Fine structure in the delayed coincidence lifetime curves for positrons in argon

Tao, S. J.; Bell, J.; Green, J H

doi:10.1088/0370-1328/83/3/312

Cited by 35 publications

(12 citation statements)

References 5 publications

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“…( j j The existence of a fast component in the spectrum is revealed if the measured values of ZcIr(t), as determined from [6] and [7], show a sharp increase as t decreases towards t = 0. It has been shown theoretically by McEachran et al (17) for Kr and Xe and by Can.…”

Section: Lifetime Parametersmentioning

confidence: 99%

“…independantly by Falk and Jones (5), Tao et rrl. (6). and Paul (7), focussed attention onto the diffusion of slow positrons in the inert gases.…”

Section: Introductionmentioning

confidence: 99%

“…The equipment consists of a pressure vessel containing the gas and a "Na positron source of a few microcuries strength. Timing pulses are derived froni two fast plastic scintillation y-ray detectors coupled to fast photomultiplier tubes which trigger 6 ARGON at 6 zero-crossing or constant fraction discriminators. One detector responds to the 1.28 MeV y-ray emitted simultaneously froni the source with the positron whilst the second detector is set to detect one of the annihilation y-rays.…”

Section: Introductionmentioning

confidence: 99%

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Positron lifetime spectra for gases

Heyland¹,

Charlton²,

Griffith³

et al. 1982

Can. J. Phys.

101

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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...

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Section: Lifetime Parametersmentioning

confidence: 99%

“…independantly by Falk and Jones (5), Tao et rrl. (6). and Paul (7), focussed attention onto the diffusion of slow positrons in the inert gases.…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Positron lifetime spectra for gases

Heyland¹,

Charlton²,

Griffith³

et al. 1982

Can. J. Phys.

101

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“…Beyond τ ∼ 200-300 ns amg epithermal annihilation occurs at momenta below the momentum-transfer cross-section minimum. Integrating over , this 'knee' in the spectrum becomes the characteristic shoulder region observed in the lifetime spectrum [34][35][36]. It is known to be relatively insensitive to the initial positron momentum distribution [16,37].…”

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confidence: 99%

Probing Positron Cooling in Noble Gases via Annihilation γ Spectra

Green

2017

Phys. Rev. Lett.

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γ spectra for positron annihilation in noble-gas atoms are calculated using many-body theory for positron momenta up to the positronium-formation threshold. These data are used, together with time-evolving positron-momentum distributions determined in the preceding Letter [Phys. Rev. Lett. 119, 203403 (2017)PRLTAO0031-9007], to calculate the time-varying γ spectra produced during positron cooling in noble gases. The γ spectra and their S[over ¯] and W[over ¯] shape parameters are shown to be sensitive probes of the time evolution of the positron momentum distribution and thus provide a means of studying positron cooling that is complementary to positron lifetime spectroscopy.

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“…The annihilation spectrum (neglecting positronium formation) is given by where r a (t>) is the velocity-dependent direct annihilation rate, related to the annihilation cross section, a a , by (1) r -Nvo , a a (2) where N is the atomic density of the gas, and f(v, t) is the positron velocity distribution func- tion. The latter is obtained by solving the diffusion equation 7 at v 2 av \l3r (…”

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confidence: 99%

Effect of Electric Field on Positron Lifetimes in Argon and Helium

1965

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Several groups 1 " 8 have recently observed a nonexponential form for the direct component of positron annihilation in the noble gases. In particular, the annihilation spectrum [ Fig. 2(a)] possesses a marked shoulder followed by an exponential decay. This effect has been interpreted 1 "" 3 in terms of a velocity dependence of the positron annihilation rate, implying that the cross section for direct annihilation rises more rapidly at low positron energies than does the simple inverse velocity dependence.The results presented in this paper indicate the velocity dependence in a more direct way by displaying the effect of an applied electric field on the lifetime characterizing the exponential portion of the annihilation spectrum. The observed annihilation rate is thus the velocitydependent annihilation rate averaged over the positron equilibrium velocity distribution determined by the strength of the applied electric field. The effect of such fields on the energy-distribution functions of electrons in gases has been the subject of extensive investigation, 4 * 5 whereas the only consideration of such effects for the case of positrons in gases is contained in the work of Marder et al., 8 which was concerned with an investigation of the effect of applied electric fields on the fraction of positrons forming positronium.The experimental work described in this paper was performed using the chamber illustrated in Fig. 1. It is an aluminum chamber demon VOLTAGE CONNECTOR COPPER RINGS

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Fine structure in the delayed coincidence lifetime curves for positrons in argon

Cited by 35 publications

References 5 publications

Positron lifetime spectra for gases

Positron lifetime spectra for gases

Probing Positron Cooling in Noble Gases via Annihilation γ Spectra

Effect of Electric Field on Positron Lifetimes in Argon and Helium

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