Work at the Japanese KEK Laboratory has shown how to obtain highly polarized elec­ tron beams from a laser-irradiated treated semiconductor photocathode. Shown here is the variation in polarization with laser wave­ length. commissioned at the Hamburg DESY Laboratory. Zeus' iron magnet yoke and the superconducting solenoid and com­ pensator have been in place for some time and successfully tested. Uranium-scintillator calorimeter ele­ ments began to go in last No­ vember, initially for the forward (proton direction) and rear mo­ dules, and installation of the central barrel is now underway. All propor­ tional-tube chambers for the back­ ing calorimeter in the iron yoke are in place. The toroids of the forward muon detector have been mounted and operated. All drift chamber planes have been installed as well as two of the four limited-streamer tube planes for the trigger. The inner panese KEK Laboratory. Despite a being left-handed, polarized parti­ chambers of the muon barrel are in relatively recent appearance on the cles provide a powerful probe of place and mounting of the outer scene, work on a polarized (spin these effects. layer has begun. oriented) electron source has Thus polarized beams are a ma­ The inner tracking system - ver­ nevertheless made significant pro­ jor goal in electron-positron collid­ tex detector, central drift chamber gress. ers. In conventional storage rings, and planar drift chambers alternat­ After a brief but intensive spell orbiting particles become trans­ ing with transition radiation ele­ of work to find the optimal super- versely polarized due to radiation ments in the forward direction, to­ lattice photocathode structure, an emission - for example, polariza­ gether with its special section of impressive polarization level, 71 tions of about 40 per cent were beam pipe - is taking shape. per cent, has been achieved for a observed at 29 GeV at KEK's TRIS­ After initial installation to ensure solid-state photocathode at room TAN ring (December 1990, page its compatibility with HERA ma­ temperature by a team of research­ 1 1) and 10 per cent at 50 GeV at chine components, the luminosity ers from KEK, Nagoya and NEC CERN's LEP (November 1990, page monitor has been removed await­ Corp (see also page 6). 3). But this effect depends strongly ing the availability of well-con­ This should go on to pay divi­ on machine parameters and is diffi­ trolled beams. dends in a future research pro­ cult to control. gramme. In the electroweak picture In contrast, in a linear collider of electromagnetism and the weak such as JLC, once polarized elec­ nuclear force, 'handedness' plays a trons are injected, the spin could KEK vital role. Particularly when the be maintained through to an inter­ Producing highly weak force is in action. Nature action point, provided depolarizing polarized electrons (1) cares about the direction in which effects are avoided. Thus a key re­ things happen, and a reaction open quirement is a highly polarized elec­ to left-handed electrons can be to­ tron source. With the Japan Linear Collider tally blocked for their right-handed The conventional source is bulk (JLC) electron-positron project hav­ counterparts. gallium arsenide with a negative el­ ing highest priority in Japanese With particles spinning in their ectron affinity surface, illuminated high energy physics planning, many direction of motion (clockwise) be­ by circularly polarized monochro­ associated research and develop­ ing right-handed, and those spin­ matic photons from a laser. But the ment tasks are underway at the Ja­ ning against the direction of motion polarization obtainable this way

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Courier, May 1991 Highly polarized electron sources at Stan­ ford. Left, a thin (0.1 micron) indium gallium arsenide layer on a gallium arsenide sub­ strate, when irradiated, shows increasing el­ ectron polarization, eventually attaining 71 per cent. At about 1.34 eV, the spectrum shows the onset of the selective pumping of electrons from a higher energy valence band to the conduction band (see text). Right, a 'thick' 1.14 micron layer with no crystal strain shows no similar effects. never exceeds 50 per cent because of spin degeneracy. One way to eliminate this intrinsic limit is to re­ move the degeneracy by a suitably arranged periodic potential in a su- perlattice structure. If the level splitting is made larger than the thermal noise level, selective pumping from a single state will be possible. Subsequently the pumped electrons need to be efficiently transported from superlayer to su- perlayer, with minimal depolariza­ tion in transit. The important first step was an optimization study of GaAs-AI- GaAs superlattice parameters such as layer thicknesses and Al con­ tent. The first photocathode result­ with bulk gallium arsenide, while ing from this study was tested last STANFORD thin epitaxial layers can approach year with a titanium/sapphire tuna­ Producing highly the theoretical 50 per cent limit. ble laser, and 53 per cent polariza­ For over a decade much effort tion was quickly seen. polarized electrons (2) has gone into investigating other Careful examination of this result semiconductors to avoid this inher­ led to a second sample with thin­ Electron spin polarization above 70 ent limit due to valence band de­ ner superlattice layers. First per cent by photoemission from a generacy. This degeneracy can be measurements gave 71 per cent specially prepared semiconductor broken by deforming the crystal polarization at a wavelength of has been achieved by T. Maruyama structure or by engineering suitable 802nm. Greatly encouraged by this and E. Garwin of the Stanford Li­ quantum wells or superlattice achievement, the collaboration is near Accelerator Center (SLAC), R. structures, opening up the possibil­ aiming for even better production Prepost and G. Zapalac of Wiscon­ ity of 100 per cent polarizations by polarized electron sources for JLC. sin, and J. Walker and S. Smith of selective pumping of the higher en­ Berkeley. ergy valence band. Since the first use of a gallium The SLAC/Wisconsin/Berkeley arsenide photocathode at SLAC for group has looked at indium gallium the historic 1978 experiment which arsenide layers grown epitaxially on Polarized electron saw left-right asymmetry in elec­ a gallium arsenide substrate, with tron scattering, semiconductor the indium giving about a one per sources photoemitters have become stan­ cent lattice mismatch. If this top dard for linear accelerators. These layer is thin enough, the resulting These two articles, from the sources give high peak currents crystal has a compression strain Stanford Linear Accelerator and short pulses, for example sa­ that splits the valence band degen­ Center (SLAC) in California, tisfying the 16 ampere/2.5 nanose­ eracy by some 50 meV. and from the Japanese KEK cond pulse requirement for Stan­ Two samples were studied - an Laboratory, highlight the ford's SLC linear collider. 0.1 micron indium gallium arsenide world-wide effort underway The conventional cathode mater­ layer thin enough to give a high to develop new techniques ial, gallium arsenide, has a theoreti­ quality strained structure, while a for the next generation of el­ cal maximum polarization of 50 per 'thick' 1.14 micron layer without ectron-positron linear acceler­ cent due to its crystal structure. In strain provided a control. ators. practice, polarization levels of The thin strained sample about 40 per cent are achieved showed a dramatic increase in el-

6 CERN Courier, May 1991