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.

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

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.

SNAQs [snacks] – Schools on Nuclear Astrophysics Questions

A new format of virtual nuclear astrophysics schools

SNAQs [snacks] – Schools on Nuclear Astrophysics Questions aims to give all students and young researchers the same, multidisciplinary knowledge about nuclear astrophysics in a period of limited “live” schools or conferences due to the pandemy. SNAQs will support this idea and strengthen the community of schools by providing a frequent lecture series to train and educate the next generation of scientist.

SNAQs will join the community of existing schools related to nuclear astrophysics that will partner with ChETEC-INFRA:

Carpathian Summer School of Physics (well established)
European Summer School on Experimental Nuclear Astrophysics (well established)
Russbach School on Nuclear Astrophysics (well established)

Those infrastructures will be networked by ChETEC-INFRA, Chemical Elements as Tracers of the Evolution of the Cosmos – INFRAstructures for Nuclear Astrophysics, a new European network of 32 partner institutions.

The kick-off event of SNAQs will discuss
What do we need to know about Nuclear Astrophysics?

and will take place on Wednesday, February 17, 2021, 14:00 – 17:30 CET (08:00 – 11:30 EST). We are very glad that following colleagues agreed to give lectures:

Aurora Tumino (Università degli Studi di Enna Kore) Chris Sneden (University of Texas)
Jordi Jose (Universitat Politècnica de Catalunya)

After registration at https://hifis-events.hzdr.de/e/snaqs-feb2021  you will get a confirmation e-mail with a ZOOM link that will allow you to join the virtual school from about 13:45 CET (07:45 EST) on February 17.

The organizing committee: Konrad Schmidt (HZDR),Marcel Heine (CNRS-IPHC), Andreas Korn (UU), Arunas Kucinskas (VU), Mohamad Moukaddam (CNRS-IPHC), Rosario Gianluca Pizzone (INFN-LNS), Olivier Sorlin (CNRS-GANIL),  Livius Trache (IFIN-HH)