The following section presents scientific highlights and news that resulted from in-house and/or collaborative activities of ELI-NP researchers, leveraging the unique ELI-NP equipment and capability that is developed in the ramp up phase towards full operation, and state of the art facilities worldwide.
Disentangeling the properties of the exotic double-gamma nuclear decay

Nature Communications vol. 11, Art. nb: 3242
(Published on 26 June 2020)

The exotic decay studied is a very rarely occurring type of decay in which a radioactive quantum state of an atomic nucleus, instead of decaying directly to a lower energy state with the emission of a single gamma-ray, emits two gamma rays and changes to an energetically lower state via a quantum double-leap through a virtual coupling to a higher-energy state. Such second-order double-decays are a unique consequence of quantum mechanics, first predicted in the doctoral dissertation of Nobel laureate Maria Göppert-Meyer under the supervision of Max Born in Göttingen University in 1931.

In this particular case, both single and double decays are allowed. Thus, this rare process is therefore called competitive double gamma decay by the scientists and is designated with the symbol "γγ/γ". On October 15, 2015, a team of nuclear physicists from Technische Universität Darmstadt announced the discovery of this process in the Ba-137 nuclide in the magazine Nature, showing that this process only occurs there with a probability of around two double-decays per one million single-decays.

Our international team, led by Julien Fuchs (LULI, France) and Sophia Chen (ELI-NP), carried out an experiment at Lawrence Livermore National Laboratory (USA) using two high-intensity short pulse laser beams: a first beam drives a broadband proton beam from a first target to probe the magnetic fields created in a second target irradiated by a second beam. We observed that the fields related to instability could be measured quite easily and that they existed over much longer than expected. It then took several years of theoretical and numerical effort, led by Laurent Gremillet et Charles Ruyer (CEA, France), using the advanced simulation codes and supercomputers to model the data (see Figure). In doing so, two variants of Weibel's instability were identified depending on the region of the plasma, namely the collisionless electromagnetic modulations can be measured as the energetic electrons traversed the dense center region and later the lower density coronal regions.

Photodisintegration of Li-7 measured by ELI-NP team

Physical Review C 101, 055801
(Published 14 May 2020)

One of the unresolved questions in nuclear astrophysics is the so-called "cosmological Lithium problem". Lithium isotopes are produced during the first 15 minutes after the Big Bang, the initial event that created the Universe. Big-Bang Nucleosynthesis (BBN) is the theory that predicts the abundances of light elements at the beginning of the Universe based on the cross sections of nuclear reactions. While for all the light elements there is a good agreement between observed and calculated primordial abundances, for Li-7 there is a discrepancy of a factor of 3-4.

Silicon array from Oak Ridge National Laboratory in USA for detecting the 4He and 3H particles from the photodisintegration of 7Li experiment at HIγS

The Charged Particle Detection group at ELI-NP proposed the measurement of the photodisintegration cross section of 7Li through the 7Li(γ,3H)4He reaction as one of the first experiments to be performed at ELI-NP using the γ-ray beam and the ELISSA detector array. However, before the ELI-NP facilities and detectors are implemented, a proof-of-principle proposal was initiated to measure the 7Li(γ,3H)4He reaction at HIγS.

Direct observation of imploded core heating via fast electrons with super-penetration scheme

Nature Communications, 10, Article number 5614
(Published 09 December 2019)

One of the promising approaches to ignite high density imploded fusion fuel (DD or DT) is fast ignition in inertial confinement laser fusion that separates the laser systems one for imploding the fuel shell to a high density and another for igniting the highly compressed core. We have tested so called "super-penetration mode" where a self-focusing fast heating beam penetrates the large corona plasma surrounding the compressed core and to heat the core via. fast electrons. Time sequences of the heating is clearly shown here. Our study indicates that the energy coupling efficiency can be improved up to 12% by optimizing the key factors in future ignition-scale plasmas.

Cu-Ka image (8 keV) with the different injection timing of fast heating laser pulse (LFEX) at 2.2, 2.6 and 4.6 nsec. Compressed core shows the time sequence of the heating by fast electrons.