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An investigation of positrons interacting with solid argon, krypton and xenon

Description: With this article we intend to shed some light on all the important characteristics of the up-to-date most efficient positron moderators, the rare gas solids. We stress on the importance of the impurities in the performance of the solid rare gas moderators. The impurity factor is linked with the crystalline changes to explain the effect of annealing, and demonstrate the role of impurities in the endurance. Significant increase in the low energy positron yield is observed after repeated anneals. The positron energy distributions from Ar, Kr, and Xe moderators are measured to be about 2 eV (FWHM).
Date: October 1, 1996
Creator: Petkov, M.P.; Roellig, L.O. & Troev, T.D.
Partner: UNT Libraries Government Documents Department

Description of the intense, low energy, monoenergetic positron beam at Brookhaven

Description: An intense (4 x 10/sup 7/ s/sup -1/), low energy (approx. =1.0 eV), monoenergetic (..delta..E approx. = 75 MeV) beam of positrons has been built at the Brookhaven National Laboratory. This flux is more than 10 times greater than any existing beam from radioactive sources. Plans are underway to increase further the flux by more than an order of magnitude. The intense low energy positron beam is made by utilizing the High Flux Beam Reactor at Brookhaven to produce the isotope /sup 64/Cu with an activity of 40 curies of positrons. Source moderation techniques are utilized to produce the low energy positron beam from the high energy positrons emitted from /sup 64/Cu. 31 refs., 7 figs.
Date: January 1, 1985
Creator: Lynn, K.G.; Mills, A.P. Jr.; Roellig, L.O. & Weber, M.
Partner: UNT Libraries Government Documents Department

A high intensity positron beam at the Brookhaven reactor

Description: We describe a high intensity, low energy positron beam utilizing high specific activity /sup 64/Cu sources (870 Ci/g) produced in a reactor with high thermal neutron flux. Fast-to-slow moderation can be performed in a self moderation mode or with a transmission moderator. Slow positron rates up to 1.6 x 10/sup 8/ e/sup +//s with a half life of 12.8 h are calculated. Up to 1.0 x 10/sup 8/ e/sup +//s have been observed. New developments including a Ne moderator and an on-line isotope separation process are discussed. 21 refs., 9 figs.
Date: January 1, 1987
Creator: Weber, M.; Lynn, K.G.; Roellig, L.O.; Mills, A.P. Jr. & Moodenbaugh, A.R.
Partner: UNT Libraries Government Documents Department

2D ACAR momentum density study of the nature of the positron surface state on Al(100)

Description: The two-dimensional angular correlation of the 2..gamma.. annihilation radiation (2D ACAR) has been measured from an Al(100) surface bombarded by 200-eV positrons. After removing the contribution of fast para-positronium annihilation, the spectrum from positrons annihilating at the surface exhibits a nearly isotropic conical shape with a (7.1 +- 0.5) mrad FWHM. 5 refs., 6 figs.
Date: January 1, 1985
Creator: Berko, S.; Canter, K.F.; Lynn, K.G.; Mills, A.P.; Roellig, L.O. & West, R.N.
Partner: UNT Libraries Government Documents Department

A positronium beam and positronium reflection from LiF(100)

Description: We report results obtained from a variable energy monoenergetic beam of positronium (Ps). Ps was formed by a charge transfer reaction of positrons colliding with Ar gas atoms in a ''gas cell'' with an efficiency of about 3 /times/ 10/sup /minus/4/. The beam was used to measure the fraction, R, of specularly reflected ortho-Ps from LiF(100). We found R to peak at about 30% for 7 eV Ps at a specular scattering angle of 50/degree/ to 60/degree/. At higher energies R drops sharply to values around 0.5% near 60 eV Ps.
Date: January 1, 1988
Creator: Weber, M.; Tang, S.; Berko, S.; Brown, B.L.; Canter, K.F.; Lynn, K.G. et al.
Partner: UNT Libraries Government Documents Department

A positronium beam and positronium reflection from LiF

Description: A positronium (Ps) beam has been constructed by transmitting monoenergetic, low energy positrons through a gas cell containing either Ar or He which provide an electron to form positronium. The positrons were obtained by magnetically deflecting positrons in the straight section of the positron beamline into a beamline which contained the gas cell and a Ps detection chamber. The two beamlines provide the capability of switching from an experiment which uses positrons (a study of the angular correlation of annihilating radiation - ACAR) to one which uses Ps atoms without breaking vacuum, nor moving equipment. The authors report the characteristics of the two beams and state that very preliminary information has been obtained on the characteristics of the Ps beamline and on the reflection of Ps from a LiF crystal.
Date: January 1, 1987
Creator: Weber, M.; Berko, S.; Brown, B.L.; Canter, K.F.; Lynn, K.G.; Mills, A.P. Jr. et al.
Partner: UNT Libraries Government Documents Department

Positronium reflection and positronium beams

Description: We have observed specular reflection of positronium, Ps, and established that there is adequate intensity at higher energies to make further study worthwhile. The scattering appears to be restricted to the outermost surface with a mean free path of (0.75 {plus minus} 0.15){Angstrom} for Ps in LiF(100). With a greater intensity Ps beam one should see higher order diffraction beams as the result of the periodicity of the surface. Ps diffraction thus offers the possibility of being a novel and valuable probe to study the outermost surface and to study adsorbents on it. Two methods for producing Ps beams are described. 29 refs., 11 figs.
Date: January 1, 1989
Creator: Weber, M.; Tang, S.; Khatri, R.; Roellig, L.O.; Viescas, A.J. (City Coll., New York, NY (USA)); Berko, S. et al.
Partner: UNT Libraries Government Documents Department