Threshold Energy Region Research Articles

Electron collision strengths for transitions within the 1s22s2sp2configuration of Si IX

New collision strengths for the electron impact excitation of the fine-structure transitions within the 1s22s22p2 configuration of Si IX have been calculated. These have been obtained from a suitable transformation of LS coupling reactance matrix elements computed with the R-matrix program. The calculations have been done at a large number of energy points in an energy region below 90.0 Ryd. The collision strengths exhibit a complex pattern of resonances in almost the entire threshold energy region. The effective collision strengths have been obtained after averaging the entire collision strength spectrum over a Maxwellian distribution of electron velocities. The results are tabulated in an electron temperature range below 2.5*106K.

Electron impact ionization cross sections and rates for highly ionized atoms

Electron impact ionization cross sections and rates for hydrogen-, helium- and lithium-like ions are presented in simple analytical form, based on a set of parameters derived from distorted wave exchange cross section calculations. Isoelectronic fits to the parameters used in the formulate allow rapid generation of cross sections and rates for any ion in these sequences with a minimum of computational effort. The use of accurate Bethe slopes in the cross section-fitting algorithm yields good accuracy both in the near threshold and high energy regions. The rate coefficient is given in the form of a correction to the commonly used Seaton formula and is thus readily adaptable to existing plasma modeling computer codes. Comparisons of the present results with other theoretical and experimental data are made.

Neutron Inelastic Scattering Cross Sections for Natural Chromium Determined from the (n,n′γ) Reaction

Absolute 125-deg differential gamma-ray production cross sections have been measured for 21 gamma rays produced in natural chromium by the (n,n′γ) reaction in the incident neutron energy range from 0.84 to 3.97 MeV. The pulsed beam time-of-flight technique was employed for background reduction. The data were corrected for neutron multiple scattering and neutron and gamma-ray attenuations in the scattering sample. Angle-integrated gamma-ray production cross sections were inferred from the differential measurements using gamma-ray angular distributions obtained from compound nucleus statistical model calculations. On the basis of the angle-integrated cross sections and measured branching ratios, neutron inelastic scattering cross sections were deduced for 22 energy levels in the four naturally occurring isotopes of chromium. These results are compared to previous measurements and the Evaluated Nuclear Data File (ENDF/B-IV, MAT 1191). The present measurements suggest that in the threshold energy region for inelastic neutron scattering to each of the first excited 2+-states in 50,52,54Cr, the cross sections are significantly overestimated in ENDF/B-IV.

The electromagnetic form factor of the pion

The superpropagator method is used to calculate the electromagnetic form factor of the massive pion in a theory of the chiral type. The calculated values for the pion radius 〈r 2〉 ≈ 0.65 fm and the scattering wavelength, a 1 1 = 0.031 and the absolute values for the form factor in the threshold energy region are in satisfactory agreement with the experimental data.

Radiation damage in copper near the threshold energy

Abstract Radiation damage and annealing studies of copper samples have been performed for the near threshold energy region. A nonlinear behavior of the damage rate vs. irradiation fluence is found. This effect is related to damage which does not anneal below 300 K and is independent of subthreshold damage.

A study of electron-field pair production by photons near the threshold with a streamer chamber technique

Recent papers (2.~) show the fundamental importance of an experimental study of triplet production in the threshold energy region from 2 to 6 MeV. The validity of Mork's (z) corrections to the Borsellino (4) cross-section could be tested; in particular, experimental data on the total cross-section would give an evaluation of the u interaction and the exchange effects involving the two electrons. Now, to our knowledge, no experimental data are available below 6 MeV, as shown in Fig. 1. We have performed a preliminary experiment with the 2.62 MeV y-rays from 2~STh. Pairs and triplets were produced in the gas of a streamer chamber filled with helium; we must indeed work with a low-atomic-number target to increase the ratio of the triplet production rate to the pair production rate. Moreover, at the energy considered here, the momentum of the recoil electron is much larger than each momentum of the helium atomic electrons; therefore, these ones can be considered as free electrons and Mork's (2) theoretical calculation is valid.

Quantum mechanical computational studies of chemical reactions: I. Close-coupling method for the collinear H + H2reaction

A fully converged, close-coupling calculation, using natural collision coordinates, is presented for the collinear H + H2 system, on a Porter-Karplus potential energy surface in the energy range of 9 to 35 kcal/mole. Eight closed channels were required for convergence in the threshold region. Inclusion of the closed channels alters considerably the numerical results. The effects of the vibrational non-adiabatic coupling terms were demonstrated. Particularly pronounced is the influence of the (energy dependent) inertial centrifugal energy due to the curvature of the reaction path. No evidence for tunnelling was obtained. The considerable difference between the quantal and classical reaction probabilities in the threshold energy region is entirely due to the non-adiabatic coupling terms, which tend to reduce the kinetic energy available along the reaction path, more so in the classical case, thereby leading to a higher dynamic threshold. The Arrhenius plot of the collinear rate constant shows more curvature...

Energy Dependence of Electron Damage in Copper

Electron irradiation below 1-Mev and subsequent recovery in copper have been investigated. The minimum threshold displacement energy is found to be less than 22 ev. The details of the recovery in stage I, as a function of electron energy, are found to be more complex than anticipated. Specifically, ${\mathrm{I}}_{a}$ decreases, ${\mathrm{I}}_{b}$ increases slowly, and ${\mathrm{I}}_{c}$ increases rapidly, respectively, in relative importance as the electron energy is reduced. A calculation of the cross section for displacement of copper atoms by electrons is also presented. A Born-Mayer potential with standard parameters is used. Only displacements in or near the close-packed directions are evaluated in detail. Other directions do not appear to contribute significantly in the near threshold energy region being considered. The agreement between this theory and experiment is found when a minimum threshold displacement energy between 19 and 20 ev and a Frenkel resistivity between 2 and 3 \ensuremath{\mu}ohm cm per atomic percent of interstitial-vacancy pairs is assumed.

Ionization Cross Sections near Threshold by Electron Impact

Ionization curves near threshold have been taken with a mass spectrometer employing a monoenergetic electron beam. These curves have been fitted to previously published data from total ionization experiments in order to obtain absolute cross sections for the threshold energy region. Curves for helium, neon, argon, mercury, carbon monoxide, and nitrogen are given.