Recoil-ion and electron momentum spectroscopy is a rapidly developing technique that allows one to measure the vector momenta of several ions and electrons resulting from atomic or molecular fragmentation. In a unique combination, large solid angles close to 4π and superior momentum resolutions around a few per cent of an atomic unit (a.u.) are typically reached in state-of-the art machines, so-called reaction-microscopes. Evolving from recoil-ion and cold target recoil-ion momentum spectroscopy (COLTRIMS), reaction-microscopes—the `bubble chambers of atomic physics'—mark the decisive step forward to investigate many-particle quantum-dynamics occurring when atomic and molecular systems or even surfaces and solids are exposed to time-dependent external electromagnetic fields. This paper concentrates on just these latest technical developments and on at least four new classes of fragmentation experiments that have emerged within about the last five years. First, multi-dimensional images in momentum space brought unprecedented information on the dynamics of single-photon induced fragmentation of fixed-in-space molecules and on their structure. Second, a break-through in the investigation of high-intensity short-pulse laser induced fragmentation of atoms and molecules has been achieved by using reaction-microscopes. Third, for electron and ion-impact, the investigation of two-electron reactions has matured to a state such that the first fully differential cross sections (FDCSs) are reported. Fourth, comprehensive sets of FDCSs for single ionization of atoms by ion-impact, the most basic atomic fragmentation reaction, brought new insight, a couple of surprises and unexpected challenges to theory at keV to GeV collision energies. In addition, a brief summary on the kinematics is provided at the beginning. Finally, the rich future potential of the method is briefly envisaged.

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Recoil-ion and electron momentum spectroscopy: reaction-microscopes

Cold Target Recoil Ion Momentum Spectroscopy: a &momentum microscope' to view atomic collision dynamics

High-energy beams of charged nuclear particles (protons and heavier ions) offer significant advantages for the treatment of deep-seated local tumors in comparison to conventional megavolt photon therapy. Their physical depth-dose distribution in tissue is characterized by a small entrance dose and a distinct maximum (Bragg peak) near the end of range with a sharp fall-off at the distal edge. Taking full advantage of the well-defined range and the small lateral beam spread, modern scanning beam systems allow delivery of the dose with millimeter precision. In addition, projectiles heavier than protons such as carbon ions exhibit an enhanced biological effectiveness in the Bragg peak region caused by the dense ionization of individual particle tracks resulting in reduced cellular repair. This makes them particularly attractive for the treatment of radio-resistant tumors localized near organs at risk. While tumor therapy with protons is a well-established treatment modality with more than 60 000 patients treated worldwide, the application of heavy ions is so far restricted to a few facilities only. Nevertheless, results of clinical phase I-II trials provide evidence that carbon-ion radiotherapy might be beneficial in several tumor entities. This article reviews the progress in heavy-ion therapy, including physical and technical developments, radiobiological studiesmore » and models, as well as radiooncological studies. As a result of the promising clinical results obtained with carbon-ion beams in the past ten years at the Heavy Ion Medical Accelerator facility (Japan) and in a pilot project at GSI Darmstadt (Germany), the plans for new clinical centers for heavy-ion or combined proton and heavy-ion therapy have recently received a substantial boost.« less

Heavy-ion tumor therapy: Physical and radiobiological benefits

High-resolution recoil-ion momentum spectroscopy (RIMS) is a novel technique to determine the charge state and the complete final momentum vector of a recoiling target ion emerging from an ionizing collision of an atom with any kind of radiation. It offers a unique combination of superior momentum resolution in all three spatial directions of with a large detection solid angle of . Recently, low-energy electron analysers based on rigorously new concepts and reaching similar specifications were successfully integrated into RIM spectrometers yielding so-called `reaction microscopes'. Exploiting these techniques, a large variety of atomic reactions for ion, electron, photon and antiproton impact have been explored in unprecedented detail and completeness. Among them kinematically complete experiments on electron capture, single and double ionization in ion - atom collisions at projectile energies between 5 keV and 1.4 GeV have been carried out. Double photoionization of He has been investigated at energies close to the threshold up to . At the contributions to double ionization after photoabsorption and Compton scattering were separated kinematically for the first time. These and many other results will be reviewed in this paper. In addition, the experimental technique is described in some detail and emphasis is given to envisaging the rich future potential of the method in various fields of atomic collision physics with atoms, molecules and clusters.

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Recoil-ion momentum spectroscopy

When an atom is ionized by ion impact, the electron is ejected into a final continuum state of a two-centre potential due to the Coulomb fields of the projectile and ionized atom. The related effects on the electron yield or energy and angular distributions are referred to as two-centre electron emission (TCEE). The present report is devoted to a discussion of experimental and theoretical evidence of this TCEE. The use of heavy ions or antiprotons as projectiles allows to unravel these effects by monitoring the two centre potential. On the theoretical side, the continuum distorted wave-eikonal initial state theory (CDW-EIS) accounts for the TCEE thus allowing a detailed interpretation of the experimental findings.

http://iopscience.iop.org/article/10.1088/0953-4075/24/14/005/pdf

Two-centre effects in ionization by ion impact

1. Introduction.- 2. Overview of Ionization Mechanisms.- 3. Theoretical Work.- 4. Experimental Methods.- 5. Ionization by Bare Projectiles.- 6. Ionization by Dressed Projectiles.- 7. Multiple Electron Processes.- 8. Concluding Remarks.- A. Summary of Analytic Cross Section Formulae.- B. Kinematic Effects on Electron Emission.- C. References to Double Differential Cross Section Studies.- References.

Electron Emission in Heavy Ion-Atom Collisions

Cross sections for electron emission in the energy range from 2–300 eV were measured for 60 MeV/u Kr34+ ions impacting on H2 Model calculations are introduced to guide the search for interference effects in the electron spectra produced by the coherent emission of electrons from the two H atoms in analogy with Young’s two-slit experiment Experimentally, a full sinusoidal-like oscillation was observed in the energy range up to 250 eV in good agreement with the calculations The oscillatory structure is found to be similar for the observation angles 20°, 30°, 150°, and 160°

Evidence for interference effects in electron emission from H2 colliding with 60 MeV/u Kr34+ ions

The continuum-distorted-wave-eikonal-initial-state model is extended to describe single-electron ionisation by impact of bare projectiles on multielectronic targets. Applications are given for collisions between multicharged ions and helium. Double differential, single differential and total cross sections are calculated. Experimental data and present theoretical results show deviations from the square of the projectile charge dependence predicted by the first Born approximation.

A theoretical model for ionisation in ion-atom collisions. Application for the impact of multicharged projectiles on helium

Absolute doubly differential cross-sections for electron emission in 25 MeV/u Mo40+ + He collisions were measured as a function of the electron energy and emission angle. The data show enhancement at forward angles and reduction at backward angles with respect to results derived using the Born approximation. Comparison with a continuum distorted-wave approximation suggests that this observation can be explained by the combined influence of the projectile and target Coulomb fields on the ejected electron. This two-centre effect is found to be particularly important for electron emission at backward angles.

http://iopscience.iop.org/article/10.1209/0295-5075/4/8/007/pdf

Roberto D. Rivarola

Papers

Two-centre effects in ionization by ion impact

Electron Emission in Heavy Ion-Atom Collisions

Evidence for interference effects in electron emission from H2 colliding with 60 MeV/u Kr34+ ions

A theoretical model for ionisation in ion-atom collisions. Application for the impact of multicharged projectiles on helium

Evidence for Two-Centre Effects in the Electron Emission from 25 MeV/u Mo40+ + He Collisions: Theory and Experiment