ELI-NP Thematics - Photo Nuclear Physics

Nuclear Astrophysics

Several key reactions important for the astrophysical p-process, Big Bang Nucleosynthesis, and supernova explosion, including the pivotal reaction ⁷Li(γ, ³H) 4He associated with the longstanding "Cosmological Li problem" are currently under investigation. Preliminary experiments were performed using the HIγS facility at Duke University. Complementary reactions ⁷Li(p, α)⁴He, and ⁶Li(p, α)³He which are also linked to the "Cosmological Li problem" are being studied with the scaled-down version of the ELISSA detector array at the IFIN-HH 3 MV Tandem. The Extreme Light Infrastructure Silicon Strip Array (ELISSA) is a 4π silicon strip detector array implemented at the ELI-NP facility to detect charged particles from the photodissociation reactions using high-brilliance, quasi-monoenergetic gamma beams. This array can provide charged particle detection with low thresholds, high energy, and angular resolution over an almost 4π solid angle. Additionally, measurement of (γ, p) and (γ, α) reactions on ¹¹²Sn and, ¹⁰²Pd important for the astrophysical p-process are also of interest.

a) A scaled-down version of the ELISSA detector array in the mini-ELISSA chamber. b) Schematic detector setup for the ⁷Li(γ, ³H) ⁴He experiment. c) Simulated version of the ELISSA array

Gamma strength above and around the neutron threshold

The electromagnetic dipole response of nuclei is often referred to as gamma-ray strength, showing several characteristics features that are explored at ELI-NP and will be further investigated in detail with the gamma-ray beams. The Giant Dipole Resonance contains the majority of the electromagnetic response and appears when an impulse from a high-energy electromagnetic field, for example from gamma-rays, disturbs the nucleus, often interpreted as causing the protons to oscillate with the resonant frequency against the neutrons that do not interact with the field. A similar but weaker situation can occur with the magnetic component of the field, generating a Magnetic Dipole Resonance. Another, still not unambiguously understood, feature occurs at lower energies appearing a small resonant-like structure. This is sometimes referred to as the Pygmy Dipole Resonance but the exact nature is the subject of broad experimental and theoretical investigations within the community. The underlying structure details of the Giant Dipole Resonance, Magnetic Dipole Resonance, and Pygmy Dipole Resonance will be studied using a large array of high-resolution LaBr3:Ce and CeBr3 scintillator detectors for gamma rays and liquid scintillators and lithium glass scintillators for neutrons. These setups intend to measure the details of the wave functions of these resonances by comparing the decay branching of gamma rays and neutrons into ground and excited states. The LaBr3:Ce and CeBr3 scintillator detectors are also used to gain further insight into the Pygmy Dipole Resonance region with the 9MV Tandem accelerator facilities at the IFIN-HH campus.

a) Gamma strength function for ¹¹²Sn. b) ELIGANT-TN setup. c) ELIGANT-GN setup

Materials science with fast positrons

When a positron annihilates with an electron form a material, the annihilation photons carry information of the state of the electron. With HPGe detectors it is possible to obtain information on the electron momentum distribution from the shape of the annihilation line measured for many electron-positron pairs. The Doppler Broadening of the Annihilation Line (DBAL) is useful in studying the defect structure of lattice materials. Positrons are very sensitive not only to negatively charged defects but also to open volume defects in the lattice. In the case in which they are trapped in open-volume defects, the fraction of valence electrons taking part in the annihilation process increases compared with that of core electrons.

The most commonly used source of positrons is the radioisotope ²²Na. The β⁺ decay of ²²Na results in the creation of a positron with almost simultaneous (only with 3 ps delay) emission of a 1274 keV photon. The detection of this photon by a fast scintillation detector can be used for time tagging the birth of a positron. Detection of one of the annihilation photons can be used for tagging the positron death. This is the base of the Positron Annihilation Lifetime Spectroscopy (PALS). The electron density at the annihilation site determines the positron lifetime, t. The lower the density, the longer the lifetime. The electron density in open-volume defects, such as vacancies and their agglomerates, is locally reduced when compared to a defect-free sample. Thus, the type of defect can be determined from the positron lifetime, while the defect concentration is deduced from the intensity of the corresponding lifetime component in the PALS spectrum.

a) Electron momentum information carried by the annihilation photons. b) Combined DBAL and PALS setup for positron spectroscopy with fast positrons at ELI-NP

Nuclear reactor materials studied by slow positrons

An important aim of the designing process of a nuclear reactor power plant is to ensure its safe operation. The core of any nuclear reactor is subjected to high irradiation due to the nuclear fission processes. The materials involved in the construction of a nuclear reactor core, e.g. these of the pressure vessel, fuel cladding, neutron absorbers, and fuel pellets, are subjected to strong irradiation and work at a designed high temperature, usually few hundred degrees centigrade depending on the reactor type. The behavior of the materials under these severe irradiation conditions and high-temperature regimes is necessary to be predicted with high accuracy. The processes of accumulation, segregation, diffusion, and release of the fission products as a function of the thermal and irradiation conditions are strongly dependent on material defects. The Positron Annihilation Spectroscopy (PAS) with tunable energy mono-energetic slow positrons is able to provide valuable information on the defect structure of the studied samples.

a) Advanced Sodium Technological Reactor for Industrial Demonstration. b) Doppler broadening dedicated branch of the slow positron beamline at ELI-NP. ELIGANT-TN setup. c) Depth profile of S parameter measured for B₄C samples implanted with 170-keV Li⁺ at fluence of 10¹⁴ ions/cm² (EB1) and 10¹⁶ ions/cm² (EF1) compared to the non-implanted sample ERM.

Active interrogations for nondestructive testing

Experimental nuclear physics has been driving force for multiple applications that have reached mainstream use in various topics from medical to industrial applications. On the medical side, it has played an important role in the development of diagnostic imaging techniques such as PET and SPECT. On the industrial side, it has significantly impacted material inspections, border security, and nuclear non-proliferation efforts. Neutron and gamma-ray detection capabilities and processing algorithms have evolved significantly, enabling new possibilities for security and material inspection.

Our research focuses on improving and expanding such methods by building on three pillars: detector development, simulations and advanced data processing, and method development. Past efforts have focused on potential applications of high-energy gamma-rays for material inspection. With general approaches like effective Z evaluation in cargo scanning to more specific measurement protocols such as zero-knowledge verification of nuclear warheads. Current projects are aimed towards developing detectors designed for applications that demand specific attributes, such as position sensitivity or the ability to operate in high-flux environments.

Investigating the effects of ionizing radiation on human cells

Particle therapy has significantly improved cancer treatment, especially in cases where conventional radiation therapy may be limited. Particle therapy can lead to better local control, highlighting the importance of understanding the cellular response to aid in optimizing treatment outcomes by increasing the radiation dose to the tumors while reducing exposure to healthy tissues. This approach works by inducing cell inactivation and cell death through the induction of DNA double-strand breaks. Ongoing efforts to investigate particle irradiation in vitro can be strengthened by simulations of direct and indirect actions of ionizing radiation against human cells. By evaluating parameters such as reactive oxygen species production, DNA strand breaks, fragment analysis, repair mechanisms, and overall cell survival, one can understand the biological impact of particles with varying energies on the human cell.

Photo Nuclear Physics