arXiv subjects

A. Costescu

Publications and source records attributed to A. Costescu.

3 recordsLinked to original sources

arXiv2006

The total cross-sections for the photoeffect for 2S-subshell bound electrons and pair production with the created electron in the 2S subshell for photon energies above 1 MeV

Considering the contributions of the main term of the relativistic Coulombian Green function given by Hostler [J. Math. Phys, 5, 591 (1964)] to the second order of the S-matrix element and taking into account only the large components of Dirac spinor of the 2S subshell, we have obtained the imaginary part of the Rayleigh amplitudes in terms of elementary functions. Thereby, simple and high accurate formulae for the total cross-sections for photoeffect and pair production with the electron created in the 2S subshell are obtained via the optical theorem. Comparing the predictions given by our formulae with the full relativistic numerical calculations of Kissel et al [Phys. Rev. A 22, 1970 (1980)] and Scofield [LLRL, Internal Report (1973)], a good agreement is found for low and intermediate Z values if screening effects are taken into account, for photon energies above the pair production threshold energy. We present our numerical results for the total photoeffect and pair production cross-sections for the 2S subshell of Zn and Ag atoms, for various photon energies.

physics.atom-ph↗

arXiv2006

The total cross-sections for the photoeffect for K-shell bound electrons and pair production with the created electron in the ground state for photon energies above 1 MeV

Considering the contributions of the main term of the relativistic Coulombian Green function given by Hostler to the second order of S matrix element and taking into account only the large components of the Dirac spinor of the ground state we obtain the imaginary part of the Rayleigh amplitudes in terms of elementary functions. Thereby simple and high accurate formulae for the total cross-sections for photoeffect and pair production with the electron created in the K-shell are obtained \textit{via} the optical theorem.Comparing the predictions given by our formulae with the full relativistic numerical calculations of Kissel \textit{et al} [Phys. Rev. A 22, 1970 (1980)] and Scofield [LLRL, Internal Report,1973], a good agreement is found for photon energies above the pair production threshold up to 5 MeV for any Z elements. We present our numerical results for the total photoeffect and pair production cross-sections, for various photon energies for the K-shell of Ag and Pb.

physics.atom-ph↗

arXiv2006

The nonrelativistic limit of the second order S-matrix element for the elastic scattering of photons by K-shell bound electrons

In this paper, the right expressions of the fully nonrelativistic K-shell Rayleigh scattering amplitudes and cross-sections are obtained by using the Coulomb Green function method. Our analytical result has no spurious poles as the old nonrelativistic result with retardation [M. Gavrila and A.Costescu Phys. Rev. A 2, 1752 (1970)] presents. Taking into account the exact expression of the second order S-matrix element in the case of the elastic scattering of photons by K-shell bound electrons we obtain its nonrelativistic limit (valid for photon energies omega up to alpha*Z*m, for intermediate Z), by removing in a consistent and appropriate way the higher order terms in alpha*Z and omega/m. We obtain the imaginary part of the Rayleigh amplitudes in terms of elementary functions. Thereby the exact nonrelativistic total photoeffect cross-section is obtained via the optical theorem. Comparing the predictions given by our formulae with the full relativistic numerical calculations of Kissel et al [Phys. Rev. A 22, 1970 (1980)], and with experimental results, a fairly good agreement within 10% is found for the angular distribution of Rayleigh scattering for photon energies up to alpha*Z*m, for intermediate Z, and up to twice the photoeffect threshold energy for high Z elements. Considering the right relativistic location for the photoeffect threshold very good predictions are found below the first resonance, even in its vicinity. We present our numerical results for Rayleigh angular distribution and the photoeffect total cross-section, for various photon energies, scattering angles and Z, compared with other theoretical and experimental results.

physics.atom-ph↗