Mass and life-time measurement of the 1.7ms 215-Po isotope : a crucial test of the novel concept of the cryogenic ion catcher for the Super-FRS at GSI-FAIR

In modern nuclear- and nuclear astrophysics exotic nuclei are central research topics. Especially close to the driplines these nuclei show novel and unexpected properties compared to the well known stable isotopes. The obtained information from these nuclei can explore the knowledge of stellar nucleosynthesis. The international Facility for Anti-proton and Ion Research (FAIR-GSI) has the probability to provide important contributions using the Super Fragment Separator (Super-FRS). Novel experimental concepts for measurements with short-lived nuclei are presented in this thesis together with pilot experiments performed at the current Fragment Separator FRS at GSI. In the context of this doctoral thesis a method to measure very short-lived nuclei has been successfully developed. Therefore projectile fragments have been produced, separated in-flight and thermalised in a cryogenic gas-filled stopping cell. After a fast extraction the stopped projectile fragments are investigated with a high resol- ution mass spectrometer. The ion’s kinetic energy is in the order of several eV for the transport through an RFQ beam line (10−2 mbar) and maximal 1.3 keV in the analyser of the mass spectrometer. To reduce the phase space of the ions, they are either He- or N2-gas cooled. The efficient stopping of a separated exotic ion beam, produced of a 1000 MeV/u 238U projectiles, is challenging due to their large range distribution. Even though a mono-energetic degrader system reduces the range distributions, they are still bigger than the current areal gas density of the cryogenic stopping cell (CSC). An important goal for future experiments is a higher areal density to achieve the complete and thus efficient stopping of the interesting nuclei. Effects like space charge or enlarged extraction and transport times have to be minimized because they are limiting the system’s performance. If space charge is built up inside the buffer gas, the electrical transport field will be attenuated and a decrease in efficiency will be the result. In this work, extraction times of 2 ms have been achieved using a different geometry of the CSC. Important parameter dependencies on gas pressure and electrical fields have been tested with success for the next generation CSC that will be built for the Super-FRS at FAIR. The current CSC shows extraction with full efficiency up to 3 × 1011 He3+ ions produced by the incoming fragments, which corresponds an energy loss equivalent of 104 221Ac ions. Higher beam rates cause a drop in extraction efficiencies. The future CSC will provide more than three orders of magnitude higher rate capability as the current system, which enables experiments with even higher background rates. The obtained results now pave the way for the technical realization of the next generation Ion Catcher for the Super-FRS. Furthermore, a new concept has been developed to perform accurate mass and life-time measurements of the 215Po isotope. The measurements have been per- formed under conditions which are foreseen for the Low-Energy Branch of the Super- FRS [Dickel et al., 2015b]. It has been the first direct mass measurement of the 215Po isotope, even though the mass value is well known due to α-spectroscopy and system- atics. The obtained mass value of the 215Po isotope is (214.9993276 ± 8.07 · 10−05) u, which is in agreement with the literature value given by (214.9994201±2.7×10−6) u. The result of the MR-TOF-MS assisted decay spectroscopy of 215Po performed in course of this work is (1.7735 ± 0.055) ms. MR-TOF assisting decay spectroscopy simplifies the spectroscopy measurement by mass identification and separation. It also denotes that there will be no other decay channels but 215Po and its daughter decays to populate the 215Po decay channel undesirably. Concluding the experimental and simulated results evince that the new concepts for experiments at the Super-FRS are based on solid principles. A completely new generation of accurate measurements of rare isotopes are enabled.Exotische Kerne stehen weltweit im Zentrum der modernen Kern- und nuklearen Astrophysik. Diese Kerne zeigen in der Nähe der Driplinien neue, unerwartete Ei- genschaften im Vergleich zu den gut bekannten stabilen Kernen und geben außer- dem wichtige Informationen zum Verständnis der Synthese der Elemente in den Sternen. Das internationale Zentrum für Antiprotonen und Ionen Forschung (FAIR- GSI) kann mit dem Separator Super-FRS wichtige Beiträge in dieser Forschungs- richtung leisten. Neuartige experimentelle Entwicklungen für Messungen mit kur- zlebigen Kernen am Super-FRS wurden in dieser Arbeit in Pilotexperimenten am bestehenden Fragmentseparator FRS erfolgreich erprobt. Im Rahmen dieser Doktorarbeit wurde erfolgreich an einer neuen experimentellen Methode zur Untersuchung von sehr kurzlebigen Kernen gearbeitet. Dabei wird der im Flug separierte Strahl von Projektilfragmenten in einer mit gekühltem Helium- gas gefüllten Kammer (CSC) zunächst gestoppt und dann möglichst schnell wieder extrahiert, um die interessanten Kerne in Präzisionsmessungen mit einem hochau- flösenden Flugzeitmassenspektrometer zu untersuchen. Die kinetische Energie der Fragmente ist bei diesem Transport und Messungen maximal 1300 keV, wobei der Phasenraum durch Stöße mit Helium- oder Stickstoff-Gas im RFQ-Fallensystem stark reduziert wird. Die Erzeugung und Separation der exotischen Nuklide mit einem 1000 MeV/u 238U- Projektilstrahl am Eingang des FRS stellt eine große Herausforderung für die effiz- iente Abbremsung in der kryogenen Stoppkammer dar, weil die Reichweiteverteilung der Fragmente, trotz Einsatz eines monoenergetischen Degradersystems am FRS, wegen der hohen kinetischen Energie noch breiter ist als die momentan erreichbare Flächendichte des Heliumgases in der CSC. Somit ist es ein Ziel, dass man eine möglichst große Flächendichte im Heliumgas anstreben muss, um die interessanten Kerne vollständig im Gasvolumen abzubremsen. Dabei müssen störende Effekte, wie auftretende Raumladung oder längere Extrak- tionszeiten, vermieden werden, weil sie einen effizienten Transport der gestoppten Ionen verhindern. Ausgebildete Raumladungszonen schwächen die elektrischen Felder zum Extrahieren der Ionen ab, sodass nur noch ein Bruchteil der thermischen Ionen die Stoppzelle verlässt. In dieser Arbeit konnten Extraktionszeiten von 2 ms mit einer neuen Geometrie in der Stoppzelle erreicht werden. Es wurden dabei Abhängigkeiten von wichtigen Parameter, wie Gasdruck und Feldstärken, für die geplante FAIR Stoppzelle erfol- greich getestet. Die gegenwärtige CSC hat noch die volle Extraktionseffizienz bei einem erzeugten Plasma von 3 × 1011 He3+ Ionen im Abbremsgas. Dieser Ionisationsdichte entsprechen etwa einem Energieverlust von 104 221Ac Ionen. Die zukünftige Stoppzelle wird eine um mehr als 1000-fache Ratenfestigkeit haben als die gegenwärtige Anordnung und kann somit auch bei höheren Untergrundraten noch zuverlässig arbeiten. Mit diesen experimentellen Ergebnissen steht nun der tech- nischen Realisierung des zukünftigen Ion-Catchers für die Super-FRS Anlage im Prinzip nichts mehr im Wege. Zur weiteren Demonstration des neuen Konzeptes wurden die Masse und die Halb- wertszeit 215Po Ionen erfolgreich unter Bedingungen gemessen, die für die neue Stop- pzelle [Dickel et al., 2015a] am Super-FRS vorgesehen sind. Die Masse von ein- fach geladenen 215Po1+ Ionen wurde mit der neuen Testanordnung erstmals direkt gemessen, denn die vorhandenen Werte in der Literatur basieren auf Messungen, die mittels α-Spektroskopie gewonnen wurden. Massenbestimmungen, die aus der Zer- fallsspektroskopie abgeleitet wurden, haben stets die schwierige Voraussetzung, dass Anfangs- und Endzustände genau bekannt sein müssen. Der gemessene Massenwert beträgt (214.9993276 ± 8.07 · 10−05) u und ist somit innerhalb der Messfehler in sehr guter Übereinstimmung mit dem Literaturwert von (214.9994201 ± 2.7 × 10−6) u. Die in dieser Arbeit gemessene Lebensdauer von 215Po Ionen beträgt (1.7735 ± 0.055) ms. Die Lebensdauermessung war erst durch die Bedingung der koinzidenten Massenidentifikation ermöglicht. Ohne diese experimentelle Bedingung würde die Bevölkerung von 215Po Ionen als Tochterprodukt von 219Rn Ionen den zeitlichen Verlauf dominieren. Mit den experimentellen und simulierten Ergebnissen dieser Arbeit konnte überzeu- gend gezeigt werden, dass die geplanten Konzepte für die Experimente am Super- FRS auf soliden experimentellen Grundlagen basieren

The science case of the FRS Ion Catcher for FAIR Phase-0

The FRS Ion Catcher at GSI enables precision experiments with thermalized projectile and fission fragments. At the same time it serves as a test facility for the Low-Energy Branch of the Super-FRS at FAIR. The FRS Ion Catcher has been commissioned and its performance has been characterized in five experiments with 238U and 124Xe projectile and fission fragments produced at energies in the range from 300 to 1000 MeV/u. High and almost element-independent efficiencies for the thermalization of short-lived nuclides produced at relativistic energies have been obtained. High-accuracy mass measurements of more than 30 projectile and fission fragments have been performed with a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) at mass resolving powers of up to 410,000, with production cross sections down to the microbarn-level, and at rates down to a few ions per hour. The versatility of the MR-TOF-MS for isomer research has been demonstrated by the measurement of various isomers, determination of excitation energies and the production of a pure isomeric beam. Recently, several instrumental upgrades have been implemented at the FRS Ion Catcher. New experiments will be carried out during FAIR Phase-0 at GSI, including direct mass measurements of neutron-deficient nuclides below 100Sn and neutron-rich nuclides below 208Pb, measurement of β-delayed neutron emission probabilities and reaction studies with multi-nucleon transfer.Peer reviewe

Removal of molecular contamination in low-energy RIBs by the isolation-dissociation-isolation method

Experiments with low-energy rare ion beams often suffer from a large amount of molecular contaminant ions. We present the simple isolation-dissociation-isolation method to suppress this kind of contamination. The method can be applied to almost all types of low-energy beamlines. In a first step, a coarse isolation of the massto-charge ratio of interest is performed, e.g. by a dipole magnet. In a second step, the ions are dissociated. The last step is again a coarse isolation of the mass-to-charge ratio around the ion of interest. The method was tested at the FRS Ion Catcher at GSI with a radioactive ion source installed inside the cryogenic stopping cell as well as with relativistic ions delivered by the synchrotron SIS-18 and stopped in the cryogenic stopping cell. The isolation and dissociation, here collision-induced dissociation, have been implemented in a gas-filled RFQ beamline. A reduction of molecular contamination by more than 4 orders of magnitude was achieved.peerReviewe

A novel method for the measurement of half-lives and decay branching ratios of exotic nuclei

A novel method for simultaneous measurement of masses, Q-values, isomer excitation energies, half-lives and decay branching ratios of exotic nuclei has been demonstrated. The method includes first use of a stopping cell as an ion trap, combining storage of mother and daughter nuclides for variable durations in a cryogenic stopping cell (CSC), and afterwards the identification and counting of them by a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS). We utilized our method to record the decay and growth of the 216Po and 212Pb isotopes (alpha decay) and of the 119m2Sb isomer ( t1/2=850±90 ms) and 119gSb isotope (isomer transition), obtaining half-lives consistent with literature values. The amount of non-nuclear-decay losses in the CSC up to ∼10 s is negligible, which exhibits its extraordinary cleanliness. For 119Sb isotopes, we present the first direct measurements of the mass of its ground state, and the excitation energy and decay branching ratios of its second isomeric state (119m2Sb). This resolves discrepancies in previous excitation energy data, and is the first direct evidence that the 119m2Sb isomer decays dominantly via γ emission. These results pave the way for the measurement of branching ratios of exotic nuclei.peerReviewe

Isomer studies in the vicinity of the doubly-magic nucleus Sn-100: Observation of a new low-lying isomeric state in Ag-97

Long-lived isomeric states in Ag-97 and In101-109 were investigated with the FRS Ion Catcher at GSI. In the isotope Ag-97, a long-lived (1/2(-)) isomeric state was discovered, and its excitation energy was determined to be 618(38) keV. This is simultaneously the first discovery of a nuclear isomeric state by multiple-reflection time-of-flight mass spectrometry. The measured excitation energies were compared to large-scale shell-model calculations, which indicated the importance of core excitation around Sn-100. Furthermore, advanced mean-field calculations for the Ag-97 nucleus and relevant neighboring nuclei were performed, which have contributed to a better understanding of the repetitive appearance of certain isomeric structures in neighboring nuclei, and which have supported the discovery of the isomeric state in Ag-97 in a global shell-evolution scheme. (C) 2020 The Author(s). Published by Elsevier B.V

Mass tagging:Verification and calibration of particle identification by high-resolution mass measurements

The access to exotic nuclei at radioactive ion beam facilities is crucial for the state of the art research across several fields of physics such as in nuclear structure, the understanding of fundamental interactions and nuclear astrophysics. The particle identification is of high importance, besides the challenging production of these rare and short-lived nuclei. At in-flight facilities, the particle identification is based on measuring the time-of-flight, energy-deposition and magnetic rigidity. These quantities are calibrated to convert them into A/Q and Z of the ions. To ensure a correct calibration, the unambiguous identification, also called tagging, of one species is necessary. Here, we present a novel tagging method by high-resolution mass measurements using an MR-TOF-MS after thermalization of the ions in a cryogenic stopping cell. The method was successfully established and tested at the fragment separator FRS at GSI with the FRS Ion Catcher in experiments using different FRS operation modes.</p

Studying Gamow-Teller transitions and the assignment of isomeric and ground states at N=50

Direct mass measurements of neutron-deficient nuclides around the N = 50 shell closure below 100Sn were performed at the FRS Ion Catcher (FRS-IC) at GSI, Germany. The nuclei were produced by projectile fragmentation of 124Xe, separated in the fragment separator FRS and delivered to the FRS-IC. The masses of 14 ground states and two isomers were measured with relative mass uncertainties down to 1 x 10-7 using the multiple-reflection time-of-flight mass spectrometer of the FRS-IC, including the first direct mass measurements of 98Cd , 97Rh. A new QEC = 5437 +/- 67 keV was obtained for 98Cd, resulting in a summed Gamow-Teller (GT) strength for the five observed transitions (0+ --> 1+) as B(GT) = 2.94+0.32 -0.28. Investigation of this result in state-of-the-art shell model approaches accounting for the first time experimentally observed spectrum of GT transitions points to a perfect agreement for N = 50 isotones. The excitation energy of the long-lived isomeric state in 94Rh was determined for the first time to be 293 +/- 21 keV. This, together with the shell model calculations, allows the level ordering in 94Rh to be understood.(c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/). Funded by SCOAP3.Peer reviewe