Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
1999-07-02
2003-07-08
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
C372S073000, C372S070000, C372S069000, C372S034000
Reexamination Certificate
active
06590923
ABSTRACT:
TECHNICAL FIELD
The present invention relates to systems and methods for producing a flow of preferred-spin-polarization-direction electrons, and more particularly is an optically pumped direct extraction electron spin filter system and method of utilizing a predominately single handedness, preferably laser system produced, beam of photons to optically pump electrons in atoms, (typically alkali atoms), to a dark-ground state therewithin, in a preferred-spin-polarization direction. Said method is practiced in the presence of a magnetic field which is oriented essentially co-linear with said beam of photons and said system and method serve to, in a below atmospheric pressure ambient, convert a multiplicity of typically internal electric discharge generated, “random-spin” electrons into a multiplicity of directly extracted preferred-spin-polarization-direction electrons via pumped dark-ground state atom—electron collision mediated exchange mechanism(s). Said system preferably comprises a single chamber essentially enclosed space and the presence of helium in an electron polarization direction enhancing “buffer gas” contained therewithin.
BACKGROUND
As described in a book by J. Kessler, titled “Polarized Electrons”, 2nd Ed. (Springer, Berlin 1985), while the production of electrons with a preferred-spin-polarization-direction for use as a probe of spin-dependent phenomena is known, electrons with a preferred-spin-polarization-direction are difficult to produce. Further references which document that polarized electrons are an indispensable probe of spin-dependent phenomena in many areas of physics include “Polarized Electrons In Surface Physics” Edited by R. Feder, (World Scientific, Singapore, 1985); and “Polarized Gas Targets And Polarized Beams”, (Seventh International Workshop), Edited by Holt and Miller, AIP Conference Proceedings Series CP421, (AIP New York 1998). Note, the above identified references are incorporated herein by reference to provide general background.
Continuing, the development of sources of polarized electrons began with Bohr's rejection of magnetic spin filters as described in “The Scattering Of Fast Electrons By Atomic Nuclei” Mott, Proc. R. Soc., London, Ser. A 124, 425 (1929) and “Stern-Gerlach Effect For Electron Beams”, Batelaan, Gay, Schwendiman, Phys. Rev. Lett. 79, 4517 (1997). The use of Mott scattering to produce small currents of polarized electrons is discussed in “Electron Polarization”, Shull et al., Phys. Rev. 63, 29 (1943) and in “A Method Of Measuring The Gyromagnetic Ratio Of The Free Electron”, Louisell et al., Phys. Rev. 91, 475 (1953).
As described in the previously cited Holt and Miller reference titled “Polarized Gas Targets And Polarized Beams”, the best of said polarized electron spin sources can produce high currents, (eg. microamps), and polarizations approaching unity, where Polarization (P) is defined as:
P
=(
N
+
−N
−
)/(
N
+
+N
−
)
where N
+(−)
represent the number of electrons with “up” and “down” spins, respectively.
Said Holt and Miller reference also discloses that state-of-the-art polarized electron sources use either photoemission from negative-affinity GaAs, (or variants of the GaAs basic structure), or chemi-ionization of optically pumped metastable He (He*); and that both techniques can produce electron polarizations in excess of seventy (70%) at current levels of several hundred microampers. As described in “A source Of Highly Spin-Polarised Slow Electrons Based On The ‘Fano Effect’ On Caesium Atoms”, Mollenkamp et al., J. Phys. E 15, 692, (1982), it is noted that earlier polarized electron sources which utilized, for instance, the “Fano Effect”, produced currents which were four orders of magnitude lower, with at best a similar polarization, and with much larger beam emittance.
Unfortunately, GaAs and H* sources are technically complex and pose difficulties in operation. GaAs sources must be operated under ultrahigh vacuum/low contamination conditions, produce only picoamp current levels, and production of a negative electron affinity photocathode is currently technically difficult, as described in “GaAs Spin Polarized Electron Source”, Pierce et al., Rev. Sci. Instrum. 51, 478 (1980) and the previously cited Holt and Miller reference. Helium H* sources are easier to operate, but are large, mechanically complex, and require high-throughput vacuum pumps to achieve optimum performance. Helium H* sources are described in “Improved Source Of Polarized Electrons Based On A Flowing Helium Afterglow”, Rutherford et al., Rev. Sci. Instrum. 61, 1460 (1990), and in “The Orsay Polarized Electron Source From A Flowing Helium Afterglow”, Arianer et al., Nucl. Instrum. Meth. A 382, 371 (1996).
Work by McCusker et al., reported in an article titled “Cumulative Ionization In Optically Pumped Helium Discharges: A Source Of Polarized Electrons”, Phys. Rev. A 5, 177 (1972), resulted in provided a He* source in which electrons are extracted directly from a discharge instead of being produced by chemi-ionization in a flowing discharge afterglow. While the design of system which utilizes chemi-ionization in a flowing discharge afterglow is more complex than systems which utilize direct extraction, systems which utilize flowing discharge afterglow produce much higher currents and electron polarizations.
As alluded to, systems that utilize direct extraction are inherently less complex that are systems which utilize flowing discharge afterglow and therefore offer utility. However, efforts by the inventors herein to develop an improved direct extraction source have met with limited success. The reason for the low polarization achieved from the direct extraction source investigated by the inventors herein is believed to be that unpolarized electrons are produced by ionization of ground state atoms, which effect competes with polarized electron production by exchange collisions and associated ionization involving spin-polarized metastable atoms. The problem with direct extraction is that it relies on the existence of a discharge with a relatively high ratio of metastable atoms to ground state atoms. The flowing afterglow approach overcomes this problem, but involves, as a trade-off, greater cost and complexity.
At this point it is disclosed that the present invention is based in the insight that if the key-electron-polarizing collisions mechanism involved ground-state atoms, instead of excited atoms, the problem of unpolarized electrons being produced by ionization of ground state atoms, would be overcome. The present invention then provides that free electrons diffuse under the action of an electric field through Rb vapor that has been spin polarized by optical pumping, and through spin-polarizing “electron-exchange-mechanism” collisions with the Rb, the free electrons become spin-polarized and are directly extracted as a beam.
Continuing, the idea of utilizing spin-exchange collisions to polarize ensembles of electrons is not new. Articles by:
Fargo et al., titled “The Production Of Polarized Electron Beams by Spin Exchange Collision”, Phys. Lett. 20, 279 (1966) and “On A Sourcve Of Polarized Electrons”, Proc. R. Soc. Edinb. A, Math. 70, 15 (1971/72); and by
Krisciokaitis-Krisst et al., titled “Theoretical Consideration Of Spin-Polarized Electron Source Based On Elastic Electron-Hydrogen Spin-Exchange Collisions”, Nucl. Instrum. Methods Phys. Res. (Netherlands) 83, 45 (1970); and “Prototype Polarized-Electron Source Through Electron-Hydrogen Spin Exchange With Teflon Containment Of Hydrogen And A Longitudinal Magnetic Trap”, Nucl. Instrum. Methods Phys. Res. (Netherlands) 118, 157 (1974);
describe use of spin-exchange collisions to polarize ensembles of electrons, Said articles describe pulsed sources of polarized electrons which operate by directing beams of polarized Rb and H through electron traps. The Rb atoms are polarized by passage through a hexapole magnet. In turn, electrons become polarized and are periodically dumped. Said procedure provides between forty (40%) and sixty
Batelaan Hermanus
Birdsey Benjamin B.
Gay Timothy J.
Hitt Brooks A.
Jr. Leon Scott
The Board of Regents of The University of Nebraska
Welch James D.
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