Positronium formation in s-wave positron–hydrogen scattering

1982 ◽  
Vol 60 (4) ◽  
pp. 591-596 ◽  
Author(s):  
J. W. Humberston
Keyword(s):  
Cross Section ◽  
Atomic Hydrogen ◽  
Positronium Formation ◽  
Excitation Threshold ◽  
Matrix Elements ◽  
S Wave ◽  
Formation Cross Section ◽  
Positron Energy ◽  
R Matrix ◽  
A Value

A two channel version of the Kohn variational method is used with trial functions containing up to 120 linear variational parameters to calculate the R-matrix elements and the positronium formation cross section for the scattering of s-wave positrons by atomic hydrogen. Results are obtained in the energy range between the positronium formation threshold and the lowest excitation threshold of the atom. The positronium formation cross section rises very steeply to a value of [Formula: see text] just above the positronium formation threshold and then rises approximately linearly with positron energy to a value of [Formula: see text] just below the excitation threshold. Owing to the weakness of the coupling between the positron and positronium channels, the elastic scattering cross section for the positron continues smoothly across the positronium formation threshold.

scholarly journals Positron?Lithium Inelastic Scattering with Polarisation Potentials

1991 ◽  
Vol 44 (6) ◽  
pp. 677 ◽  
Author(s):  
M EI-Shabshiry ◽  
SY EI Bakry ◽  
AH Moussa ◽  
MA Abdel-Raouf
Keyword(s):  
Inelastic Scattering ◽  
Cross Section ◽  
Cross Sections ◽  
Excitation Cross Section ◽  
Positronium Formation ◽  
Elastic Channel ◽  
Formation Cross Section ◽  
Positron Energy ◽  
Maximum Value

Positron-lithium inelastic scattering is studied at positron energies ranging from 0�005 to 20 eV using the coupled static model. Two channels are open namely the elastic and positronium formation. The polarisation potentials of the Li atom in the first channel and that of the positronium atom in the second channel are taken into consideration in calculating the corresponding cross sections. The partial cross sections in each channel are calculated for eight values of the total angular momentum (0 ~.e ~ 7). In the elastic channel the total cross section Ull has its maximum value at the lowest energy, and decreases with an increase in positron energy Ki. The total positronium formation cross section has a small dip at 0�1 eV, and maximum value at Ki = 1 �4 eV, and then decreases smoothly with e+ energy. We compare our results with those of Ward et al. (1989) where positronium formation is ignored. The agreement in elastic cross sections improves with e+ energy, while the total collisional cross sections have the closest agreement at 1�0 eV. From this comparison, we find that positronium formation is important in the very low energy region, and the role of the excitation cross section increases steadily with e+ energy.

Positronium formation in low-energy positron–helium scattering

1996 ◽  
Vol 74 (7-8) ◽  
pp. 335-342 ◽  
Author(s):  
J. W. Humberston ◽  
P. Van Reeth
Keyword(s):  
Elastic Scattering ◽  
Cross Section ◽  
Cross Sections ◽  
Wave Functions ◽  
Positronium Formation ◽  
Recent Analysis ◽  
S Wave ◽  
Small Magnitude ◽  
Formation Cross Section ◽  
Positron Collisions

Detailed investigations were made of positron collisions with helium atoms in the Ore gap using a two-channel version of the Kohn variational method with very flexible trial wave functions and accurate correlated helium-target wave functions. Accurate values of the s-wave elastic scattering and positronium-formation cross sections are presented, together with preliminary values of the cross sections for s-wave scattering. The s-wave positronium-formation cross section in helium displays a remarkable similarity in both magnitude and energy dependence to the corresponding cross section in positron–hydrogen scattering, with a rapid rise from the positronium-formation threshold and a more gradual rise thereafter. Its small magnitude is consistent with the findings of a recent analysis of experimental measurements of the positronium-formation cross section. Calculations of the s-wave elastic-scattering cross section above and below the positronium-formation threshold reveal a Wigner "rounded step" at the threshold itself.

scholarly journals The 12C(a,g)16O Cross Section at Low Energies

1991 ◽  
Vol 44 (4) ◽  
pp. 369 ◽  
Author(s):  
FC Barker ◽  
T Kajino
Keyword(s):  
Cross Section ◽  
Ground State ◽  
State Transitions ◽  
Correct Treatment ◽  
R Matrix ◽  
A Value ◽  
S Factor ◽  
Low Energies ◽  
Cascade Transitions ◽  
Matrix Formula

An R-matrix formula for the cross section for radiative capture reactions is developed and applied to fit recently measured 12C(oc,y)160 data, for both ground-state transitions and cascade transitions through the 6�92 and 7 �12 MeV levels. The correct treatment of the channel contributions is significant for the E2 cascade transitions. Consistent fits of the cascade and ground-state data suggest a value of the channel radius larger than those previously used, and consequently a value of the low-energy astrophysical S-factor appreciably larger than that adopted recently.

scholarly journals Positronium Formation in e+ ?Na Scattering

1992 ◽  
Vol 45 (6) ◽  
pp. 757
Author(s):  
N Ranjit Singh ◽  
Keka Basu Choudhury ◽  
PS Mazumdar ◽  
S Brajamani
Keyword(s):  
Cross Section ◽  
Energy Region ◽  
Intermediate Energy ◽  
Positronium Formation ◽  
Distorted Wave ◽  
Formation Cross Section ◽  
Wave Approach

The positronium formation cross section in positron-sodium scattering has been evaluated by employing a distorted wave approach in the low and intermediate energy region. The effect of the positronium� formation cross section on the total cross ection for positron-sodium scattering has also been discussed.

scholarly journals New and Old Measurements of Electron Scattering in Atomic Hydrogen

1998 ◽  
Vol 51 (4) ◽  
pp. 633 ◽  
Author(s):  
J. F. Williams
Keyword(s):  
Cross Section ◽  
Cross Sections ◽  
Differential Cross ◽  
Atomic Hydrogen ◽  
Intermediate Energy ◽  
Absolute Calibration ◽  
Absolute Cross Section ◽  
Close Coupling ◽  
R Matrix ◽  
Correlation Parameters

A critical look is made of discrepancies and agreements between new and old measurements and theories for elastic and n = 2 excitation of atomic hydrogen by electron impact, mainly at 16·5, 54 and 100 eV. A discussion of earlier work indicates the contributions of Weigold and colleagues. The diffculties of observing and modelling small scattered fluxes at backward scattering angles and of making absolute cross section calibrations are noted. New measurements of elastic scattering at 16·5 eV confirm earlier measured angular distributions. An absolute calibration of the differential cross section at 16·5 eV gives agreement within one standard deviation with intermediate energy R-matrix and multi-pseudostate close coupling values. At 16·5 eV, measurements of the separate 2s and 2p differential cross sections and the lambda, R and I correlation parameters again support the values from those theories.

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