Alpha spectroscopy of purified beams of exotic nuclei at the FRS Ion Catcher

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Alpha spectroscopy of purified beams of exotic nuclei at the FRS Ion Catcher © 2024 The Authors. Published by Elsevier B.V. Published version Tortorelli, Nazarena; Reiter, Moritz Pascal; Rink, Ann Kathrin; Purushothaman, Sivaji; Ayet, San Andrés Samuel; Bergmann, Julian; Dickel, Timo; Diwisch, Marcel; Ebert, Jens; Geissel, Hans; Greiner, Florian; Haettner, Emma; Hornung, Christine; Kelic-Heil, Aleksandra; Knoebel, Ronja; Lippert, Wayne; Miskun, Ivan; Moore, Iain D.; Pietri, Stephane; Plaß, Wolfgang R.; Pohjalainen, Ilkka; Prochazka, Andrej; Scheidenberger, Christoph; Takechi, Maya; Thirolf, Peter G.; Weick, Helmut; Winfield, John; Xu, Xiaodong Tortorelli, N., Reiter, M. P., Rink, A. K., Purushothaman, S., Ayet, S. A. S., Bergmann, J., Dickel, T., Diwisch, M., Ebert, J., Geissel, H., Greiner, F., Haettner, E., Hornung, C., Kelic-Heil, A., Knoebel, R., Lippert, W., Miskun, I., Moore, I. D., Pietri, S., . . . Xu, X. (2025). Alpha spectroscopy of purified beams of exotic nuclei at the FRS Ion Catcher. Nuclear Physics A, 1053, Article 122967. https://doi.org/10.1016/j.nuclphysa.2024.122967 2025 Nucl. Phys. A 1053 (2025) 122967 Available online 3 October 2024 0375-9474/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Contents lists available at ScienceDirect Nuclear Physics A journal homepage: www.elsevier.com/locate/nuclphysa Alpha spectroscopy of purified beams of exotic nuclei at the FRS Ion Catcher Nazarena Tortorelli a,c, ,∗, Moritz Pascal Reiterb,f, Ann Kathrin Rinkb, Sivaji Purushothamanc, Samuel Ayet San Andrés c, Julian Bergmann b, Timo Dickel c,b, Marcel Diwisch b, Jens Ebert b, Hans Geissel c,b,1, Florian Greiner b, Emma Haettnerc, Christine Hornungb, Aleksandra Kelic-Heilc, Ronja Knoebelc, Wayne Lippert b, Ivan Miskunb, Iain D. Moore d, Stephane Pietri c, Wolfgang R. Plaßc,b, Ilkka Pohjalainend, Andrej Prochazka c, Christoph Scheidenbergerc,b,e, Maya Takechi c, Peter G. Thirolf a, Helmut Weick c, John Winfieldc,1, Xiaodong Xuc aLudwig-Maximilians-Universität München, 85748 Garching, Germany bII. Physikalisches Institut, Justus-Liebig-Universität Gießen, 35392 Gießen, Germany cGSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany dUniversity of Jyväskylä, 40014 Jyväskylä, Finland eHelmholtz Research Academy Hesse for FAIR (HFHF), GSI Helmholtz Center for Heavy Ion Research, Campus Gießen, 35392 Gießen, Germany fUniversity of Edinburgh, United Kingdom A R T I C L E I N F O A B S T R A C T Keywords: FRS-IC 𝛼-spectroscopy Nuclear structure Half-life The FRS Ion Catcher (FRS-IC) is located at the final focal plane of the Fragment Separator FRS at GSI. The FRS-IC setup is well known for high-precision experiments with stopped exotic nuclei produced by projectile fragmentation and fission. The facility consists of the cryogenic gas-filled stopping cell (CSC), an RFQ-based beamline (DISTRICT), and a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS). This paper illustrates how alpha spectroscopy performed at this facility has emerged as a promising tool to unveil the nuclear structure of exotic nuclei, i.e., halflive and decay energy measurements. First studies of that kind were performed on the decay chains of 218Rn, 219Rn, 221Ac, 220Fr, and 223,224Th produced by projectile fragmentation of 238U. The 𝛼 decay energy measurements performed and the deduced Q𝛼values confirm the known maximum at N= 128 and the values of Q𝛼at N= 132 − 133 follow the predicted increasing in Q𝛼values compared to the values for At isotopes at the same neutron number N. Further, the production rate ratio of the isomer to the ground state of 211Po was measured. It allows an estimate of the angular momentum distribution of 211Po fragments following fragmentation of 238U in a 9Be target at relativistic energies. In addition, the potential of mass-selected decay spectroscopy behind the MR-TOF-MS was demonstrated with short-lived 215Po ions (𝑡1∕2 =1.78 ms). This demonstrates that the FRS-IC is a reliable setup for 𝛼spectroscopy studies and related nuclear structure studies. * Corresponding author. E-mail addresses: [email protected], [email protected] (N. Tortorelli). 1Deceased. https://doi.org/10.1016/j.nuclphysa.2024.122967 Received 26 July 2024; Received in revised form 30 September 2024; Accepted 30 September 2024 Nuclear Physics, Section A 1053 (2025) 122967 2 N. Tortorelli, M.P. Reiter, A.K. Rink et al. 1. Introduction Ever since the discovery of the phenomenon in 1899 by Rutherford [1], and the empirical evidence of the relationship between the half-lives and decay energies in 1911 by Geiger and Nuttall [2], nuclear alpha decay has been used in a large number of applications. The pioneering work by Rutherford studying the interaction of alpha radiation with matter led to the discovery of the atomic nucleus. The role of alpha spectroscopy as an important experimental nuclear physics technique was strengthened with the theoretical explanation of the alpha decay mechanism using quantum-mechanical tunneling by Gamov in 1928 [3]. Starting from the second half of the last century, alpha spectroscopy has been employed in various scenarios as a powerful identification method of radioactive isotopes [4], especially as a diagnostic method in radioactive ion beam facilities. High-resolution alpha spectroscopy can provide a unique fingerprint of an alpha-decaying nuclide in terms of the characteristic alpha energies and branching ratios. The identification of long alpha chains has been heavily used in the search for new super-heavy elements [5]and studies of actinides [6]. The population of direct alpha decaying nuclear isomers has been employed in characterizing the angular momentum in various nuclear reactions: fragmentation [7], fusion [8], and recently multi-nucleon transfer reactions [9]. In addition, the characterization of alpha-decaying nuclides has aided nuclear structure studies in the lead region in combination with high-resolution laser spectroscopy [10]or time-of-flight mass spectrometry [11]. This paper discusses the implementation of an alpha spectroscopy setup at the FRS Ion Catcher (FRS-IC) at GSI [12]. The FRS-IC has been designed for high-precision experiments with stopped exotic nuclei produced by projectile fragmentation and fission. The FRS Ion Catcher consists of the cryogenic gas-filled stopping cell (CSC) [13–15], an RFQ transport and diagnostics beam line (DISTRICT), and a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) [16–18]. Alpha spectroscopy of the stopped exotic nuclei using silicon detectors has been employed as one of the key tools for physics measurements as well as a systematic characterization tool of the FRS-IC. The first studies were performed by investigating the decay chains of 218Rn, 219Rn, 221Ac, 220Fr and 223,224Th produced by projectile fragmentation of 238U. Another key physics result from the FRC-IC employing alpha spectroscopy was the determination of the angular momentum distribution of 211Po following the fragmentation reaction 238U+ 9Be at relativistic energies [19]. Including the mass-selection method shown in that paper, it is possible to perform decay spectroscopy of short-lived nuclides having a parent’s background-free decay, exemplified by the half-life measurement for the very short-lived isotope 215Po (𝑡1∕2 =1.78 ms) which we report here. 2. FRS Ion Catcher (FRS-IC) The FRS-IC is located at the final focal plane of the FRagment Separator FRS at GSI [20]. It is an experimental facility where the advantages of ISOL and in-flight separator techniques are elegantly combined to enable low-energy, high-precision experiments of exotic ions produced at relativistic energies. Ions extracted from the heavy-ion synchrotron SIS-18 impinge on a production target at the FRS to produce projectile or fission fragments. The spatial separation of the fragments is performed by two-fold magnetic rigidity analysis in front of and behind a mono-energetic degrader placed at the central focal plane. The very short flight time of a few hundred nanoseconds through the FRS gives access to very short-lived nuclides and allows isotopes to be identified using the in-flight particle detectors of the FRS on a ion-by-ion (event-by-event) basis. The in-flight separation technique is particularly challenging for nuclei produced at relativistic energies because of the large longitudinal and transverse emittance of the ions after production and slowing down. The ions are first range-bunched [21]using a dispersive magnetic stage and a mono-energetic degrader system to reduce the range spread of the ions by up to an order of magnitude, which makes stopping accordingly more efficient. After production and separation, the ions are then slowed down and finally stopped and thermalized in a gas-filled stopping cell. Typically, the stopping cell is filled with helium gas at a pressure on the order of 100 mbar at cryogenic temperature. The thermalized ions are then guided to the exit side of the cell using static DC electric fields, focused to an exit nozzle using an RF carpet applying AC and DC fields, and extracted as an ion beam with low kinetic energy, see details in [13–15]. The ions extracted from the stopping cell may be pre-filtered for a certain mass-over-charge range of interest via an extraction RF quadrupole (RFQ) operated as quadrupole mass filter (QMF) [22]. The ions then enter a low-energy (∼eV) RFQ beamline which provides first diagnostics tools, calibration ion sources, transport, isolation as well as cooling and trapping; from here on referred to as the DISTRICT module (DISTRICT, Diagnostics, Ion Sources, Transport, Isolation, Cooling and Trapping module). The first alpha spectroscopy devices of the FRS-IC are situated in the first DISTRICT vacuum chamber. DISTRICT is equipped with a remote-controlled translation stage that accommodates two silicon detectors, a channeltron detector and an RFQ segment such that the ions can either be identified and quantified by their alpha-decay energy and half-life or transmitted downstream to the multiple-reflection time of flight mass spectrometer (MR-TOF-MS). The MRTOF-MS itself is a powerful and universal mass spectrometer with single-ion sensitivity, a mass resolving power (FWHM) as high as 106and mass accuracy of low 10−8 [23]. In the MR-TOF-MS, the ions are accumulated, cooled by collisions with helium buffer gas in an RF trap system, and then injected as bunches into the time-of-flight analyzer, where they fly for a certain number of turns until they exit the analyzer and impinge on a time-of-flight detector for mass measurements or a Bradbury-Nielsen gate for mass selection followed by another silicon detector. This is the second alpha spectroscopy device of the FRS-IC. To illustrate this description a sketch of this facility is shown in Fig. 1. However, an in-depth description of the experimental setup and the working principle of the full FRS-IC setup is not within the scope of this work and is described in detail elsewhere [12,15]. The following presentation of the experimental setup will focus on the alpha spectroscopy systems of the FRS-IC. Nuclear Physics, Section A 1053 (2025) 122967 3 N. Tortorelli, M.P. Reiter, A.K. Rink et al. Fig. 1. Schematic layout of the fragment separator FRS together with the FRS Ion Catcher. 3. FRS-IC alpha spectroscopy systems The alpha spectroscopy setup installed in the first DISTRICT module contains two identical detector setups on a remote-controlled translation stage. This redundancy additionally facilitates fast switching between detectors and reduced decay background by switching to a clean detector. The Si detectors used are silicon ion-implanted charged particle detectors manufactured by ORTEC from the ULTRA series with 100 μm depletion depth and 150 mm2active area (Ortec Ultra 𝐵𝑈 − 016 − 150 − 100 bakeable). The detector and its data acquisition (DAQ) system (CAEN DT5780SDM) are optimized to achieve optimum energy resolution, typically sigma between 30 to 60 keV are achieved. In order to characterize the performance and optimize the system offline, an internal source emitting alpha-decaying nuclei is needed. For this purpose, several ion sources have been installed, a 223Ra alpha recoil source with a half-life of ≈11days has been used initially, whereas more recently a 228Th source with a half-life of ≈2years was installed at the entrance side of the CSC. As alpha recoils are typically emitted with ≤100 keV energy, they can easily be stopped in the CSC and delivered to the DISTRICT setup. In addition a 3-line alpha source containing 239Pu, 241Am and 244Cm is installed directly in DISTRICT at an off-axis location. Ions from the internal sources and those from online production are transported to the setup and the energy of the alpha particles emitted can be measured. However, in order to not accumulate long-lived activity directly on the detector surface, an aluminum collection foil with thickness of 100 μg/cm2or 200 μg/cm2is installed in front of each detector. The foils are placed 5mm in front of the detector surface, which limits the maximum solid angle covered by the Si detector. In our case the detector covers a solid angle of 𝑑Ω∕4𝜋=0.21, assuming a point-like implantation spot at the center of the foil. Simulations suggest an implantation spot size of less than 1 mm; thus, the point-like spot is a valid assumption. The performance of the alpha detector permanently installed as part of DISTRICT will be reported in the following paragraph. The alpha detection system located at the end of the MR-TOF-MS will be discussed in Sec. 5. 3.1. Performance characterization Recoiling ions from the 223Ra source were extracted and collected on the foil in front of one of the Si detectors. Fig. 2shows typical alpha energy spectra as obtained with DISTRICT under three different scenarios: no foil installed, 100 μg/cm2and 200 μg/cm2foils installed. Without a foil installed, the setup reaches an alpha energy resolution of about 14 keV in sigma, about a factor of two lower compared to the manufacturer’s specifications. However, with a foil installed the energy resolution of the alpha spectroscopy detectors is dominantly limited by the collection foils themselves, as those cause additional peak broadening due to two effects. Firstly, due to direct energy straggling 𝜎𝑑𝑖𝑟𝑒𝑐𝑡 of the alpha particles penetrating through the foil and, secondly, due to peak broadening 𝜎𝑎𝑛𝑔𝑙𝑒 caused by alpha particles penetrating the foil under different angles given by the solid angle coverage of the detector, which results in a variation of effective thickness when penetrating the foil [24]. As shown in Fig. 2, a peak width of 𝜎𝑒𝑥𝑝 =26keV and 𝜎𝑒𝑥𝑝 =48keV could be achieved for 100 μg/cm2and 200 μg/cm2thick aluminum foils, respectively. The peak broadening by the foil setup was further estimated via SRIM energy loss calculations. In Table 1, the experimental resolution is compared to the estimates. The 𝜎𝑑𝑖𝑟𝑒𝑐𝑡 and 𝜎𝑎𝑛𝑔𝑙𝑒 values are quadratically added to the intrinsic detector resolution of 14 keV achieved without foil, which gives an estimate for the expected total peak width 𝜎𝑡𝑜𝑡𝑎𝑙 to be obtained when using collection foils. The calculated peak widths are in good agreement with the experimental results and confirm the limits set by the foils; no measurement was performed for a foil thickness of 50 μg/cm2. It is further possible to employ even thinner foils, such as e.g. 10 μg/cm2as recently demonstrated in [25]. Nuclear Physics, Section A 1053 (2025) 122967 4 N. Tortorelli, M.P. Reiter, A.K. Rink et al. Fig. 2. Alpha decay spectrum of 219Rn, originating from the internal 223Ra source and measured using the silicon detector of the DISTRICT with Al collection foil thicknesses of 200 μg/cm2and 100 μg/cm2, respectively, placed in front of the detector surface in comparison to the resolution achievable with the bare detector. The measurements are performed under different cleanliness and extraction conditions, which results in different elements to be extracted and transported with different efficiencies. The three datasets were not acquired during identical acquisition times and no normalization was applied, such that the number of counts is plotted in arbitrary units. Table 1 Comparison of the peak broadening induced by aluminum foils placed in front of the silicon detector surface to prevent long-lived activity deposition on the detector. The silicon detector setup has an intrinsic alpha energy resolution of 14 keV. 𝜎𝑑𝑖𝑟𝑒𝑐𝑡 refers to the contribution due to direct energy straggling of the alpha particles penetrating through the foil, while 𝜎𝑎𝑛𝑔𝑙𝑒 takes into account the peak broadening contribution caused by the dependency for the alpha particles on the energy from the penetration angle into the foil. The quadratic sum of these two contributions is given as 𝜎𝑡𝑜𝑡𝑎𝑙 . 𝜎𝑒𝑥𝑝 refers to the experimentally measured energy resolution. Al foil 𝜎𝑑𝑖𝑟𝑒𝑐𝑡 𝜎𝑎𝑛𝑔𝑙𝑒 𝜎𝑡𝑜𝑡𝑎𝑙 𝜎𝑒𝑥𝑝 μg/cm2(keV) (keV) (keV) (keV) 200 8.837 40.948 100 6.118 24.226 50 4.311.319.3However, thicker foils are typically considered more stable and the reliability of the setup in DISTRICT, a standard commissioning and characterization tool, was considered to be more important. As such, foils with a thickness of 100 μg/cm2were chosen for standard operation and were used during all measurements reported in this paper. Under these conditions alpha spectroscopy of separated secondary beams can be performed with energy resolutions of about 60 keV sigma. 4. Optimization of the FRS-IC performance The following section is devoted to offline and online procedures and technical developments to optimize the performance of the FRS Ion Catcher setup. 4.1. Offline optimization of the FRS-IC To prepare the setup to transport and then study the ions of interest in a certain mass range, preparatory offline measurements are required to characterize and optimize individual subsystems. The FRS-IC facility consists of multiple filtering and analysis stages composed of detectors and mass filters [22]. Offline characterization of the ion extraction and detection performance from the gas cell is enabled by the internal alpha recoil ion source installed in the entrance region (attached to the first electrode ring) of the DC electrode system of the CSC. Ions traversing the CSC on off-axis trajectories are guided to the extraction nozzle at the exit side by the RF carpet. The RF carpet provides a DC gradient towards its axial center and a repelling RF potential. Thus, it guides ions towards the exit nozzle, where they are dragged via a supersonic gas jet out of the CSC into a subsequent RFQ, acting as an ion guide and mass filter, with a mass resolving power of 10 (using FWHM), while achieving a transmission efficiency of about 80% [26]. The mass filter allows us to select the mass region of interest or a certain charge state and to break apart the molecular ions during the transport. Depending on the setting of the RFQ as an ion guide or mass filter, the extracted ions arrive at DISTRICT either Nuclear Physics, Section A 1053 (2025) 122967 5 N. Tortorelli, M.P. Reiter, A.K. Rink et al. Fig. 3. Stopping range distribution of 223Th ions in the cryogenic stopping cell determined as a function of the variable degrader thickness. The black solid curve shows a Gaussian fit to the experimental normalized total efficiency. The expected range distribution of a monoenergetic beam in red is shown with straggling in the variable degrader. The hatched region indicates the areal density (AD) covered by the stopping cell and it is arbitrary placed in this plot to indicate the CSC thickness in relation to the momentum spread of the incoming beam. (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.) in a wide or narrow mass range. Here, alpha spectroscopy can be performed using the silicon detectors previously introduced. The alpha spectroscopy is of primary importance, as it enables system optimization through the utilization of radioactive atomic species due to the precise knowledge we possess regarding the generating sources and intrinsic properties of radioactive species. Using this detector, a scan of the mass-over-charge range of a certain species extracted from the CSC can be done. This allows the identification and quantification of the charge-state distribution and as such helps to prepare optimum mass-over-charge settings for the downstream beam line. Furthermore, in the case when there is no alpha emitter in the mass region of interest for the measurement, the identification of masses is done at the MR-TOF-MS once the calibration to the mass filter and the extraction is done with the internal alpha recoil ion source. 4.2. FRS-IC as alpha tagging setup In case the ion of interest stems from a secondary beam with charge 𝑍and mass 𝐴being far away from the primary beam, the in-flight identification based on measuring 𝐴∕𝑄vs 𝑍may become difficult. A non-negligible difference between calibration and operational settings may emerge due to the large difference in energy loss between the primary beam and secondary fragments, rendering the possibility of a wrong assignment of 𝑍. In such a scenario, the FRS-IC can correct the calibration by identifying a single nuclide closer in Z to the ion of interest. This can be done either via “𝛾-tagging” ([27], [28]), via “mass tagging” using precision mass spectrometry with the MR-TOF-MS [29]or as “alpha tagging” using alpha spectroscopy. At the FRS-IC two separate setups have been used for alpha tagging, a single DSSD detector placed in front of the CSC and the silicon detectors within DISTRICT. In later method, the ions are thermalized in the stopping cell and measured using a Si detector in DISTRICT. Based on available alpha decay energies the stopped ions can be identified and the in-flight identification can be updated. The method was used during multiple online experiments and demonstrates the feasibility of using the FRS-IC as a tagger for alpha decaying nuclides with half-lives from few milliseconds extended to few minutes. 4.3. Online optimization of the FRS-IC After completing the isotope identification at the FRS, the following steps have to be taken to maximize the amount of the beam stopped in the CSC so that the maximum beam can then be transported to downstream setups for example to the MR-TOF-MS. The approach involves optimizing the homogeneous degrader of variable thickness (variable degrader) installed just before the CSC, see Fig. 1. By adjusting the thickness of the degrader, it is possible to center the mean range of the beam in the middle of the stopping cell. The ideal thickness depends on the characteristics of the species to be stopped (mainly energy and atomic number Z) and on the density of the helium gas filling the CSC. Depending on the specific experiment, the CSC is filled with helium gas with an areal density from 2to 9mg/cm2. Ions were injected into the CSC, extracted and subsequently sent to DISTRICT, where they were collected on the Si detector setup. Here, they were spectroscopically identified via their characteristic alpha decay energies. To obtain the number of extracted ions of the nuclide of interest, the number of registered decay events, taking the solid angle of the detector into account, were compared to the number of isotopes identified In-Flight via the FRS detectors. This optimization procedure allows to set the degrader to a thickness that maximizes the number of ions stopped in the CSC. Fig. 3illustrates such a stopping range distribution (normalized) of 223Th ions versus the areal thickness of the variable degrader at a CSC areal density of 4.9mg/cm2. The black curve represents a Gaussian fit to the experimental data points; the red curve shows a distribution expected for a monoenergetic beam, while the hatched region indicates the areal density covered by the stopping cell. Nuclear Physics, Section A 1053 (2025) 122967 6 N. Tortorelli, M.P. Reiter, A.K. Rink et al. Fig. 4. Alpha-energy spectra of 221,220Ra produced by 238U projectile fragmentation at 1000 MeV/u at the FRS Ion Catcher, measured with the Si detector in the DISTRICT. Once the optimal settings for stopping in the CSC have been determined, it is possible to scale the degrader thickness to stop arbitrary ions of interest, which indeed might have a different energy and Z. 5. Results In the following section several offline and online experiments will be discussed based on data acquired using the alpha spectroscopy setups. 5.1. Alpha spectroscopy of 238U projectile fragments The alpha tagging method explained in 4.2 has been applied during a study of short-lived nuclides produced by 238U projectile fragmentation at 1000 MeV/u. Combining the separation of the FRS and the selective stopping in the CSC, the decay chains of several decay chains could be measured with the Si detectors in DISTRICT. In Fig. 4, the alpha decay lines of 221Ra and 220Ra and their daughters are shown. The silicon detectors were initially calibrated using the 223Rn decay chain from the internal source inside the CSC and alpha lines from the 3-line alpha source installed at DISTRICT. This showed that, a linear calibration function with only a small quadratic correction, contributing to systematic shift of about <2keV, could be used. In order to determine the alpha energies of the projectile fragments all peaks were fitted with Gaussian and exponentially modified Gaussian (HyperEMG) line shapes [30]. As during the online data collection decays of long-lived 211Bi, previously collected on the foil, were also recorded, those were used to re-calibrate and adjust for gain drifts of the pre-amplifiers through out the experiment. To the uncertainty obtained via the fitting and calibration procedure a systematic uncertainty of 5keV was added accounting for the difference in energy loss in the aluminum foil between different isotopes and alpha energies. The uncertainty due to the recoil distribution of the alpha decaying daughters has not been added since the energy difference was negligible. The new measured Q-alpha values are listed in Table 2. Fig. 5shows the deviation between the measured Q-alpha values from 238U projectile fragments and previous literature. The measured Q-alpha values reproduce known literature values well. The deviation is characterized by a normal distribution, with a Birge ratio of 0.5, and as a consequence no further additional systematic uncertainties were considered. Noteworthy is the measurement of 221Ac, whose 𝑄𝛼measurement was done in the past as part of the decay chain of 225 Pa [6]. There, two close-lying, about 50 keV apart, peaks were identified in the energy spectrum and due to the limited pre-separation capabilities (using a He-jet technique without mass resolution and mass identification) it could not be ruled out which peak should belong to the 221Ac ground state or might belong to different species. Moreover, also the evaluator of NNDC [31]puts the existence of this second alpha line as belonging to the decay of 225 Pa in doubt. However, already in [32], and more recently in [33], suiting mass separated samples, the gs-gs alpha decay was reported with a good precision and the existence of the spurious peak, as belonging to the decay of 225 Pa, was refuted. In the work presented here, the direct In-Flight production and separation combined with the selective stopping in the CSC has been applied and this strongly suppresses neighboring isotopes [34]. In our case there is no identification of a secondary peak in our spectrum. Further, we were able to rule out possible long-lived contamination via a time-resolved alpha measurement, see Sec. 5.2. Our 𝑄𝛼energy of 7778(10) keV is in good agreement with the 𝑄𝛼energy reported in [32]and [33] providing a confirmation of the reliability of the FRSIC data. The 𝑄𝛼values for the isotopic chains of Th, Ac, Ra, Fr, Rn, At and Po are shown in Fig. 6as a function of the neutron number 𝑁 using open symbols. Values of isotopes determined in this work are shown as solid symbols. The well known characteristic maximum of the 𝑄𝛼values around 𝑁= 128 can be seen. The known crossings of the 𝑄𝛼values of the astatine and radon chains at 𝑁= 131 and radium chains at 𝑁= 133 are reproduced in our measurements, where the open symbol (literature values) are overlapped by the Nuclear Physics, Section A 1053 (2025) 122967 7 N. Tortorelli, M.P. Reiter, A.K. Rink et al. Table 2 Alpha decay energies (𝐸𝛼) and Q-alpha values (𝑄𝛼) of radioactive nuclei measured using the FRS Ion Catcher produced online via 238U projectile fragmentation at 1000 MeV/u. ‘Level𝑙𝑖𝑡’ refers to alpha transitions into excited states, energy values were taken from literature [31]. Isotope Level𝑙𝑖𝑡 𝐸𝛼𝑄𝛼 𝐴X (keV) (keV) (keV) 211Po 0 7450 (7) 7595 (7) 211𝑚Po-1 1633 7273(5) 9047 (5) 211𝑚Po-2 0 8877 (13) 211𝑚Po-3 897.8 78000 (28) 212Po 0 8801 (13) 8970 (13) 213Po 0 8387 (46) 8548 (47) 214Po 0 7706 (37) 7853 (38) 216At 0 7792 (16) 7941 (16) 215Rn 0 8692 (19) 8849 (19) 216Rn 0 8055 (15) 8207 (15) 217Rn 0 7750 (26) 7896 (26) 218Rn 0 7143 (26) 7277 (26) 217Fr 0 8313 (10) 8469 (10) 219Ra-1 0 7990 (15) 8135 (11) 219Ra-2 315.8 7674 (13) 220Ra 0 7457 (9) 7595 (9) 221Ac-1 0 7640 (11) 7778 (10) 221Ac-2 209 7432 (13) 223Th 140.0 7297 (11) 7571 (11) 224Th-1 0 7164 (13) 7296 (12) 224Th-2 178.4 6996 (20) Fig. 5. 𝑄𝛼values of 238U fragmentation products measured at the FRS Ion Catcher in comparison to their literature values, taken from [31]. An updated value for 221Ac was recently reported in [33], indicated by the green data point. solid symbol (measured values). A detailed discussion is beyond the scope of this paper, but it demonstrates the potential of FRS-IC alpha spectroscopy system for nuclear structure studies of actinides. 5.2. Half-life measurement of 221Ac and 223Th Combined alpha and half-life measurements of 221Ac and 223Th were preformed. To do so, the DISTRICT data acquisition system was set to record energy and time of each event, whereas the start signal for the clock was given by a trigger signal from the FRS data acquisitions. The spill length / bunch length of the 238U primary beam was set to less then 10 ms. Under those conditions, only the extraction time from the CSC to the silicon detector setup causes additional delays and limits the technique to nuclei with half-lives of above a few tens of ms. In the analysis one can then gate on a specific alpha energy range and evaluate the temporal evolution of those decays. In Fig. 7, the results of the time-resolved decays curves of 223Th and 221Ac are plotted after gating on their characteristic dominant alpha decay branch. The observed pattern can be well described by a Gaussian peak convoluted with an exponential decay function. Nuclear Physics, Section A 1053 (2025) 122967 8 N. Tortorelli, M.P. Reiter, A.K. Rink et al. Fig. 6. The 𝑄𝛼values of the decay of the radio-isotopes measured at the FRS Ion Catcher after their production via projectile fragmentation of 238U at 1 GeV/u are plotted as a function of the neutron number N. The 𝑄𝛼values are represented as solid symbols. The literature values are the open symbols. Fig. 7. Half-life measurement of 223Th and 221Ac. The number of 223Th (221 Ac) decay events is plotted as a function of the time elapsed after the injection of the ions into the CSC. The fit function consists of a Gaussian plus an exponential decay function used to determine the half-life. It considers the initial activity increase dominated by the ion extraction from the CSC and the exponential decay dominating after all ions have been extracted and collected on the foil in front of the silicon detector. The solid line represents the fit to the data. The half-lives of 223Th and 221Ac can then be determined in two ways: (i) by gating directly on the alpha decay line of the isotope as shown in Fig. 7or (ii) by gating on the alpha decay energies of much shorter lived direct daughter isotopes. The method of filtering on the nuclide daughters originates from the need to validate the half-live and identification of 221Ac. For 223Th, it was possible to determine its half-life by filtering on the alpha decay of 223Th and, in addition, filtering the alpha decay of its daughter nuclides 219Ra and 215Rn. Since the daughter nuclides 219Ra and 215Rn have a very short half-life compared to that of 223Th the evolution of their decays is dominated by the slower decay rate of 223Th. Similarly, for 221Ac, it was possible to derive its half-life by filtering the alpha decay of the daughter nuclides 217Fr and 213At. As for the case above, these daughter nuclides have significantly shorter half-lives than 221Ac. This indirect method agrees well with direct determination of the half-life of 221Ac. In the case of 221Ac an abundance of events beyond 300 − 400 ms is not well described by the fit and may have suggested the presence of a long-lived contaminant. However, a similar long-lived component did not appear in the decay patterns of 217Fr and 213At and as such could be ruled out. The half-lives of 223Th and 221Ac determined here, are listed in Table 3. The half-lives of 221Ac is in good agreement with the known literature values from the ENSDF [31] compilation and with a recent result of [33]. Our result for 229Th further reinforce a half-life of around 600 ms as suggested by [35]and [36], which had been in conflict with a half-life of 900(10) ms as obtained by [37]. 5.3. Angular momentum distribution of 211Po Measuring the fraction of the nuclei produced in their ground state and isomeric states can provide helpful information about the angular momentum distribution populated during the fragmentation process. At the FRC-IC, 211Po was chosen as a suitable candidate to be studied in this context. 211Po has a rich level scheme, including multiple high-spin isomeric states. Its first isomeric state with