Giersch-Excellence-Award 2015 for Paul Scharrer

Paul Scharrer, PhD student in the SHE Chemistry Research Section of the Helmholtz Institute Mainz (HIM) has been awarded "Giersch Excellence Award" for outstanding scientific work in the past years and is invited to join the Graduiertenschule Giersch.

Paul Scharrer in front of the gas-stripper section of the UNILAC accelerator at GSI.
Photo: Ch. Düllmann / GSI

The topic of his thesis project is the fundamental investigation of electron stripping processes of slow heavy ions in gaseous media. Besides being of key importance for the study of superheavy elements in gas-filled recoil separators like TASCA at GSI, which was successfully used for the identification of elements 114, 115, and 117 as well as for sensitive searches for the new elements 119 and 120, such processes are exploited to produce highly charged ions suitable for heavy ion beam acceleration at GSI and at the future FAIR accelerator facility. The focus of Paul Scharrer's work is on the electron stripping of heavy projectiles used as heavy ion beams at GSI and at heavy ion accelerator centers around the world. Typically, projectiles like 238U are initially produced in a comparatively low charge state (4+ at GSI), which is not well suited for acceleration to high energies. Therefore, after having reached 1.4 MeV/u in the first accelerator stage, the projectiles pass through a gas-filled region, where they are stripped of electrons, which increases their charge state.

Together with his colleagues from the SHE Chemistry Department at GSI and HIM and the Linac and Operations (L&O) Department within the FAIR@GSI division, Paul Scharrer developed a new gas stripper setup, which exploits the low duty cycle of the FAIR facility. The new setup employs pulsed gas injection, delivering gas only while beam is passing. This allowed reducing the gas load dramatically, allowing for significantly higher gas densities during beam passage to be achieved, despite the limited pumping capacity and the strict vacuum requirements in the adjacent accelerator sections. Furthermore, the new setup allows use of any gas, unlike the previously used stripper, which was exclusively based on N2. Guided by theoretical studies from Prof. V. Shevelko from the Lebedev Physical Institute in Moscow, Russia, who was a HIM Visiting Fellow for several months in 2013-2015 to support the work, systematic studies showed a pulsed hydrogen-based stripper to be superior. The efficient stripping process in hydrogen gas allowed achieving a new record 238U28+ intensity at the UNILAC, exceeding the previous highest values by more than 50%, and already reaching more than 65% of the FAIR design beam brilliance. Besides the perspective to achieve yet higher average charge states for most of the ion species at higher H2-density, the new setup offers opportunities for operation as a pulsed stripper, where every pulse can be tailored to different projectiles from the two ion source terminals feeding this accelerator line. "This significantly enhances the versatility of the UNILAC accelerator and is also a critical step towards the FAIR facility" explains Dr. Winfried Barth from GSI's L&O department. Christoph Düllmann, professor at Johannes Gutenberg University Mainz and head of the SHE Chemistry department at GSI and HIM adds "Paul's work highlights the close connection of basic research like studies on production and properties of superheavy elements, and technical advances that arise, sometimes in fields that appear rather remote at first glance"