[en] The ratio, R, of cross sections for double and single ionization of helium induced by photons as a function of their energy is of particular interest as a measure of the relative importance of electron correlation effects. Calculations of this quantity are highly sensitive to assumptions about correlations between the two escaping electrons. At photon energies above ∼6 keV the major contributing process comes from Compton scattering. Recent theories differ widely in their predictions of an asymptotic value of R. The authors have made a measurement at a photon energy significantly higher than those explored previously. For this they chose the 60-keV x-ray beam at ID15 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. To minimize background problems, they employed the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) technique and a supersonic cold jet of helium gas. Using a position-sensitive channel-plate detector and time-of-flight measurement, the authors were able to cleanly identify the He ions and determine a value of R. They discuss how this result serves to discriminate between several theoretical calculations of the asymptotic limit

[en] The probabilities for cusp and δ-electron production were measured as a function of the impact parameter and of the outgoing projectile charge state for collisions of 0.5 MeV u^-1 H⁺, He⁺, He²⁺ on Ne and He targets. The experimental results for H⁺ projectiles are presented here and compared with both quantum mechanical and classical calculations. The results give strong evidence that in collisions between fast light ions and atoms a post-collision Coulomb focusing effect of the outgoing projectile ionic charge on the ionized electrons is of great importance for capture to the continuum. (author)

No abstract available

[en] We investigate the method of an indirect detection of a MCP charge avalanche projected onto a resistive layer (G. Battistoni, et al., Nucl. Instr. and Meth., 202 (1982) 459). If the sheet resistance is favourable one can detect the charge cloud by the capacitive coupling to an anode structure a few millimetres behind the layer. The anode structure can be, for example, a wedge-and-strip electrode pattern (M. Unverzagt, Diplomarbeit, Universitaet Frankfurt 1992, private communication) as it is used for directly collecting the electron avalanche from a MCP. Detection of the induced charge is beneficial in several respects. Firstly, image distortions produced by secondary electron mediated charge redistribution are eliminated. Secondly, the noise component due to quantized charge collection, commonly referred to as partition noise, is not present. In addition, the dielectric substrate can function both as an element of the vacuum enclosure and HV insulator, making the electrical connections easily accessible and the pattern operable at ground potential, independently of detector operating voltages. This technique can be used to simplify the electronic design requirements where varying high voltages are required at the detector input face such as plasma analysers, etc. It also has application in the manufacture of intensifier tubes (J. Barnstedt, M. Grewing, Nucl. Instr. and Meth., these proceedings) where the inclusion of a readout pattern inside the intensifier body with associated electrical feed-throughs can prove problematic. We will present data on the performance of such detection geometries using several types of charge division anode, and discuss the advantages compared with the 'traditional' charge collecting method

[en] Double ionization of an atom by a single photon is the simplest and most fundamental many-electron process. The ejection of two electrons following the absorption of one photon is strictly prohibited in an independent electron approximation. Thus determining the probability of double photoionization alone is already a challenging test of the understanding of electron-electron correlation. Furthermore, in the slow breakup of a bound system into three charged particles, the final state wave function must represent a high degree of few-body Coulomb correlation involving the simultaneous interaction of all three particles. The case of double photoionization is again particularly well suited to study this problem as the energy and the angular momentum delivered to the system can be very well controlled. Helium, as the most basic three body system, has been the target of extensive studies over the past decades. The purpose of this project has been to study double and single ionization using cold target recoil ion momentum spectroscopy (COLTRIMS). This technique has been widely applied within the area of ion-atom collisions to study the dynamics of energy and momentum transfer in collisions between few-electron systems, and the entire technical machinery has been transferred to photon-atom collisions. The technique uses space- and time-imaging of He⁺ and He⁺⁺ recoil ions created in photon-He collisions to measure the full momentum vector of each ion produced. Event-mode recording is used and a solid angle of nearly 4π is realized, allowing an extremely high data-collection efficiency. In order to reduce the initial momentum spread of the He target a precooled supersonic He jet is used

[en] Many particle dynamics in atomic and molecular physics has been investigated by using the COLTRIMS method. The method and its power is discussed. The COLTRIMS technique visualizes many-particle fragmentation processes in the eV and sub milli-eV regime and reveals like the bubble chamber the complete momentum pattern in atomic and molecular particles reactions. Complete differential cross sections in momentum space have been measured for the transfer ionization channel in fast four-body (p+He→H deg. +He²⁺+e) collisions. The correlated kinematical transfer ionization channel has been used to probe the highly correlated contributions to the asymptotic parts (high momenta and large nuclear impact parameters) of the He ground-state momentum wave function. In this reaction, one electron with selected initial momentum (2.5-7.5 a.u.) in the He ground state is kinematically captured by the proton (tunneling through the two-center barrier). The measured three-particle final-state momentum distributions show well-structured patterns, which reflect special features of the three-particle initial-state momentum wave function

[en] New applications for single particle and photon detection in many fields require both large area imaging performance and precise time information on each detected particle. Moreover, a very high data acquisition rate is desirable for most applications and eventually the detection and imaging of more than one particle arriving within a microsecond is required. Commercial CCD systems lack the timing information whereas other electronic microchannel plate (MCP) read-out schemes usually suffer from a low acquisition rate and complicated and sometimes costly read-out electronics. We have designed and tested a complete imaging system consisting of an MCP position readout with helical wire delay-lines, single-unit amplifier box and PC-controlled time-to-digital converter (TDC) readout. The system is very flexible and can detect and analyse position and timing information at single particle rates beyond 1 MHz. Alternatively, multi-hit events can be collected and analysed at about 20 kHz rate. We discuss the advantages and applications of this technique and then focus on the detector's ability to detect and analyse multiple hits

[en] Absolute doubly differential cross sections for electron emission are presented for 0.5 MeV/u multi-charged ion impact on helium, neon, and argon targets. For the helium target, Bq+, Cq+ (q = 2--5) and Oq+, Fq+ (q = 3--6) projectiles were studied; for neon and argon, only Cq+ (q = 2--5) projectiles were used. Electron emission for 10 degrees ≤ Θ ≤ 60 degrees was studied. The measured cross sections were assumed to scale as the square of an effective projectile charge, Z_eff, which was determined as a function of emitted electron energy and angle. For distant collisions (low emitted electron energies), we find that Z_eff ∼ q for small q and Z_eff < q for the highest values of q investigated. For sufficiently close collisions (above the binary encounter peak), Z_eff > Z and increases as q decreases. This is true for all angles and targets investigated

[en] Vector correlation is a powerful method for the investigation of photofragmentation of molecules or clusters. It consists of measuring in coincidence the velocity vectors of the electrons and atomic fragments emitted from the same physical event, and provides detailed information about the spectroscopy and intramolecular dynamics of excited states. A new velocity spectrometer which combines imaging and time-resolved detection techniques has been developed and applied to the investigation of dissociative photoionization of diatomic molecules (NO, CO, O₂, H₂, etc.) induced by synchrotron radiation linearly polarized light (P)(AB+hν(P)→A⁺+B*+e in the VUV photon energy range (20 eV≤hν≤40 eV). The (V_A⁺,V_e) velocity vector correlation leads first to the correlation of the fragment ion and electron kinetic energies, which enables to identify and select each fragmentation process and to measure their branching ratios. The angular correlation of the ion and electron velocities for a selected process leads then to the angular distribution of the photoelectrons in the molecular frame, for each orientation of the molecular axis with respect to the linear polarization of the incident light. We illustrate the method by the recent data obtained for dissociative photoionization of NO and O₂

[en] We have developed a novel delay-line anode design based on L/2 shifted maeander-lines for the readout of open-faced and sealed MCP-based detectors. In combination with the sealed detector (image intensifier) we are able to provide position and time sensitive single photon detection from near UV to near IR. (orig.)