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The Buffer-Gas Cell and the Extraction RFQ for SHIPTRAP

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The Buffer-Gas Cell and the Extraction RFQ for SHIPTRAP The buffer-gas cell and the extraction RFQ for SHIPTRAP Dissertation der Fakultät für Physik der Ludwig-Maximilians-Universität München vorgelegt von Jürgen Benno Neumayr aus Wasserburg am Inn München, den 28.04.2004 1. Gutachter: Prof. Dr. D. Habs 2. Gutachter: Prof. Dr. G. Graw Tag der mündlichen Prüfung: 08.07.2004 ii Summary SHIPTRAP is a new ion trap system for high-precision mass measurements of transuranium recoil-ions from the SHIP facility at GSI. The system consists of a buffer-gas cell to thermalise the incoming ions, an extraction system to separate the ions from the buffer gas, an RFQ buncher to cool and accumulate the ions and a tandem Penning-trap system for isobaric purification and high-precision mass measurements. The buffer-gas cell and the extraction RFQ were developed, assembled and tested within this thesis at the MLL (Maier-Leibnitz Laboratory) in Garching and at the GSI. The main requirements given for the gas-cell/extraction-RFQ system are a fast and effective extraction of the stopped recoil ions. Therefore besides the purity of the system preventing recombination and molecule formation, ensured by the use of only non-organic material inside the cell and a highly efficient gas purification, especially the interaction between guiding electric fields and the gas flow inside the system has been optimised. The design optimisation of the different parts of the system was achieved using simulation programs like the VARJET code (gas flow), the SIMION code (ion trajectories in electric fields), a combination of both and the SRIM code (stopping ranges of ions in matter). In the actual set-up of buffer-gas cell/extraction-RFQ the ions enter the cell through a thin metallic entrance foil (~4 µm thick, Ø 60 mm) and are stopped in the helium buffer gas (~50 – 200 mbar). The thermalised ions are then accelerated by electric fields (~10 V/cm) created by a segmented DC electrode inside the gas cell towards a funnel-shaped ring electrode structure. This funnel guides the ions by the applied RF and DC fields towards the extraction nozzle (Ø 0.6 mm) which acts as an interface between the stopping chamber and the extraction chamber. At the nozzle the buffer gas expands into the extraction chamber (10-1 – 10-3 mbar) creating a supersonic gas jet and dragging the extracted ions. Inside the extraction-RFQ structure the ions are separated from the buffer gas and are cooled during the transport. In order to guarantee an optimum purity, the set-up is bakeable and a hydrocarbon-free pumping equipment is used. For characterisation measurements of the set-up both stable and radioactive ions were used. For off-line measurements stable Er ions were created inside the buffer-gas cell by using a focused pulsed laser beam. These ions were used for field optimisations and the determination of the extraction time of the gas cell. Depending on the field strength mean extraction times of around 5 ms at a He pressure of 50 mbar were achieved. A completely experimental determination of the overall efficiency of the gas- cell/extraction-RFQ system could be achieved using the characteristic α decay of 152Er ions. This isotope was produced via the reaction 121Sb(35Cl,4n)152Er at the MLL and via 116Sn(40Ar,4n)152Er at GSI. The measurements at the MLL were done in a longitudinal configuration, where the ions entered the gas cell along the extraction iii axis. A maximum efficiency, including stopping efficiency and extraction efficiency, of about 8 % could be achieved. The on-line measurements with stable Ag ions were performed in collaboration with the Gießen group and their Ortho-TOF mass spectrometer. Besides an overall efficiency of the gas-cell/extraction-RFQ system of about 4 %, including the transfer efficiency between the extraction RFQ and the TOF spectrometer, the efficiency dependence on the beam intensity stopped in the cell was studied. Space-charge limitations at intensities beyond 2.5·108 ions/s were observed. The measurements at GSI were performed in the actual SHIPTRAP configuration with an ion injection nearly perpendicular (82.5°) to the cell axis. In addition the measurements were done with and without the SHIPTRAP RFQ buncher coupled to the extraction RFQ. These measurements showed an overall efficiency of the gas- cell/extraction-RFQ system of almost 5 %, while together with the RFQ buncher around 3 % in transmission mode and in bunching mode were achieved. With this efficiency the SHIPTRAP gas cell ranges amongst the leading facilities for stopping and cooling of radioactive ions world wide, thus enabling the start of the experimental physics program at the SHIPTRAP facility. iv Zusammenfassung SHIPTRAP ist ein neues Ionenfallensystem für hochpräzise Massenmessungen an Transuranen, die am SHIP-Aufbau an der GSI hergestellt werden. Das System besteht aus einer Puffergas-Zelle zur Thermalisierung der erzeugten Ionen, einem Extraktionssystem, um die Ionen vom Puffergas zu separieren, einem RFQ-Buncher zum Kühlen und Akkumulieren der Ionen und einem Tandem-Penning-Fallensystem zur Trennung des isobaren Untergrundes und zur hochpräzisen Massenmessung. Im Rahmen dieser Dissertation wurden die Puffergas-Zelle und der Extraktions-RFQ entwickelt, gebaut und am MLL (Maier-Leibnitz-Laboratorium) in Garching und an der GSI getestet. Die Hauptanforderung an das System aus Zelle und RFQ ist eine schnelle und effektive Extraktion der gestoppten Ionen. Neben der Reinheit des Systems, die die Vermeidung von Rekombination und Molekülbildung gewährleistet und durch den ausschließlichen Gebrauch von anorganischen Materialien in der Zelle und einer sehr effizienten Gasreinigung erreicht wird, galt es in besonderem Maße das Zusammenspiel zwischen den elektrischen Führungsfeldern und dem Gasfluß im System zu optimieren. Die Optimierung des Designs der verschiedenen Systemteile wurde durch den Einsatz von Simulationsprogrammen erreicht. Dazu gehören VARJET (Gasfluß), SIMION (Ionentrajektorien in elektrischen Feldern), eine Kom- bination beider und SRIM (Ionenstoppverteilungen). Beim aktuellen Aufbau von Zelle und RFQ treten die Ionen durch eine dünne metallische Eintrittsfolie (~4 µm Dicke, Ø 60 mm) in die Zelle ein und werden im Helium-Puffergas (~50 – 200 mbar) gestoppt. Die thermalisierten Ionen werden dann durch elektrische Felder (~10 V/cm), die durch eine segmentierte DC-Elektrode innerhalb der Zelle erzeugt werden, auf eine trichterförmige Ringelektrodenstruktur (Funnel) hin beschleunigt. Dieser Funnel führt die Ionen durch die angelegten Gleich- und Wechselspannungsfelder zur Extraktionsdüse (Ø 0.6 mm), die als Verbindung zwischen Zelle und Extraktionskammer dient. An der Düse expandiert das Puffergas in die Extraktionskammer (10-1 – 10-3 mbar), wobei ein Gasstrahl mit Überschall- geschwindigkeit erzeugt wird, der die Ionen mit sich zieht und extrahiert. Im Extrak- tions-RFQ werden die Ionen vom Puffergas separiert und während des Transportes gekühlt. Um eine optimale Reinheit zu gewährleisten, ist der Aufbau ausheizbar und das verwendete Vakuumsystem kohlenwasserstofffrei. Die Messungen zur Charakterisierung des Aufbaus wurden sowohl mit stabilen, als auch mit radioaktiven Ionen durchgeführt. Für Off-line-Messungen wurden dabei stabile Er-Ionen innerhalb der Zelle mit einem fokussierten und gepulsten Laserstrahl erzeugt. Diese Ionen dienten der Optimierung der Felder und der Ermittlung von Extraktionszeiten aus der Zelle. Je nach Feld- stärke wurden so mittlere Extraktionszeiten von etwa 5 ms bei einem Heliumdruck von 50 mbar erreicht. Eine ausschließlich experimentelle Bestimmung der Gesamteffizienz des Systems Zelle/RFQ konnte mithilfe des charakteristischen Alphazerfalls von 152Er-Ionen er- reicht werden. Dieses Isotop wurde mit den Reaktionen 121Sb(35Cl,4n)152Er am MLL v und 116Sn(40Ar,4n)152Er an der GSI erzeugt. Bei den Messungen am MLL wurde eine longitudinale Ausrichtung verwendet, bei der die Ionen entlang der Extraktionsachse in die Zelle eintraten. Für die Effizienz wurde dabei ein Maximum von 8 % ermittelt, wobei dieser Wert die Stop- und die Extraktionseffizienz enthält. Die On-line-Messungen mit stabilen Silberionen wurden in Zusammenarbeit mit der Giessener Gruppe und ihrem Ortho-TOF-Massenspektrometer durchgeführt. Neben der Bestimmung der Gesamteffizienz für Zelle und RFQ von etwa 4 %, wobei hier die Transfereffizienz zwischen Extraktions-RFQ und dem TOF-Spektrometer beinhaltet ist, wurde die Effizienz in Abhängigkeit von der Intensität des in der Zelle gestoppten Strahles untersucht. Dabei wurden raumladungsbedingte Verluste oberhalb einer Intensität von 2.5·108 Teilchen/s beobachtet. Die Messungen an der GSI wurden in der aktuellen SHIPTRAP-Konfiguration durchgeführt, bei der der Ioneneinschuß nahezu senkrecht zur Extraktionsachse (82.5°) erfolgt. Zusätzlich wurden diese Untersuchungen sowohl ohne, als auch mit dem SHIPTRAP-Buncher gekoppelt an die Extraktionskammer durchgeführt. Diese Untersuchungen zeigten eine Gesamteffizienz von Zelle und RFQ von fast 5 %, während zusammen mit dem Buncher etwa 3 % sowohl im Transmissionsmodus als auch beim Betrieb als Buncher erreicht wurden. Mit diesen Effizienzen reiht sich die SHIPTRAP Zelle in die weltweit führende Gruppe von Systemen ein, die dem Stoppen und Kühlen von radioaktiven Ionen dienen. Ermöglicht wird damit der Beginn für das experimentelle Programm am SHIPTRAP- Aufbau. vi CONTENTS Chapter 1 Introduction.............................................................................................1 1.1. Production of radioactive ions ...................................................................1
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