POLARIZED ELECTRON SOURCES

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POLARIZED ELECTRON SOURCES

L. Cardman

To cite this version:

L. Cardman. POLARIZED ELECTRON SOURCES. Journal de Physique Colloques, 1990, 51 (C6),

pp.C6-473-C6-477. �10.1051/jphyscol:1990655�. �jpa-00230925�

COLLOQUE DE PHYSIQUE

Colloque C6, supplement au n022, Tome 51, 15 novembre 1990

POLARIZED ELECTRON SOURCES

Nuclear Physics Laboratory and Department of Physics. UNiversity of Illinois at Urbana-Champaign, 23 Stadium Drive, Champaign, IL 61820, and Continuous Electron Beam Accelerator Facility, 12000 Jefferson Avenue, Newport News, VA 23606, U.S.A.

Abstract - The advent of the new generation of 100% duty factor electron accelerators has led to renewed interest in electro-weak interaction physics and in experiments that exploit the spin degrees of freedom in electron scattering. A polarized electron source with the highest possible figure of merit is a crucial element in such experiments. We review the status of polarized electron sources, with particular emphasis on those best suited for use with 100% duty factor accelerators.

1 - INTRODUCTION

The ideal polarized electron source for use with cw accelerators would produce a beam of 100% polarized electrons with an intensity (after chopping and bunching) of order 100pA, low emittance, and a high level of stability and reproducibility for the hundreds of hours that are necessary for typical experiments. Finally, the polarization must be easily and rapidly reversible with negligible impact on the other characteristics of the beam to minimize systematic errors in scattering asymmetry measurements. For a general introduction to polarized electrons, their production, and their uses the reader referred to the monograph by I<essler / l / ; source development has been the topic of two workshops that have been summarized by Sinclair /2,3/.

A number of different methods have been devised /2,3/ for producing polarized electron beams, including:

atomic beams of Li and of He; the Fanno effect in Rb and in CS; field emission from EuS; and photoemission from GaAs. All polarized electron sources now in use with accelerators are based on photoemission from GaAs or related materials because only the GaAs-type sources have been able t o produce both the high peak currents required by the low duty factors of the previous generation of accelerators and the rapid polarization reversal necessary for eliminating systematic errors in parity violation measurements. The advent of cw electron accelerators, with their reduced peak current requirements, has lead to a renewed interest in the flowing helium afterglow source, which has achieved higher polarizations than GaAs at very useful beam currents. In the sections that follow we discuss both GaAs and flowing helium afterglow sources, outlining the principles of their operation, and indicating present performance levels and the outlook for improvements for each of these technologies.

2 - THE FLOWING HELIUM AFTERGLOW SOURCE

The flowing helium afterglow source, developed /4,5/ at Rice University, represents the only approach available with a demonstrated capability for providing very high (82%) polarization electron beams with a current over 1 pA. Its operation can be understood from Figure 1. Helium gas is injected through a Pyrex nozzle into a 10 cm diameter flow tube which is exhausted by Roots pump that maintains the pressure in the flow tube in the range of 65-200 microns. The nozzle passes through the center of a small rf cavity which excites a microwave discharge in the helium gas, generating helium ions, free electrons, and helium atoms in many different excited states. Most of the excited species have sub- millisecond lifetimes and decay within the nozzle, leaving in the gas that passes down the main flow tube only neutral helium atoms in their ground state, helium ions, electrons, and helium atoms excited to the metastable singlet (He(2 'So)) and triplet (IIe(2 3S1)) states.

The triplet metastables are optically pumped by circularly polarized 1.08pm light which enters the flow tube along an externally-applied, uniform, one gauss magnetic field, which defines the quantization axis for the pumping. Right circular polarized light will preferentially populate the He(2 3S1) (mj=+l) level; reversing the polarization preferentially populates the (mj=-l) level.

(l)Supported, in part, by the NSF under Grant No. PHY-89-21146 and by CEBAF under Contract No. SURA-88-C8531LD.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990655

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EXTRACTION

1

LENS SYSTEM,

f

Figure 1: A Schematic Plan View of the Rice Flowing Helium Afterglow Polarized Electron Source /4/.

The polarized electrons are freed from the flowing helium atoms by a Penning chemi-ionization reaction which conserves angular momentum:

The free polarized electrons are then extracted through an electrostatically biased, differentially pumped aper- ture.

The Rice group has recently completed /5/ an extensive effort exploring the limits of polarization and intensity achievable with the flowing helium afterglow source; their progress is evident in Figure 2, which compares the performance of the source in its earlier /4/ and present /5/ configurations. A major improvement was the use of an LNA laser with-sufficient power to saturate the polarization; the extraction efficiency has also been improved. They showed that polarization losses due to the presence of singlet metastables in the afterglow are 52%, and it was demonstrated that P-state mixing effects are not the cause of the polarization degradation at high extracted currents. Significant polarization degradation due to unpolarized electrons produced in the microwave discharge was identified, and measures to suppress these electrons by rf heating were successful.

Finally, it was demonstrated that fluorescent 3P1 --t3S photons that are produced in the optical pumping cycle and re-absorbed by other He(2 3S) atoms in the flow tube can seriously degrade the spin polarization. High spin polarization thus requires that the fluorescent photon mean free path be larger than the typical flow tube dimensions (-10 cm). On the other hand, high current requires high He(2 3S) density. This radiation trapping limited the current that could be attained while preserving high spin polarization.

A group at Orsay /6/ has begun the construction of a source of this type, and will investigate additional improvements in the Rice design (in particular, ideas for overcoming the radiation trapping limits) as well as the modifications necessary to adapt it for use with an accelerator (raising the source potential to -1OOkV and adding chopping and bunching).

3

-

The operation of semiconductor photoemission polarized sources has been described in detail in the liter- ature /7,8/. There are two fundamental aspects of these sources: 1) the polarized electrons are produced by optically pumping electrons from the top of the valence band to the bottom of the conduction band of a suitable direct-bandgap semiconductor (usually GaAs); and 2) these electrons are able to escape from the semiconductor because its surface has been atomically cleaned and then covered with a monolayer of an alkali metal (typically CS) and oxidized (most often using NF3) to lower the work function of the surface to the point where the energy of the electron in the vacuum outside the material is lower than its energy in the bulk material, a condition referred to as negative electron affinity (NEA). A typical GaAs surface that has been treated in this manner will have a quantum efficiency of order a few percent. One watt of laser power will produce 6.5 mA of beam

Current (A)

Figure 2: Polarization vs extracted (dc) beam current for different configurations /4,5/ of the Rice flowing helium afterglow source.

current from a photocathode having a 1% quantum efficiency, so very reasonable laser power can provide very high electron currents. Achieving and maintaining this efficiency requires that the surface of the semiconductor be free of sorbed gases at the sub- monolayer level; this implies vacuums of order 10-'l torr, and represents the major difficulty in this type of source. By careful attention to materials preparation and vacuum techniques, GaAs sources have demonstrated /g/ the ability to deliver hundreds of Coulombs of charge over lifetimes of over 1000 hours.

The optical pumping produces polarization because the electronic states near the bandgap minimum have well-defined, single-electron-like quantum numbers. The sense of the polarization can be reversed by reversing the helicity of the pumping photons. The photon energy used must be just above the bandgap energy; if higher photon energies are used the electron states become less well described by single electron quantum numbers, and, eventually, contributions from other bands appear. Both of these effects will reduce the electron polarization.

While most photocathode sources have used GaAs, the Mainz group has used /10/ and suggest that its larger bandgap leads to longer cathode lifetimes. In addition, the SLAC group /11/ has used the combination of GaA1xAsl-x with thin samples to obtain higher polarization at the wavelength of their high power laser.

In the semiconductors that have been used in photocathode sources (GaAs, and the alloys GaAlxAs(l-x) and G ~ A S ~ P ( ~ - ~ ) ) , a degeneracy at the top of the valence band limits the theoretically-achievable polarization to 50% but the polarization of operating has ranged from 25 to 43%. Recent experiments /12,13/ indicate that this loss of polarization is due to spin dilution as the electrons diffuse to the semiconductor surface; the absorption depth for 1.4 eV photons in GaAs is about lpm. The CEBAF/Illinois/SLAC/Wisconsin measurements /13/

indicate that a sample thickness less than 0.4pm will give a polarization close to the theoretical maximum, while the polarization obtainable from a 0.9 pm thick sample is virtually indistinguishable from that obtainable from bulk GaAs. The quantum efficiency of the 0.4pm sample was -1%.

Of course even the thinnest conceivable sample of GaAs would not have a polarization greater than 50%, so further increases in polarization will require a different approach that somehow removes the degeneracy in GaAs. Among the possibilities that have been discussed /7/ are: the application of uniaxial stress to the crystal;

the construction of artificial structures with the bandgap energy varying in the direction perpendicular to the emitting surface; and the use of direct bandgap materials other than GaAs in which the degeneracy is absent due to the lack of symmetry in the crystal structure. To date none of these efforts have been successful.

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1 O-=

GoAs

7

Current (A)

Figure 3: The figures of merit, P21, for the flowing helium afterglow and GaAs sources in accelerator applications.

The application of uniaxial strain to a crystal of GaAs removes the band degeneracy by breaking the symmetry of the crystalline structure. Calculations 1141 of this effect suggest that polarizations of order 70%

can be achieved, but the strain required is of order 6 X 10' dynes/cm2. The generation of stress of this magnitude in the laboratory by conventional mechanical means is far from trivial, and the crystal is more likely to break than serve as a useful photocathode. The most promising way to generate this strain is to grow the material of interest on a substrate having a different lattice constant; GaAs and Si, for example, have lattice constants that differ by 4%. Calc~~lations of polarization enhancements with strain have been partially confirmed by measurements /14,15/ of the polarization of the recombination light emitted after optical pumping of the samples; but the polarization measured 1151 for electrons extracted from a photocathode made from GaAs on Si was only 50%; the reason for this discrepancy is not known at this time.

Multi-layer heterostructures, such as alternating layers of GaAs and GaAlxAsl-x, have been shown to produce high polarization in conduction band /16/, but the polarization of the beams extracted from a similar sample was low /l?'/; this has been attributed /15/ to the relatively thick surface layer of GaAs on the samples, but the feasibility of the technique remains to be demonstrated.

A third avenue approach is the use of alternate direct-bandgap materials which naturally lack the valence band degeneracy present in GaAs; of course it must also be possible to fabricate an NEA photocathode from the material by appropriate treatment of the surface. A particularly promising class of materials are the so-called 11-IV-V2 chalcopyrites, which are ternary analogues of the 111-V compounds (such as GaAs). The valence band degeneracy has been removed (by about 100 meV) by the fact that they have two cations instead of one and, in most cases, significant tetragonal distortion. There have been very few attempts /3/ to measure the polarization of electrons emitted from photocathodes prepared on chalcopyrite materials; none of them have been successful.

These efforts have been hampered significantly by the poor quality of the chalcopyrite materials available, and, quite possibly, by the low band gaps of the materials available to date. Recently, the RTI group /18/ has grown excellent quality epitaxial samples of ZnGeAs2 using the MOCVD technique. They have also succeeded 1191 in growing an alloy of ZnGeAsz with phosphorous, Z ~ G ~ ( A S ~ . ~ P ~ . ~ ) ~ . This material has a bandgap between 1.22 and 1.25 eV, a value significantly higher than the 1.15 eV of ZnGeAs2, and the Illinois/CEBAF group has successfully fabricated photocathodes on this material. The emitted electron polarization has not yet been measured, and we eagerly await the results. CdSiAs2 has an even higher bandgap, 1.74 eV; this material is probably the most promising chalcopyrite from a theoretical point of view, but good samples are not available.

The status of polarized electron sources for cw accelerators is summarized in Figure 3. The current-available from the existing Rice source (see Fig. 2) has been reduced by 50% to account for the losses associated with

the chopping and bunching necessary for acceleration using a linac; for GaAs we assume a polarization of 49%:

The figures of merit for GaAs and the flowing helium afterglow sources at their present stage of development are comparable for beam currents S30pA; GaAs is superior above that current, and the flowing helium afterglow source is superior below it.

For some experiments, such as scattering polarized electrons from polarized targets, the allowable beam currents are reduced to the range of one to one hundred nanoamperes, depending on the details of the target and cooling system. For such cases the figure of merit for the He source is a factor of 3.4 higher. At the other extreme, for a parity violation measurement on hydrogen, where 100pA currents could be used, the GaAs source has a figure of merit a factor of 3.5 higher. In addition, the GaAs source has demonstrated the stability and reproducibility necessary for 10-s asymmetry measurements in parity violation studies; the stability of the He source is unknown. It will be interesting to watch the evolution o-f both of these technologies.

REFERENCES

J. Kessler, Polarized Electrons, 2nd edition (Springer

-

C. K. Sinclair, Jour. de Physique 46, C2-669 (1985), and references therein.

C. K. Sinclair, in High Energy Spin Physics, p. 1412, ed. K. J. Heller, (AIP, New York, 1989), and references therein.

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