B:photonuclearreactions

B: Photonuclear reactions

Laser quanta bouncing off of relativistic electrons may get their energy boosted into the gamma-ray regime. Experiments at the S-DALINAC complement research foreseen to be carried out at ELI-Nuclear Physics. Picture: Jan-Christoph Hartung, all rights reserved.
Laser quanta bouncing off of relativistic electrons may get their energy boosted into the gamma-ray regime. Experiments at the S-DALINAC complement research foreseen to be carried out at ELI-Nuclear Physics. Picture: Jan-Christoph Hartung, all rights reserved.

The back-scattering of intense laser pulses from an ultra-relativistic electron beam provides high-energy electromagnetic radiation in the X-ray and gamma-ray regimes. These novel radiation sources allow to probe atomic nuclei with intense polarized gamma-ray beams whose energy is precisely known. The projects of project area B focus on the investigation of collective excitations of medium-mass and heavy nuclei, i.e., where a large number of the protons and neutrons making up the atomic nucleus participate in a joint motion when excited by a gamma-ray. With the planned experiments, the response of the nuclear system to electromagnetic radiation at or above the particle separation threshold will be probed: How will the nucleus decay? What (and how many) quantum states are populated? In case of very heavy nuclei, will they burst into large fission fragments? Until the intense, monochromatic gamma-ray beam at ELI-Nuclear Physics in Romania comes online, experiments will be carried out with bremsstrahlung at TU Darmstadt’s S-DALINAC and at other facilities such as HIgS at Duke University in North Carolina, USA.

Project area coordinator: Professor Dr. Joachim Enders ()

Project B1: Nuclear dipole response at the particle-separation threshold

(PIs: Professor Dr. Thomas Aumann, Professor Dr. Dr. h.c. mult. Norbert Pietralla)

Knowing how a nucleus responds to gamma rays right at the energy where it starts to fall apart is particularly interesting for nuclear structure, understanding nuclear matter, and as input for nuclear astrophysics. B1 studies in collaboration with neutron-detector development in C1 this energy range in medium-mass nuclei.

Project B2: Nuclear level densities and gamma strength functions

(Pis: Dr. Johann Isaak, Professor Dr. Dr. h.c. mult. Norbert Pietralla)

Bound quantum systems are characterized by discrete states. But how many are there? Precise answers to this question are known usually only right at the particle separation threshold and at very low energies. B2 investigates an innovative approach using self-absorption.

Project B3: Fragmentation and decay of collective dipole excitations

(PIs: Professsor Dr. Dr. h.c. mult. Norbert Pietralla, Dr. Johann Isaak)

When driving a system of two masses connected by a spring with an external periodic force, one finds a resonance phenomenon. Similarly, a prominent resonance of protons and neutrons vibrating against each other is observed when nuclei are “driven” by electromagnetic radiation. B3 – using the spectrometer R&D from project C3 – investigates how the nuclei decay after having carried out such a resonant vibration.

Project B4: Photofission

(PIs: Professor Dr. Joachim Enders, Professor Dr. Thorsten Kröll)

That heavy excited nuclei fall apart into usually two major fragments is known since about 80 years ago when Otto Hahn discovered nuclear fission and Lise Meitner and Otto Frisch subsequently explained the observations. B4 – complementary to the neutron-induced experiments in A4 – studies for the first time in one experiment how the nuclei break apart, in what directions the fragments are released and what energy the fragments carry.