No abstract available

[en] For the new high-flux reactor FRM II, the fission fragment accelerator MAFF is under design. MAFF will supply intense mass-separated radioactive ion beams of very neutron-rich nuclei with energies around the Coulomb barrier. A central part of this accelerator is the ion source with the fission target, which is operated at a neutron flux of 1.5x10¹⁴ cm^-2 s^-1. The target consists of typically 1 g of ²³⁵U dispersed in a cylindrical graphite matrix, which is encapsulated in a Re container. To enable diffusion and extraction of the fission products, the target has to be maintained at a temperature of up to 2400 deg. C during operation. It has to stand this temperature for at least one reactor cycle of 1250 h. Comprehensive tests are required to study the long-term behaviour of the involved materials at these conditions prior to operation in the reactor. The present paper gives details of the target conception and the projected tests

[en] Published in summary form only

[en] The elusive ‘thorium isomer’, i.e. the isomeric first excited state of ²²⁹Th, has puzzled the nuclear and fundamental physics communities for more than 40 years. With an exceptionally low excitation energy and a long lifetime it represents the only known candidate so far for an ultra-precise nuclear frequency standard (‘nuclear clock’), potentially able to outperform even today’s best timekeepers based on atomic shell transitions, and promising a variety of intriguing applications. This tutorial reviews the development of our current knowledge on this exotic nuclear state, from the first indirect evidence in the 1970s, to the recent breakthrough results that pave the way towards the realization of a nuclear clock and its applications in practical fields (satellite based navigational systems and chronometric geodesy) as well as fundamental physics beyond the standard model (the search for topological dark matter and temporal variations of fundamental constants). (tutorial)

[en] The 229Th nucleus assumes a unique role in the nuclear landscape for its low-lying isomeric first excited state 229mTh with an excitation energy of 8.338±0.024 eV [1], accessible with modern VUV-laser systems. A nuclear clock based on this thorium isomer holds promise not only to push the limits of high-precision metrology with a fractional uncertainty expected in the range of 10^-19 [2], but also to contribute to dark matter and other fundamental physics research as a novel quantum sensor. It will also be able to contribute to the search for theoretically expected temporal fluctuations of fundamental constants like the fine-structure constant α [3]. The cryogenic Paul-trap experiment currently operated at the LMU Munich is primarily designed for long ion-storage times, allowing for the measurement of the still unknown ionic lifetime of the isomer. The lifetime is expected to be several thousands of seconds and its determination is essential for the realization of a nuclear frequency standard. In a second step, the setup will be a platform for VUV-comb spectroscopy of the 229Th nuclear transition, paving the way towards a first nuclear clock prototype. In this poster, the building blocks of the experimental setup for trapping and sympathetic laser cooling of 229mTh3+ by 88Sr+ are presented and the status of first measurements, such as trapping, storage, and Doppler-laser cooling of 88Sr+, are discussed. This work was supported by the European Research Council (ERC) (Grant agreement No. 856415) and BaCaTec (7-2019-2). [1] Kraemer, S., Moens, J., Athanasakis-Kaklamanakis, M. et al., Observation of the radiative decay of the 229Th nuclear clock isomer, Nature 617, 706–710 (2023) [2] C. J. Campbell et al., Single-Ion Nuclear Clock for Metrology at the 19th Decimal Place, Phys. Rev. Lett. 108, 120802 (2012) [3] E. Peik et al., Nuclear clocks for testing fundamental physics, Quantum Sci. Technol. 6, 034002 (2021)

[en] The 229Th nucleus has the unique property of a very low-lying isomeric first excited state with an excitation energy of only 8.338(24) eV [1], which is addressable with state-of-the-art VUV frequency comb laser systems. Storage in the isolated environment of a cryogenic ion trap will allow for lifetime measurements of the excited isomeric state (expected in the range of a few 10 seconds), precise spectroscopy of the nuclear transition, and eventually the creation of the first nuclear clock with an estimated systematical uncertainty approaching 10^-19 [2]. For loading of 229Th3+ ions into the ion trap, a compact version of the very successful He-buffer-gas-cell ion source used in [3-5] has been designed, built, and commissioned at LMU. The compactness of the setup will allow the installation on the laser table next to the ion trap where 229Th3+ will be trapped and sympathetically cooled by laser-cooled Sr ions. The challenging boundary conditions of 32 mbar He in the buffer gas cell and <10^-8mbar in the ion trap require several stages of differential pumping, which have been implemented. In this contribution, we report on the commissioning of the compact He-buffer-gas-cell ion source, including the demonstration of the fulfilment of the differential pumping requirements, and first experiments towards transferring 229Th3+ to and trapping of 229Th3+ in the ion trap. In addition, we report on efforts to integrate an ablation ion source for 88Sr+ into the ion guide between the buffer gas cell and the ion trap for combined extraction of 229Th3+ and 88Sr+. This work was supported by the European Research Council (ERC grant agreement No. 856415) and BaCaTec (7-2019-2). [1] S. Kraemer et al., Observation of the radiative decay of the 229Th nuclear clock isomer, Nature 617, 706–710 (2023). [2] C. Campbell et al., Single-ion nuclear clock for metrology at the 19th decimal place, Phys. Rev. Lett. 108, 120802 (2012). [3] L. von der Wense et al., Direct detection of the 229Th nuclear clock transition, Nature 533, 47–51 (2016). [4] B. Seiferle et al., Lifetime Measurement of the 229Th Nuclear Isomer, Phys. Rev. Lett. 118, 042501 (2017). [5] B. Seiferle et al., Energy of the 229Th nuclear clock transition, Nature 573, 243–246 (2019).

[en] The target for the Munich Fission Fragment Accelerator (MAFF) consists of typically 1 g of the fission material ²³⁵U in the form of UC₂, dispersed homogeneously in a cylindrical graphite matrix, which is encapsulated in a protective Re container. This special type of target is currently under development. The problems related to its manufacture are discussed. To enable diffusion and extraction of the fission products, the target has to be maintained at a temperature of up to 2700 K during operation. Extensive tests are required to study the long-term behaviour of the involved materials at these conditions. For this purpose a resistively heated high vacuum furnace has been set up, which allows high-temperature heat treatment of target samples for a period of up to 1000 h

[en] At the future MAFF/FRM-II facility in Munich an ion trap system MAFFTRAP will be built. Its goal is to investigate the neutron-rich, in particular heavy, nuclei from thermal neutron fission and fusion of fission products with heavy target nuclei. Basic aims of these investigations are: radioactive beam cooling and purification for nuclear spectroscopy experiments, precise nuclear mass measurements and charged particle spectroscopy 'in-trap'

[en] The target for the Munich Fission Fragment Accelerator consists of typically 1 g of the fission material ²³⁵U dispersed homogeneously in form of UC₂ in a cylindrical graphite matrix, which is encapsulated in a protective Re sheath. Preparation methods for this special type of target are described along with the appropriate diagnostic methods. Results for different types of graphite are presented

[en] High-power, short pulse lasers have emerged in the last decade as attractive tools for accelerating charged particles (electrons, ions) to high energies over mm-scale acceleration lengths, thus promising to rival conventional acceleration techniques in the years ahead. In the first part of the article, the principles of laser-plasma interaction as well as the techniques and the current status of the acceleration of electron and ion beams will be briefly introduced. In particular with the upcoming next generation of multi-PW class laser systems, such as the one under construction for the ELI-Nuclear Physics project in Bucharest (ELI-NP), very efficient acceleration mechanisms for brilliant ion beams like radiation pressure acceleration (RPA) come into reach. Here, ultra-dense ion beams reaching solid-state density can be accelerated from thin target foils, exceeding the density of conventionally accelerated ion beams by about 14 orders of magnitude. This unique property of laser-accelerated ion beams can be exploited to explore the scenario of a new reaction mechanism called ‘fission-fusion’, which will be introduced in the second part of the article. Accelerating fissile species (e.g. ²³²Th) towards a second layer of the same material will lead to fission both of the beam-like and target-like particles. Due to the close to solid-state density of the accelerated ion bunches, fusion may occur between neutron-rich (light) fission products. This may open an access path towards extremely neutron-rich nuclides in the vicinity of the N=126 waiting point of the astrophysical r process. ‘Waiting points’ at closed nucleon shells play a crucial role in controlling the reaction rates. However, since most of the pathway of heavy-element formation via the rapid-neutron capture process (r-process) runs in ‘terra incognita’ of the nuclear landscape, in particular the waiting point at N=126 is yet unexplored and will remain largely inaccessible to conventional nuclear reaction schemes even at next-generation radioactive beam facilities, underlining the attractive perspectives offered, e.g., by ELI-NP