Muonic atom spectroscopy is a method that can determine absolute nuclear charge radii with typical relative precision of $$10^{-3}$$ . Recent developments have enabled to extend muonic atom spectroscopy to microscopic target quantities as low as $$5\,{\upmu }\text {g}$$ . This substantial reduction from the traditional limit of the order of $$100\,\text {mg}$$ is based on a transfer mechanism in a high-pressure hydrogen gas cell, which transports the muon to the surface of the target material rather than stopping it over a broad depth range. This approach enables the measurement of absolute nuclear charge radii of long-lived radioactive isotopes (half-life above $$\sim$$ 20 years), but the production of appropriate targets for the technique has presented some major challenges, such as the formation of organic layers on the substrate. This study presents a systematic investigation of the stopping efficiency for different target preparation methods: ion implantation, drop-on-demand printing, and molecular plating. Notable differences between the three methods were discovered in terms of their performance allowing to further fine tune the method of choice for future target preparations. Our findings show that implantation provides appropriate targets for our method with negligible losses. This achievement opens the landscape of potential measurements to isotopes where high mass separation is required not achievable with other methods. Furthermore, molecular plated targets performed substantially better than those prepared using drop-on-demand printing.

Measured binary quasifission mass spectra in reactions with actinide nuclides show a large peak in yield near the doubly-magic 208Pb. This has generally been attributed to the enhanced binding energy of 208Pb causing a valley in the potential energy surface, attracting quasifission trajectories. To investigate this interpretation, binary quasifission mass spectra and cross-sections have been measured at near-barrier energies for reactions of 50Ti with actinide nuclides from 238U to 249Cf. Cross-sections have also been deduced for sequential fission (a projectile-like nucleus and two fragments from fission of the complementary target-like nucleus). Binary cross-sections fall from ∼70% of calculated capture cross-sections for 238U to only ∼40% for 249Cf, with a compensating increase in sequential fission cross-sections. The data are consistent with the 208Pb peak originating largely from sequential fission of heavier fragments produced in more mass-asymmetric primary quasifission events. These are increasingly suppressed as the heavy quasifission fragment mass increases above 208Pb. The important role of sequential fission calls for re-interpretation of quasifission characteristics and dynamics in superheavy element synthesis reactions.

We have revisited the region of the actinides in the vicinity of the neutron number N=152 and conducted high-precision mass measurements using the newly implemented Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) technique. The masses of ^244 Pu and ^249 Cf were found to deviate by 5.4 and 2.9 combined sigmas, respectively, from our previous results published in 2014. This indicates the presence of systematic errors in the earlier measurements. Consequently, we decided to remeasure all the nuclides from our 2014 study, along with ^248 Cm, to ensure accuracy and reliability. With our greatly improved apparatus, we have measured the masses of ^244 Pu, ^241 Am, ^243 Am, ^248 Cm, and ^249 Cf, using ^208 Pb and ^238 U as mass references. The masses of these reference ions were recently determined with ultra-high precision at Pentatrap. Our results were implemented in the latest Atomic Mass Evaluation (AME), showing good consistency. The region related to the masses measured in this study, especially for isotopes near the N=152 deformed shell gap, is discussed in terms of two-neutron separation energies, first excited 2 ^+ energy levels and their differentials, as well as δ V_pn values, the average proton-neutron interaction of the most loosely bound two nucleons.

The radioactive nuclide 225Ac is one of the few promising candidates for cancer treatment by targeted- α -therapy, but worldwide production of 225Ac faces significant limitations. In this work, the Isotope Separation On-Line method was used to produce actinium by irradiating targets made of uranium carbide and thorium carbide with 1.4-GeV protons. Actinium fluoride molecules were formed, ionized through electron impact, then extracted and mass-separated as a beam of molecular ions. The composition of the mass-selected ion beam was verified using time-of-flight mass spectrometry, α - and γ -ray decay spectrometry. Extracted quantities of ^225Ac^19F_2^+ particles per C of incident protons were 3.9(3)× 10^7 from a uranium carbide target and 4.3(4)× 10^7 for a thorium carbide target. Using a magnetic mass separator, the long-lived contamination 227 Ac is suppressed to <5.47× 10^-7 (95 A ^-1 h ^-1 of directly produced ^225Ac^19F_2^+ .

Muonic atom spectroscopy–the measurement of the x rays emitted during the formation process of a muonic atom–has a long standing history in probing the shape and size of nuclei. In fact, almost all stable elements have been subject to muonic atom spectroscopy measurements and the absolute charge radii extracted from these measurements typically offer the highest accuracy available. However, so far only targets of at least a few hundred milligram could be used as it required to stop a muon beam directly in the target to form the muonic atom. We have developed a new method relying on repeated transfer reactions taking place inside a 100 bar hydrogen gas cell with an admixture of 0.25 g gold target.

The phase-imaging ion cyclotron resonance technique (PI-ICR) has been implemented at TRIGA-Trap together with a newly built five-pole cylindrical trap. In PI-ICR the total phase of trapped ions is measured by projecting the ion motion onto a position-sensitive delay-line micro-channel plate detector. The systematic uncertainties have been investigated and first mass measurements on stable Pb isotopes have been performed with PI-ICR. The new technique offers higher mass-resolving power, allows checking for the presence of contaminant ion species, and it proved useful in tuning the harmonicity of the trapping potential as well as in aligning the trap symmetry axis with respect to the magnetic field axis by visualizing the radial ion motion. This is a non-scanning technique where every detected ion contributes equally, therefore it is more sensitive than the previously used time-of-flight ion-cyclotron-resonance (ToF-ICR) technique, which is based on the scanning of the sideband-frequency of trapped ions and recording their time of flight after ejection. It will enable us to carry out high-precision mass measurements in the actinide region with uncertainties on the ppb level.

Laser resonance ionization spectroscopy was performed on the rare einsteinium isotopes $^{253\ensuremath{-}255}\mathrm{Es}$ at the RISIKO mass separator in Mainz. With low sample sizes ranging down to femtograms, the prominent 352 nm-ground-state transition was measured in all three einsteinium isotopes, and four additional ground-state transitions were measured in $^{254}\mathrm{Es}$. Hyperfine-structure analysis resulted in assigned spin values of $I(^{254}\mathrm{Es})=7$ and $I(^{255}\mathrm{Es})=7/2$. From the extracted coupling constants, nuclear magnetic dipole moments of ${\ensuremath{\mu}}_{I}(^{254}\mathrm{Es})=3.42(7)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{N}$ and ${\ensuremath{\mu}}_{I}(^{255}\mathrm{Es})=4.14(10)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{N}$ as well as spectroscopic electric quadrupole moments of ${Q}_{s}(^{254}\mathrm{Es})=9.6(1.2)\phantom{\rule{0.16em}{0ex}}e\mathrm{b}$ and ${Q}_{s}(^{255}\mathrm{Es})=5.1(1.7)\phantom{\rule{0.16em}{0ex}}e\mathrm{b}$ were derived. Our value for $^{254}\mathrm{Es}$ deviates from the value of $|{\ensuremath{\mu}}_{I}(^{254}\mathrm{Es})|=4.35(41)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{N}$ extracted from the angular anisotropy of $\ensuremath{\alpha}$-radiation emitted by $^{254}\mathrm{Es}$.

The ECHo experiment aims at determining the effective electron neutrino mass by analyzing the endpoint of the 163 Ho electron capture spectrum. High energy resolution detectors with a well-tailored detector response are the essential ingredient for the success of the ECHo experiment. Metallic magnetic calorimeter arrays enclosing 163 Ho have been chosen for the ECHo experiment. The first MMC array, ECHo-1k, showed excellent performances with an average energy resolution of 5.5 eV FWHM @ 5.9 keV. Based on the results obtained with the ECHo-1k array, optimization studies have paved the way towards a new detector design for the next experimental phase, ECHo-100k. The ECHo-100k chip features an optimized single pixel design to improve the detector performance as well as an upgraded on-chip thermalization layout. The newly fabricated ECHo-100k detectors have been fully characterized at room temperature, at 4 K and at millikelvin temperature. The obtained results show that the ECHo-100k array achieved the expected performance with an average energy resolution of 3.5 eV FWHM @ 5.9 keV, fulfilling the requirements for the ECHo-100k experimental phase.

The heaviest elements can exclusively be produced in actinide-target based nuclear fusion reactions with intense heavy-ion beams. Ever more powerful accelerators deliver beams of continuously increasing intensity, which brings targets of current technology to their limits and beyond. We motivate efforts to produce targets with improved properties, which calls for a better understanding of targets produced by molecular plating, the current standard method. Complementary analytical methods will help shedding more light on their chemical and physical changes in the beam. Special emphasis is devoted to the aspect of the optimum target thickness and the choice of the backing material.

We introduce a novel thick-target concept tailored to the extraction of refractory 4d and 5d transition metal radionuclides of molybdenum, technetium, ruthenium and tungsten for radioactive ion beam production. Despite the more than 60-year old history of thick-target ISOL mass-separation facilities like ISOLDE, the extraction of these most refractory elements as radioactive ion beam has so far not been successful. In ordinary thick ISOL targets, their radioisotopes produced in the target are stopped within the condensed target material. Here, we present a concept which overcomes limitations associated with this method. We exploit the recoil momentum of nuclear reaction products for their release from the solid target material. They are thermalized in a carbon monoxide-containing atmosphere, in which volatile carbonyl complexes form readily at ambient temperature and pressure. This compound serves as volatile carrier for transport to the ion source. Excess carbon monoxide is removed by cryogenic gas separation to enable low pressures in the source region, in which the species are ionized and hence made available for radioactive ion beam formation. The setup is operated in batch mode. Initially, we investigate the feasibility of the approach with isotopes of more than 35s half-life. At the cost of reduced efficiency, the concept could also be applied to isotopes with half-lives of at least one to 10s. We report parameter studies of the key processes of the method, which validate this concept and which define the parameters for the setup. This would allow for the first time the extraction of radioactive molybdenum, tungsten and several other transition metals at thick-target ISOL facilities.

Measurements of mass and angular distributions have been made for fission-like outcomes in reactions forming isotopes of flerovium (Z=114), using 48Ca, 50Ti, and 54Cr projectiles. The dominant fast quasifission process, which masks the presence of fusion-fission, has minimum yield at the most backward angles, where the sensitivity to fusion-fission is thus highest. In fitting the backward angle mass spectra, only weak evidence for a component of super-asymmetric fission was found, but a near-symmetric fission component was consistently required for the 48Ca + 244Pu reaction, giving upper limit to the fusion probabilities PCN of ∼10−2, ∼5 times lower than previous results. PCN for the 50Ti reaction was lower than 48Ca, whilst no evidence of fusion-fission was found for the 54Cr reaction.

The SHE departments devoted to the research of superheavy elements, operate the recoil separators SHIP and TASCA and their ancillary installations including SHIPTRAP and a laser spectroscopy setup at SHIP as well as chemistry and nuclear spectroscopy setups at TASCA. In 2019, the activities at GSI focused on the UNILAC beamtime within the FAIR Phase-0 program and on the analysis of data obtained in prior beamtimes. At HIM, the advancement of actinide sample preparation, manipulation, and characterization for various applications was most central. In addition, technical developments, for example for single-ion mass measurements, have been performed.

Direct high-precision mass spectrometry of the heaviest elements with SHIPTRAP, at GSI in Darmstadt, Germany, requires high efficiency to deal with the low production rates of such exotic nuclides. A second-generation gas stopping cell, operating at cryogenic temperatures, was developed and recently integrated into the relocated system to boost the overall efficiency. Offline measurements using 223Ra and 225Ac recoil-ion sources placed inside the gas volume were performed to characterize the gas stopping cell with respect to purity and extraction efficiency. In addition, a first online test using the fusion-evaporation residue 254No was performed, resulting in a combined stopping and extraction efficiency of 33(5)%. An extraction time of 55(44)ms was achieved. The overall efficiency of SHIPTRAP for fusion-evaporation reaction products was increased by an order of magnitude to 6(1)%. This will pave the way for direct mass spectrometry of heavier and more exotic nuclei, eventually in the region of superheavy elements with proton numbers Z⩾104.

We report on the successful coupling of the Penning-trap mass spectrometry setup TRIGA-TRAP to the research reactor TRIGA Mainz. This offers the possibility to perform direct high-precision mass measurements of short-lived nuclei produced in neutron-induced fission of a 235U target located near the reactor core. An aerosol-based gas-jet system is used for efficient transport of short-lived neutron-rich nuclei from the target chamber to a surface ion source. In conjunction with new ion optics and extended beam monitoring capabilities, the experimental setup has been fully commissioned. The design of the surface ion source, efficiency studies and first results are presented.

Background: Cross sections for the formation of superheavy elements (SHE) by heavy ion fusion are suppressed by the competing quasifission process. This results in a fissionlike decay after capture but before formation of a compact compound nucleus. Fast quasifission is evident from very mass-asymmetric fission, focused in angle. In contrast, slow quasifission shows no significant mass-angle correlation, and a mass distribution peaked at symmetry. However, it shows angular distributions more anisotropic than those calculated for fission following fusion. Following fusion, low excitation energies should increase SHE survival through reduced competition from fission. However, in reactions with deformed actinide target nuclei, subbarrier fusion is highly suppressed by both fast and slow quasifission. Purpose: To investigate the threshold for quasifission by investigating signatures of slow quasifission in both fission angular and mass distributions, as a function of beam energy with respect to the capture barrier, for the projectiles 9 Be, 12 C , and 16 O that form the neighboring compound nuclei 258 , 260 No. Methods: Fission mass and angular distributions have been measured from below to above-barrier energies using the kinematic coincidence method for the reactions 9 Be + 249 Cf, 12 C + 248 Cm, and 16 O + 244 Pu. Fission following transfer reactions can significantly contaminate fission events that follow capture, and must be rejected. Existing methods to reject transfer-induced fission have been refined to allow quantitative subtraction of the transfer fission component. Results: The capture-fission mass-angle distributions show no evidence for fast quasifission, as might be expected. However, measured fission fragment angular anisotropies are larger than transition state model (TSM) calculations for fusion fission. The deviations increase with larger projectile charge and for bombarding energies below the mean capture barrier energy. Even for the 9 Be + 249 Cf reaction, the subbarrier angular anisotropy significantly exceeds the TSM calculation. Fission mass distributions measured at the same excitation energies also show a consistent dependence on the projectile charge. Conclusions: New refined analysis techniques have been developed to enable reliable separation of fission following capture from sequential fission following transfer reactions. For fission following capture at above-barrier energies, the 9 Be angular anisotropies are close to the TSM predictions, supporting the validity of TSM calculations of fusion-fission for such heavy elements. At subbarrier energies the angular anisotropy data indicate a component of slow quasifission even for 9 Be, and a probability that increases rapidly with projectile charge. It is concluded that the probability of slow quasifission changes smoothly with projectile charge, having no sharp threshold.

A good knowledge of the radiative capture cross section of 242Pu is required for innovative nuclear reactor studies, especially for MoX fuel reactors. However, the experimental data available show discrepancies in the energy regions of interest: the thermal point and the keV region. Previous experimental results of the thermal cross section deviate from each other by 20% and these discrepancies are reflected also in the evaluated libraries, each of them giving more credit to different data sets. A recent measurement by Genreith et al. did not succeed to solve the existing discrepancy due to the large uncertainties and correction factors in the analysis. This work presents a new measurement of the thermal capture cross section of 242Pu carried out in the Budapest Research Reactor using the same thin targets of a previous measurement at n_TOF-EAR1, each containing 30mg of 99.995% pure 242Pu . The combined analysis of the full prompt $ \gamma$-ray spectrum and the 243Pu decay has led to three compatible values for the thermal cross section. Their average value, 18.9(9)b, has an improved accuracy compared to recent measurements. Leaving aside the activation value of Genreith using an outdated intensity value for the 84 keV decay line of 243Pu , our average result is in very good agreement with the JEFF-3.2 evaluation and all the previous measurements, with the exception of the highest value 22.5(11)b reported by Marie et al., which has a strong influence in the ENDF evaluation.

J. Khuyagbaatar,1,2,* A. Yakushev,2 Ch. E. Düllmann,1,2,3 D. Ackermann,2,† L.-L. Andersson,1 M. Asai,4 M. Block,2 R. A. Boll,5 H. Brand,2 D. M. Cox,6,‡ M. Dasgupta,7 X. Derkx,1,3 A. Di Nitto,3 K. Eberhardt,1,3 J. Even,1,§ M. Evers,7 C. Fahlander,8 U. Forsberg,8 J. M. Gates,9 N. Gharibyan,10 P. Golubev,8 K. E. Gregorich,9 J. H. Hamilton,11 W. Hartmann,2 R.-D. Herzberg,6 F. P. Heßberger,1,2 D. J. Hinde,7 J. Hoffmann,2 R. Hollinger,2 A. Hübner,2 E. Jäger,2 B. Kindler,2 J. V. Kratz,3 J. Krier,2 N. Kurz,2 M. Laatiaoui,2 S. Lahiri,12 R. Lang,2 B. Lommel,2 M. Maiti,12,‖ K. Miernik,5 S. Minami,2 A. Mistry,6,¶ C. Mokry,1,3 H. Nitsche,9,** J. P. Omtvedt,13 G. K. Pang,9 P. Papadakis,6,14 D. Renisch,3 J. Roberto,5 D. Rudolph,8 J. Runke,2 K. P. Rykaczewski,5 L. G. Sarmiento,8 M. Schädel,2,4 B. Schausten,2 A. Semchenkov,13 D. A. Shaughnessy,10 P. Steinegger,15,16 J. Steiner,2 E. E. Tereshatov,10,†† P. Thörle-Pospiech,1,3 K. Tinschert,2 T. Torres De Heidenreich,2 N. Trautmann,3 A. Türler,15,16 J. Uusitalo,14 D. E. Ward,8 M. Wegrzecki,17 N. Wiehl,1,3 S. M. Van Cleve,5 and V. Yakusheva1 1Helmholtz Institute Mainz, 55099 Mainz, Germany 2GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany 3Johannes Gutenberg-Universität Mainz, 55099 Mainz, Germany 4Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 5Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 6University of Liverpool, Liverpool L69 7ZE, United Kingdom 7The Australian National University, Canberra, ACT 0200, Australia 8Lund University, 22100 Lund, Sweden 9Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 10Lawrence Livermore National Laboratory, Livermore, California 94551, USA 11Vanderbilt University, Nashville, Tennessee 37235, USA 12Saha Institute of Nuclear Physics, Kolkata 700064, India 13University of Oslo, 0315 Oslo, Norway 14University of Jyväskylä, 40351 Jyväskylä, Finland 15Paul Scherrer Institute, 5232 Villigen, Switzerland 16University of Bern, 3012 Bern, Switzerland 17The Institute of Electron Technology, 02-668 Warsaw, Poland

We report on the development and characterization of the first radioactive boron beams produced by the isotope mass separation online (ISOL) technique at CERN-ISOLDE. Despite the long history of the ISOL technique which exploits thick targets, boron beams have up to now not been available. This is due to the low volatility of elemental boron and its high chemical reactivity which make the definition of an appropriate production target unit difficult. In addition, the short half-lives of all boron radioisotopes complicate tracer release studies. We report here on dedicated offline release studies by neutron capture and alpha detection done with implanted 10B in prospective target materials, as well as molecule formation and ionization tests, which suggested the use of multiwalled carbon nanotubes (CNT) as target material and injection of sulfur hexafluoride SF6 to promote volatile boron fluoride formation. Two target units equipped with an arc discharge electron impact ion source VADIS coupled to a water cooled transfer line to retain non-volatile elements and molecules were subsequently tested online. The measured yield of these first 8B ISOL beams increases in the series ${}^{8}\mathrm{BF}_{3} < {}^{8}\mathrm{BF} < {}^{8}\mathrm{B} < {}^{8}\mathrm{BF}_{2}$, reaching a maximum yield of $ 6.4 \times 10^{4}$8BF2+ ions per μC of protons.

The production of selenium ion beams has been investigated at the CERN-ISOLDE facility via two different ionization methods. Whilst molecular selenium (SeCO) beams were produced at ISOLDE since the early 1990s, recent attempts at reliably reproducing these results have so far been unsuccessful. Here we report on tests of a step-wise resonance laser ionization scheme for atomic selenium using the ISOLDE Resonance Ionization Laser Ion Source (RILIS). For stable selenium an ionization efficiency of 1% was achieved. During the first on-line radioisotope production tests, a yield of $ \approx 2.4 \times 10^4$ ions/μC was measured for 71Se+, using a ZrO2 target with an electron impact ion source. In parallel, an approach for extraction of molecular carbonyl selenide (SeCO) beams was tested. The same ion source and target material were used and a maximum yield of $ \approx 3.6\times 10^5$ ions/μ C of 71SeCO+ was measured.