Nuclear Reactions

Gains @ Gelina
Figure 1. Gains @ GELINA
847 keV
Figure 2. 847-keV γ production cross section from the 56Fe(n,nγ)56Fe reaction
Neutron Induced Reactions Cross Section Measurements
Contact persons: Alexandru NEGRET, Catalin BORCEA, Adina OLACEL, Marian BOROMIZA, Andreea OPREA, Liviu STOICA

High precision cross section data are required for the development of the next generation of nuclear reactors, the so-called Generation IV fission reactors. These are fast reactors, more secure than the current available reactors, producing a smaller quantity of radioactive waste and, hence, offering a reliable solution to the energetic needs of humanity while minimizing the quantity of CO2 emissions. A significant international effort is, therefore, made for the measurement of the neutron-induced reactions on all the isotopes of interest for the development of such facilities.

We are involved in high-precision neutron inelastic cross section measurements performed at the neutron time-of-flight facility GELINA operated by JRC-Geel in Belgium. The experimental setup consists of an array of 12 HPGe detectors operated together with a 235U fission chamber used to integrate the neutron flux (see Figure 1). These are located at the end of a 100-m flight path, allowing cross section measurements by combining the γ spectroscopy and the time-of-flight techniques.

The resulting data are unique in the world for their accuracy, neutron energy resolution and reliability. Figure 2 (taken from Ref. [3]) displays the production cross section for the first γ transition in 56Fe compared to other previous measurements, emphasizing the quality of our results.

A second experimental facility we use to measure neutron-induced reaction cross section is n_TOF, the neutron time-of-flight facility of CERN [7]. The neutron flux and the experimental setups installed there are complementary to the ones available at GELINA. They are particularly fit for neutron capture and neutron-induced fission measurements. Being part of the very broad n_TOF collaboration, we are primarily involved in two research topics: the effort of adapting the HPGe detectors to the particularly challenging experimental environment available at n_TOF and neutron capture cross section measurements on several target nuclei. For more details please see the references below or contact one of us.

[1] A. Negret, L.C. Mihailescu, C. Borcea, Ph. Dessagne, K.H. Guber, M. Kerveno, A.J. Koning, A. Olacel, A.J.M. Plompen, C. Rouki, and G. Rudolf, Cross section measurements for neutron inelastic scattering and the (n, 2n) reaction on 206Pb, Phys. Rev. C91, 064618 (2015).
[2] A. Olacel, C. Borcea, P. Dessagne, M. Kerveno, A. Negret, and A. J. M. Plompen, Neutron inelastic cross-section measurements for 24Mg, Phys. Rev. C 90, 034603 (2014).
[3] A. Negret, C. Borcea, Ph. Dessagne, M. Kerveno, A. Olacel, A. J. M. Plompen, and M. Stanoiu, Cross-section measurements for the 56Fe(n,xnγ) reactions, Phys. Rev. C 90, 034602 (2014).
[4] A. Negret, C.Borcea, A.J.M.Plompen, Neutron inelastic scattering measurements for background assessment in neutrinoless double beta decay experiments, Phys. Rev. C88, 027601 (2013).
[5] M. Kerveno, J. C. Thiry, A. Bacquias, C. Borcea, P. Dessagne, J. C. Drohe, S. Goriely, S. Hilaire, E. Jericha, H. Karam, A. Negret, A. Pavlik, A.J.M. Plompen, P. Romain, C. Rouki, G. Rudolf, and M. Stanoiu, Measurement of 235U(n, nγ) and 235U(n, 2nγ) reaction cross sections, Phys. Rev. C87, 024609 (2013).
[6] D.Deleanu, C.Borcea, Ph.Dessagne, M.Kerveno, A.Negret, A.J.M.Plompen, J.C.Thiry, The gamma efficiency of the GAINS spectrometer, Nucl. Instrum. Meth. Phys. Res. A624, 130 (2010).

Figure 3.
Charged Particle Induced Reactions Cross Section Measurements
Contact persons: Alexandru NEGRET, Catalin BORCEA, Adina OLACEL, Marian BOROMIZA, Liviu STOICA

The charged particle-induced reactions play an important role in many fields: nuclear structure investigations, astrophysics, medicine applications, etc. But our primary interest in investigating such reactions goes towards understanding the reaction mechanisms and the nucleon optical model potentials (OMPs), and to exploit this understanding in developing a procedure able to infer neutron-induced cross sections from charged particle ones. This implies, apart from measuring the corresponding charged-particle-induced cross sections, also a very serious reaction theory input. If successful, such an approach would then allow one to calculate the neutron-induced cross sections starting from simpler-to-measure reactions, like those induced by proton beams.

Proton and neutron optical model potentials are very alike from a formal point of view: up to the parametrisations employed, they are identical except for the Coulomb term (for protons) and the inverted sign of the isospin-breaking term [1]. As such, one can exploit this formal similarity to devise a procedure able to infer the neutron inelastic channel from the proton corresponding one. In doing so, a new type of projectile is introduced – which is neither fully a proton nor is it a neutron – whose optical potential is defined starting from the one of the proton. More precisely, starting from the proton we aim at making this new projectile as similar as possible to a neutron so that it would induce very similar reaction theory dynamics and, consequently, very similar inelastic cross sections.

The most important causes generating differences between neutron and proton inelastic cross sections are the Coulomb barrier (i.e. the Coulomb term in the OMP) followed by nuclear structure effects and very small isospin breaking terms. If, for example, one removes the Coulomb term from the proton OMP, the resulting projectile – which, consequently, does not feel any Coulomb barrier - generates fairly similar inelastic cross sections to the ones induced by neutrons. References [2,3] show how this and other changes of the proton OMP can be implemented and what results they yield for the particular case of the 16O, 28Si and 58Ni target nuclei (see Figure 3, taken from Ref. [2]). The same references also discuss important issues/open questions still to be addressed by our procedure aiming to relate the two nucleon-induced inelastic channels. If you wish to know more about this topic and about other present and future investigations of our group, please contact one of us.

The experiments measuring charged-particle cross sections are carried out at the 9-MV Tandem Accelerator of IFIN-HH and they make use of similar γ-spectroscopy techniques to the ones mentioned above for neutrons. Various setups are used, from a simple one using two HPGe detectors to the employment of the entire ROSPHERE array.

[1] Theory of Nuclear Reactions, P. Frobrich and R. Lipperheide, CLARENDON PRESS, OXFORD (1996)
[2] M. Boromiza, A. Negret, C. Borcea, A. Olacel et al., Nucleon inelastic scattering cross sections on 16O and 28Si, Phys. Rev. C101, 024604 (2020).
[3] A. Olacel , M. Boromiza, A. Negret, C. Borcea, et al., Nucleon inelastic scattering cross sections on 58Ni, submitted for publication to Physical Review C (2020).