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Atomic hydrogen cleaning of semiconductor photocathodes

Description: Negative Electron Affinity (NEA) semiconductor photocathodes are widely used for the production of polarized electron beams, and are also useful for the production of high brightness electron beams which can be modulated at very high frequencies. Preparation of an atomically clean semiconductor surface is an essential step in the fabrication of a NEA photocathode. This cleaning step is difficult for certain semiconductors, such as the very thin materials which produce the highest beam polarization, and those which have tightly bound oxides and carbides. Using a small RF dissociation atomic hydrogen source, the authors have reproducibly cleaned GaAs wafers which have been only degreased prior to installation in vacuum. They have consistently prepared very high quantum efficiency photocathodes following atomic hydrogen cleaning. Details of their apparatus and most recent results are presented.
Date: June 1, 1997
Creator: Sinclair, C. K.; Poelker, B. M. & Price, J. S.
Partner: UNT Libraries Government Documents Department

Exposure of GaAs to atomic hydrogen for cleaning prior to NEA photocathode activation

Description: Creating an atomically clean semiconductor surface is an essential step in preparing negative electron affinity (NEA) photoemission cathodes. While bulk GaAs can be satisfactorily cleaned by chemical etching and in situ heat cleaning, many high polarization electron source materials are either much too thin, or have oxides and carbides which are too tightly bound, to be cleaned by these methods. Some polarized source candidate materials may be degraded during the heat cleaning step. It is well established that the exposure of many III-V, II-VI, and elemental semiconductors to atomic hydrogen, typically at elevated temperatures, produces semiconductor surfaces free of contamination. Furthermore, this cleaning, possibly followed by thermal annealing, leaves surfaces which show sharp LEED patterns, indicating good stoichiometry and surface order. Atomic hydrogen cleaning should eliminate the chemical etching step, and might reduce the temperature and/or temperature-time product presently used in forming NEA cathodes. The process is readily adaptable to in situ use in ultrahigh vaccum.
Date: December 31, 1998
Creator: Sinclair, C.K.; Poelker, B.M. & Price, J.S.
Partner: UNT Libraries Government Documents Department

5 MeV Mott polarimeter for rapid precise electron beam polarization measurements

Description: Low energy (E{sub k} = 100 keV) Mott scattering polarimeters are ill-suited to support operations foreseen for the polarized electron injector at Jefferson Lab. One solution is to measure the polarization at 5 MeV where multiple and plural scattering are unimportant and precision beam monitoring is straightforward. The higher injector beam current offsets the lower cross-sections; measured rates scale to 1 kHz/{mu}A with a 1 {mu}m thick gold target foil.
Date: November 1, 1997
Creator: Price, J.S.; Poelker, B.M. & Sinclair, C.K.
Partner: UNT Libraries Government Documents Department

5 MeV Mott Polarimeter Development at Jefferson Lab

Description: Low energy (E{sub k}=100 keV) Mott scattering polarimeters are ill- suited to support operations foreseen for the polarized electron injector at Jefferson Lab. One solution is to measure the polarization at 5 MeV where multiple and plural scattering are unimportant and precision beam monitoring is straightforward. The higher injector beam current offsets the lower cross-sections. Recent improvements in the CEBAF injector polarimeter scattering chamber have improved signal to noise.
Date: January 1, 1997
Creator: Price, J. S.; Sinclair, C. K.; Cardman, L. S.; Haanskneccht, J.; Mack, D. J.; Piot, P. et al.
Partner: UNT Libraries Government Documents Department

Advances in DC photocathode electron guns

Description: At Jefferson Lab, a DC photoemission gun using GaAs and GaAs-like cathodes provides a source of polarized electrons for the main accelerator. The gun is required to produce high average current with long operational lifetimes and high system throughout. Recent work has shown that careful control of the parameters affecting cathode lifetime lead to dramatic improvements in source operation. These conditions include vacuum and the related effect of ion backbombardment, and precise control of all of the electrons emitted from the cathode. In this paper, the authors will review recent results and discuss implications for future photocathode guns.
Date: July 1, 1998
Creator: Dunham, Bruce M.; Heartmann, P.; Kazimi, Reza; Liu, Hongxiu; Poelker, B. M.; Price, J. S. et al.
Partner: UNT Libraries Government Documents Department

A load-locked gun for the Jefferson Lab polarized injector

Description: Construction is underway at Jefferson Lab on a load-locked polarized electron source. The design incorporates all of the essential features of the existing non load-locked gun and improves on the designs of existing load-locked guns operating at other labs. When complete, the authors expect the new load-locked gun to enhance the versatility of the JLAB polarized injector.
Date: April 1, 1999
Creator: Schneider, W. J.; Adderley, P.; Clark, J.; Day, A.; Dunham, B.; Hansknecht, J. et al.
Partner: UNT Libraries Government Documents Department