14 June 2017
MAP-Newsletter Special Edition on CALA
Who isn’t fascinated by the view of the night sky? And, at the same time, as a scientist, to be driven by the desire for a deeper understanding of the connections in the birth and death of countless cosmic objects? The fundamental question here is how, following its hydrogen beginnings, the universe has been enriched with heavier elements in the course of its development.
It is not for nothing that the question of the conditions and mechanisms of the formation of heavy elements in the universe ranks among the most pressing mysteries in physics today. We already have quite clear theoretical ideas about the underlying mechanisms of the cosmic nucleosynthesis of heavy elements by fast neutron capture with subsequent beta decay (‘r-process’).
This is constantly occurring under enormous force in the universe. But the way in which the elements are actually distributed is difficult to understand under Earth-bound conditions. The understanding of this requires experimental data on atomic nuclei which we cannot yet produce experimentally.
To bring the elementary formation in the universe to light, large-scale particle accelerators are in operation – or under construction – worldwide, with costs in the billions. Their goal is to get closer to the key nuclei important to cosmic element formation in the laboratory. Atoms with the ‘magic’ neutron number of N = 126 in the nucleus are the ultimate goal.
But the problem is: on Earth, the conditions are not extreme enough. Thus isotopes with 15 neutrons less than N = 126 currently provide the most extreme limits of nuclear physics accelerator experiments. These nuclei are produced in very small quantities and only for short fractional seconds in the laboratory in order to infer their properties. Considering that a typically 10-20-fold accelerated particle flow is required to achieve a nucleus more ‘exotic’ by just one neutron, it is quickly clear that the r-process nuclei of N = 126 will not be accessible with conventional accelerator technology for a long while.
This is precisely where new possibilities with the use of laser-driven ion acceleration open up – those which will be available in the future at CALA. In addition to the focus of biomedical research, the ‘High-Field (HF) Beamline’ offers optimal conditions for entering new territory in the field of astrophysics.
The basis is a significant difference between laser-accelerated ion pulses and those of conventional accelerator systems: in laser ion acceleration, ion pulses with an unrivalled high particle density – ideally close to the density of solid bodies – can be generated, thus achieving a density many orders of magnitude higher than by conventional acceleration. Thus, a novel nuclear reaction mechanism is possible, where heavy, fissile atomic nuclei are first detached from a thin film and accelerated by means of laser light to energies above the fission barrier of approximately 7 MeV/nucleon. These nuclei then hit a second film. There, the projectile and the target nuclei alternately undergo fission. This results in light and heavy neutron-rich fission fragments, respectively. The decisive factor is that the high projectile particle density also means that the fission fragments have a high density, as a result of which a subsequent fusion, for example, between two light fission fragments, becomes possible. This reaction of two nuclei rich in neutrons now leads into the region of the extremely neutron-rich, astrophysically eminently important nuclei with 126 neutrons in their interior. There are still a number of development steps on the way to achieving this ambitious goal. But by exploiting a unique feature of laser-driven ion acceleration, we are confident that we will approach the goal of a better understanding of the formation of heavy elements in the universe.
In this issue of our newsletter, we would like to introduce you, dear readers, to many other exciting research approaches of the scientists working at CALA. We hope you enjoy reading.
Dr. Peter Thirolf