Almost all the elements that we find on Earth and in the universe today were created by nuclear reactions in stars. Light elements are “baked together” by fusion to form heavier elements. However, this no longer works for elements that are heavier than iron. How and where exactly the heavy elements were formed in the universe is not yet fully understood. However, we already know that a variety of processes play a role.
Large microscopes
To better understand the processes involved in the formation of elements in stars, a similar state can be created for a short time in the laboratory using a particle accelerator. A large detector system (here MINIBALL @ CERN, Geneva) can be used to detect the particles and γ-rays that are produced. This provides a microscope with which one can observe the formation of the elements.
Targets
The nuclear reactions are usually generated in thin foils, the “targets”. The particle beam interacts with the target. Shown here is a foil containing deuterium. Deuterium nuclei consist of a proton and a neutron. As in the star, the neutron can be used to create a new element.
Semiconductor silicon counter
The reaction with deuterium leaves a proton, which can be precisely measured with such silicon detectors. The speed and direction of the proton can be used to learn exactly how the nuclear reaction took place. Here, hair-thin signal cables are being applied to the detector.
Let there be light!
Shown here are light waveguides. This technique is used to indicate exactly when a nuclear reaction has occurred. These detectors are used at the “SuN” spectrometer (Prof. Spyrou, Michigan State University). We collaborate with Prof. Spyrou's group to perform measurements on the formation of the elements at the new accelerator facility “FRIB”.
We are all made up of atoms, carbon, nitrogen and oxygen being among the most important to form all life on earth. Although Earth's complex organic life is governed by the rules of biology and organic chemistry, the fact that these elements are available in large quantities is a direct consequence of their nuclear properties.
Nuclear astrophysicists have a detailed understanding of how lighter elements are formed by nuclear fusion reactions in old stars. So Carl Sagan was right when he said the famous phrase:
The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.
The science of the origin of the elements in the universe is called "nucleosynthesis". However, fusion reactions are not able to explain the formation of heavier elements, whose exact origin is a complex mystery with many unanswered questions.
In fact, the creation of heavier elements is somewhat more complicated: when approaching the heavier elements in the range of iron, nuclei can no longer be effectively created by fusion reactions due to the negative energy balance (Q value) of these reactions as well as the large electrical repulsion of heavier elements.
It is now known that heavier elements can only be formed in reactions with uncharged neutrons. An initial nucleus, for example 58Fe (an iron nucleus with a total of 58 protons and neutrons), captures a neutron and forms the isotope 59Fe. This new isotope either captures another neutron and forms 60Fe or decays into the nucleus 59Fe via β decay, forming a new element! This process continues, forming heavier and heavier elements. In fact, it has been shown that the isotope 60Fe, a radioactive isotope with a half-life of around 2.6 million years, is only produced in certain types of supernova explosions. Traces of 60Fe can be found in meteorites and on Earth.
With the 10 MV accelerator mass spectrometry setup, our laboratory is one of the few places in the world that can detect these traces of 60Fe. The detection of this very rare isotope makes it possible to better understand the origin of our solar system and to determine when and how often supernovae explosions have occurred in our cosmic neighborhood. You will find more information on this interesting research shortly in a news article below.
Astrophysicists also discovered that most heavy nuclei are mainly formed in two processes: the slow neutron capture process (s-process) and the fast neutron capture process (r-process).
The s-process mainly takes place in so-called asymptotic giant branch stars (AGB). But processes at very high neutron densities are necessary to explain the observed abundance distributions in the solar system and the universe: In the r-process, neutron capture often wins out over β-decay, resulting in extremely neutron-rich nuclei that then decayed into the stable nuclei we find on Earth today.
But where in the cosmos do we find such high neutron densities? To answer these questions, we need to measure the probability of neutron captures for the extremely neutron-rich nuclei. To do this, we carry out experiments with our local accelerators, but also work on so-called Radioactive Ion Beam (RIB) facilities. At these large international laboratories such as ISAC@TRIUMF, Argonne National Laboratory (Illinois, USA) and at the new Facility for Radioactive Ion Beams FRIB(Michigan, USA), we can produce and accelerate small amounts of the exotic nuclei involved in the r-process. More details on this research can be found in upcoming news articles below.
