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Research topics


Advanced Gamma Tracking Array

Gamma-ray spectroscopy with AGATA

Our group is pursuing nuclear-structure experiments in various regions of the Segre chart. The AGATA spectrometer enables the novel detection method gamma-ray tracking with unprecedented detection sensitivity for in-beam spectroscopy. Prominent and actual examples are results from the heavy actinide region and neutron-rich nuclei around the shell closures at Z=50 and N=82 measured at LNL in Italy. At the moment, AGATA is operated at GANIL, France.

Coulomb excitation with radioactive ion beams

MINIBALL @ HIE-ISOLDE

The brand-new HIE-ISOLDE accelerator at CERN allows for investigations of very exotic nuclei which are relevant for basic nuclear physics as well as nuclear astrophysics. Our group managed a demanding measurement with accelerated ions of doubly-magic 132Sn. The gamma-transitions from excited states were recorded with the MINIBALL array. The MINIBALL spectrometer provides best detection sensitivity for studies with extreme low beam intensities.

Germanium detector technologies

Research and development of highly segmented HPGe detectors

The ultimate detector for gamma-ray spectroscopy will be a closed shell of high-purity germanium (HPGe) detectors. This project is realized within the AGATA collaboration in Europe. The AGATA detectors comprise three highly segmented Ge crystals in a common cryostat. The research and development for these advanced detection systems are performed by the Cologne AGATA detector group. New advanced technology and basic detector physics are at the core of these ultra-sensitive radiation detection devices. This is described and documented in numerous publications:

Instrumentation for FAIR

Lund-York-Cologne-Calorimeter LYCCA

The Lund-York-Cologne CAlorimeter (LYCCA) is a charged-particle detector for the FAIR/NUSTAR collaboration, to discriminate heavy ions produced in nuclear reactions of relativistic radioactive-ion beams (RIB). The charge number Z and mass number A of the reaction products can be determined by measuring their time of flight (ToF), energy loss and total energy. Employing the position sensitivity of LYCCA, the flight paths of the reaction products can be tracked event-by-event, enabling the high-resolution in-beam γ-ray spectroscopy (HISPEC) far from the line of stability. After the first successful deployment in the NUSTAR-PreSPEC campaign at GSI Darmstadt, the electronics and data-acquisition systems were upgraded at STFC Daresbury. Highly integrated AIDA front-end electronics modules with application-specific integrated circuits (ASICs) are employed to process the signals of several thousand of spectroscopy channels. LYCCA is currently set up and tested at the FN tandem accelerator at IKP Cologne.

It depends on the position ...

Pulse-shape analysis in highly segmented HPGE detectors

Measurements with the position-sensitive, highly segmented AGATA HPGe detectors rely on the gamma-ray tracking technique (GRT) which allows to determine the different interaction points of the individual γ rays hitting the detector. GRT is based on a pulse-shape analysis (PSA) of the preamplifier signals from the 36 segments and the central electrode of each detector. The measured signals are compared to simulations to determine the individual γ-ray interaction points in the detector with a position resolution of about 4 mm. An important application of the position sensitivity is the correction of charge-carrier trapping. After being irradiated with neutrons, HPGe detectors suffer from crystal defects, resulting in a reduced charge collection along with a poor energy resolution. Software-based correction mechanisms have been developed to cope with these effects.

  • B. Bruyneel, B. Birkenbach, and P. Reiter: Pulse shape analysis and position determination in segmented HPGe detectors: The AGATA detector library. Eur. Phys. J 52:70 (2016)
  • B. Bruyneel et al. Correction for hole trapping in AGATA detectors using pulse shape analysis.
    Eur. Phys. J 49:61 (2013)
Imaging of gamma rays

Compton imaging

Imaging of high-energetic γ radiation is feasible with a Compton camera. Such a device, combining a highly segmented HPGe detector and a double-sided silicon strip detector (DSSSD), is developed in our group. The imaging process, i.e. the reconstruction of the radiation source, is based on the knowledge of each individual interaction point and its corresponding energy deposition after Compton scattering in the detector material via pulse-shape analysis and γ-ray tracking. For this purpose, modern digital spectroscopy electronics and elaborate software libraries are employed. An optimum angular resolution is achieved by using the DSSSD to detect the first Compton scattering in coincidence with the full absorption within the HPGe. The HPGe detector can also be used in a high-efficiency stand-alone mode. First results were recently published:

Plunger, DSAM, Fast-Timing

Nuclear lifetimes

Lifetimes of excited states are important observables of the atomic nucleus. With the knowledge of both gamma-ray transition energies and nuclear lifetimes, reduced transition probabilities can be determined. These signatures give information on the structure, i.e. the transition's matrix element and the involved nuclear wave functions, as well as the collectivity of the investigated nuclei. Different techniques are employed to study a large range of lifetimes: (i) The Doppler-shift attenuation method (DSAM) and (ii) the recoil-distance Doppler shift method (plunger) for short-lived states (~0.1 - 1000 ps) as well as the electronic fast-timing method for longer lifetimes (<100 ns). Lifetime measurements with pulsed ion beams allow for the determination of lifetimes in the range of 100 ns up to several seconds. Experiments, yielding a variety of lifetimes, are performed at the FN-tandem accelerator of the IKP with dedicated setups to provide access to lifetimes of present interest.

Spectroscopy up to high spins and energies

Gamma-ray spectroscopy with HORUS

A new dedicated experimental setup at the Cologne HORUS γ-ray spectrometer is employed to complement results obtained with the γ-ray tracking array AGATA. Especially slightly neutron-rich Xe and Ba isotopes in the vicinity of the Z=50 and N=82 shell closures are difficult to populate via fusion-evaporation reactions with reasonable yields due to a lack of suitable beam-target combinations. Elusive charged-particle evaporation channels require clean and selective trigger conditions. A double-sided silicon strip detector (DSSSD) is employed for the detection of charged particles providing position and energy information. This setup allows for detailed spectroscopy at the HORUS γ-ray array: High-spin level schemes of various Xe and Ba nuclei were considerably extended to higher energies. The results are confronted with latest shell-model calculations based on modern effective interactions considering 132Sn as a closed core.