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Slow Positron Study of Positronium Formation and Diffusion in Crystalline and Amorphous Ice.

DE85005886

Publication Date	1984
Personal Author	Lynn, K. G.; Eldrup, M.; Vehanen, A.; Schultz, P. J.
Page Count	29
Abstract	The aim of the present work has been to study the interaction of low energy positrons and of Ps with a molecular solid. As a first example we have chosen ice which has been extensively investigated both by conventional positron annihilation methods as well as by many other techniques. Unlike in metals, positrons injected into most molecular solids may have a certain probability to form Ps in the bulk of the material. The process by which Ps is formed in condensed molecular materials has been a matter of strong debate. According to the Ore model of Ps formation, which is recognized as being valid for low density gases, the positron during its slowing down picks off an electron from a molecule with which it simultaneously forms Ps. For energies below E/sub L/ = I-6.8 eV, where I is the lowest ionisation energy and 6.8 eV is the Ps binding energy in vacuum, this process is not energetically possible. Above an energy E/sub u/ approx. I the model predicts a reduction of the yield of stable Ps, because of rapid slowing down of the positron by ionisation or by splitting up again of a Ps atom which may have been formed. The energy range E/sub L/ less than or equal to E approximately less than E/sub u/ is the so-called Ore gap in gases. In the present work both crystalline and amorphous ice have been studied using positrons of energies in the range 0 to 4.5 keV. The experimental methods used were determination of the yields of 3 gamma -annihilations as well as Doppler broadening measurements in the temperature range 45 to 150 K. The main results obtained were a determination of the Ps diffusion constant, and a demonstration that in the present experiments one contribution to the total Ps yield comes from positrons with typical Ore gap energies, and another contribution, due to spur processes, from positrons with initial energies of several hundred eV. 57 references, 9 figures. (ERA citation 10:014901)
Keywords	Ice Amorphous State Annihilation Crystal Lattices EV Range 01-10 EV Range 10-100 EV Range 100-1000 Energy Dependence KeV Range 01-10 Positron Collisions Positronium ERDA/640302
Source Agency	Technical Information Center Oak Ridge Tennessee
NTIS Subject Category	46 - Physics 99F - Physical & Theoretical Chemistry
Corporate Authors	Brookhaven National Lab., Upton, NY.; Department of Energy, Washington, DC.; Helsinki Univ. of Technology, Espoo (Finland). Lab. of Physics.; Risoe National Lab., Roskilde (Denmark).; Western Ontario Univ., London (Canada).
Supplemental Notes	International workshop on slow positrons in surface science, Pajulahti, Finland, 25 Jun 1984.
Document Type	Conference Proceedings
NTIS Issue Number	198513
Contract Number	AC02-76CH00016

Slow Positron Study of Positronium Formation and Diffusion in Crystalline and Amorphous Ice.

Slow Positron Study of Positronium Formation and Diffusion in Crystalline and Amorphous Ice.
DE85005886

Publication Date	1984
Personal Author	Lynn, K. G.; Eldrup, M.; Vehanen, A.; Schultz, P. J.
Page Count	29
Abstract	The aim of the present work has been to study the interaction of low energy positrons and of Ps with a molecular solid. As a first example we have chosen ice which has been extensively investigated both by conventional positron annihilation methods as well as by many other techniques. Unlike in metals, positrons injected into most molecular solids may have a certain probability to form Ps in the bulk of the material. The process by which Ps is formed in condensed molecular materials has been a matter of strong debate. According to the Ore model of Ps formation, which is recognized as being valid for low density gases, the positron during its slowing down picks off an electron from a molecule with which it simultaneously forms Ps. For energies below E/sub L/ = I-6.8 eV, where I is the lowest ionisation energy and 6.8 eV is the Ps binding energy in vacuum, this process is not energetically possible. Above an energy E/sub u/ approx. I the model predicts a reduction of the yield of stable Ps, because of rapid slowing down of the positron by ionisation or by splitting up again of a Ps atom which may have been formed. The energy range E/sub L/ less than or equal to E approximately less than E/sub u/ is the so-called Ore gap in gases. In the present work both crystalline and amorphous ice have been studied using positrons of energies in the range 0 to 4.5 keV. The experimental methods used were determination of the yields of 3 gamma -annihilations as well as Doppler broadening measurements in the temperature range 45 to 150 K. The main results obtained were a determination of the Ps diffusion constant, and a demonstration that in the present experiments one contribution to the total Ps yield comes from positrons with typical Ore gap energies, and another contribution, due to spur processes, from positrons with initial energies of several hundred eV. 57 references, 9 figures. (ERA citation 10:014901)
Keywords	Ice Amorphous State Annihilation Crystal Lattices EV Range 01-10 EV Range 10-100 EV Range 100-1000 Energy Dependence KeV Range 01-10 Positron Collisions Positronium ERDA/640302
Source Agency	Technical Information Center Oak Ridge Tennessee
NTIS Subject Category	46 - Physics 99F - Physical & Theoretical Chemistry
Corporate Authors	Brookhaven National Lab., Upton, NY.; Department of Energy, Washington, DC.; Helsinki Univ. of Technology, Espoo (Finland). Lab. of Physics.; Risoe National Lab., Roskilde (Denmark).; Western Ontario Univ., London (Canada).
Supplemental Notes	International workshop on slow positrons in surface science, Pajulahti, Finland, 25 Jun 1984.
Document Type	Conference Proceedings
NTIS Issue Number	198513
Contract Number	AC02-76CH00016