Research topics

Our group is dedicated to nuclear structure experiments in different regions of the nuclide map. The AGATA spectrometer with the novel γ-tracking method offers unprecedented detection sensitivity for in-beam spectroscopy. Recent examples are results on heavy actinide nuclei and neutron-rich nuclei near the shell closures Z=50 and N=82 obtained in experiments at the LNL in Italy. AGATA is currently located at the GANIL research center in France.

• L. Kaya et al. Isomer spectroscopy in ¹³³Ba and high-spin structure of ¹³⁴Ba. Phys. Rev. C 100, 024323 (2019).
• L. Kaya et al. Identification of high-spin proton configurations in ¹³⁶Ba and ¹³⁷Ba. Phys. Rev. C 99, 014301(2019).
• L. Kaya et al. Millisecond 23/2⁺ isomers in N=79 isotones ¹³³Xe and ¹³⁵Ba. Phys. Rev. C 98, 054312 (2018).
• L. Kaya et al. High-spin structure in the transitional nucleus ¹³¹Xe: Competitive neutron and proton alignment in the vicinity of the N = 82 shell closure. Phys. Rev. C 98, 014309 (2018).
• J. Dudouet et al. ⁹⁶Kr-Low-Z Boundary of the Island of Deformation at N = 60. Phys. Rev. Lett. 118, 162501 (2017).
• A. Vogt et al. Isomers and high-spin structures in the N = 81 isotones ¹³⁵Xe and ¹³⁷Ba. Phys. Rev. C 93, 054325(2017).
• K. Hadyńska-Klȩk et al. Superdeformed and Triaxial States in ⁴²Ca. Phys. Rev. Lett. 117, 062501(2016).
• Zs. Podolyák et al. Role of the Δ Resonance in the Population of a Four-Nucleon State in the ⁵⁶Fe → ⁵⁴Fe Reaction at Relativistic Energies. Phys. Rev. Lett. 117, 222302 (2016).

The ultimate detector design for γ-spectroscopy is a closed shell of high-purity germanium (HPGe). Such an approach is being pursued in the AGATA collaboration in Europe. The AGATA detectors each comprise three highly segmented Ge crystals in a common cryostat. Research and development on these complex and highly integrated detector systems is being driven forward in the AGATA detector group in Cologne. Both novel technologies and fundamental detector physics are the basis for the construction and successful operation of ultrasensitive γ-detector systems. Our research results are summarized in a large number of publications:

• A. Wiens et al. Improved energy resolution of highly segmented HPGe detectors by noise reduction. Eur. Phys. J. A 49:47 (2013).
• B. Bruyneel et al. The liquid nitrogen fill level meter for the AGATA triple cluster detector. Nucl. Inst. Meth. Phys. Res A 640 (1) 133-138 (2011).
• D. Lersch et al. The liquid nitrogen fill level meter for the AGATA triple cluster detector. Nucl. Instr. Meth. Phys. Res. A 640, 133-138 (2011).
• A. Wiens et al. The AGATA triple cluster detector. Nucl. Inst. Meth. Phys. Res A 618 (1-3) 223-233 (2010).
• B. Bruyneel et al. Crosstalk properties of 36-fold segmented symmetric hexagonal HPGe detectors. Nucl. Inst. Meth. Phys. Res A 599 (2-3) 196-208 (2009).
• B. Bruyneel et al. Characterization of large volume HPGe detectors. Part II: Experimental results. Nucl. Inst. Meth. Phys. Res A 569 (3) 774-789 (2006).
• B. Bruyneel et al. Characterization of large volume HPGe detectors. Part I: Electron and hole mobility parameterization. Nucl. Inst. Meth. Phys. Res A 569 (3) 764-773 (2006).

Experiments with the position-sensitive and highly segmented AGATA HPGe detectors are based on the novel method of gamma-ray tracking (GRT). This allows the precise determination of the individual interaction points and thus the reconstruction of the energy and all scattering paths of γ-rays in the detector array. The tracking requires a pulse-shape analysis (PSA) of the preamplifier signals of all 36 segments as well as the central electrode of each detector. The measured signals are compared with simulations to determine the individual γ-interaction points in the detector with a spatial resolution of approx. 4 mm. An important application of position sensitivity is the correction of charge carrier trapping. HPGe detectors are inevitably exposed to fast neutrons in experiments and suffer crystal defects as a result. These are noticeable in a deteriorated energy resolution due to reduced charge collection. Software-based corrections can restore the original energy resolution of the detector after neutron damage.

• L. Lewandowski et al. Pulse-Shape Analysis and position resolution in highly segmented HPGe AGATA detectors. Eur. Phys. J. A 55, 81 (2019).
• 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).

The new HIE-ISOLDE accelerator complex at the CERN research center near Geneva offers new possibilities for studying very exotic nuclei using radioactive ion beams. Experiments with nuclei far from the valley of stability are relevant for understanding the basic nuclear structure and its interactions, as well as for questions of nuclear astrophysics. Our group succeeded in a challenging measurement with post-accelerated ions of the double-magic isotope ¹³²Sn. The γ-transitions of excited states were detected with the MINIBALL spectrometer. This offers a high detection sensitivity for experiments with the smallest beam intensities.

• D. Rosiak, et al. Escape-suppression shield detector for the MINIBALL ɣ-ray spectrometer. Eur. Phys. J. A (2019) 55:48.
• D. Rosiak, et al. Enhanced quadrupole and octupole strength in doubly magic ¹³²Sn. Phys. Rev. Lett. 121, 252501 (2018).
• P. Butler, J. Cederkall, and P. Reiter: Nuclear-structure studies of exotic nuclei with MINIBALL. J. Phys. G: Nucl. Part. Phys. 44, 044012 (2017).
• J.N. Wilson et al. Anomalies in the Charge Yields of Fission Fragments from the ²³⁸U(n,f) Reaction. Phys. Rev. Lett. 118, 222501 (2017).
• E. Clément et al. Spectroscopic Quadrupole Moments in ^96,98Sr: Evidence for Shape Coexistence in Neutron-Rich Strontium Isotopes at N = 60. Phys. Rev. Lett. 116, 022701 (2016).
• B. Siebeck et al. Testing refined shell-model interactions in the sd shell: Coulomb excitation of ²⁶Na. Phys. Rev. C 91, 014311 (2015).

The Lund-York-Cologne Calorimeter (LYCCA) is a particle detector for the FAIR/NUSTAR collaboration for the detection of heavy ions produced in nuclear reactions with relativistic radioactive ion beams (RIB). The nuclear charge Z and the mass A of the reaction products can be determined by measuring the time-of-flight (ToF), energy losses and total kinetic energy. The high granularity and position sensitivity of LYCCA allows the reconstruction of the individual trajectories of the reaction products. This is necessary to perform high-resolution gamma spectroscopy on nuclei far from stability. After the successful first use during the NUSTAR-PreSPEC campaign at GSI in Darmstadt, an upgrade of the electronics and the data acquisition system was carried out at STFC Daresbury. Highly integrated AIDA front-end electronics modules with Application-Specific Integrated Circuits (ASICs) are used to process the signals from more than a thousand spectroscopy channels. After the test phase in Daresbury, the LYCCA detector with the new electronics was set up at the IKP tandem accelerator in Cologne and is currently in operation.

• D. Ralet et al. Lifetime measurement of neutron-rich even-even molybenum isotopes. Phys. Rev. C 95, 034320 (2017).
• A. Wendt et al. Isospin symmetry in the sd shell: Transition strengths in the neutron-deficient sd shell nucleus ³³Ar. Phys. Rev. C 90, 054301 (2014).

Imaging of high-energy γ-radiation is possible with a Compton camera. Our group is developing such a setup, consisting of a highly segmented HPGe detector and a double-sided segmented silicon detector (DSSSD). To reconstruct the origin of γ-radiation, the detection of the individual interaction sites and energies after Compton scattering within the Ge crystal is used. Modern digital spectroscopy electronics and extensive software libraries are used for this purpose. An optimized angular resolution is achieved by using the Si detector in coincidence with the Ge detector. The functional principle requires good energy resolution of the detectors and optimal determination of the interaction sites using pulse shape analysis and γ-ray tracking. The first results are summarized in a publication:

• T. Steinbach, R. Hirsch et al. Compton imaging with a highly-segmented, position-sensitive HPGe detector. Eur. Phys. J. A 53, 23 (2017).

Lifetimes of excited states are important observables of the atomic nucleus. With the help of gamma transition energies and nuclear lifetimes, reduced transition strengths can be determined. These signatures provide information about the structure, i.e. the transition matrix elements and the nuclear wave functions involved, as well as about the collectivity of the nuclei under investigation. Different techniques are used to study a wide range of lifetimes: The Doppler shift method (short: DSAM) and the plunger method (also: RDDS) are used for shortest lifetimes (0.1 ps - 1000 ps) while the fast timing method is used for longer lifetimes (<100 ns). Lifetime measurements with pulsed ion beams enable the investigation of lifetimes in the range from 100 ns to several seconds. Experiments to measure a wide range of lifetimes are carried out at the FN tandem accelerator of the Institute of Nuclear Physics together with specially designed setups.

• M. Droste et al. Lifetime measurements in the ground-state band in ¹⁰⁴P. Phys. Rev. C 106, 024329 (2022).
• K. Arnswald et al. Enhanced quadrupole collectivity in doubly-magic ⁵⁶Ni: Lifetime measurements of the 4⁺ and 6⁺ states. Phys. Lett. B 820, 136592 (2021).
• H. Kleis et al. Lifetime measurements of excited states in ⁵⁵Cr. Phys. Rev. C 104, 034310 (2021).
• K. Arnswald et al. Lifetime measurements in ⁴⁴Ti. Phys. Rev. C 102, 054302 (2020).
• M. Queiser et al. Cross-shell excitations from the fp shell: Lifetime measurements in ⁶¹Zn. Phys. Rev. C 96, 044313 (2017).
• K. Arnswald et al. Enhanced collectivity along the N=Z line: Lifetime measurements in ⁴⁴Ti, ⁴⁸Cr, and ⁵²Fe. Phys. Lett. B 772, 599-606 (2017).

A new dedicated experimental setup at the Cologne γ-spectrometer HORUS allows complementary and detailed measurements to results obtained with the AGATA tracking array. In particular, slightly neutron-rich Xe and Ba isotopes near the shell closures at Z=50 and N=82 are difficult to access in fusion evaporation reactions with stable beam-target combinations. Clean and selective trigger conditions are particularly necessary for evaporation channels involving charged particles. A double-sided segmented silicon detector (DSSSD) is used to detect charged particles in coincidence with the gamma spectrometer HORUS. With this setup, high-spin states in different Xe and Ba isotopes could be spectroscoped and existing term schemes could be extended to higher energies. The results are compared with the latest shell model calculations.

• A. Vogt et al. Isomers and high-spin structures in the N=81 isotones ¹³⁵Xe and ¹³⁷Ba. Phys. Rev. C 95, 024316 (2017).
• L. Kaya et al. Characterization and calibration of radiation-damaged double-sided silicon strip detectors. Nucl. Instr. Meth. Phys. Res. A 855 109-117 (2017).
• B. Fu et al. γ-ray spectroscopy of ³³P and ³³S after fusion-evaporation reactions. Phys. Rev. C 94, 034318 (2016).