Investigating the formation of elements in the universe ― Slow ion beams at the GSI experimental storage ring
Chemical elements are produced in the cosmos, e.g. in stellar explosions or on the surface of neutron stars. A key process in the formation of elements is the capture of hydrogen nuclei (protons), which transforms one element of the periodic table into another element. This process takes place at extreme temperatures – albeit at relatively low energies of the particles involved. An international research team has now succeeded in studying proton capture at the experimental storage ring of the GSI Helmholtzzentrum für Schwerionenforschung. The aim was to more precisely determine the probability of proton capture occuring in astrophysical scenarios.The results were published in the journal Physical Review Letters.
In the experiment, the researchers first brought the noble gas xenon to high speeds using the GSI accelerator in order to strip off all the electrons of the atomic shell. The leftover atomic nuclei were then fed into the experimental storage ring ESR and slowed down. The xenon nuclei were then induced to interact with hydrogen nuclei at a material sample known as the gas target, which is built into the ring. This resulted in reactions, in which xenon nuclei captured a proton and were transformed into the heavier element caesium – a process that is also expected in astrophysical scenarios.
In the investigation of these phenomena, the researchers are faced by two challenges, as Dr. Jan Glorius from the GSI Atomic Physics research department explains: "The energy interval, in which the reactions are most likely to occur under astrophysical conditions, is known as the Gamow window. Within the Gamow window, the atomic nuclei possess relatively low energies of the order of several megaelectronvolts or less. In other words: they are rather slow and thus difficult to handle in the intensity required. Furthermore, the cross-section, i.e. the probability of an interaction of both partners involved, dramatically decreases with the energy. Until now, it has been hardly possible to create suitable conditions for such reactions in the laboratory. These are the two main reasons for the fact that experimental data in this area – particularly involving heavy nuclei – has been extremely rare."
"For such an experiment a powerful accelerator system, as the chain of linear accelerator UNILAC and ring accelerator SIS at GSI, is required just to provide the heavy reaction partner as a particle beam. Subsequently, a suitable storage ring has to be available to slow down the beam to the energies of the Gamow window, to permanently store it and to facilitate the reaction with the light partner", states Professor Yuri Litvinov, head of the substantially involved ASTRUm research project at GSI, which is funded by the European Union. "In the case of the experiment conducted, we succeeded in demonstrating that the ESR storage ring — although in fact designed for higher energies — can be used for this purpose." In particular, an extremely good vacuum is necessary in the system. Otherwise, the low energy ions would capture electrons from the residual gas in the storage ring at a high rate and thereby would be lost for the experiment.
The scientists have even gone a step further and make use of this actually undesired effect. Interactions also occur between the xenon nuclei and electrons of the hydrogen gas in the target, and these can be identified from the ensuing X-ray radiation. As this atomic process is not only very dominant but also extremely well understood, it is possible to derive from it the number of potential xenon reaction partners that were available for proton capture. Their different, mass-dependent deflection in the magnetic field of the storage ring enables the newly created caesium nuclei behind the target to be separated from the remaining xenon nuclei and to be measured. From the ratio of potential reaction partners and actual reactions, it is possible to determine the probability of proton capture.
"In addition to improving the experimental technique to attain the lower energies of the heavy collision partner, the experiment also provides important restrictions of the hitherto only theoretically predicted reaction rates used to model the formation of elements", states René Reifarth, Professor of Experimental Astrophysics at the Goethe University in Frankfurt and spokesperson of the experiment. "This experiment makes a decisive contribution to furthering our understanding of nucleosynthesis in the cosmos."
As a result of the success of the experiment, further studies of similar reactions are planned in the coming experiment periods at the ESR. In order to better reproduce the conditions existing in astrophysical scenarios, even unstable elements could be created, then sorted out using the GSI fragment separator and fed into the storage ring. A further sign of progress in this research program is the imminent commissioning of the dedicated low-energy storage ring CRYRING, part of the future international particle accelerator FAIR (Facility for Antiproton and Ion Research), which is currently under construction at GSI. It is particularly suitable for making ion beams available at low energies.
The experiments were conducted within the framework of the SPARC research collaboration (Stored Particles Atomic Physics Research Collaboration), which is part of the FAIR research programme. Equipment used in this project was funded by the Collaborative Research Network of the Federal Ministry of Education and Research. (cp)