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Lista de proiecte » TRANSfer reactions Induced by Lithium Via Alpha Nuclear clusters In Astrophysics

TRANSfer reactions Induced by Lithium Via Alpha Nuclear clusters In Astrophysics
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Acronim: TRANSILVANIA
Autoritatea contractanta: UEFISCDI
Numar / Data contract: TE 161 / 2022
Program:
Director proiect: Dmitry Testov
Parteneri:
Data incepere / finalizare proiect: 2023-07-01 / 2024-12-31
Valoarea proiectului: 450.000,00 RON
Rezumat: The aim of this project is to resolve the flux of neutrons in the quiescent phases of stellar evolution responsible for the production of 50% of the chemical elements heavier than iron in our Galaxy. Neutron captures that take place on a timescale which is generally slower than the subsequent beta-decay is called the slow neutron capture process (s-process). Two helium-induced reactions, 13C(α, n) and 22Ne(α, n), provide the necessary source of neutrons, yet the cross sections at low astrophysical energies are not known sufficiently well in order to calculate precise, reliable stellar reaction rates to accurately determine the neutron flux. The goals of this project is to investigate these two reactions indirectly with a time projection chamber (TPC) which already exists at ELI-NP and was successfully characterized. Using lithium beams available at the 9 MV tandem accelerator at IFIN-HH, we propose to measure the alpha transfer cross sections to extract the asymptotic normalization coefficients, which are related to the alpha partial widths which control the resonance strengths of these (alpha,n) reactions. In the present proposal, we focus on the case of 22Ne. By performing the measurements in normal kinematics with a TPC filled with enriched 22Ne gas, we will be able to measure the outgoing charged particles with full solid angle coverage and thus obtain the angular distributions to further constrain the properties important alpha-cluster astrophysical states.

Obiective: The TRANSILVANIA project will determine the alpha particle asymptotic normalization coefficients (ANCs) of quantum states dominating the 13C(α,n) and 22Ne(α,n) stellar reaction rates with high precision. The method is well-developed in nuclear astrophysics (Mukhamedzhanov & Tribble, 1999). For states above the particle emission threshold, the reduced width, γ, controls the reaction rate, and the smallest one involved makes a predominate effect, which can be expressed as: γ ≈ ⌈α ⇐⇒ ⌈α << ⌈n, where the ⌈i are the partial widths (Rolfs & Rodney 1988), and the reduced width can be determined from the ANC using the R-Matrix method. Measuring the ANC at sub-Coulomb energies minimizes the model dependence on the theoretical distorted wave Born approximation (DWBA) calculations of the cross section on the assumed optical model potentials (Rogachev et al. 2010). An efficient experimental technique in nuclear experimental physics is called the thick target inverse kinematics (TTIK) method (Artemov et al. 1990). In the TRANSILVANIA project, we propose a modified version of this technique: the thick target normal kinematics (TTNK) method using a time projection chamber (TPC). Here the ion chamber fill gas serves as both the detector medium and the target gas simultaneously, also being called an 'active target'. Noble plus 'quenching' (greenhouse-type) gas mixtures are the preferred fill gases for ion chambers; therefore, both neon and carbon dioxide are suitable in a gas mixture. Both 6Li and 7Li beams are available at the IFIN-HH tandem accelerators. A lithium beam of ~1 MeV/u (within the capabilities of both the 9 MV FN tandem and the 3 MV tandetron) will be injected into the Mini-eTPC, which will be at a nominal pressure ~100 mbar of research gas. As the lithium beam traverses the field cage (130 mm), it will lose about 1.5 MeV in the active target gas; the energy loss will be precisely measured with a silicon photodiode and varying the gas pressure during the calibration phase of these experiments. The result is the lithium ions lose on average about 10 keV / mm. Because the scattering position can be measured with a resolution of several mm, and the scattering depth is directly proportional to the energy loss of the ion beam, it means the center-of-mass energy of each reaction vertex can be precisely determined with high accuracy. The entrance window is an extremely thin silicon nitride foil; 1 ?m was tested previously with the Mini-eTPC to be used in the proposed project, and 75 and 150 nm foils (energy loss of ~50 keV for 1 MeV/u 6Li ions) are at hand in ELI-NP. As energy loss goes as a power of the nuclear charge, Z, the resulting setup of the TTNK method minimizes beam straggling, particularly when coupled with the small beam energy spread available from tandem accelerators and the fact that the Bragg curve of the beam is measured event by event. One challenge facing the traditional TTIK method is that the heavy ion beam and light outgoing charged particles experience orders of magnitude differences in their energy loss in a TPC, requiring sophisticated techniques to deal with the dynamic range settings of the system. Conversely, a low energy, light ion beam, and the outgoing heavy charged particles from, e.g., the 13C(6Li,d)17O and 22Ne(6Li,d) 26 Mg reactions exhibit very similar absolute values for the energy losses over the same volume of gas, considering the the positive Q-values of the lithium-induced, alpha transfer reactions to be measured by TRANSILVANIA. Because the Q-values for contaminant reactions differ significantly from one another, the proposed experimental design allows us to quantify and exclude background events in the analysis. Therefore, by measuring both the incoming ion beam, the scattering location, and the energy loss, three dimensional trajectory, and residual energy of the outgoing reactant, we can uniquely and precisely determine all the information needed to reconstruct the kinematic equation for each reaction, event by event, yielding the differential cross section as a function of energy over a wide range of angles simultaneously. This cross section can then be compared with the theoretical calculations, and we can determine the ANC of the resolved resonances. By precisely measuring the ANCs with high resolution, we can then calculate accurate 13C(α,n) and 22Ne(α,n) stellar reaction rates as requested by the stellar modeling community. To achieve our goals, the following tasks shall be performed: 1) Geant4 simulations of the planned measurements (summer / autumn 2021): 1a) Optimize beam energy for each measurement 1b) Optimize gas pressure 2) Proposal of the experiment to the IFIN-HH PAC (anticipated call for September 2021) 3) Offline tests (autumn / winter 2021) 3a) For window frangibility 3b) Optimize fill gas ratios, using non-isotopically enriched CO2 , He, and Ne gases. 3c) High voltage tuning for the required gas gain 3d) Testing and debugging of new GET electronics, already under requisition 4) Primary measurement of 13C(6Li,d) at IFIN-HH in Romania with the Mini-eTPC (spring 2022) 2b) Short measurements of (7Li,t) for 13C in the same conditions, to check systematics 5) Analysis of the data, with a main focus to extract alpha-branch ANCs of populated states 6) Use of the alpha-branch ANCs of 17O in stellar evolution codes, to distinguish alternate scenarios 7) Publication of results 8a) Primary measurement of 22Ne(6Li,d) at IFIN-HH in Romania with the Mini-eTPC (spring 2023) 8b) Short measurements of (7Li,t) for 22Ne in the same conditions, to check systematics 9) Analysis of the data, with a main focus to extract alpha-branch ANCs of populated states 10) Use of the alpha-branch ANCs of 26Mg in stellar evolution codes, to distinguish alternate scenarios 11) Publication of results

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