupling. Nuclear polarizations up to 50% are theoretically possi- ble. At this conference the initial performance of 50-60 n.4 of 3He+ with -7% polarization was reported.

The laboratory having previously the most extensive polarized heavy-ion source development program, MPI- Heidelber~, has now decommissioned its polarized sources of 6Li- and 23Na- 1351. Maior comDonents of the best ~ e i d a b e r ~ source have been incorpohted into a new polarized 6Li source installa6on at siclay 1361. The emerging -35 pA dc 6Lil+ beam is injected axially into the 5.5 T solenoidal field of their electron-beam ionizer DIONE where it is further stripped. Eight extracted 25 ps long pulses of 6Li3+ are subsequently stored and accelerated I371 in MIMAS prior to injection and acceleration to 200 or 750 MeVIA in SATURNE. They are able to transport 6Li3+ nuclei to their high energy targets at the rate of 7x108 per beam pulse with a polarization of 70%.

C

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For electrons

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There is growing interest in studies of spin-dependent obsewables at new electron accelerators like CEBAF. This has renewed attention on sources of polarized electron beams 1381. Of the two competing technologies, that of illuminating a cesium-activated GaAs photocathode with circularly polarized light, and that of extracting polarized electrons from a weak discharge containing optically pumped P S 1 metastable atoms, the former now seems more sucessful. Considerable recent effort focused on under- standing why the electron beam polarization in the GaAs source has traditionally been -35-43%, i.e. well below the theoretical maximum of 50%. New layered photocathodes in which polarized photoelectrons are extracted only from a I 0.4 micron thick GaAs surface region have eliminated the loss of the emerging electron polarization during diffusion to the surface in the bulk GaAs photocathodes previously used. Atten- tion is now directed toward finding other photocathode materials which remove the intrinsic bandgap degen- eracy providing the 50% polarization limit in GaAs.

4

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P O L A m D SHORT-LIVED NUCLEAR REACTION PRODUCTS

Two laboratories report on polarization techniques and nuclear measurements for short-lived nuclei far from the line of stability. These nuclei are created as reaction products, separated on-line in an isotope separator, and then polarized. At CYCLONE 1391 three complimentary polarization methods are employed for the atoms while in flight: passage through very thin tilted foils, reflection at grazing incidence from very flat polished surfaces, and spin-exchange with an optically-pumped alkali vapor. In all three cases the resulting electron polarization is subsequently transfered to the nucleus via the hypefine interaction. To slow the polarization loss caused by spin-lattice relaxation when these atoms are then implanted in a host during measurements, the host is cooled to S 20K. This facilitates measurement of an angular distribution of the subsequent ^a,

P,

CE, or X-ray radiation for the short-lived decaying species.

At Osaka using the isotope separator CARP, recent work has optimized the tilted-foil technique I401 for polarizing fragments from high-energy, heavy ion reactions. Here the polarization depends on the tilt-angle of the foil with respect to the arriving particle, the number of foils, and the inter-foil spacing. Also multiple foils can "pump up" the polarization. Studies were conducted as a function of the number of foils and the distance between adjacent foils. Final polarized nuclei were implanted into a Pt target as host for the subsequent p- NMR experiment. Their experiments have parameterized the basic physical interaction causing the polarization. Depending on the nucleus, very large polarizations (0.8 f 0.3 for W C ) have been seen.

Both laboratories are exploring the limits of these techniques 1411. The basic question is, how far from the stability line can one obtain meaningful anisotropies in the radioactive decay before the polarization is lost to spin-lattice relaxation? For these very rare nuclei, the number of particles available to polarize becomes extremely small making the efficiencies of collection and subsequent polarization very important considera- tions. In Japan 1421 their goal is to make measurements when the flux is only -1001s. In the case of rare-earth nuclear reaction products they discovered that the polarization can be enhanced by implanting the products in an alkaline-earth fluoride host at 20K and then optically pumping the crystal. Via the magnetic circular- dichroism of the rare earth atom in the alkaline-earth fluoride host, one can polarize the atomic electrons and subsequently via the hyperfine interaction the rare-earth nucleus. Asymmety measurements for the rare-earth nucleus 164Tm were reported.

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5

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POLARIMETERS AND SPIN PRECESSION SYSTEMS

A

-

For Drotons at low energies

-

Traditional polarimeters for measurements at low energy continue to be developed; two were reported at this meeting. After study at Giessen 1431 of the 4He{p,p)4He analyzing power in the energy range 1.5 to 2.2 MeV, an efficient vane-type polarimeter was bu~lt and calibrated to monitor the proton polarization of beams at these low energies. At Tsukuba 1441 a magnetic spectrograph with a solid angle of 10 msr followed by a '2C@,p)12C polarimeter has been constructed for polarization transfer measurements in the energy range 14.0 to 18.5 MeV.

B - For Drotons at intermediate and higher e n e r w

-

As polarized beams from cyclotrons and synchrotrons are used at higher energies, it has become increasingly important to develop flexible post-accelerator systems for flexible spin-axis alignment and reliable polarization determination. Since measurement of a full compli- ment of polarization transfer coefficients has often become desirable in such laboratories 1451, one simple polarimeter for the beam at the target is often not enough to assure knowledge of the initial beam's polarization.

Traditional higher energy proton polarimeters have utilized p+p scattering. A conference contribution from Indiana 1461describes a new in-line polarimeter which measures their proton polarization via p+d elastic scattering. It is effective for proton energies between.100 and 200 MeV where p+d analyzing powers are typically larger than 0.5 and slowly varying both in energy and in angle. The usual problems of unwanted background arising from @,2p) reactions in p+p polarimeters or poor detector resolution and beam halo effects in p+I2C polarimeters are completely eliminated. In the p+d polarimeter the recoil deuterons are detected in coincidence with the elastically scattered protons, and an additional electronic cut using the deuteron-proton flight-time difference yields extremely clean spectra.

Each laboratory has its own special systems. At Indiana two of the p

+

d polarimeters described above have been installed in the transport line from their cyclotron to their target areas and the Cooler ring. They are used, in combination with two superconducting spin precession solenoids and an intermediate beam deflection magnet, to determine all components of the incident proton beam polarization on the target of the new high- resolution spectrograph. At Osaka the polarization transfer measurement program in p+p elastic and inelastic scattering has recently been enhanced by the addition of a spin precession solenoid / x d which allows the spin quantization axis to be aligned nearly along the beam axis at the target of the DUMAS/MUSASHI spectrograph/polarimeter system 1471. This allows them for the first time to measure those transfer coefficients which require an initially longitudinally polarized beam.

One of the most significant recent advances in this field has been the development of spectrometer focal-plane proton polarimeters which by ray reconstruction yields extremely clean, high-resolution spectra while maintaining both high analyzing power and high efficiency. Two such devices are reported at this conference.

At Osaka the polarimeter for the spectrograph RAIDEN 1481 operates at 65 MeV. It utilizes seven carbon analyzer targets, six vertical drift chambers, and seven plastic scintillators to accomplish an effective analyzing power of 0.85 at 65 MeV and and efficiency of IxIO-~. At Indiana 1491 the high-resolution polarimeter also employs p

+

12C scattering but operates at the higher energy of 120 to 200 MeV. The high density graphite target for the polarimeter is located behind two focal plane vertical drift chambers and a scintillator, which define the trajectory of the incoming particle. Following this target two sets of paired X-y multiwire proportional chambers and two planes of plastic scintillator define the scattered particle trajectory.

The result is a device with an -8% momentum bite, high efficiency, and large effective analyzing power.

Mention should be made also of the simple techniques realized 146,501 for the calibration of such proton polarimeters. When a spin-112 particle is scattered from a spin-0 target, simple equations relate the normal components of the incoming and outgoing proton polarizations when the spectrometer target analyzing power Ay = 0 or 1. This can leaddirectly to knowledge of the polarization of the beam which strikes the focal plane polarimeter and thus to an easy absolute calibration.

C

-

For very high-energ accelerators

-

Another topic which has not received attention at this conference is the recently completed "Siberian Snake" I511 proof-of-principle experiment. These devices were conceived to allow depoldzing resonances in circular accelerators to

bk

croised without loss of beam poIarization. They operate to precess the horozontal component of the proton spin by 180' on each pass around the accelerator.

Thus any unwanted precesion occuring because of horizontal magnetic field components elsewhere in the ring is completely cancelled out on the next pass. Snakes will be essential if polarization physics is ever to be

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pursued with beams accelerated to very high energies in strong focusing machines like the SSC. Recently at Indiana, 1521 using a polarized beam in the Cooler ring, it has been shown that a prototype Snake works to eliminate the effects of the Gy = 2 depolarizing resonance at 108 MeV.

D

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For other nuclei

-

Also at Osaka a new H(n,n)H polarimeter for 200-400 MeV neutrons is under construction 1531. It will be used with the new neutron time-of-flight facility to study the polarization transfer coefficient in (p,n) reactions. To achieve the efficiency desired, the polarimeter consists of two successive lOxlOOxl00 cm3 liquid scintillator planes followed by two plastic scintillators of similar size. On each of the four comers of each plane are placed photomultiplier tubes. These allow determination of the scattering event location within the individual plane via timing differences. The first three of these detection planes serve as the targets in which the H(n,n) reaction occurs. Each following detection plane serves as a "catcher" in which a second scattering event provides the additional information to determine the polar and azimuthal scattering angle. The system has been tested using 63 MeV neutrons and awaits the completion of the new time-of- flight facility before final installation and first use for experiments.

There are also vigorous programs using polarized deuteron beams. Since deuterons are characterized by both vector and tensor polarization, polarimetry is more complex than for nucleons. Nevertheless, very high quality polarimeters 1541 using the 3He(d,p)4He reaction can monitor both the vector and tensor polarization of 10-30 MeV deuterons. For the higher energies at SATURNE 1551, one relies on monitoring the beam polarization at 400 keV before acceleration using the D(d,p)T reaction and on the belief that there are no depolarizing resonances during acceleration. This has been verified by measuring the deuteron beam's vector polarization at high energies using a well-calibrated nucleon polarimeter 1561 to measure the left-right asymmetry in d-p quasielastic scattering.

6

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FUTURE TECHNICAL TRENDS

The work of the last five years points already toward technical advances we can expect before our next conference. A prime example is last month's success with rf stacking injection of a polarized beam in ^the Indiana University Cyclotron Facility's Cooler ring, storage of the beam for up to -10 minutes without Ioss of polarization, and the first experiment there to measure the analyzing power in H@,p)H elastic scattering at 185 MeV 1571 using an unpolarized gas jet target. Polarized gas-jet-target techniques are also advancing quickly, so we can expect that the initial tests in the VEPP-3 ring will be followed soon by several others.

With any luck in four years, we will hear reports of similar storage-ring-based polarization experiments with both a polarized beam and a polarized target.

Polarized beams of higher intensity, wider variety, and greater polarization flexibility are just becoming available at both lower and higher energies. New experimental facilities already provide intense polarized 20

-

100 keV H and D beams which can be used, for example to explore the detailed nuclear structure of few- body systems such as 3He. We can expect to see efforts to prepare thicker targets of polarized 3He for experiments with polarized GeV electron beams to study the nucleon spin structure function 1581. With the completion of the FILTEX jet target, we may be given a chance to use it to produce the first polarized anti- proton beam at CERN.

There is then no lack of new ideas nor lack of energy to pursue them. If there is any difficulty facing us, it is the increasing sophistication and cost of our projects. The recent trend is one of forming ever larger technical collaborations to share the project development effort, cost, and benefits. This seems likely to continue.

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1551 J. Arvieux et al., Nucl. Phys. (1984) 613.

I561 J. Bysmcky et al., Nucl. Instr. and Methods (1985) 131.

I571 W.K. Pitts et al., contribution 31F to this conference.

1581 The Hermes collaboration, post-deadline contribution 24C to this conference.