Bremsstrahlung from gas atom targets: Study of background processes

Recent absolute bremsstrahlung cross section experiments on gas targets of Ne, Ar, Kr and Xe at 28 and 50 keV have shown a significant polarization bremsstrahlung (PB) contribution, in contrast with previous thin-film experiments where no PB has been seen. Recently, Obolensky and Pratt have considered ways to improve the PB model, but the theory is still about 20% below the data. While a more complete theoretical calculation is certainly needed, we consider two additional background processes, not corrected in the experiment, that depend on the background photon spectrum in the beam line produced by electron interaction with collimators in the beam. We compare an estimate of both backgrounds with that from beam electrons elastically scattered by the gas into the cell window or wall and discuss initial efforts to measure these backgrounds.     

Photo-neutron cross-section of 100Mo

The ¹⁰⁰Mo(γ, n) reaction cross-section was experimentally determined at end point bremsstrahlung energy of 10 and 12.5 MeV using off-line γ-ray spectrometric technique. It was also found that ¹⁰⁰Mo(γ, n) reaction cross-section increases sharply from the end point bremsstrahlung energy of 10 MeV to 12.5 MeV, which may be because of GDR around the energy region of 12–16 MeV. The ¹⁰⁰Mo(γ, n) reaction cross-section as a function of photon energy was calculated theoretically using TALYS 1.2 computer code. The flux-weighted average values of ¹⁰⁰Mo(γ, n) reaction cross-section for bremsstrahlung having end point energy of 10 and 12.5 MeV were also calculated using the experimental and theoretical data of mono-energetic photon. The present experimental ¹⁰⁰Mo(γ, n) reaction cross-sections were compared with the bremsstrahlung flux-weighted average values of experimental and theoretical data and found to be in the lower side for 10 MeV and in the higher side for 12.5 MeV.     

Comparison of theory and experiment for polarizational bremsstrahlung

Portillo and Quarles have recently observed significant deviations from ordinary bremsstrahlung predictions for bremsstrahlung from gas targets, after not seeing such effects in solids. These deviations are interpreted as an observation of the polarizational bremsstrahlung component. We review previous discussion of this component. No adequate theory is presently available for the angular dependence of the bremsstrahlung cross section being measured in these new experiments. We examine the validity of the simple estimates which Portillo and Quarles have tried to use.     

Bremsstrahlung cross-section with screening and Coulomb corrections at high energies

Using analytical formulae which are exact in Born approximation, the doubly differential bremsstrahlung cross-section with form-factor screening is calculated. For the atomic form factor parameters are applied which approximate self-consistent Dirac–Hartree–Fock–Slater calculations. The evaluation of the bremsstrahlung spectrum requires a single numerical integration. The results are superior to the customary Bethe approximation, in particular at the high-energy part of the spectrum. At high energies the screening correction can be added to any Coulomb-corrected cross-section without screening. In the present work, we are using a cross-section calculated by means of Sommerfeld–Maue functions with additional higher-order terms.     

Enhancements in dissociative electron attachment to CF4, chlorofluorocarbons and hydrochlorofluorocarbons adsorbed on H2O ice

We report that the absolute cross sections for dissociative attachment of approximately 0 eV electrons to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are strongly enhanced by the presence of H2O ice. The absolute cross sections for CFCl3, CHF2Cl, and CH3CF2Cl on water ice are measured to be approximately 8.9 x 10(-14), approximately 5.1 x 10(-15), and approximately 4.9 x 10(-15) cm2 at approximately 0 eV, respectively. The former value is about 1 order of magnitude higher than that in the gas phase, while the latter two are 3-4 orders higher. In contrast, the resonances at electron energies > or = 2.0 eV are strongly suppressed either for CFCs and HCFCs or for CF4 adsorbed on H2O ice. The cross-section enhancement is interpreted to be due to electron transfer from precursor states of the solvated electron in ice to an unfilled molecular orbital of CFCs or HCFCs followed by its dissociation. This study indicates that electron-induced dissociation is a significant process leading to CFC and HCFC fragmentation on ice surfaces.     

Total ionization and electron attachment cross sections of CCl2F2 by electron impact

The absolute total ionization cross sections from threshold to 250 eV and dissociative attachment cross sections from zero to 10 eV have been measured for the CCl₂F₂ (dichloro-difluoro-methane) molecule by using a parallel plate condenser type ionization chamber. The maximum of the ionization cross-section curve was found to be at an energy of about 90 eV with a cross section of 1.44 × 10^?19 m². The attachment cross-section curve shows three peaks, the most intense being at zero electron energy with a cross-section value of 1.80 × 10^?20 m², and the other two at energies of 0.6 eV and 3.5 eV, respectively. The maximal relative error in cross-section values is 0.08, for electron energies larger than 0.4 eV.     

Monte Carlo Simulation of Electron Transport and X-Ray Generation. II. Radiative Processes and Examples in Electron Probe Microanalysis

Theoretical models for Monte Carlo simulation of radiative processes, i.e. bremsstrahlung and characteristic x-ray emission, are presented. Possible strategies for simulating electron transport are briefly described. For mechanisms involving energy loss and angular deflections, difficulties for strict implementation of accurate numerical differential cross sections still remain due to the strong correlations between these variables. Practical solutions for the case of inelastic collisions and bremsstrahlung emission are described. Comparisons of simulation results with experimental data for several problems of interest in electron probe microanalysis are presented.     

Asymmetry of bremsstrahlung from polarized electrons

The triply differential cross section of unpolarized bremsstrahlung produced by polarized electrons in the Coulomb field of a nucleus is calculated with the aid of Sommerfeld-Maue wave functions. The photon asymmetry originating from transversely polarized electrons is calculated for various values of energies and angles, in particular for the parameters of recent experiments. As a function of the atomic number Z of the target nucleus, the asymmetry shows a distinct deviation from a linear dependence, in contrast to a previous calculation exact to lowest order in the Coulomb parameter Z/137. The cross-section formula is integrated numerically over the angles of the outgoing electrons to obtain the photon asymmetry observed in non-coincidence experiments.     

Photo-neutron cross-section measurement in the 8 and 10 MeV bremsstrahlung induced reaction of 238U

The ²³⁸U(γ, n)²³⁷U reaction cross-section at the end point bremsstrahlung energies of 8 and 10 MeV was measured by using an activation technique. Induced gamma ray activities were measured by high resolution gamma-ray spectrometer with high-purity germanium detector. The photo neutron cross section on ²³⁸U is also calculated theoretically using TALYS 1.2 computer code. The experimentally obtained reaction cross sections were compared with the flux weighted average values from the literature data based on mono-energetic photon as well as the value from TALYS. It was found that the cross section of ²³⁸U(γ, n)²³⁷U reaction increases with increase of bremsstrahlung energy and were closer to the flux-weighted experimental data from literature and the values from TALYS based on mono-energetic photons.     

Polarizational bremsstrahlung from atomic clusters

We consider a photon emission in collisions of electrons with atomic clusters under conditions in which the radiation of atomic electrons is significant or even dominates the total spectrum. This occurs for example, when the frequency of the emitted photon is comparable with the energy of a plasmon resonance in a cluster. We consider general results of the theory of polarizational bremsstrahlung, and we present the results of recent calculations of the cross sections of this process for a number of targets.     

Bremsstrahlung during electron capture to continuum

Theoretical approaches for radiative ionization (RI) in energetic collisions of highly stripped projectiles with target atoms are reviewed and set into context with related processes. The interrelation between RI and electron-nucleus bremsstrahlung is displayed with the help of inverse kinematics. Particular emphasis is laid on the forward-peak region of the electron spectrum resulting from target electrons which are slowed down to approximately zero velocity in the projectile frame of reference. The forward-peak intensity and shape for RI is contrasted to the one obtained from nonradiative capture to continuum. The ridge in the photon spectrum related to forward-peak electrons can unambiguously be identified in measured doubly differential photon emission cross sections resulting from ion–atom collisions at relativistic impact energies.     

K-shell ionization induced by 30 MeV/u argon and neon beams

We have studiedK-shell ionization induced by 30 MeV/u Ne and Ar projectiles on target atoms with atomic numbers ranging from 27 to 90. X-ray production cross sections and energy shifts were measured with Si(Li) detectors. In most cases satisfactory agreement between measurements and theoretical direct ionization cross sections is obtained when the contribution of electron capture is included. The influence of multiple ionization on the fluorescence yield ω _K is discussed.     

L X-ray emission from fast highly charged Cu ions in collisions with gaseous targets: Saturation effect in excitation and transfer

We have measured L X-ray production cross sections for highly charged 156 MeV Be-like Cu ions in collisions with gaseous targets of H₂, Ne, Ar, Kr and Xe. In the present collision systems, measured projectile L X-ray intensity is contributed by the excitation as well as electron transfer processes. The projectile L X-ray production cross sections are found to increase initially and then saturate with increasing target atomic number. The charge state dependence of projectile L X-ray production cross sections have been measured with Kr target.     

Review of structures in low-energy bremsstrahlung: Classical and quantum descriptions

A brief review of recent advances in studying structures in energy dependence of the bremsstrahlung cross sections for low incident electron energies is presented. Examples of structures are given in both classical and quantum formalisms. It is shown that the origin of the structures can be formulated as a lack of contribution to the radiation from electrons with certain angular momenta at certain energies. In quantum mechanics the lack of contribution to the total cross section from certain electron angular momenta is due to zeroes in corresponding dipole matrix elements. In classical mechanics summation over angular momentum is replaced by integration and structures are due to suppressed or enhanced contribution from certain intervals of angular momentum. A survey of the known properties of the matrix elements’ zeroes is given.     

Preparation of 240Pu and 242Pu targets to improve cross-section measurements for advanced reactors and fuel cycles

²⁴⁰Pu and ²⁴²Pu targets were prepared by the so-called “molecular plating” on an Al backing for cross-section measurements. The total activity of the actinide layer was determined by low-geometry alpha-particle counting. The atomic abundances of the two Pu materials were determined via thermal ionization mass spectrometry. A radiochemical separation was performed by anion exchange prior to the preparation of the layers to prevent interferences with the ingrown daughter nuclide during the characterization and cross-section measurements. The targets were prepared to be used in the project “Metrology for New Generation Nuclear Power Plants” (MetroFission), within the frame of the European Metrology Research Programme, directed to improve the knowledge on neutron cross sections through metrological approaches. For measurements of the ²⁴⁰Pu(n,f) and ²⁴²Pu(n,f) cross sections ²⁴⁰Pu and ²⁴²Pu targets were produced and mounted in fission chambers. In parallel, ²⁴⁰Pu and ²⁴²Pu targets were prepared for the ANDES project (Accurate Nuclear Data for Nuclear Energy Sustainability) for cross-section measurements at the CERN neutron time-of-flight facility n_TOF, and the Van de Graaff laboratories of IRMM and CNRS/CENBG.     

Cold and ultracold NH-NH collisions: the field-free case

We present elastic and inelastic spin-changing cross sections for cold and ultracold NH(X (3)Σ(-)) + NH(X (3)Σ(-)) collisions, obtained from full quantum scattering calculations on an accurate ab initio quintet potential-energy surface. Although we consider only collisions in zero field, we focus on the cross sections relevant for magnetic trapping experiments. It is shown that evaporative cooling of both fermionic (14)NH and bosonic (15)NH is likely to be successful for hyperfine states that allow s-wave collisions. The calculated cross sections are very sensitive to the details of the interaction potential, due to the presence of (quasi)bound state resonances. The remaining inaccuracy of the ab initio potential-energy surface therefore gives rise to an uncertainty in the numerical cross-section values. However, based on a sampling of the uncertainty range of the ab initio calculations, we conclude that the exact potential is likely to be such that the elastic-to-inelastic cross-section ratio is sufficiently large to achieve efficient evaporative cooling. This likelihood is only weakly dependent on the size of the channel basis set used in the scattering calculations.     

Resonant vibrational excitation and de-excitation of N2(v) by low-energy electrons

We have calculated cross sections and rate coefficients for low-energy electron impact excitation of the nitrogen molecule from vibrationally excited levels N2(v) 1-8. Calculations are performed in the 2Pig shape resonance energy region, from 0 to 5 eV. The cross sections are determined by using our recent integral cross section measurements of the ground level vibrational excitation and the most recent cross sections for elastic electron scattering, applying the principle of detailed balance. The rate coefficient calculations are performed for the Maxwellian electron energy distribution. By using extended Monte Carlo simulations, the electron energy distribution functions (EEDF) and the rate coefficients are also determined for the nonequilibrium conditions, in the presence of the homogeneous external electric field for the typical, moderate values of the electric field over gas number density ratios, E/N.     

Photon polarization and spin asymmetry in the elementary process of bremsstrahlung

Experimental and theoretical results are reviewed concerning the photon polarization and spin asymmetry in the elementary process of bremsstrahlung. In electron–photon coincidence experiments using an unpolarized primary beam (300 keV) the electron–nucleus bremsstrahlung was found to be almost completely linearly polarized. The same behavior was found in electron–electron bremsstrahlung. By using a transversely polarized electron beam the photon emission asymmetry was measured for fixed direction of the outgoing electrons.     

Delta-electron emission in fast heavy ion — atom collisions: observations of new phenomena and breakdown of common scaling laws

Double differential cross sections for the emission of Delta-electrons have been measured in fast uranium-rare gas collisions. The well-known Binary Encounter peak reveals unexpected structures for certain observation angles and its intensity increases towards smaller angles, which is in contradiction to results and scaling laws obtained by experiments with light ion impact. The observed dependencies are fairly well described by recent calculations in the framework of IA and CTMC. From systematic experimental as well as theoretical studies we can derive that the potential of the partially stripped projectile ion gives rise to rainbow and glory scattering of the target electron in the field of the projectile. The rainbow scattering is observed in the laboratory frame as pronounced interference structures, whereas the glory scattering is responsible for the steep increase of the cross sections for binary-encounter electrons towards small laboratory ejection angles. The observed effects have a dramatic influence on the commonq ² scaling laws derived from experiments with light ions. Furthermore, since the binary-encounter electrons ejected at forward angles have approximately twice the projectile velocity, these new phenomena have an important influence on the electronic stopping power of heavy ions and therefore have to be taken into account for the investigation of radiation damage by these ions e.g. in biological matter.     

Calculated electron impact cross sections for the K-shell ionization of Fe,Co, Mn,Ti, Zn,Nb, and Mo atoms using the DM formalism

We used the Deutsch-Märk (DM) formalism to calculate atomic K-shell electron impact ionization cross sections for the elements Fe, Co, Mn, Ti, Zn, Nb, and Mo. The calculated K-shell ionization cross sections are compared with recently measured K-shell ionization cross sections. Good to satisfactory agreement was found for all atoms with the exception of Ti. Moreover, when compared to other available K-shell ionization cross sections for these atoms, calculated using other theoretical methods and semiempirical formulae, the predictions of the DM formalism achieve a level of agreement with experimental data that is as good or better than the predictions from the other methods.