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Nuclear Spectroscopy Department

The team after a successful beamtime


The Nuclear Spectroscopy Department aims to study the structure of atomic nuclei by performing decay and in-flight gamma spectroscopy experiments at GSI/FAIR and other accelerator facilities all over the world. With comprehensive high-resolution gamma-ray and charged particle spectroscopy of selected key isotopes, the evolution of the shell structure and exotic nuclear shapes near the limits of nuclear existence and its relevance to the nucleosynthesis of heavy nuclei are being investigated. A recent overview of past, present and future research by the department is provided in ref [1]. A kaleidoscope of main scientific topics is shown in figure 1. The department is continuously developing necessary detectors and instrumentation for these spectroscopic investigations as well as the associated experimental methodology. The transfer of technologies derived from the development work is actively pursued for the benefit of society. Many activities are performed together with international partners in the collaborations HISPEC/DESPEC, AGATA, MINIBALL, PARIS, PANDA and others. The department operates a special HISPEC/DESPEC group coordinating the activities of the collaboration and developing and building the related infrastructure for the experimental campaigns at GSI and FAIR.

High-resolution In-flight SPECtroscopy (HISPEC) experiments are possible thanks to the unprecedented characteristics of AGATA, the Advanced GAmma Tracking Array. The nuclei of interest are secondary reaction products following Coulomb excitation, direct reactions, or fragmentation reactions of relativistic radioactive ion beams provided by the FSR and in future by the Super-FRS at energies of 100-200 MeV/u. AGATA allows to disentangle the complex structure of excited states for the most exotic nuclei ever studied. The department is one of the key partners in the development of AGATA and hosted the early implementation of the spectrometer already from 2012 to 2014 [2].

Slowing down the rare isotope beams from FRS/Super-FRS to energies around the Coulomb barrier enables exploitation of classical reaction types like multiple Coulomb excitation, transfer reactions and compound nuclear reactions. This method is particularly useful for beam species not easily available at ISOL facilities. The slowing-down method and a suite of dedicated beam detectors is being developed by the department. Tying in with previous work, complementary close to the Coulomb barrier experiments at the UNILAC stable beam facility are planned for the near future as well.

Several DEcay SPECtroscopy (DESPEC) setups are being commissioned at different international accelerator laboratories. They are now available at GSI/FAIR for FAIR/NUSTAR Phase-0 experiments. These set-ups will deliver comprehensive decay information from identified key nuclei at secondary beam yields as low as one ion per hour. The department leads the development work for the DEGAS spectrometer, the most efficient dedicated high-resolution gamma spectrometer for decay experiments worldwide. Furthermore, a fast, position-sensitive plastic detector for β-particle measurements in conjunction with the active AIDA implanter has been developed. 

In the coming years the department will concentrate on decay studies employing DESPEC set-ups at the FRS within the FAIR/NUSTAR Phase-0 campaign. These experiments will be complemented by investigations with stable beams at barrier energies from the UNILAC accelerator, as well as by experiments at other international facilities.

Fig.1: Nuclear shell structure, nuclear shapes, nuclear symmetries, nuclear astrophysics

Highlights/Projects/Results/Activities in 2018

In 2018 activities concentrated on the preparation of the first DESPEC experiments within the FAIR/NUSTAR Phase-0 campaign. The early implementation of the so-called Fast Timing Setup was successfully installed on a movable platform at the s4 focal plane of the FRS. Figure 2 shows the detector arrangement including the AIDA active implanter coupled to the b-Plast timing detector, the fast-timing gamma-array FATIMA, and the high-resolution gamma-array DEGAS/GALILEO. All these sub-systems are coupled together for the first time by the White Rabbit time stamping system.

The DEGAS detector system including its active shielding was modelled and simulated in detail [3] and the first Ge triple prototype has been produced and its characterization started. To overcome the limited time resolution of the AIDA implanter a novel plastic detector b-Plast has been designed, built and successfully tested. It relies on a novel light readout scheme with SiPMs [4] instead of conventional photomultipliers.

The HISPEC/DESPEC infrastructure TDR was finally submitted to the ECE of FAIR for evaluation. This TDR is a comprehensive description of the principle experiment setups, which will be realized at FAIR. New collaborators attracted by the unique GSI/FAIR facility have expressed their interest and will add to the versatility of HISPEC/DESPEC. For instance, recently the gSPEC project has been launched together with Australian and French groups aiming at the measurement of nuclear moments.

The analysis of previous PRESPEC AGATA experiments was on going and important results, e.g. ref. [5], are continuously emerging.

The VIDEO-2 gamma radiation localizing system employing six CeBr3(Tl) scintillator crystals has been assembled and the testing has started. Figure 3 shows a representation of the graphical user interface with an environmental background spectrum including the typical 40K line (green line). The software (red lines) identified an “artificial” 60Co source deployed in the test environment and determined its direction.

Fig.2: Experimental set-up with FATIMA, AIDA and GALILEO

Outlook for 2019

In 2019 it is planned to commission the DESPEC Fast Timing Setup at the FRS and optimize it for experiments starting in 2020. The production of the DEGAS Ge detector array will start after thoroughly testing the prototype. These tests include characterization on the 3D gamma-scanner, a sophisticated instrument developed and operated by the department. Upgrade measures for the scanner to speed up the calibration process by employing a novel active grid are planned in this context.

To enlarge the experimental basis it is planned to refurbish the old X7 cave in order to become operational for UNILAC experiments from 2020 onwards. Meanwhile nuclear spectroscopy experiments are planned to continue at other international facilities.

The VIDEO-2 direction sensitive gamma-detector developed for security applications will be finalized. This novelty will have unprecedented sensitivity and a direction accuracy of <3 degrees, enabling the localization of enhanced gamma radiation by triangulation.

Fig.3: VIDEO-2 gamma radiation localizing system

Selected publications

[1] J. Gerl, M. Gorska, H.J. Wollersheim. Towards detailed knowledge of atomic nuclei - The past, present and future of nuclear structure investigations at GSI, DOI: 10.1088/0031-8949/91/10/103001

[2] E. Clément, A. Gadea and J. Gerl. The AGATA Campaigns at GSI and GANIL. Nuclear Physics News 28, 3 (2018) 16, DOI: 10.1080/10619127.2018.1495478.

[3] G.S. Li et al., Simulated characteristics of the DEGAS γ -detector array, DOI: 10.1016/j.nima.2018.02.062

[4] M.L. Cortés et al., Silicon photomultipliers as readout for a segmented Time-of-Flight plastic detector, DOI: 10.1016/j.nima.2018.05.031

[5] N. Lalovic, et al..Study of isomeric states in 198,200,202,206Pb and 206Hg populated in fragmentation reactions, J. Phys. G 45 No. 3 (2018), DOI:10.1088/1361-6471/aaa9df