Electron Coincidence Research Articles

The discovery of a prolate–oblate–spherical shape triple of spin 0 + states in the atomic nucleus 186Pb

Two excited J(pi)=0(+) states in Pb-186 populated in the a-decay of Po-190 have been identified through alpha -particle/conversion electron coincidences in an experiment at the velocity filter SHIP ...

Electron Kinetic Energies in Multicapture Processes in Xe30+ – C60 Collisions

Electron multicapture processes at large impact parameters have been studied for collisions between 150 keV Xe30+ (v ≈ 0.2 a.u.) and C60. By detecting the number of ejected electrons in coincidence with an outgoing Xe28+ ion and a C60r+ ion (3 ≤ r ≤ 6) we have obtained electron spectra corresponding to the ejection of (r - 2) electrons in the continuum. Experiments have been performed for several extraction fields and the electron collection probability has been extracted from each spectrum. A charge trajectory simulation allows an estimation of the kinetic energy distribution of the ejected electrons. We find that most of the detected electrons have energies less than 100 eV.

Interaction of Multicharged Ions with Molecules (CO2,C60) by Coincident Electron Spectroscopy

First results for the investigation of electron capture processes in collisions between multicharged ions and molecule targets using electron spectroscopy in coincidence with charged fragments, are presented. It is shown that a much more detailed investigation of the capture reaction can be achieved using molecular instead of heavy atomic targets provided that an analysis of the target dissociation is made. The collisional systems 18O8+ + Ar, CO2 and C60 have been studied at 80 keV. Non coincident electron spectra as well as first results of double or triple coincidence experiments are discussed. Kinetic energy distributions of the Cn+ fragments (n = 1 to 8) produced in multiple capture processes from C60 target are given. A detailed investigation of the double capture process with CO2 molecule allows the measurement of kinetic energy release distributions (KERD) which characterize the dissociation of CO22+ molecular ions; our results are found to be very similar to those measured in double photoionisation experiments.

Secondary Electron/Reflected Particle Coincidence Studies during Slow Highly Charged Ion-Surface Interactions

We have measured the secondary electron emission statistics (ES) for 5 keV Nq+ (q = 1–4) ions incident at 10° on polycrystalline aluminium, in coincidence with specularly reflected N0. In this arrangement the kinetic contribution to secondary electron emission is minimised. The experimental data shows that the coincident electron yield, γ, increases linearly with incident ion charge state. The kinetic emission contribution has also been determined from this data. The ES due to 2 and 4 keV He2+ impact on polycrystalline aluminium in coincidence with specularly reflected He+ and He0 have also been determined. The process He2+ → He0 yields a larger γ value than the process He2+ → He+.

Angle-resolved electron–electron and electron–ion coincidence spectroscopy: new tools for photoionization studies

Abstract The development of two new experimental methods in gas phase photoionization studies is reviewed. These are angle-resolved electron–electron coincidence spectroscopy and angle-resolved electron–fragment ion coincidence spectroscopy. The review concentrates on systems which employ time-of-flight techniques for both the electron and ion detections. In addition to a technical description of such a system for both methods, showcase examples of their application in synchrotron radiation-based atomic and molecular physics are presented and discussed.

What can we learn using X-ray fluorescence and Auger electron coincidence spectroscopy?

Abstract X-ray fluorescence and Auger electron coincidence spectroscopy (XAECS) is formulated in a systematic manner by treating the initial ionization (excitation) and subsequent core-hole decays on an equal footing. For this purpose the unitary expansion method is used. XAECS and Auger photoelectron coincidence spectroscopy (APECS) are compared. The unique possibility of the deexcitation electron spectroscopy (DES) measurement in coincidence with the X-ray fluorescence from the resonantly excited core-hole state using the XAECS technique, is discussed, in particular from the viewpoint of the competition between the charge transfer core-hole screening and the core-hole decay.

Effects of electron correlation on the level widths, the photoelecton, Auger electron and ion-yield spectra above the 4d threshold in Ba

Electron correlation has been studied in the singly ionized 5p-1, 5s-1 and 4d-1 states and the double ionized (5s, 5p, valence)-2 states of Ba by carrying out extensive configuration interaction Hartree-Fock calculations including the mixing of configurations with outermost valence electrons (nl = 6s,5d,6p,4f, ... ). The results of calculations have been compared with existing experimental photoelectron, ion-yield, electron-ion coincidence, and Auger electron spectra.

Scintillating-Glass-Fiber Neutron Sensors, their Application and Performance for Plutonium Detection and Monitoring

Most neutron detection sensors presently employ 3He gas-filled detectors. Despite their excellent performance and widespread use, there are significant limitations to this technology. A significant alternative neutron sensor utilizing neutron-active material incorporated into a glass scintillator is presented that offers novel commercial sensors not possible or practical with gas tube technology. The scintillating optical fiber permits sensors with a multitude of sizes ranging from devices of a single fiber of 150µm to sensors with tens of thousands of fibers with areas as large as 5 m2 depending on the neutron flux to be measured. A second significant advantage is the use of high-speed electronics that allow a greater dynamic range, not possible with gas detectors. These sensors are flexible, conformable and less sensitive to vibration that optimizes the source-to-detector geometry and provides robust performance in field applications. The glass-fibers are sensitive to both gamma-rays and neutrons. However, the coincidence electronics are optimized for neutron to gamma-ray discrimination allowing very sensitive measurements with a low false-alarm rate. Applications include SNM surveillance, material control and accountability (MC&A), safeguard inspections, Pu health physics/bioassay and environmental characterization.

Alpha decay of the new isotopes 188,189Po

New neutron-deficient isotopes 188,189Po have been produced in the complete fusion reaction of 52Cr ions with a 142Nd target at the velocity filter SHIP. The evaporation residues were separated in-flight and subsequently identified on the basis of α-γ and α-conversion electron coincidence measurements and of α-α position and time correlations. In 189Po a ground state to ground state α decay with Eα1= 7540(20) keV, T1/2= 5(1) ms and two fine structure α-decays at Eα2= 7264(15) keV and Eα3= 7316(15) keV have been observed. In 188Po (T1/2= 400+200 −150μs) a ground state to ground state α decay at Eα= 7915(25) keV and a fine structure α decay at Eα= 7350(40) keV have been found. Improved data on the α-decay of 189Bi were obtained.

Postcollision Interaction in the Coincident Emission of Photoelectrons and Auger Electrons at Small Relative Angles

Postcollision interaction effects have been investigated for sequential two-electron emission in xenon for particular kinematical conditions, small relative angles between the emitted electrons, and selected energies ${\ensuremath{\epsilon}}_{a}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}{\ensuremath{\epsilon}}_{b}$ which are halfway in between the nominal values of the ``photo''-electron and the ``Auger'' electron. The experimental results are in good agreement with the theoretical prediction. Our successful technique to measure two coincident electrons of nearly the same energy and the same directions opens a plethora of possibilities to probe for different cases the Coulomb interaction of three charged particles and the impact of the intermediate resonance state.

Cross section for the production of N 2+ ( X2 Σ g+) ions by electron impact on N 2

A combination of electron scattering and laser-induced fluorescence (LIF) techniques has been employed in the direct experimental determination of the N 2 + ( X 2Σ g +) ionization cross section as a function of electron energy from threshold to 200 eV. Electron impact on N 2 produces N 2 + ground-state ions that are detected by pumping the X 2Σ g + → B 2Σ u + (0,0) vibrational transition at 391 nm with a tunable dye laser and detecting the subsequent LIF of the B 2Σ u + → X 2Σ g + (0,1) vibrational transition at 428 nm. LIF spectra obtained at different electron energies yield the relative N 2 +( X) cross section which is put on an absolute scale by normalization to the absolute cross section value of Doering and Yang [J. Geophys. Res. 102 (1996) 9683] obtained from electron–electron coincidence ( e,2 e) experiments at 100 eV.

What can be learned from photoelectron Auger electron coincidence spectrometry ?

A comprehensive review is given for well-defined sequential processes of photoelectron and Auger electron emission, concentrating on the aspects what information can be extracted from energy- and angle-resolved coincidence measurements between these emitted electrons. In particular, attention is paid, with increasing complexity, to the spectroscopic aspect, the angular correlation of both electrons obtained at different electron energies, and the particular features which occur if the energies of both electrons are nearly or exactly equal.

Observation of angular correlation between subsequently emitted Auger electrons

In an exploratory feasibility study we have measured the angular correlation between Auger electrons that were emitted in a cascadelike decay process after resonant photoexcitation, using synchrotron radiation from the Brookhaven National Synchrotron Light Source. While the monochromator was tuned to the argon $1\stackrel{\ensuremath{\rightarrow}}{s}4p$ resonance (3203.5 eV) we recorded Ar $\mathrm{LMM}$ Auger electrons in coincidence with ${\mathrm{KL}}_{2,3}{L}_{2,3}{,KL}_{1}{L}_{2,3},$ and ${\mathrm{KL}}_{2,3}{M}_{1,2,3}$ Auger electrons. We found different nonisotropic angular correlations between distinct energy regions of the $\mathrm{LMM}$ group of Auger lines and the ${\mathrm{KL}}_{2,3}{L}_{2,3}$ Auger electrons, while for other kinetic energies the $\mathrm{LMM}$ Auger electrons exhibit isotropy. Because the $\mathrm{KLL}$ and $\mathrm{LMM}$ Auger energies are so different, we believe that the nonisotropic angular correlation observed is due to an alignment effect rather than a dynamical postcollision interaction effect.

Correlations in Electron - Atom Collisions involving Polarised Electrons and Photons

More sophisticated experimental methods to study inelastic electron–atom collisions include polarisation-correlation measurements such as electron–photon (e, eγ) and electron–electron (e, 2e) coincidence studies or stepwise electron–photon (eγ, γ) correlations. Electron correlations and spin effects can be explored on the most fundamental level in such experiments. In this paper recent (e, eγ) studies with Hg and Xe, (eγ, γ) studies with Ne and (e, 2e) studies with Xe using polarised electrons and/or polarised photons are reviewed.

Fine structure in -decay and shape coexistence in

Excited states in populated in the -decay of have been identified through -particle-conversion electron coincidences. The level scheme has been interpreted using a configuration mixing calculation, providing estimates of the mixing matrix elements, mixing amplitudes and the energies of unperturbed and unobserved levels.

Spherical mirror analyzer as an instrument for electron coincidence spectroscopy

The time-of-flight characteristics of a spherical-mirror electrostatic energy analyzer of charged particle fluxes are investigated under conditions of ideal solid-angle focusing for a point source located on the symmetry axis of the spectrometer. It is shown that the time it takes a particle to move from the source to the ideal focus, also located on the symmetry axis, is, to first order, independent of particle direction for directions near the normal to the axis. This time-of-flight focusing realized in this way enables efficient use of the spherical-mirror analyzer in electron spectroscopy methods in which each emission event is registered separately.

Multiple Capture Investigated by Coincident Electron Spectroscopy in X7+ + Ar, at 70 keV

The multiple electron capture in N7+ + Ar and F7+(1s2) + Ar systems is investigated at 70 keV with a new electron-recoil ion charge coincidence experiment. The whole electron energy range has been studied. Up to six electrons are found to be captured into autoionizing states. The recoil ion charge distribution associated with the emission of electrons is similar for both systems and found to be in good agreement with the prediction of Niehaus's model roughly adapted to take into account autoionizing cascades. New findings for the coincident double and triple captures are briefly discussed. A capture of an inner L-shell electron of Ar into the K-shell of the projectile is also observed in N7+ + Ar collisions.

Predissociation of optically forbidden states in electron-hydrogen collisions as studied by coincident electron energy-loss spectroscopy

The electron energy-loss spectra of in coincidence with H(2p) formation have been measured in the range just above the dissociation limit of H(1s) + H(2p) at the incident electron energies of 30 and 150 eV and the scattering angle of . The ordinary energy-loss spectra at the same incident energies and scattering angle have also been measured. It is clearly seen that optically-allowed states dominate in the ordinary energy-loss spectra while optically-forbidden states contribute noticeably to the coincident energy-loss spectrum at the 30 eV incident energy. The forbidden states predissociating into H(1s) + H(2p) are assigned and the branching ratio of the predissociation, the intensity ratio of the coincident energy-loss spectrum to the corresponding ordinary one, is obtained. The predissociation pathways into H(1s) + H(2p) from those forbidden states are discussed in detail.

Double Photoionization of Spatially AlignedD2

The four-body breakup of spatially aligned ${\mathrm{D}}_{2}$ by 58.8 eV photons from the Advanced Light Source has been investigated by measuring the three dimensional momentum vectors of both fragment ions and one of the two electrons in coincidence. Energy and angular correlation between ions and electrons is discussed. We find rotational symmetry of the electron angular distribution around the polarization vector of the light and significant differences between helium and ${\mathrm{D}}_{2}$ as well as between molecular alignment parallel and perpendicular to the polarization axis.

Site-Selective Photochemistry of Core Excited Molecules: Role of the Internal Energy

A further insight into the photochemistry of a core excited polyatomic molecule has been obtained using a new electron-ion coincidence setup. Selecting the kinetic energy of Auger electrons in coincidence with fragment ions we observed a strong site selective fragmentation for the lowest states of the ${\mathrm{ClCH}}_{2}{\mathrm{Br}}^{2+}$ ion, following selective Br $3d$ or Cl $2p$ core photoionization. At higher energy the selectivity diminishes and is completely lost for the highest states. The pure photoselective effect has been emphasized by subtracting the thermodynamical, naturally selective, fragmentation pattern.