Positron scattering from hydrocarbons

The total cross sections for positron impact on hydrocarbons have been calculated using the additivity rule in which the total cross section for a molecule is the sum of the total cross section for the constituent atoms. The energy range considered is from a few eV to several thousand eV. The total cross sections for positron impact on an atom are calculated by employing a complex spherical potential which comprises of a static, polarization and an absorption potential. We have good agreement with the experimental results for hydrocarbons for positron energy ⩾100 eV. Our results also agree with the available calculations for CH₄ and C₂H₂ which employed full molecular wavefunctions beyond 100 eV. Our absorption cross sections also agree with molecular wave-function calculations for C₂H₂ and CH₄ beyond 100 eV. We have shown the Bethe plots fore ⁺−C ande ⁺−H scattering systems and Bethe parameters have been extracted. We have fitted the cross section for positron impact on hydrocarbons in the formσ _t(C _n H _m )=naE ⁻ b+mcE ⁻ d in the energy range 300–5000 eV wherea=195.0543,b=0.7986,c=371.1757 andd=1.1379 withE in eV andσ _t in 10⁻¹⁶ cm².     

Total electron scattering cross-sections for O atoms: O2 and O3 molecules

Total (elastic+inelastic) cross sections fore ⁻-O,e ⁻-O₂ ande ⁻-O₃ scattering have been calculated at sample energies between 100 and 1000eV. The basice ⁻-O atom scattering amplitudes are obtained from the partial wave analysis with a complex optical potential. Thee ⁻-O₂ ande ⁻-O₃ cross-sections are obtained through the independent atom model. Oure ⁻-O₂ cross-sections reproduce the experimental data quite well. Adequate comparisons are made in all the three cases.     

Elastic scattering of positrons by xenon atoms

Summary Partial and total cross-sections of positron scattering by Xe atoms are calculated at twenty values of incident energy ranging from 0.01 Ry to 4.00 Ry. The calculations are carried out via model potential (describing the positron target interaction) and a polarization potential introduced previously by Lane and Geltmann (1967). The comparison between our results and those obtained by other authors shows significant agreement and supports our simple model scattering process. The author of this paper has agreed to not receive the proofs for correction.     

Elastic scattering of electrons by argon atoms

The partial wave method with a central potential has been applied to investigate the elastic scattering of electrons by the argon atoms in the intermediate energy range (100 eV − 1 keV). The central potential includes the effects of the static field, exchange and polarization. The results are in good accord with recent experimental data. A brief account of this investigation was presented at the X Int. Conf. on the Physics of Electronic and Atomic Collisions held at Paris in July 1977.     

Differential elastic cross-sections of excited exotic hydrogen atoms

Differential cross-sections for the elastic scattering of excited exotic hydrogen atoms by hydrogenic atoms are calculated for the first time. The calculations are performed for exotic atoms (μ a,π a; a=p,d,t) in the states with the principal quantum numbers n=2-10 at relative motion energies E=0.001-15 eV and at scattering angles ϑ =0–180^00B0;. This revised version was published online in August 2006 with corrections to the Cover Date.     

Positron scattering from alkaline-earth elements

The total (elastic + inelastic) cross sections fore ⁺ impact on alkaline-earth elements from Be to Ra are calculated by employing a complex spherical optical potential. This potential has static, polarization and absorption components. The positron energy range is from a few eV to several thousand eV. We have compared our elastic cross sections for Mg and Ca with the other available results and the agreement is good for energies above 100eV. We have also compared our absorption cross sections withe ⁻ ionization cross sections at high energies where our absorption cross sections are in good accord. We have made Bethe plots fore ⁺ scattering on these elements.     

Total and transport elastic cross-sections for excited muonic and pionic hydrogen atoms

The results of an experiment on muon catalyzed dd-fussion in HD gas are presented. The experiment was performed at the muon beam of PSI using a high-pressure ionization chamber filled with pure HD-gas of low D₂ concentration on the level 1%, at temperatures 50, 150 and 300 K. The non-resonant character of ddμ-molecule formation on HD molecules was confirmed by measuring the ratio of yields of the two ddμ-fusion channels, R=Y(³He,n)/Y(³H,p), which proved to be close to unity. The ddμ formation rate was found to vary from λ_ddμ-HD=0.05· 10⁶ s^-1 at T=50 K to λ_ddμ-HD=0.12· 10⁶ s^-1 at T=300 K, in agreement with the theoretical prediction. A prominent peak at t<60 ns was observed in the time spectrum of fusion neutrons indicating a resonant contribution of ddμ formation from epithermal dμ atoms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Elastic scattering of electrons and positrons by cadmium atoms

ABSTRACT

The differential, integrated elastic, total and momentum transfer cross sections along with Sherman function for the elastic scattering of electrons and positrons by cadmium atoms have been evaluated from the partial wave solution of the Dirac relativistic scattering equations for a projectile-atom complex potential at the energy range 6.4 eV < E < 1.0 keV. For various scattering quantities, a comparison of our results exhibits better agreement with the experimental data than the other available theoretical values.     

Positron spin relaxation in solids

Summary The positron spin relaxation in a longitudinal magnetic field was investigated in some solids where Ps formation occurs. The experimental results indicate that this relaxation manifests itself in those organic compounds where the positron can excite low-lying triplet states. The values of positron slowing-down time, obtained by adopting a schematic model, range from 13 to 28 ps. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero della Pubblica Istruzione, and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Positron implantation profile in kapton

Summary Measurements of positrons' implantation profile were made with geometry as in the majority of PAT experiments, making use of the difference in values of mean lifetimes of positrons in the absorber and in the detector. The function describing the absorption of positrons in the absorber taking into account measurement geometry was fitted to the experimental data. The correction to the exponential relation occurring in this function is the dominating factor for small thicknesses of the absorber. In this analysis various values of positrons' backscatter coefficients of the nickel and of the kapton were also taken into account.

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Riassunto Sono state effettuate misure del profilo di impianto dei positroni per mezzo della geometria come nella maggioranza degli esperimenti PAT, usando la differenza nei valori delle vite medie dei positroni nell'assorbitore e nel rilevatore. La funzione che descrive l'assorbimento di positroni nell'assorbitore, considerando la geometria di misurazione, è stata adattata ai dati sperimentali. La correzione alla relazione esponenziale che si veriffica in questa funzione è il fattore dominante per spessori ridotti dell'assorbitore. Sono stati presi in esame in quest'analisi anche vari valori dei coefficienti di backscatter dei positroni del nickel e del kapton.

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Резюме Проведены измерения профиля имплантации позитронов в случае геометрии, как в большинстве экспериментов PAT, используя различия в величинах средних времен жизни позитронов в поглотителе и в детекторе. Функция, описывающая поглощение позитронов в поглотителе с с учетом геомтрии измерения, удовлетворяет экспериментальным данным. Поправка к экспоненциальному отношению, появляющаяся в этой функции, представляет основной коэффициент для малых толщин поглотителя. В этом анализе учитываются различные величины для коэффициентов обратного рассеяния от никеля и картона.

     

Positron polarization in chiral media

Summary The positron spin depolarization, relative to the direction of a static magnetic field, was investigated in some chiral media where Ps formation occurs. The results indicate that helical positrons are able to distinguish one enantiomer from the other: the remaining polarizations—just before Ps is formed—differ by about 20% on passing from one enantiomer to the other of the same chiral species. Moreover the existence of a process—strictly connected with chirality and able to hinder any depolarizing process more efficiently than the field itself—has been made evident at low fields. This work was supported by INFM (Consorzio Nazionale di Fisica della Materia).     

Positron spin relaxation in naphthalene

Summary The remaining spin polarizationP of positrons was measured in naphthalene against longitudinal magnetic-field strength. The measuring method was based on the relative yield of Ps in triplet and singlet state. In a zero fieldP is reduced to 50% of the initial polarization and approaches 100% upon increasing the applied field to larger values. This effect, which in teflon is quite absent, appears to be determined by the specific behaviour of positrons in naphthalene. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) of Ministero Pubblica Istruzione and by GNSM (Gruppo Nazionale di Struttura della Materia) of CNR.     

Elastic scattering of electrons by helium atoms in high energy higher order Born approximation

The differential cross-sections forē-helium elastic scattering are calculated by using Yateshigh-energy higher order Born approximations, through 0 (K _i ^{Emphasis>−2} ) of the incident electron momentum, and comparisons have been made with the recent theoretical and experimental results.     

On the interaction of exotic atoms with their environment

A theoretical study is made of the effect of collisions on the two-photon 2s → 1s transition of the fully-ionized muonic boron migrating in helium. Data on atomic processes are indicated which may be extracted from measurements of the yield of this transition. This revised version was published online in August 2006 with corrections to the Cover Date.     

High energy elastice + − O2 scattering

Theoretical differential cross-sections are obtained for the elastic scattering of fast (E _i ⩾ 200 eV) positrons by oxygen molecules. Employing the independent atom model, the atomic scattering amplitudes are calculated with the (static + polarization) model potentials, in the partial wave analysis (PWA). Comparisons made with the theory and experiments on the incident electrons, show significant differences at small angles.     

Positron scattering by phonons in metals

Processes involved in positron-matter interaction are studied. The stopping power and mean free path are calculated for positrons with an energy of about 1 eV, which are scattered mostly by phonons. The mean free path and range of positrons in tungsten, as well as the stopping power of tungsten, are calculated for positron energies between 0.025 and 10 eV.     

Positron annihilation in ionic crystals

Positron lifetime measurements were made on Ca²⁺ doped NaCl, Ba²⁺ doped NaCl and X-ray- and additively-coloured KCl crystals. In the NaCl(Ca) system a correlation which could be approximated by a power equation was found between the dopant concentration and the positron annihilation parameters. In the NaCl(Ba) system the decay parameters do not change significantly as expected and no significant difference in the decay parameters of the coloured and uncoloured crystals could be found. Paper C 4 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Positron systematics in annealed metals

Abtract Deconvoluted Doppler-broadened annihilation energy spectra in annealed metals are fitted by a parabola and a Gaussian function whose width and intensity vary systematically with the atomic number.