Search Results (19 total)

Spectra of L x rays emitted from thick solid targets containing elements with atomic numbers ranging from 49 (In) to 67 (Ho) were measured in high resolution using a curved crystal spectrometer. The projectiles included heavy ions (Ne, Ar, Kr, and Xe) ranging in energy from 6 to 15  MeV∕amu. A scaling rule was established for the apparent average fraction of M vacancies at the time of L x-ray emission and the result was compared to one obtained previously for the apparent average fraction of L vacancies at the time of K x-ray emission. The apparent degree of L-shell ionization at the time of L x-ray emission was examined as a function of target atomic number (for 6  MeV∕amu Kr projectiles), projectile atomic number (for an ₅₁Sb target), and projectile energy.

Cross sections have been measured for Kα x-ray production in targets of Al, Ti, Cu, Zr, Ag, Sm, and Ta by Ar, Kr, and Xe ions ranging in energy from 2.5  to  25  MeV∕amu. In addition, the degree of simultaneous L-shell ionization and the enhancement of the Kα diagram lines due to secondary ionization processes were assessed by performing high-resolution spectral measurements on Al, Ti, V, Co, and Cu targets. This information was used to correct for the Kα x-ray yield produced by electron bombardment and photoionization, and to calculate the fluorescence yields needed to convert the Kα x-ray production cross sections into K-vacancy production cross sections. The resulting cross sections were compared with the predictions of the perturbed stationary state approximation with corrections for projectile energy loss, Coulomb deflection, and relativistic effects (ECPSSR theory). Also, the scaling properties of the Kα x-ray production cross sections were examined and a semiempirical (universal) curve was deduced that reproduces the measured cross sections to within ±30% on average.

Spectra of K x rays emitted from a variety of solid targets (atomic number Z₂=17–32) under bombardment by fast heavy ions (atomic number Z₁=6–83) at 2.5–25  MeV∕amu were measured in high resolution using a curved crystal spectrometer. The spectra were analyzed in order to examine the dependence of Kα x-ray satellite structure on the Geometrical Model’s universal variable X_n. Scaling rules were established for the apparent average L-shell spectator vacancy fraction at the time of Kα x-ray emission, the satellite peak centroids, and the satellite peak widths.

Spectra of L x rays emitted from Ho targets bombarded by 10 keV electrons and 6  MeV∕amu C, Ne, Ar, Kr, and Xe ions were measured in high resolution using a curved crystal spectrometer. The spectra were analyzed in order to examine the systematic evolution of the L x-ray satellite structure as a function of projectile atomic number. Scaling rules are established for the apparent average M-shell spectator vacancy fraction and apparent average double to single L-vacancy population ratio at the time of L x-ray emission. The results are compared to those obtained for K-shell ionizing collisions and with predictions of the geometrical model.

The dependence of electron loss by 6-MeV/amu Xe¹⁸⁺ on target atomic number was investigated by measuring single-collision cross sections for loss of one to eight electrons in targets of the noble gases He, Ne, Ar, Kr, and Xe. The total-electron-loss cross sections were found to increase linearly with target atomic number, but an abrupt slope change was observed to occur between Ne and Kr. Calculated total-loss cross sections obtained using the n-body classical trajectory Monte Carlo method were in good agreement with the measurements. The dependence of the individual cross sections on the number of electrons lost was reasonably well represented by a semiempirical fitting procedure utilizing the independent-electron approximation. Additional measurements performed with a variety of molecular targets provided a rigorous test of cross-section additivity. It was found that the additivity rule works well in this collision regime and that the molecular nature of the target has remarkably little influence on the cross sections.

Beams of 10-MeV/amu Ne, Ar, Cr, Kr, Xe, and Bi ions have been employed to examine the dependence on projectile atomic number (Z₁) of Al K-shell vacancy production in nearly symmetric (Z₁/Z₂=0.8) to highly asymmetric (Z₁/Z₂=6.4) collisions. Aluminum K x-ray yields were measured with a Si(Li) detector as a function of target thickness to establish cross sections for charge equilibrated projectiles. Average target atom L-shell vacancy distributions excited in K-shell vacancy-producing collisions and x-ray yield corrections for contributions from photoionization and secondary electron impact were deduced from high-resolution spectral measurements performed with a curved-crystal spectrometer. Knowledge of the average L-shell vacancy distributions provided the means for estimating the appropriate fluorescence yields needed to convert the x-ray yields to K-shell vacancy production cross sections. Theoretical cross sections greatly overestimate the experimental results for Z₁>10. The Z₁ dependences of the K-shell vacancy production cross sections for both Al and Cu (from a previous study) are well characterized by a simple empirical scaling law.

The dependence of Cu (target atom) K-shell vacancy production cross sections on projectile atomic number was investigated in collision systems for which the ratio of projectile-to-target atomic numbers (Z₁/Z₂) ranged from 0.34 to 2.86. A combination of energy and wavelength dispersive x-ray spectrometry was used to measure Cu K x-ray production cross sections and to determine the appropriate fluorescence yields for converting them to K-vacancy production cross sections. The high-resolution spectra also revealed the presence of sizable contributions from predominately single-ionization mechanisms not directly associated with ion-atom interactions. The role of electron capture to the projectile was examined by observing the dependence of the cross sections on target thickness. The Cu K-vacancy production cross sections determined for equilibrated projectiles display a plateau centered in the region of symmetric collisions (Z₁/Z₂∼1) and they become essentially constant beyond Z₁=54. The cross sections for Z₁>24 fall far below a Z₁² scaling law and are greatly overestimated by the perturbed stationary-state theory with energy loss, Coulomb deflection, and relativistic corrections.

Spectra of L x rays emitted by 5.2- to 14.4-MeV/nucleon Xe ions traveling in gaseous targets of He, Ne, and Ar have been measured with a curved crystal spectrometer. Detailed spectral analysis provided estimates of the average projectile charges and the average L- and M-electron populations inside the gases. In comparisons of the present results for gases with those obtained previously for solids, it was found that Xe ions emitting L x rays in solids have, on average, many more L vacancies than those that emit L x rays in gases. Average charges deduced for Xe ions traveling in Ar gas were 2.3 units lower than the average charges of Xe ions traveling in solid KCl.

L x-ray emission from Xe ions incident on a variety of solid targets at energies ranging from 6 to 15 MeV/u has been investigated using a curved crystal spectrometer of moderate resolution. Analysis of the spectra provided estimates of the average charges and (in some cases) the charge distributions of those ions emitting L x rays inside the targets. Calculations employing theoretical electron capture and loss cross sections were used to examine the dependence on depth within the target of contributions to the spectra from Xe ions having different average numbers of L and M electrons. Average charges and charge distributions deduced from the x-ray spectra were found to agree quite well with those predicted by the model calculations.

Spectra of K x rays from Ar ions (incident energy 15 MeV/u) traveling in thick solid targets of Be, C, NaF, and KCl were measured with a curved crystal spectrometer for the purpose of investigating the dependence of the H-like Ar 2p-state population fraction on projectile energy and target atomic number. Information pertaining to the energy dependence of the 2p fraction was obtained from the Doppler-shifted average energies and peak shapes associated with the Lyman-α lines. Model calculations that took into consideration the fraction of ions with one 2p electron and x-ray absorption as a function of depth in the target were performed. Generally good agreement between the experimental results and the predictions of the model were observed for all of the targets except NaF.

The dissociation of CO^Q+ molecular ions (Q=4 to 9) produced in multiply ionizing collisions accompanied by the transfer of an electron to the projectile has been studied using time-of-flight techniques. Analysis of the coincident-ion-pair flight-time-difference distributions yielded average values of the kinetic-energy releases for the various dissociation reactions. These values were found to be as much as a factor of 2 greater than the kinetic-energy releases expected for dissociation along Coulombic potential curves. The average kinetic-energy release observed for a given ion pair with charges q₁ and q₂ are nearly equal to the point-charge Coulomb potential energies for an ion pair with charges q₁+1 and q₂+1, suggesting that the parent molecular ion is formed with two electrons, on average, in highly excited states that do not contribute to the screening of the nuclei. © 1996 The American Physical Society.

Spectra of L x rays emitted by 6- and 8-MeV/u Xe ions interacting with a variety of solid targets have been measured with a curved-crystal spectrometer. The spectra displayed an overall structure consisting of six strong peaks which were fairly well resolved for low-Z targets (Z₂=3–6), but broadened into two featureless groups for higher-Z targets. The main components were identified as Lα₁ and Lβ₁ pairs associated with initial-state configurations having one to six L-shell vacancies. Detailed spectral analysis provided estimates of the average projectile charges and the average L- and M-electron populations inside the solids.

Data on the production and dissociation of CO^Q+ molecular ions (where Q=1 through 7) obtained by ion-ion coincidence time-of-flight measurements were analyzed to determine production yields, dissociation fractions, and branching ratios. A detailed comparison of the dissociation fractions for CO⁺ and CO²⁺ for several collision systems in the same perturbative regime revealed them to be quite similar, whereas the dissociation fraction for CO⁺ produced by valence-electron photoionization is a factor of 1.8 to 3.6 larger. The results for Q≥2 indicated a preference for dissociation channels leading to symmetric or nearly symmetric charge division. An enhancement of the total ionization yields for Q>4 was observed, and it suggests that electron transfer followed by LMM Auger decay plays an important role in determining the final charges of the dissociation products.

Cross sections for K x-ray production in solid targets (Z=13, 22, 26, 29, 32, 40, 42, 46, and 50) by 30-MeV/amu beams of H, N, Ne, and Ar were measured. The cross sections were determined by recording the spectra of K x rays with a Si(Li) detector in coincidence with beam particles detected in a microchannel plate assembly. The K-shell-ionization cross sections deduced from these data agree quite well with the predictions of the perturbed-stationary-state theory with energy loss, Coulomb deflection, and relativistic corrections. Detailed analyses of the projectile and target Z dependences of the cross sections were performed. Also, the relative intensities of K x rays from double K-shell ionization of the higher-Z targets are presented for N, Ne, and Ar projectiles.

The decay of silver and hafnium atoms with a vacancy in the K shell by the emission of photon pairs which continuously share the transition energy was studied. Radioactive decay of ¹⁰⁹Cd and of ¹⁷⁹Ta was used to generate K-shell vacancy states in silver and hafnium atoms, respectively. A pair of germanium detectors in a 180° geometry and a fast-slow coincidence system with a (128×512×512)-channel three-parameter pulse-height analyzer were used in the measurements. Two-dimensional spectra of numbers of events as functions of amplitudes of coincident pulses from the two detectors were analyzed. Surface fitting of two-dimensional spectra and curve fitting of sum spectra were applied to deduce the number of events. The differential transition probabilities of 2s→1s, 3s→1s, 3d→1s, and 4sd→1s two-photon decay per decay of a K-shell vacancy have been determined. For silver they are compared with the results of relativistic self-consistent-field calculations of Mu and Crasemann [Phys. Rev. A 38, 4585 (1988)] and of Tong, Li, Kissel, and Pratt [Phys. Rev. A 42, 1442 (1990)]. The results for silver and hafnium are compared with the results of nonrelativistic calculations for hydrogenic silver and hafnium ions. The 3d→1s two-photon decays show the resonance effect, in accordance with the theoretical predictions.

Transient molecular ions of CO^q+ (where q=2–7) were produced in single collisions of 97-MeV Ar¹⁴⁺ projectiles with neutral CO molecules. The resulting dissociation products were identified by coincidence time-of-flight spectroscopy in which the time of flight of the first ion to reach the detector and the time difference between the first ion and its partner were recorded event by event. An iterative matrix-transformation procedure was employed to convert the time-difference spectra for the prominent dissociation channels into total-kinetic-energy distributions. Analysis of the total-kinetic-energy distributions and comparisons with the available data for CO²⁺ and CO³⁺ from synchrotron radiation experiments led to the conclusion that ionization by Ar-ion impact populates states having considerably higher excitation energies than those accessed by photoionization.

Decay of double-K-shell-vacancy states in xenon atoms, created in the decay of ¹³¹Cs, was investigated. The measurements were performed with a pair of germanium detectors, a fast-slow coincidence system, and a three-parameter pulse-height analyzer. In the analysis of the two-dimensional E₁-E₂ spectrum, improved least-squares routines were applied. The following results were derived: the probability of creation of a double K-shell vacancy per ¹³¹Cs decay, P_KK=(1.48±0.35)×10^-5; the hypersatellite energy shifts Δ^h(Kα)=(653±20) eV, Δ^h(Kβ₁)=(834±39) eV, and Δ^h(Kβ₂)=(903±81) eV; the average values of the satellite energy shifts due to the presence of an L₃- or L₂-shell spectator vacancy Δ^s(KαL^-1)=(80±15) eV, Δ^s(Kβ₁L^-1)=(169±34) eV, and Δ^s(Kβ₂L^-1)=(261±81) eV; the intensity ratios of the hypersatellite transitions, I(Kα₂^h)/I(Kα₁^h)=0.94±0.18, I(Kβ₁^h)/I(Kα₁^h)=0.36±0.06, and I(Kβ₂^h)/ I(Kα₁^h)=0.09±0.04; the intensity ratios of the satellite transitions I(Kα₂L^-1)/I(Kα₁L^-1)=0.44±0.10 and 0.44±0.09 for an L₃ and L₂ spectator vacancy, respectively; and the intensity ratios of some other satellite transitions.

Decay of xenon atoms with a vacancy in the K shell by the emission of photon pairs which continuously share the transition energy was investigated. The results show the expected dominance of transitions of electrons from the 2s, 3s, and 3d states, for which the single-x-ray transitions are strongly forbidden. In the energy range of the present measurements the resonance effect due to the intermediate 2p (and 3p) states for transitions of electrons from higher shells has not been observed.

A pair of germanium detectors and a three-parameter analyzer were applied in an experimental study of the creation and of the decay of atomic states with a double K-shell vacancy, created in the decay of ¹⁰⁹Cd. Assuming a variable intensity ratio of the Kα₁ and Kα₂ hypersatellite lines, their shift with respect to the diagram lines, Δ_Kα^h=546±20 eV, and the shift of the Kα satellite lines with an initial L-shell vacancy, Δ_Kα^s(L^-1)=54±6 eV, were obtained. The former result is in agreement with the previous experimental result of van Eijk et al. and with the theoretical result of Chen et al., but the latter result is significantly lower than the theoretical value of 73 eV. For the I(Kα₁ ^h)/I(Kα^h) intensity ratio, a value of 0.54±0.11 was obtained. From the numbers of counts in the hypersatellite-satellite peaks the intensity ratios of the hypersatellite lines I(Kβ_1’ ^h)/I(Kα^h)=0.195 ±0.016, and I(Kβ_2’ ^h)/I(Kα^h)=0.055 ±0.008 were obtained. The former value seems to be larger than the theoretical value 0.168, while the latter value is significantly larger than the theoretical value 0.029. Also the I(Kβ_1’ ^s)/I(Kα^s) and I(Kβ_2’ ^s)/I(Kα^s) intensity ratios of the satellite lines for the L-, M-, and N-shell initial spectator vacancies were derived. Within the errors the results are in agreement with the corresponding values for the diagram lines. The probability of creation of double K-shell vacancies per ¹⁰⁹Cd decay was determined to be (6.07±1.12)×10^-5, in accord with the result (6.25±0.54)×10^-5 of van Eijk et al.