The ASTRUm project aims at performing experiments in laboratory which shall unveil still unknown details of the synthesis of chemical elements in stars.
Such nucleosynthesis processes involve exotic nuclei, that is radioactive nuclei that do normally not exist on Earth and have to be produced artificially.
One of the obvious thoughts is to try to store such exotic nuclei which are produced in tiny quantities only.
The main goal of ASTRUm is to employ stored radioactive ions for forefront nuclear astrophysics research.
Our experiments are based at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, where the worldwide unique combination of accelerator facilities is in operation.
The latter includes the heavy-ion cooler-storage ring ESR coupled to the radioactive-ion beam facility FRS.
The ASTRUm project was focused on three main experimental goals which were successfully reached.
Within ASTRUm we have developed a method to measure charge-particle capture reactions in inverse kinematics on secondary beams.
Proton capture reaction on stable 124Xe in the center-of-mass energy range from 6 to 8 MeV/u was measured.
The lowest energy is just at the upper edge of the Gamow window of the p-process of stellar explosive nucleosynthesis.
Recently, we dramatically enhanced the sensitivity of the experiment by physically removing the background
enabling us to measure for the first time the proton capture reaction on radioactive 118Te.
This result is a milestone step forward to perform reaction studies on radioactive ions directly in the Gamow window of the p-process.
The second goal was to measure the bound-state beta decay of fully-stripped 205Tl.
205Tl is stable as neutral atom and is present on Earth.
However, if all bound electrons are removed, an exotic bound-state beta decay becomes energetically possible.
In an ordinary beta-minus decay one of the neutrons in the nucleus is transmuted to a proton with an emission of electron and electron antineutrino.
In the bound-state decay, the electron is not emitted to continuum but occupies one of the free atomic orbitals thus saving the binding energy.
By measuring the bound state beta decay of 205Tl, the constraints on the origin of the matter of our solar system can be made.
The experiment has been successfully conduced in spring 2020, though the restrictions due to COVID19 were tough.
The analysis of the data is completed and the publication is in preparation.
The third goal relates to the development of highly sensitive non-destructive particle detectors for storage rings.
A prototype detector was installed into the ESR.
Based on its excellent performance, a search for 0+->0+ transitions in fully-stripped ions became possible.
Such transition with an emission of a single gamma-quantum is forbidden due to conservation of angular momentum.
Only a much slower, second-order process, in which two photons are emitted coupled to the total angle momentum 0, is allowed.
The corresponding experiment has been successfully conducted in Spring 2021.
The half-life of the 0+ state in 72Ge could be measured. No indications for a first 0+ state in 70Se were seen in the online analysis.
An unprecedented mass resolving power for isochronous storage-ring mass spectrometry has been achieved:
An isomeric state with an excitation energy of merely 100 keV in 72Br has been clearly resolved.
The data analysis is ongoing.
