Fission of238U induced by136Xe for energies close to the Coulomb barrier

Excitation functions and angular distributions have been measured for fission of²³⁸U induced by¹³⁶Xe ions with bombarding energies between 4.5 and 5.9 MeV/N. Structures expected theoretically as characteristic for Coulomb fission have not been observed. The Q-value of ?(7.5±1.0) MeV measured for bombarding energies below 4.7MeV/N, however, appears to be compatible with inelastic scattering (e.g. Coulomb excitation) rather than subcoulomb transfer followed by fission. The total kinetic energy of deep inelastic events at 5.9 MeV/N is consistent with the current knowledge about mass diffusion, but also (for the highest excitation energies) with a fast fission process in the presence of the projectile.     

Generation of stable, low-divergence electron beams by laser-wakefield acceleration in a steady-state-flow gas cell

Laser-driven, quasimonoenergetic electron beams of up to approximately 200 MeV in energy have been observed from steady-state-flow gas cells. These beams emitted within a low-divergence cone of 2.1+/-0.5 mrad FWHM display unprecedented shot-to-shot stability in energy (2.5% rms), pointing (1.4 mrad rms), and charge (16% rms) owing to a highly reproducible gas-density profile within the interaction volume. Laser-wakefield acceleration in gas cells of this type provides a simple and reliable source of relativistic electrons suitable for applications such as the production of extreme-ultraviolet undulator radiation.     

Controlled transport and focusing of laser-accelerated protons with miniature magnetic devices

This Letter demonstrates the transporting and focusing of laser-accelerated 14 MeV protons by permanent magnet miniature quadrupole lenses providing field gradients of up to 500 T/m. The approach is highly reproducible and predictable, leading to a focal spot of (286 x 173) microm full width at half maximum 50 cm behind the source. It decouples the relativistic laser-proton acceleration from the beam transport, paving the way to optimize both separately. The collimation and the subsequent energy selection obtained are perfectly applicable for upcoming high-energy, high-repetition rate laser systems.     

Dense laser-driven electron sheets as relativistic mirrors for coherent production of brilliant X-ray and γ-ray beams

Several techniques exist to obtain brilliant X-ray beams by coherent reflection from relativistic electrons (E _e=γ mc ²) with Doppler frequency upshift of 4γ ². We describe a new approach starting with an ultra-thin solid target. Larger ‘driver’-laser intensities with high contrast are required to produce dense electron sheets. Their acceleration in vacuum results in a transverse momentum component besides the dominant longitudinal momentum component. The counter-propagating ‘production’ laser for optimum Doppler boost in X-ray production by reflection has to be injected opposite to the electron direction and not opposite to the driver laser. Different measures to increase the reflectivity of the electron sheet via laser trapping or free-electron-laser-like micro-bunching are discussed, extending the photon energy into the MeV range. Here, first-order estimates are given.     

The REX-ISOLDE project

The Radioactive Beam Experiment REX-ISOLDE [1–3] is a pilot experiment at ISOLDE (CERN) testing the new concept of post acceleration of radioactive ion beams by using charge breeding of the ions in a high charge state ion source and the efficient acceleration of the highly charged ions in a short LINAC using modern ion accelerator structures. In order to prepare the ions for the experiments singly charged radioactive ions from the on-line mass separator ISOLDE will be cooled and bunched in a Penning trap, charge bred in an electron beam ion source (EBIS) and finally accelerated in the LINAC. The LINAC consists of a radiofrequency quadrupole (RFQ) accelerator, which accelerates the ions up to 0.3 MeV/u, an interdigital H-type (IH) structure with a final energy between 1.1 and 1.2 MeV/u and three seven gap resonators, which allow the variation of the final energy. With an energy of the radioactive beams between 0.8 MeV/u and 2.2 MeV/u a wide range of experiments in the field of nuclear spectroscopy, astrophysics and solid state physics will be addressed by REX-ISOLDE. This revised version was published online in July 2006 with corrections to the Cover Date.     

Status of the Penning trap project in Munich

The MLLTRAP at the Maier-Leibnitz-Laboratory (Garching) is a new Penning trap facility designed to combine several novel technologies to decelerate, charge breed, cool, bunch and purify the reaction products and perform high-accuracy nuclear and atomic mass measurements. It is now in the commissioning phase, achieving a mass-resolving power of about 10⁵ in the purification trap for stable ions.     

Signatures of the Unruh effect via high-power, short-pulse lasers

The ultra-high fields of high-power short-pulse lasers are expected to contribute to understanding fundamental properties of the quantum vacuum and quantum theory in very strong fields. For example, the neutral QED vacuum breaks down at the Schwinger field strength of 1.3×10¹⁸ V/m, where a virtual e⁺e^- pair gains its rest mass energy over a Compton wavelength and materializes as a real pair. At such an ultra-high field strength, an electron experiences an acceleration of a_S=2×10²⁸g and hence fundamental phenomena such as the long predicted Unruh effect start to play a role. The Unruh effect implies that the accelerated electron experiences the vacuum as a thermal bath with the Unruh temperature. In its accelerated frame the electron scatters photons off the thermal bath, corresponding to the emission of an entangled pair of photons in the laboratory frame. While it remains an experimental challenge to reach the critical Schwinger field strength within the immediate future even in view of the enormous thrust in high-power laser developments in recent years, the near-future laser technology may allow to probe the signatures of the Unruh effect mentioned above. Using a laser-accelerated electron beam (γ～300) and a counter-propagating laser beam acting as optical undulator should allow to create entangled Unruh photon pairs (i.e., signatures of the Unruh effect) with energies of the order of several hundred keV. An even substantially improved experimental scenario can be realized by using a brilliant 20 keV photon beam as X-ray undulator together with a low-energy (γ≈2) electron beam. In this case the separation of the Unruh photon pairs from background originating from linearly accelerated electrons (classical Larmor radiation) is significantly improved. Detection of the Unruh photons may be envisaged by Compton polarimetry in a 2D-segmented position-sensitive germanium detector.     

Space-Charge Effects with REXTRAP

The beam quality of radioactive ion beams produced by present target ion source technology is often not sufficient for direct post-acceleration. Furthermore, pulsed beams insure a more efficient use of an accelerator. In the case of REX-ISOLDE, the post accelerator at the CERN ISOLDE facility, a gas-filled Penning trap (REXTRAP) has been chosen for accumulation of the radioactive ions and conversion into cooled bunches. Radial centering of the ions is achieved by applying an rf field with a frequency equal to the cyclotron frequency of the desired ion species. The efficiency achieved in the first tests with different isotopes covering nearly the entire mass range was already >20%. Going to total numbers of >10⁵ stored ions in the trap a shift of the centering frequency could be observed, which is most likely due to space charge effects. Despite this, it was possible to accumulate up to 10⁷ ions and deliver them as cooled bunches. This revised version was published online in July 2006 with corrections to the Cover Date.     

Status of the SHIPTRAP Project: A Capture and Storage Facility for Heavy Radionuclides fromSHIP

The ion trap facility SHIPTRAP is being set up to deliver very clean and cool beams of singly-charged recoil ions produced at the SHIP velocity filter at GSI Darmstadt. SHIPTRAP consists of a gas cell for stopping and thermalizing high-energy recoil ions from SHIP, an rf ion guide for extraction of the ions from the gas cell, a linear rf trap for accumulation and bunching of the ions, and a Penning trap for isobaric purification. The progress in testing the rf ion guide is reported. A transmission of about 93(5)% was achieved. This revised version was published online in July 2006 with corrections to the Cover Date.