Periodic Reporting for period 4 - ASTRUm (Astrophysics with Stored Highy Charged Radionuclides)
Période du rapport: 2020-10-01 au 2021-09-30
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.
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.
From the beginning of the project we concentrated on three goals.
1) Charge-particle capture reactions for p-process nucleosynthesis.
The proton capture reaction 124Xe(p,g)125Cs was measured for the first time.
The results were published in a high-level peer-reviewed Physical Review Letters journal.
In this measurement we could reach center-of-mass energy as low as 6 MeV/u which is at the upper edge of the Gamow window.
Measuring at lower energies turned out problematic due to exponentially decreasing reaction rates and a huge background due to elastic scattering of primary beam particles off the target atoms.
Therefore, before addressing radioactive ions, which inevitably have orders of magnitude smaller beam intensities, the technique had to be improved.
The sensitivity of the method was boosted by basically eliminating the elastic scattering background via blocking the corresponding orbits in the storage ring.
Owing to these developments, in spring 2021 we have measured proton capture reaction on stored radioactive beam, namely the 118Te(p,g) reaction, at 7 and 6 MeV/u.
Thus this goal of the project has been accomplished.
The analysis is being finalised.
2) Bound state beta decay of 205Tl
The experiment which has been proposed more than three decades ago has finally been accomplished in Spring 2020.
It has been conducted under complicated restrictions due to COVID19 which has been discussed in a dedicated press release of the GSI laboratory.
The data analysis has been completed.
The determined decay rate is a factor of two smaller than predicted by theory, which affects the termination of the s-process nucleosynthesis.
Dedicated s-process calculations are being performed.
3) Highly sensitive non-destructive Schottky detectors
The novel highly-sensitive detector was installed in the ESR (published in Reviews of Scientific Instruments).
The excellent performance of the detector was decisive for studies of 0+->0+ transitions in fully-ionised atomic nuclei.
The rate of 0+->0+ transition in 72Ge could be measured. No indication for a first 0+ state in 70Se was found online.
With this successful measurement, a new technique, combined isochronous and Schottky mass spectrometry, has been established at the ESR.
The data analysis is ongoing.
All results as well as technical developments are published in peer-reviewed journal articles.
1) Charge-particle capture reactions for p-process nucleosynthesis.
The proton capture reaction 124Xe(p,g)125Cs was measured for the first time.
The results were published in a high-level peer-reviewed Physical Review Letters journal.
In this measurement we could reach center-of-mass energy as low as 6 MeV/u which is at the upper edge of the Gamow window.
Measuring at lower energies turned out problematic due to exponentially decreasing reaction rates and a huge background due to elastic scattering of primary beam particles off the target atoms.
Therefore, before addressing radioactive ions, which inevitably have orders of magnitude smaller beam intensities, the technique had to be improved.
The sensitivity of the method was boosted by basically eliminating the elastic scattering background via blocking the corresponding orbits in the storage ring.
Owing to these developments, in spring 2021 we have measured proton capture reaction on stored radioactive beam, namely the 118Te(p,g) reaction, at 7 and 6 MeV/u.
Thus this goal of the project has been accomplished.
The analysis is being finalised.
2) Bound state beta decay of 205Tl
The experiment which has been proposed more than three decades ago has finally been accomplished in Spring 2020.
It has been conducted under complicated restrictions due to COVID19 which has been discussed in a dedicated press release of the GSI laboratory.
The data analysis has been completed.
The determined decay rate is a factor of two smaller than predicted by theory, which affects the termination of the s-process nucleosynthesis.
Dedicated s-process calculations are being performed.
3) Highly sensitive non-destructive Schottky detectors
The novel highly-sensitive detector was installed in the ESR (published in Reviews of Scientific Instruments).
The excellent performance of the detector was decisive for studies of 0+->0+ transitions in fully-ionised atomic nuclei.
The rate of 0+->0+ transition in 72Ge could be measured. No indication for a first 0+ state in 70Se was found online.
With this successful measurement, a new technique, combined isochronous and Schottky mass spectrometry, has been established at the ESR.
The data analysis is ongoing.
All results as well as technical developments are published in peer-reviewed journal articles.
Within the Project we have measured for the first time the rates for 124Xe(p,g)125Cs and 118Te(p,g)119I reactions.
It is important to emphasise that 118Te is a short-lived secondary nuclide.
In these measurements we reached the center-of-mass energies as low as 6 MeV/u, which is to date the lowest energy for in-ring nuclear reaction studies.
Dramatic reduction of background was achieved which boosted the sensitivity of the measurements dramatically.
Thus, reaction studies on low-intensity secondary beams became possible which opens a way to investigations of a multitude of key astrophysical reactions considered unfeasible before.
Bound state beta decay of 205Tl has been measured.
This measurement was pursued for more than 30 years and has now been completed.
The obtained decay rate is a factor of two different from theoretical estimations.
Astrophysical calculations to estimate the effect of the new result on the s-process nucleosynthesis are being conducted.
Thanks to the excellent performance of the developed non-destructive detector, a new highly-sensitive high-precision mass spectrometry technique has been established.
It enables life-time measurements of short lived species and furthermore boosts mass precision.
In Spring 2021, measurements of de-excitation of 0+ state in fully-ionised 72Ge were performed illustrating the power of the method.
It is the first time that 2nd order electromagnetic transitions could be studied under clean conditions, when all other decay channels are disabled.
The data analysis is ongoing. However, a range of new physics cases is already proposed relying on this new simultaneous mass and lifetime spectroscopy technique.
It is important to emphasise that 118Te is a short-lived secondary nuclide.
In these measurements we reached the center-of-mass energies as low as 6 MeV/u, which is to date the lowest energy for in-ring nuclear reaction studies.
Dramatic reduction of background was achieved which boosted the sensitivity of the measurements dramatically.
Thus, reaction studies on low-intensity secondary beams became possible which opens a way to investigations of a multitude of key astrophysical reactions considered unfeasible before.
Bound state beta decay of 205Tl has been measured.
This measurement was pursued for more than 30 years and has now been completed.
The obtained decay rate is a factor of two different from theoretical estimations.
Astrophysical calculations to estimate the effect of the new result on the s-process nucleosynthesis are being conducted.
Thanks to the excellent performance of the developed non-destructive detector, a new highly-sensitive high-precision mass spectrometry technique has been established.
It enables life-time measurements of short lived species and furthermore boosts mass precision.
In Spring 2021, measurements of de-excitation of 0+ state in fully-ionised 72Ge were performed illustrating the power of the method.
It is the first time that 2nd order electromagnetic transitions could be studied under clean conditions, when all other decay channels are disabled.
The data analysis is ongoing. However, a range of new physics cases is already proposed relying on this new simultaneous mass and lifetime spectroscopy technique.