Successful installation of the first three AGATA triple clusters at Legnaro National Laboratories: our eyes to look into the nucleus

The first experimental campaign with the gamma-ray tracking array AGATA will start at Legnaro National Laboratories (LNL) this spring. The campaign is led by the GAMMA group in the framework of a large international collaboration involving the main European countries and it will use the intense stable beams provided the Tandem-ALPI-PIAVE accelerator complex. AGATA is the most advanced gamma-ray detector in the world. It is made of segmented hyper-pure germanium crystals, and it is conceived as a modular detector array which, once completed, will host 60 triple clusters. This instrument allows one, from the analysis of the electric signal shapes coming from the germanium crystals, to track the single gamma ray inside the crystal with a spatial resolution of few mm.

AGATA will enable the exploration of exotic nuclei produced in heavy-ion collision with unparalleled efficiency and sensitivity. In a first phase at LNL, AGATA will be coupled to the PRISMA large-acceptance magnetic mass spectrometer and to other ancillary detectors for charged particles, neutrons and high-energy gamma rays or for the measurement of lifetimes of excited nuclear states. The first three triple clusters of AGATA have been successfully installed together with the system to support their functioning and the electronics for signal treatment. By the beginning of the experimental campaign, up to 13 triple clusters will be installed, covering about one fourth of the solid angle. This number will then be extended in coming years until arriving at a coverage of half of the solid angle for the subsequent phases of the experimental campaign.

LABORATORI NAZIONALI DEL GRAN SASSO INFN: NEW HINTS ON THE SYNTHESIS OF HEAVY ELEMENTS IN STARS

For decades, physicists and astrophysicists have been wondering about the origin of the elements heavier than iron. To produce them for the stars, neutrons are needed, which, being uncharged, are easily captured by other ions, allowing the synthesis of elements such as Cadmium, Tugsten or Lead.

The most important source of neutrons in stars is the capture that generates a nucleus of 16O and releases a neutron by the process, ¹³C(a,n)¹⁶O, after a long experimental campaign of about 4 years, the international collaboration LUNA (Laboratory for Underground Nuclear Astrophysics https://luna.lngs.infn.it ), which operates at the Underground National Laboratory (LNGS) of INFN, measured the speed of this process with very high precision directly at stellar temperatures.

The results of this work were published in the American Physical Society's Physical Review Letters:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.152701

Gianluca Imbriani, spokesperson for the LUNA collaboration underlines that "our knowledge of the cross section of the process of capturing an alpha particle by a 13C nucleus, in the range of astrophysical energies, has so far been based on extrapolations from measurements to higher and on experiments based on indirect techniques”.

"About 50% of the elements heavier than iron present on Earth and in the Solar System were produced by stars with a mass slightly larger than that of the Sun, which lived before its formation 4.5 billion years ago - says Oscar Straniero, astrophysicist of INAF and collaborator of LUNA -. When they become red giants, ideal conditions develop within these stars for the activation of the ¹³C(a,n)¹⁶O  process, which initiates the synthesis of heavy nuclei ".

"Thanks to the greatly reduced background level in the Gran Sasso underground laboratory - add Alba Formicola and Andreas Best, who coordinated the work for this measurement - LUNA is the only experiment to date that has managed to directly measure the process ¹³C(a,n)¹⁶O in the energy window of astrophysical interest, drastically reducing the uncertainties. This will have a great impact on the prediction of the formation of a series of heavy elements, the synthesis of which strongly depends on the speed of this process. "

"The LUNA experiment will continue its scientific activity in the next decade thanks to the LUNA-MV project - concludes Matthias Junker, local responsible for the Gran Sasso National Laboratories of the LUNA collaboration -, which will focus on the study of the key processes to determine the evolution of massive stars, important for understanding the chemical composition of the universe. "

About 50 scientists from universities and research institutes from Italy, Germany, Hungary and the United Kingdom collaborate with LUNA. For this measurement a contribution in particular should be underlined, and it is that of the Atomki laboratory in Debrecen, Hungary, where targets necessary for this experiment have been developed.

For further information, please contact: Gianluca Imbriani This email address is being protected from spambots. You need JavaScript enabled to view it.

Toward a solution to the missing lithium problem

If there is something missing that should be there, what is the cause? ― It has been known for decades that there is a significant discrepancy between the theoretical and observed lithium abundance in the early universe; the primordial ⁷Li abundance is overestimated and/or underobserved by a factor of 3 – 4. Such an inconsistency is called the “cosmological lithium problem (CLP)”, although the standard Big Bang theory is known as a very “successful” model in comparison with most of other observational facts.

Researchers now found experimental evidence that the theoretical ⁷Li abundance should be corrected downward by about 10%. This is not yet a complete solution, but a necessary contribution for the further theoretical approaches to the CLP solution with slightly less dauting tasks.

The first lithium is created during the Big Bang nucleosynthesis (BBN) period as well as hydrogen and helium from a jumble of protons and neutrons. The BBN is a complex network of nuclear reactions, and the abundance of a nuclide depends not only on the reaction directly to produce it but also on the reaction to destroy it and often on other reactions along the way. For example, the abundance of ⁷Li is mainly dominated by the production and reduction processes of ⁷Be. This falls in the context of the primordial nucleosynthesis studied performed by the ASFIN group and in particular to the investigations on ⁷Be destruction which has been examined in several papers.

Project Assistant Professor Seiya Hayakawa from the Center for Nuclear Study – the University of Tokyo collaborating with the nuclear astrophysics group ASFIN of the INFN – LNS, Sungkyunkwan University, and others leads an experimental project aiming at measuring the nuclear reactions responsible for the ⁷Be reduction in BBN. Recently, there are several attempts to measure these reactions ⁷Be(n, p)⁷Li and ⁷Be(n, α)⁴He by other research groups, but the data in the BBN-relevant energy region were still somewhat scarce.

These reactions are very difficult to observe directly since both ⁷Be and neutron are unstable. The research group proposed to use deuteron as a target instead of a bare neutron, and a ⁷Be beam produced by the CRIB (Center-for-Nuclear-Study Radioactive Ion Beam separator, https://www.cns.s.u-tokyo.ac.jp/crib/crib-new/home-en/index.html). This is a unique technique known as the Trojan Horse method, developed by the nuclear astrophysics group at INFN – LNS. By this method, the deuteron is like the Trojan horse in Greek myth, and the neutron is the soldier who sneaks into the impregnable city of Troy, namely, the reactions of interest, ⁷Be(n, p)⁷Li and ⁷Be(n, α)⁴He. The experimental result shows a significant contribution of the transition to the first excited state of ⁷Li for the first time, which offers a further ⁷Be reduction during the BBN and ultimately less ⁷Li abundance by about 10%.

Experimental setup and conceptual diagram of the Trojan Horse method

Link to the original paper: https://iopscience.iop.org/article/10.3847/2041-8213/ac061f

Press release of The University of Tokyo (in Japanese): https://www.s.u-tokyo.ac.jp/ja/info/7456/

Press release of The University of Tokyo (in English): https://www.u-tokyo.ac.jp/focus/en/press/z0508_00184.html

For more details:

Seiya Hayakawa
Center for Nuclear Study, The University of Tokyo
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Marco La Cognata
INFN – LNS
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

A new analysis of nuclear reactions raises questions about the evolution of the oldest stars

The oldest stars, which date back more than 13 billion years, show surprisingly high calcium abundances. Among the various models suggested by astrophysicists to explain this curious behaviour, one of the most accredited is based on the fact that these old stars originated from material coming from a generation of primeval massive stars, formed shortly after the big bang. These primeval stars would end their existence in a faint-supernova process and would produce elements up to the calcium region. To do this, the primeval stars should burn hydrogen into calcium through a series of so-called breakout reactions.
Among all these reactions, those involving a proton and a fluorine-19 nucleus represent a critical turning point on the path toward calcium: if the reaction leads to the emission of a gamma ray and a neon-20 nucleus, the nucleosynthesis process goes on, while if it leads to the production of an alpha particle and an oxygen-16 nucleus, the process takes a backward step.
Given the particularly delicate role of these nuclear reactions, a group of American, Canadian, and Italian researchers collected and analysed, through refined quantum models of nuclear reactions, the experimental data that have been accumulating in the literature for over seventy years on collisions between protons and fluorine-19 nuclei. In this way, new estimates have been obtained on the rates of these nuclear reactions in stars and on their uncertainties, which have been used to perform complex calculations of stellar nucleosynthesis. The obtained calcium abundance is much lower than that observed experimentally, despite the considerable uncertainties due to the poor knowledge of the nuclear structure of some states in neon-20. This new tension between theoretical predictions and experimental observations on calcium in the oldest stars casts doubts on the consistency of faint supernova processes and draws attention to the need to obtain new data on low-energy p+¹⁹F nuclear reactions.
Because of the impact of the results obtained on one of the most debated astrophysical scenarios today, the work was reported as Editors' Suggestions of the Physical Review C journal and was the subject of a synopsis in the prestigious magazine Physics of the American Physical Society.
On the Italian side, Ivano Lombardo, researcher of the INFN Section of Catania, contributed to this research work. In the past years he has carried out various low energy measurements at the INFN Laboratori Nazionali di Legnaro and at the Tandem accelerator of Federico II University of Naples on the p + ¹⁹F nuclear reaction channels with the emission of an alpha particle and an oxygen-16 nucleus.
For more information on this research topic, the reader is referred to the original article:
R. J. deBoer, O. Clarkson, A. J. Couture, J. Görres, F. Herwig, I. Lombardo, P. Scholz and M. Wiescher, ¹⁹F(p, γ)²⁰Ne and ¹⁹F(p,α)¹⁶O reaction rates and their effect on calcium production in Population III stars from hot CNO breakout, PHYSICAL REVIEW C 103, 055815 (2021)
and to the synopsis in Physics:
M. Schirber, Uncertainty over First Stars, May 26, 2021 • Physics 14, s66