Free-electron Cross Sections Research Articles

Buildup with Bremsstrahlung in the Martian Atmosphere

Photon buildup is a function of energy, medium, and geometry and therefore must be specifically calculated for the case of interest. The Martian atmosphere, mostly comprising carbon dioxide, is becoming more relevant to radiation researchers and therefore warrants the study of this gas mixture’s buildup properties for ionizing photon flux resulting from the secondary effects of galactic cosmic rays and solar flares. Specifically, this work uses the MCNP6 code to develop energy absorption buildup factors in finite slab models for energies ranging from 40 keV to 15 MeV with Martian regolith as the backscattering medium. The Martian carbon dioxide cycle is accounted for by determining maximum and minimum mean densities as a function of orbital position. An isotropic point source model for the atmosphere is also developed using the geometric progression fitting function. Buildup is bounded to a factor of approximately 23 at 100 keV for normally incident photons at the top of the atmosphere. For conservatism, the design problem neglects coherent scattering but assumes bremsstrahlung effects and uses Klein-Nishina free-electron cross sections for Compton scattering.

Absolute cross-section measurements for ionization of He Rydberg atoms in collisions with K.

Absolute cross sections for ionization of 1.5--10.0 keV/amu Rydberg helium atoms in principal quantum states 12\ensuremath{\le}n\ensuremath{\le}15 due to collisions with potassium have been measured. The data are compared with the free-electron cross section at equal velocity. Our results for the collisional ionization cross sections (${\mathrm{\ensuremath{\sigma}}}_{\mathit{i}}$) agree both in shape and absolute magnitude with the data available for the total electron-scattering cross sections (${\mathrm{\ensuremath{\sigma}}}_{\mathit{e}}$) and support recent theoretical models for ionization of Rydberg atoms with neutral perturbers.

Comparison of rate coefficients for rydberg electron and free electron attachment

New experimental results on attachment reactions involving free electrons at sub-meV resolution allow for the first time a conclusive comparison of measured rate coefficients for Rydberg electron attachment with those calculated from the measured free electron cross sections on the basis of the quasi-free electron model for Rydberg electron collisions. Using classical velocity distributions for the highn Rydberg electrons and our measured free electron attachment cross sections, we calculate Rydberg electron attachment rate coefficientsknl for the two cases SF6 and HI for Rydberg binding energies |En| of 0.1–40 meV. We find a significant increase inknl towards lower binding energies, especially for HI, which is due to the deviation of the free electron cross section from the limitings-wave behaviour σ0∼E−1/2. The increase at |En|≲2 meV is in qualitative agreement with our highn Rydberg data (n≳80) if l-mixing due to residual electric fields is taken into account. For low l, Rydberg rate coefficientsknl(|En|) are significantly larger than free electron rate coefficientske(E=|En|), while for circular orbits (l=n−1) they agree. On average, attachment reactions of Rydberg electrons in low l orbits proceed with an effective collision energy substantially smaller than the binding energy |En|.

Single electron capture in He + on SF 6, collisions at low keV energies

We have measured the absolute differential and total cross sections for single electron capture from He + ions on SF 6 in the energy range 1.0 to 5.0 keV. The total collisional destruction cross sections of He Rydberg atoms on SF 6 has been compared with the sum of the available free electron cross sections (for equal velocity) and the present absolute total cross sections. The results support the validity of the independent particle model for the destruction of Rydberg atoms over this range of energies.

Integral cross sections for ion pair and free electron production in collisions with hydrogen atoms

Detailed integral cross sections of processes H + M → H −, H + + e + M, H + e + M + (M  Ne,Ar,Kr,Xe,O 2) are reported in the low energy region ( E < 100 eV or E < 500 eV for M + + e). In general, ion pair formation is the dominating process at low energy and has a stongly oscillating cross section; free electron cross sections are monotonously rising.