Positron lifetime spectra for gases

Abstract

Recent observations on the lifetimc spectra for gases are presented and discussed. There is little to report on the inert gases helium. neon, and argon and thc spectra for these gases are thought to be understood. Ncw lifctime data for krypton and xenon have revealed two fast components which, although probably connected with the low positronium fractions for these gases, have yet to receive a satisfactory interpretation.The polarized orbital calculations for ZCll's and momentum transfer cross sections for all the inert gases by the York group are now complete and arc generally in good agreement with experiment.For the molecular gases some information on rotationaiexcitation cross sections has been obtained from thermalization times for nitrogen, hydrogen, and deuterium. In general, the Z,,-, parameter is dependant on gas density and temperature with some gascs showing a pronounced maximum in the density dependance. The gases C3Hx, CaHlo, and CH3CI have very large values of ZCir which indicate localization or capture of the positron by one or more molecules. These three gases exhibit a maximum in the instantaneous decay rate of the "free" positron component at low densities from which a "capture" cross section can be estimated.The positronium fraction, F, generally increases with the gas density, usually approaching a limiting value asymptotically, but for nitrogen this density dependance has a maximum at -140 amagat. No satisfactory explanation has been offered for this behaviour.Positronium formation cross sections for several gases have been deduced from measurements of the variation of F with the concentration of the gas in helium. These cross sections are similar in magnitude to those deduced from the total cross section beam measurements.On prtsente et discute des rtsultats rtcents concernant les spectres de vie moyenne des positrons dans les gaz. I1 y a peu de choses a rapporter dans le cas des gaz inertes htlium. neon et argon, et I'on pense que les spectres pour ces gaz sont bien compris. Des nouvellcs donnees pour la vie moyenne dans le krypton et le xtnon ont revel6 I'existence de deux composantes rapides pour lesquelles, bien qu'clles soient probablernent relites au fait que la fraction positronium est faible dans ces gaz, il n'y a pas encore d'interprttation satisfaisante.Les calculs par orbitales polarisCes des Z,,,et des sections efficaces de transfert d'impulsion effectuts par le groupe de York sont maintenant complct et sont gtnCralernent en bon accord avec les donnCes exptrimentales.Dans le cas des gaz moleculaires, quelqucs renseignements sur les sections efficaces d'excitation rotationneJle ont t t i obtenus a partir des temps de thermalisation pour I'azote, I'hydrogkne et le deuttrium. En gtntral, le paramktre ZCfr dtpend de la densit6 et la temperature du gaz, avec pour certains gaz, un maximum pcononcC dans la courbe de variation en fonction de la densitt. Les gaz C3H8, C4Hltr et CH3CI ont des valeurs trks grandes de Z,,-,-, ce qui est une indication de localisation ou de capture du positro...

Spin-Orbit Quenching of Positronium during Atomic Collisions

“…As the Ps atoms slow down, so k decreases and the slope of the curve decreases. The fast component in the experimental Xe spectrum also shows this feature and is fitted to two exponentials [7].…”

“…3 in Ref. [7]. A scattering length of 2:5a 0 (recent calculations and analysis of experimental data suggest a value between 1.5 and 3:2a 0 [9,14]) results in a momentum transfer cross section of 25 a 2 0 .…”

“…Experiments using H 2 :Xe mixtures have also resulted in a larger Ps fraction [10]. [7] were digitized and plotted on the same scale as the present simulation. Those parts of the spectrum traditionally ascribed to free positron annihilation are identified.…”

“…The existence of spin-orbit quenching provides an explanation to one of the oldest conundrums in positron annihilation physics, namely, the question: why is the Ps fraction so small for xenon and to a lesser extent krypton [7][8][9]? An examination of the tail of the annihilation spectrum for Xe reveals that only 3% of the positrons have survived the thermalization process as o-Ps.…”

“…The parameters used in the simulation were initially estimated using criteria that were deemed to be physically sensible. However, some of the parameters P H Y S I C A L R E V I E W L E T T E R S week ending 9 MAY 2003 (e.g., F so , , 1 Z eff , E 0 ) were then fixed by manual adjustment to give a reasonable fit to the spectrum for pure Xe [7] while ensuring that the parameters did not become unphysical. The density of the gas was chosen as 9.64 amagats (1 amagat 2:69 10 25 atoms=m ÿ3 ) as this is the same density as Fig.…”