Recombination and electron impact excitation of S¹⁴⁺ and S¹⁵⁺ ions was measured at the Stockholm refrigerated electron beam ion trap. The collision energy range was 1.4-3 keV, where we covered the KLL, KLM, KLN, and KLO dielectronic recombination resonances resulting in S¹³⁺ and S¹⁴⁺ ions. The recombination rates were obtained by detecting the charge state distribution with a newly developed time-of-flight technique. Resonance energies and cross sections calculated within the relativistic many-body perturbation theory for S¹⁵⁺ agree well with the experimental data. The temperature dependent rate coefficients have been extracted from the measured rates and compared with calculations from literature used for studies of collisionally ionized astrophysical plasmas. A good agreement for S15+ was obtained, while the plasma rates for S¹⁴⁺ were 23% lower than the so far published values. In addition to the time-of-flight spectra, the x-ray spectra, produced mainly by photo-recombination and excitation, have been also collected. The combination of these two measurements allowed us to separate the photo-recombination and the excitation spectra, and the excitation rate coefficients for summed intensities with known fractions of S¹⁴⁺ and S¹⁵⁺ ions were extracted.

Measurement of circular polarization of gamma-rays with position sensitive solid state detectors is proposed. It is based on the transfer of the photon spin to the recoiled electron in the process of Compton scattering and subsequent detection of the electron spin polarization. Bremsstrahlung polarization correlations, in particular the left-right asymmetry of the photon emission from transversally polarized electrons, are used for this purpose. Compton scattering and electron bremsstrahlung events as well as the scattered and the bremsstrahlung photon absorptions take place inside the active volume of the position sensitive detector. These events are identified and their complete kinematics is reconstructed by means of gamma-ray tracking. No spin polarized targets and no inactive materials are needed for this technique. The proposed method is naturally integrated into the concept of a Compton telescope, allowing for the first time to build an imaging gamma-ray circular polarimeter. It should work in the energy region of several 100 key up to several 10 MeV.

Electron-ion recombination of sulfur ions with electrons in the energy range of 1.6-3 keV was studied at the Stockholm Refrigerated Electron Beam Ion Trap. We obtained the KLn dielectronic recombination (DR) cross sections up to n = 5 for H-like and He-like sulfur ions by observing the x-rays from the trapped ions. A fully relativistic distorted wave approximation method was used for calculating the DR cross sections, while the resonance energies were obtained with a multiconfiguration Dirac-Fock approach using the GRASP II code. The calculations agree with the experimental results within the experimental error bars. Additionally, the obtained total DR resonance strengths were used to check the behaviour of a scaling formula for low-Z, He-like iso-electronic sequence (Watanabe et al 2001 J. Phys. B: At. Mol. Opt. Phys. 34 5095).

Atomic-field bremsstrahlung has been studied with a longitudinally polarized electron beam. The correlation between the initial orientation of the electron spin and the angle of photon polarization has been measured at the photon high energy tip region. In the time reversal this corresponds to a so-far unobserved phenomenon of production of longitudinally polarized electrons by photoionization of unpolarized atoms with linearly polarized photons. The results confirm the fully relativistic calculations for radiative recombination and suggest a new method for electron beam polarimetry.

Equipment at the Stockholm Refrigerated Electron Beam Ion Trap (R-EBIT) was developed for photo-recombination studies. A LabView-based event mode data acquisition and R-EBIT control system was implemented. The energies of KLL dielectronic recombination resonances in Li- to C-like argon ions were determined and compared with theoretical calculations performed using a distorted wave approximation. The theoretical and experimental peak positions for Li-, Be-, and C-like argon ions agree within the error bars. For B-like argon we observe an energy shift of 9 eV between the experimentally obtained peak position and the calculated result.

Tracking of gamma-rays in position sensitive germanium and silicon detectors is a relatively new concept. In the energy region of a few hundred keV up to a few MeV it aims at identification of the sequence and positions of photon scattering points inside a detector. This is achieved by verifying the energy and momentum conservation laws for each Compton interaction as well as the scattering and absorption probabilities. By calculating multiple figures of merit, each favoring a specific class of events (e.g. full-energy, escape and bremsstrahlung), one can identify these events and increase the peak/total ratio of the full-energy spectrum. As an example, events where bremsstrahlung photons are produced by Compton recoiled electrons are identified. A further extension of this technique may allow identification of pair production events and extend the tracking analysis into a higher energy region of several MeV where pair production dominates.

We describe a refrigerated EBIT (R-EBIT) commissioned at the AlbaNova Research Center at Stockholm University. As an innovative solution, the superconducting magnet and the trapping drift tubes of the R-EBIT are cooled to a temperature of 4 K by a set of two cooling heads connected to helium compressors. This dry, i.e. liquid helium and liquid nitrogen free, system is easily operated and creates highly charged ions at a fraction of the cost of traditional liquid-cooled systems. A pulsed and continuous gas injection system was developed to feed neutral particles into the electron beam in the trap region. This improves significantly the highly charged ion production and R-EBIT performance. Fast extraction of ions from the R-EBIT yields very short ( < 100 ns), charge-separated ion bunches which can be either analysed using a straight time-of-flight section or sent to experimental beam lines following selection in a bending magnet. An emittance meter was used to measure the emittance of the ions extracted from the R-EBIT. The extracted ions were also re-trapped in a cylindrical Penning trap and properties of the re-trapped ions have been measured using the emittance meter. Results of these measurements are reported in this publication.

For spectroscopy, polarimetry and imaging purposes a new gamma-ray tracking algorithm has been developed featuring identification of Compton escape events. The rejection of these events results in a significant increase of the Peak/Total ratio. The initial photon energy is restored for these events. Although the energy resolution in the spectrum reconstructed from the escape events is lower than the one from the full-energy events, the Monte-Carlo simulations show that the combined spectrum has an increased detector full-energy efficiency of up to 130% compared to its intrinsic full-energy efficiency. The assumed geometrical origin of the photons is verified event-by-event. This enables separation of photons emitted from a target and from background sources. A linear polarization analysis of the gamma-lines can be performed. The efficiency of the algorithm and the Peak/Total ratio depending on the detector properties is discussed along with the proposed optimization schemes. The influence of the intrinsic properties of the scattering process like Compton profile and electron recoiling is discussed as well. The described algorithm deals with single photon events with energies of approximate to 100 keV up to a few MeV.

A new laboratory for highly charged ions is being built up at Stockholm University. A fully refrigerated electron beam ion trap (R-EBIT, 3 T magnet, 30 keV electron energy) was installed. It was used for spectroscopic studies, ion cooling experiments, electron ion collisions, and highly-charged ion surface studies. Here we report on an upgrade of this EBIT to a ``Super EBIT'' (S-EBIT, 4 T magnet, 260 keV electron energy). The high-voltage trapping system, the ion injection as well as the extraction scheme of S-EBIT and the LabView based operational system of S-EBIT are described.

A new approach provides charge-state selective recombination rate coefficients from TOF spectraof highly charged ions extracted from an EBIT. Experimentally derived dielectronic recombinationspectra are compared with results of relativistic many body perturbation theory calculations. Electronimpact excitation rate coefficients are also obtained from the experiment through the separationof the DR contribution in the X-ray spectra. The combination of TOF and X-ray measurementsoffers a powerful tool for the simultaneous extraction of atomic quantities for several charge states.