Astral matter from the Paul Scherrer Institute
Processes in stars recreated with isotopes from PSI
To understand why chemical elements exist in certain amounts on Earth, processes that take place in exploding stars – supernovae – need to be studied. After all, much of the matter that makes up the earth was formed in a supernova billions of years ago and hurled into space by the explosion. Researchers looking to study the processes in a lab on Earth often face a problem: the substances, particular isotopes, which play a key role in the supernovae, do not exist on Earth and thus are not available for the experiments. One solution comes from the Paul Scherrer Institute (PSI): these isotopes are formed in sufficient amounts at the institute’s experimental facilities and can be used for studies. For instance, an international team of researchers used the titanium isotope Ti-44 to study an important process at CERN in Geneva. They discovered that it is less effective than was previously believed and therefore the existing theoretical calculations of processes in the stars need to be corrected. The results were published in the journal Physics Letters B.
Supernovae – exploding stars – rank among the most spectacular phenomena in the universe. In the space of a few days, they expand many times over and hurl the majority of their matter into the universe. Many elements such as silicon, sodium, iron or even trace elements that are vital on Earth stem from supernova explosions, which propelled them into our solar system. They formed inside the stars or during the actual explosion. Consequently, the research into the processes in supernovae not only helps us to understand stars, but also why some chemical elements are found more frequently than others on Earth.
Many complicated processes govern what happens in a supernova. Different isotopes of different elements form and disappear again by transforming into other elements, which can happen in many different ways. Therefore, if we want to decode what goes on in the stars, we also have to understand the individual processes – for instance, how likely the process is to produce or destroy a certain isotope is important. Studying these processes in a lab on Earth often falters because of a simple problem: the corresponding isotopes don’t exist naturally on Earth and thus are not available for studies. However, some of these isotopes can be produced artificially. A sufficient amount of these isotopes, which otherwise only exist inside stars, form during experiments at the Paul Scherrer Institute, which means that they can be used to conduct studies. Nonetheless, these experiments are primarily about other issues; the isotopes used are effectively by-products.
Processes in supernovae simulated
The results of one such experiment, where researchers from an international cooperation recreated a process that otherwise only occurs in exploding supernovae, were recently published. It involved the titanium isotope Ti-44, which can help astrophysicists to understand the processes that take place during the explosions. It is one of the few isotopes that can be observed from Earth because it generates radiation as it decays, which reaches the earth. And if we understand how it behaves in similar conditions to those of the explosion, we will also be able to understand supernovae better in general. In concrete terms, the researchers studied a reaction in which the titanium captures a helium nucleus (the core of a helium atom, composed of two protons and two neutrons) and transforms into the vanadium isotope V47 and a proton.
Knowing the rate of this reaction is vital to enabling any firm conclusions to be drawn from the observations being made of Ti-44 in supernovae ejecta by satellites such as INTEGRAL and NuSTAR, explains Alexander Murphy from the University of Edinburgh, who headed the research project.
In order to determine this rate, the researchers sent a beam of rapid Ti-44 atoms through a helium-filled chamber at CERN’s REX ISOLDE experimental facility, which enabled them to observe how often the desired reaction took place.
At CERN’s REX ISOLDE complex, we managed to produce a particularly pure Ti-44 beam from the available sample material, stresses Thierry Stora, head of the ISOLDE facility’s Target and Ion Source group. The experiments reveal that the process examined takes place considerably more slowly than was previously believed.
If the result we have found holds more generally, then it will explain why up until now computer models have been unable to explain the amounts of Ti-44 that the satellites have seen. To confirm this we will need to conduct more studies over a wider range of beam energies, says Murphy.
The rare titanium isotope that made this study possible is actually a by-product of other experiments. Researchers had been studying how steel changes if pieces of it are exposed to intensive proton and neutron radiation in the neutron source SINQ at PSI.
Different isotopes formed, including Ti-44, which tended to be a hindrance for the experiments on how the mechanical properties of steel change, explains Rugard Dressler, a scientist in the Heavy Elements research group at PSI.
Specifically producing such isotopes is very expensive and time-consuming. Here, they simply form by the wayside. And because the PSI accelerator is the most powerful in the world, large amounts of the isotope are formed. Using the radiochemical method available at PSI, the isotope Ti-44 could then be extracted from the pieces of steel and supplied for the experiments at CERN.
Range of rare isotopes
Many different, rare isotopes that are useful for a wide range of experiments in fields such as nuclear astrophysics, basic physical research or environmental research and otherwise could only be produced at great expense are produced at the PSI facilities. PSI has devised a suitable method to separate the isotopes and make them available for the experiments, stresses Dorothea Schumann, head of the research group RadWasteAnalytics at PSI, which is concerned with the isotopes produced at the PSI facilities.
Text: Paul Scherrer Institute/Paul Piwnicki
The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 1900 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 350 million.