Past news

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

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

lombardo

 

 

 

 

The CLAS experiment sheds light on the internal structure of protons

The CLAS experiment sheds light on the internal structure of protons

In an experiment performed with the CLAS detector at Jefferson Lab (USA), by using as a probe a beam of polarized electrons accelerated to intermediate energies (of the order of the proton mass), it was possible to measure some global properties of protons polarized in a strong magnetic field. The measurement allows to verify the effective theories derived from quantum chromodynamics (QCD), the theory that describes the fundamental strong force. This allows to improve the understanding of the internal structure and the global properties of the nucleons, that is of the protons and neutrons comprising atomic nuclei, describing the dynamics between their constituents (quarks), and the mediators of the strong force (gluons). The results of the experiment, with a decisive contribution by INFN and some of the spokespersons being Italian, have been published on Nature Physics. New measurements now under way with the new apparatus CLAS 12 (see photo), will complete the picture in the upcoming years, providing more details on the complex interactions among quarks and gluons and on the way they influence the spin of the nucleons.

Hall B 1 

 

 

 

 

 

 

UNCOVERING THE MECHANISM OF ANGULAR MOMENTUM GENERATION IN NUCLEAR FISSION

Uncovering the mechanism of angular momentum generation in nuclear fission

 

The NU-BALL collaboration sheds light on an outstanding mystery of Nuclear Physics

Nuclear fission, in which a heavy nucleus splits in two and releases energy, was discovered at the end of the 1930s by the chemists Otto Hahn and Fritz Strassmann, and the physicists Lise Meitner and Otto Frisch. This physical phenomenon still has fascinating unknown aspects to be revealed. In the fission process, the fragments are observed to emerge spinning. This observation has been an outstanding mystery in Nuclear Physics for decades: the internal generation of around 6-7 units of angular momentum (or spin) in each fragment is particularly puzzling for systems which start with zero, or almost zero, spin.

A series of experiments at the Irène-Joliot-Curie Laboratory in Orsay, France, has now revealed, unexpectedly, that the fragments resulting from nuclear fission obtain their intrinsic angular momentum after fission and not before, contrary to what most theories have hypothesized thus far. This surprising result was made possible by the NU-BALL collaboration, an international group of nuclear physicists which has measured, with high precision, the gamma radiation emitted by the fast-neutron-induced fission of uranium 238U and thorium 232Th isotopes, in an experimental campaign that lasted 7 weeks.

These new insights into the role of angular momentum in nuclear fission are of fundamental important for a profound understanding of the fission process, with relevant consequences for other research areas, such as the study of the structure of neutron-rich isotopes, the synthesis and stability of super-heavy elements and, in applied fields, on the gamma-ray heating problem in nuclear reactors.

The results of NU-BALL have been published in Nature on 25/02/2021, https://doi.org/10.1038/s41586-021-03304-w

The NU-BALL collaboration has used a high-granularity gamma spectrometer made of more than 100 high-purity and large-volume Germanium detectors from the European GAMMAPOOL network (http://gammapool.lnl.infn.it). The collaboration includes researchers from 37 institutes and 16 countries – among them scientists from the University of Milan and the National Institute of Nuclear Physics (belonging to the GAMMA experiment from the Nuclear Physics Committee 3), who have actively contributed to the setting up of the detectors, to the data analysis and interpretation of the results, now published in Nature.

For further info:

Prof. Silvia Leoni, This email address is being protected from spambots. You need JavaScript enabled to view it.

NU BALL

 

 

 

 

 

 

 

 

The installation of AGATA at the Legnaro National Laboratories is underway

The installation of AGATA at the Legnaro National Laboratories is underway

AGATA is a gamma ray spectrometer fruit of a European collaboration, made up of segmented hyper pure germanium crystals. It is the most sophisticated detector for gamma rays, completely innovative because it allows to trace the path of the single photon inside the germanium crystal with a resolution of a few millimeters. This allows to considerably increase the detection efficiency of the AGATA spectrometer and to identify with high precision the direction of the photon incident on the detector. The high positional accuracy is obtained thanks to the analysis of the shape of the electronic signals produced by gamma rays.
Thanks to these unprecedented characteristics, the AGATA detector is a real "eye" capable of looking inside the atomic nuclei produced in the collisions between accelerated ions, the type of experiments that will be carried out at the Legnaro National Laboratories (LNL) starting from 2022. With these measurements it will be possible to study in detail the properties of the excited states of atomic nuclei, thus helping us to understand the structure of the nucleus and the forces that bind protons and neutrons in it, making up the world around us. Not only that: the experiments with AGATA will also allow us to understand how the nucleosynthesis of elements occurs in stellar processes, such as mergers of neutron stars.
The arrival of AGATA at LNL therefore fits perfectly with the entry into operation at LNL, in the coming years, of the new SPES accelerator system that will allow us to study nuclear reactions using exotic, i.e. unstable, nuclear beams, thus getting closer and closer to what happens in the universe in the astronomical sites where the elements that make up our world are generated.
The Italian scientific community, represented by the staff of the National Institute of Nuclear Physics (INFN), is one of the pillars of this frontier European project for gamma spectroscopy and, not surprisingly, an important phase in the development and use of AGATA will take place in Italy in the next few years

agata installation

 

 

 

 

 

 

 

 

 

Pulsed production of antihydrogen

Pulsed production of antihydrogen

Cold antihydrogen atoms (with K or sub-K temperature) are a powerful tool to precisely probe the validity of fundamental physics laws. The design of highly sensitive experiments needs antihydrogen with controllable and well defined conditions: energy, position, quantum number and time of the production. The paper of the AEgIS collaboration "Pulsed production of antihydrogen" published on 8 Feb. 2021 on Communications Physics (https://www.nature.com/articles/s42005-020-00494-z)  presents experimental results on the production of antihydrogen in a pulsed mode in which the time when 90% of the atoms are produced is known with an uncertainty of ~250 ns, about 1000 times  more accurate than previously attained. This is the first time in which we know when the antihydrogen atoms are produced.  Previous experimentally demonstrated schemes of H¯ production did not allow tagging the time of the formation with accuracy.

The results are based on the data collected at CERN during the 2018 with antiprotons delivered by the Antiproton Decelerator.
The pulsed source is generated by the charge-exchange reaction between Rydberg positronium atoms—produced via the injection of a pulsed positron beam into a nanochanneled Si target, and excited by laser pulses—and antiprotons, trapped, cooled and manipulated in electromagnetic traps (see figure).
The results have been obtained during the last year of data taking with antiproton at CERN in 2018. They mark a milestone in the field of trapping, manipulating and detecting charged particles, producing and exciting positronium and forming and detecting antihydrogen.
The number of observed events is 79, while only 33.4 ± 4.6 events are expected under the hypothesis of absence of antihydrogen formation and the probability that the observation is not antihydrogen is then only about 1 in 3.5 million.
The agreement between the observed and expected number of antihydrogen allows predicting that a significantly larger (by some orders of magnitude) flux of anti-atoms will be available in an optimized experimental geometry and with an increased number of antiprotons and positronium atoms.
The result is then  a major landmark in the first phase of the  AEgIS experiment aiming, on the long term,  to perform direct measurements of the validity of the Weak Equivalence Principle for antimatter.

 

figure1b.jpg

 

 

 

 

 

 

CSN3 Grants and job openings

The position of head of the nuclear science and instrumentation laboratory of the IAEA Laboratories in Seibersdorf (Austria) is open. More details at this link

 

 

Login Form

csn2 csn3 csn4 csn5 infn uffcom amministrazione-trasparente