Periodic Reporting for period 3 - NECTAR (Nuclear rEaCTions At storage Rings)
Berichtszeitraum: 2024-01-01 bis 2025-06-30
Our body, the Earth, the Sun and everything we know in the Universe is composed of chemical elements: hydrogen, oxygen... One of the fundamental questions is to understand how the heaviest elements found on Earth, such as uranium, were formed. NECTAR (NuclEar reaCTions At storage Rings) is designed to provide the scientific community with data to answer this question.
Chemical elements are produced by nuclear reactions in stars. The heaviest elements are produced in nuclear reactions where a neutron is captured by a nucleus. When the target nucleus is heavy, it can break in to two lighter nuclei, i.e. fission, after capturing the neutron. These reactions also take place in a nuclear reactor, thus their importance also for industrial applications. Today, we know rather well the probability for neutron induced reactions on stable target nuclei, but these data are often not known for unstable target nuclei. The reason is the difficulty to produce samples containing the radioactive nuclei of interest. Neutron-induced reactions are also very difficult to calculate theoretically, mainly because we are not able to describe accurately how the nucleus de-excites, i.e. how it releases the internal energy acquired after the capture of a neutron. The excited nucleus may decay by the emission of energetic photons (known as gamma rays), neutron emission or fission. These three different de-excitation modes have different probabilities, which depend on fundamental properties of the nucleus such as level densities, particle and gamma transmission coefficients or fission barriers, which are very difficult to calculate.
NECTAR aims to circumvent these problems by using the surrogate reaction method in inverse kinematics. In standard measurements a beam of neutrons interacts with a heavy, radioactive nucleus at rest. In NECTAR, the kinematics of the nuclear reaction is inverted and the heavy, unstable nucleus is to be put in the beam to bombard a light nucleus. Since free neutron targets are not available, we use targets of light nuclei such as protons or deuterons (a nucleus made of a proton and a neutron). By appropriately choosing the projectile nucleus we can produce the nucleus that is formed in the neutron-induced reaction of interest. The probabilities for the different de-excitation modes, which can be measured with the alternative or surrogate reaction, are particularly useful to constrain the models describing the fundamental nuclear properties mentioned above and eventually to inform much more accurate theoretical predictions for neutron-induced reactions. One of the figures shows the principle of the surrogate reaction method.
NECTAR experiments are to be performed with the heavy-ion storage rings of the GSI/FAIR facility (https://www.gsi.de/en/(öffnet in neuem Fenster)) in Germany, which offer the ideal conditions for investigating surrogate reactions. The storage rings at GSI/FAIR provide high-quality radioactive beams, which can be used in conjunction with ultra-thin gas-jet targets. This and the capability of storage rings to separate the beam-like residues produced after the nuclear reaction will enable for measurements of decay probabilities with unrivalled accuracies. However, the ultra-high vacuum (UHV) conditions inside the storage rings pose severe constraints to in-ring detection systems. NECTAR proposes a completely new solution to cope with this issue, which is to use solar cells.
Solar cells, the devices that are routinely used to convert sunlight into electricity, have significant advantages with respect to the traditionally-used silicon detectors. First, they are much more resistant to radiation damage, which represents a huge advantage in the UHV environment of the storage rings as it reduces significantly the need for breaking the UHV for detector troubleshooting during the beam period. Moreover, solar cells are much more cost effective.
The objective of NECTAR is to develop a set-up and a methodology to simultaneously measure fission, gamma- and neutron-emission probabilities induced by surrogate reactions in inverse kinematics at the storage rings of the GSI/FAIR facility.
Chemical elements are produced by nuclear reactions in stars. The heaviest elements are produced in nuclear reactions where a neutron is captured by a nucleus. When the target nucleus is heavy, it can break in to two lighter nuclei, i.e. fission, after capturing the neutron. These reactions also take place in a nuclear reactor, thus their importance also for industrial applications. Today, we know rather well the probability for neutron induced reactions on stable target nuclei, but these data are often not known for unstable target nuclei. The reason is the difficulty to produce samples containing the radioactive nuclei of interest. Neutron-induced reactions are also very difficult to calculate theoretically, mainly because we are not able to describe accurately how the nucleus de-excites, i.e. how it releases the internal energy acquired after the capture of a neutron. The excited nucleus may decay by the emission of energetic photons (known as gamma rays), neutron emission or fission. These three different de-excitation modes have different probabilities, which depend on fundamental properties of the nucleus such as level densities, particle and gamma transmission coefficients or fission barriers, which are very difficult to calculate.
NECTAR aims to circumvent these problems by using the surrogate reaction method in inverse kinematics. In standard measurements a beam of neutrons interacts with a heavy, radioactive nucleus at rest. In NECTAR, the kinematics of the nuclear reaction is inverted and the heavy, unstable nucleus is to be put in the beam to bombard a light nucleus. Since free neutron targets are not available, we use targets of light nuclei such as protons or deuterons (a nucleus made of a proton and a neutron). By appropriately choosing the projectile nucleus we can produce the nucleus that is formed in the neutron-induced reaction of interest. The probabilities for the different de-excitation modes, which can be measured with the alternative or surrogate reaction, are particularly useful to constrain the models describing the fundamental nuclear properties mentioned above and eventually to inform much more accurate theoretical predictions for neutron-induced reactions. One of the figures shows the principle of the surrogate reaction method.
NECTAR experiments are to be performed with the heavy-ion storage rings of the GSI/FAIR facility (https://www.gsi.de/en/(öffnet in neuem Fenster)) in Germany, which offer the ideal conditions for investigating surrogate reactions. The storage rings at GSI/FAIR provide high-quality radioactive beams, which can be used in conjunction with ultra-thin gas-jet targets. This and the capability of storage rings to separate the beam-like residues produced after the nuclear reaction will enable for measurements of decay probabilities with unrivalled accuracies. However, the ultra-high vacuum (UHV) conditions inside the storage rings pose severe constraints to in-ring detection systems. NECTAR proposes a completely new solution to cope with this issue, which is to use solar cells.
Solar cells, the devices that are routinely used to convert sunlight into electricity, have significant advantages with respect to the traditionally-used silicon detectors. First, they are much more resistant to radiation damage, which represents a huge advantage in the UHV environment of the storage rings as it reduces significantly the need for breaking the UHV for detector troubleshooting during the beam period. Moreover, solar cells are much more cost effective.
The objective of NECTAR is to develop a set-up and a methodology to simultaneously measure fission, gamma- and neutron-emission probabilities induced by surrogate reactions in inverse kinematics at the storage rings of the GSI/FAIR facility.
During the first 30 months of NECTAR, we have performed an experiment to test solar cells at GANIL (https://www.ganil-spiral2.eu/fr/(öffnet in neuem Fenster)). This experiment has allowed us to demonstrate the good performances of solar cells and their high resistance to radiation damage. We have also investigated the ultra-high-vacuum (UHV) compatibility of solar cells and performed realistic simulations of the curent generation in the cell after the interaction with a heavy ion.
We have also been able to successfully conduct the first NECTAR experiment at the ESR storage ring of the GSI/FAIR facility. In this experiment a 208Pb beam at 30.77 A MeV interacted with a gas-jet target of hydrogen leading to the excitation of the 208Pb projectiles via inelastic scattering reactions. We measured the scattered protons with a Si telescope and the beam residues produced after deexcitation of 208Pb via gamma-rays and neutron emission with a position sensitive Si strip detector placed behind one of the dipoles of the ring. These detection systems were designed and developed by the NECTAR team. These detection systems are shown in the pictures. The data analysis has been completed and we have been able to measure for the first time high-precision gamma and neutron-emission probabilities.
We have also been able to successfully conduct the first NECTAR experiment at the ESR storage ring of the GSI/FAIR facility. In this experiment a 208Pb beam at 30.77 A MeV interacted with a gas-jet target of hydrogen leading to the excitation of the 208Pb projectiles via inelastic scattering reactions. We measured the scattered protons with a Si telescope and the beam residues produced after deexcitation of 208Pb via gamma-rays and neutron emission with a position sensitive Si strip detector placed behind one of the dipoles of the ring. These detection systems were designed and developed by the NECTAR team. These detection systems are shown in the pictures. The data analysis has been completed and we have been able to measure for the first time high-precision gamma and neutron-emission probabilities.
Solar cells have been used so far with heavy ions with energies of about 1 A MeV. Thanks to NECTAR we have investigated the response of solar cells in a much broader range of energies, namely from 1 to 15 A MeV. We have made the first simulations of the current generation in solar cells and investigated for the first time the UHV compatibility of solar cells.
In the first NECTAR experiment at the ESR we could disentangle the heavy residues produced after gamma ad neutron emission from the intense elastic-scattered-beam background. The left figure representing the spectra of the beam residues shows the latter background, while the right figure shows the same spectrum in coincidence with the detected protons. The latter figure shows that we could separate very well the residues produced after gamma emission from those produced after neutron emission. This allowed us to measure for the first time simultaneously the probabilities that the excited 208Pb decays by emitting gamma-rays or a neutron. This is very complicated with usual methods, as it requires detecting gamma rays and neutrons, for which the detection efficiencies are very low. With NECTAR we do not need to detect neutrons and we were able to measure this decay channel with 100% efficiency. Moreover, we measured the gamma emission channel probability with efficiencies 10 to 20 times larger than in conventional experiments. Thanks to these exceptional efficiencies we were able to determine the decay probabilities with high precision, even though the number of events measured in our experiment was rather low. In addition, we have achieved excitation energy resolutions of about 250 keV, which is outstanding for experiments in inverse kinematics with heavy ions.
In the next years we foresee to include an additional detection system for fission fragments made of solar cells. We will then perform a first experiment with a 238U beam interacting on a deuterium target and simultaneously measure fission, gamma and neutron emission probabilities.
In the first NECTAR experiment at the ESR we could disentangle the heavy residues produced after gamma ad neutron emission from the intense elastic-scattered-beam background. The left figure representing the spectra of the beam residues shows the latter background, while the right figure shows the same spectrum in coincidence with the detected protons. The latter figure shows that we could separate very well the residues produced after gamma emission from those produced after neutron emission. This allowed us to measure for the first time simultaneously the probabilities that the excited 208Pb decays by emitting gamma-rays or a neutron. This is very complicated with usual methods, as it requires detecting gamma rays and neutrons, for which the detection efficiencies are very low. With NECTAR we do not need to detect neutrons and we were able to measure this decay channel with 100% efficiency. Moreover, we measured the gamma emission channel probability with efficiencies 10 to 20 times larger than in conventional experiments. Thanks to these exceptional efficiencies we were able to determine the decay probabilities with high precision, even though the number of events measured in our experiment was rather low. In addition, we have achieved excitation energy resolutions of about 250 keV, which is outstanding for experiments in inverse kinematics with heavy ions.
In the next years we foresee to include an additional detection system for fission fragments made of solar cells. We will then perform a first experiment with a 238U beam interacting on a deuterium target and simultaneously measure fission, gamma and neutron emission probabilities.