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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 that is 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 form 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 and reliable stellar reaction rates so as to accurately determine the resulting neutron flux. The goal of this project is to investigate these two reactions indirectly, using a time projection chamber (TPC) which already exists at ELI-NP and has already been successfully characterized. Using the lithium beams available at the 9 MV Tandem accelerator of 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 controlling the resonance strengths of these (α, 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 of important alpha-cluster astrophysical states.

Obiective: The TRANSILVANIA project will determine the α-particle asymptotic normalization coefficients (ANCs) of quantum states dominating the ¹³C(α, n) and ²²Ne(α, n) stellar reaction rates with high precision via a well-developed method in nuclear astrophysics [1]. For states above the particle emission threshold, the reduced width - γ - controls the reaction rate, with the smallest one involved having a predominate effect that can be expressed using partial widths decay [2]. This 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 [3].
An efficient experimental technique in nuclear experimental physics is known as the thick target inverse kinematics (TTIK) method [4]. 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 as well as the target gas simultaneously, which is also called an "active target". Noble plus "quenching" (i.e. greenhouse-type) gas mixtures are the preferred fill gases for ion chambers, therefore, both neon and carbon dioxide are suitable in a gas mixture. Since both ⁶Li and ⁷Li beams are available at the IFIN-HH Tandem accelerators, a lithium beam of ~1 MeV/u will be injected into the Mini-Electronic Time Projection Chamber (Mini-eTPC) of ELI-NP, filled with the target gas at a nominal pressure of ~100 mbar.
As the lithium beam traverses the field cage (130 mm), it will lose about 1.5 MeV in the active target gas. This energy loss will be precisely measured using a silicon photodiode while varying the gas pressure during the calibration phase of these experiments. The result is that the lithium ions lose on average about 10 keV per mm of traversed gas. Since 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, this means the center-of-mass energy of each reaction vertex can be precisely determined with high accuracy.
The entrance window to the Mini-eTPC interior is an extremely thin silicon nitride foil: 1 ?m was tested previously with the setup that will be used in the proposed project, while 75- and 150 nm foils (resulting in an energy loss of ~50 keV for 1 MeV/u of ⁶Li ions) are also available at ELI-NP. As energy loss works as a power of the nuclear charge - Z - the proposed 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 inside a TPC, requiring sophisticated techniques to deal with the dynamic range settings of the system. Conversely, by using a low energy, light ion beam, the outgoing heavy charged particles resulting from the ¹³C(⁶Li, d)¹⁷O and ²²Ne(⁶Li, d)²⁶Mg reactions exhibit very similar absolute values for their energy losses over the same volume of gas due to the positive Q-values of the lithium-induced, alpha transfer reactions aimed to be studied as part of 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, as well as the energy loss, 3D trajectory, and residual energy of the outgoing reactant, we can uniquely and precisely determine all of the information needed to reconstruct the kinematic equation for each reaction, event by event, thus 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 therefore calculate accurate ¹³C(α, n) and ²²Ne(α, n) stellar reaction rates, as requested by the stellar modeling community.
In order to achieve our goals, the following tasks shall be performed:
1a. Geant4 simulations of the planned measurements (Summer/Autumn 2021);
1b. Optimizing the beam energy for each measurement;
1c. Optimizing the gas pressure inside the Mini-eTPC;
2. Proposing the experiment to the IFIN-HH PAC during the anticipated call for September 2021;
3a. Offline tests (Autumn/Winter 2021);
3b. Test for window frangibility;
3c. Optimizing the fill gas ratios, using non-isotopically enriched CO₂, He, and Ne gases;
3d. High voltage tuning for the required gas gain;
3e. Testing and debugging of new GET electronics, already under requisition;
4a. Primary measurement of the ¹³C(6Li, d) reaction at IFIN-HH with the Mini-eTPC (Spring 2022);
4b. Short measurements of the (⁷Li, t) reaction for ¹³C under similar conditions, to check systematics;
5. Analysis of the data, with a main focus to extract alpha-branch ANCs of populated states;
6. Using the alpha-branch ANCs of ¹⁷O in stellar evolution codes, to distinguish alternate scenarios;
7. Publication of results; 8a. Primary measurement of the ²²Ne(⁶Li, d) reaction at IFIN-HH with the Mini-eTPC (Spring 2023);
8b. Short measurements of the (⁷Li, t) reaction for ²²Ne under similar 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.

Modification of the reaction of the interest: The "Transfer reactions induced by lithium via α nuclear-clusters in astrophysics (TRANSILVANIA)" proposal was timely submitted and presented to the IFIN-HH PAC. In total, 10 days of beam time were allocated to perform the measurements of the ⁶Li + ²²Ne and ⁶Li + ¹³C reactions. However, the original plan of the project has been massively affected by the geo-political events that could not be foreseen at the moment when the project was designed. Since the Russian Federation is the only supplier of the required ²²Ne target gas and due to the trade sanctions imposed by the European Union, its acquisition thus became impossible.
Under these circumstances, we were not able to deliver the results on the proposed ⁶Li + ²²Ne reaction, which aimed to be the main component of the TRANSILVANIA project. Additionally, performing measurements for the reference reaction ⁶Li + ¹³C would also not have helped us achieve the goals of the present proposal. Therefore, we had to carefully reconsider the project plan and identify a compelling alternative physics case. The wide range of beams available at the IFIN Tandem provides multiple opportunities to this end, but considering the neutron-induced reaction mechanism, especially in reactions involving light nuclei, following additional theoretical computations we finally decided to concentrate our efforts on a reaction involving a neutron beam.
The combination of a quasi mono-energetic neutron beam and the Mini-eTPC experimental setup will allow us to study the ¹³C(n, α) reaction, which holds similar importance for the nuclear astrophysics program proposed within ELI-NP. This reaction will be studied using a secondary neutron beam produced via the ⁷Li(p, n) reaction performed at the 9 MV Tandem accelerator of IFIN-HH, for which we have already been approved beam time.
Indeed, when the direct approach to the study of (α, n) reactions proves particularly difficult, indirect or inverse reactions are considered as a valid method for constraining the reaction cross sections of astrophysical interest [5, 6]. For instance, the challenging ¹³C(α, n)¹⁶O measurement can benefit from experimental information from the time-reversed reaction ¹⁶O(n, α)¹³C [7]. In particular, by using the detailed balance (i.e. time-reversal invariance theorem), the reaction cross section of the ¹³ C(α, n)¹⁶O reaction can be deduced from measurements in the reverse direction.

REFERENCES:
[1] A. M. Mukhamedzhanov and R. E. Tribble Phys. Rev. C 59 (1999) 3418-3424.
[2] C.E. Rolfs and W.S. Rodney (1988) Cauldrons in the Cosmos: Nuclear Astrophysics. University of Chicago Press, Chicago, 338.
[3] G. V. Rogachev, E. D. Johnson, J. Mitchell et al,, Resonance scattering and α-transfer reactions for nuclear astrophysics, Fifth European Summer School on Experimental Nuclear Astrophysics, volume 1213 of American Institute of Physics Conference Series (edited by C. Spitaleri, C. Rolfs & R. G. Pizzone), 137-148.
[4] K. P. Artemov, O. P. Belyanin, A. L. Vetoshkin et al., Soviet Journal of Nuclear Physics 52 (1990) 408-411.
[5] C. Guerrero, A. Tsinganis, E. Berthoumieux et al., Eur. Phys. J. A (2013) 49: 27.
[6] C. Massimi, S. Altstadt, J. Andrzejewski e al., Phys. Lett. B 768 (2017).
[7] R. J. deBoer, M. Febbraro, D. W. Bardayan et al., Phys. Rev. C 132 062702 (2024).

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