Speaker
Description
Abstract
Thermal neutron cross sections of hydrogenous materials are a fundamental ingredient for the simulation and design of, amongst other things, neutron moderators at research centres and nuclear reactors. However, experimental thermal cross sections are available for just a handful of materials (see, e.g., Ref. 1]). In order to match the level of detail in modern Monte Carlo nuclear transport
codes, the possibility to calculate thermal cross sections from simplified models, molecular dynamics, and ab initio calculations has been a topic of recent development (see, e.g., Refs. [1, 2]).
We present new applications of the recently developed Average Functional Group
Approximation (AFGA) for the determination of total neutron scattering cross sections at thermal neutron energies [3]. While using only a limited number of effective vibrational densities of states, representing the hydrogen dynamics of functional groups within organic molecules, the model allows an accurate reproduction, within the incoherent approximation, of the total cross sections from a vast series of polymers, including materials for neutron moderation or phantoms for calculations related to radiation protection.
The model was developed based on neutron transmission measurements at the VESUVIO
spectrometer of the ISIS Neutron and Muon Source [4] to determine the total neutron cross section of all proteinogenic amino acids. Each molecular system was then simulated using phonon calculations, and the hydrogen-projected vibrational density of states within different functional groups in the molecule was isolated. For each functional group, the vibrational density of states was averaged over all amino acids containing it. This procedure effectively corrected each function
(representing a perfect single crystal) for the disorder and correlation phenomena that the same functional group experiences in a macromolecule or an organic polymer. Such large systems, whose modelling with atomistic simulations is prohibitively challenging, can be simply rationalised in the AFGA as a combination of contribution from our average functional groups.
Figure 1 reports the total cross section of hydrogen within several functional groups from the AFGA, showing the importance of taking into account the chemistry of the material at thermal neutron energies. In order to facilitate its use, the AFGA is now avail able within the NCrystal environment [2].
References
[1] Macfarlane, R., Muir, D. W., Boicourt, R. M., Kahler, III, A. C., and Conlin, J. L. The njoy nuclear data processing system, version 2016. Los Alamos National Laboratory report LAUR-17-20093 (2017).
[2] Cai, X.-X., and Kittelmann, T. Ncrystal: A library for thermal neutron
transport. Computer Physics Communications 246 (2020), 106851.
[3] Romanelli, G., Onorati, D., Ulpiani, P., Cancelli, S., PerelliCippo, E., Damián, J. I. M., Capelli, S. C., Croci, G., Muraro, A., Tardocchi, M., Gorini, G., Andreani, C., and Senesi, R. Thermal neutron cross sections of amino acids
from average contributions of functional groups. Journal of Physics: Condensed
Matter 33, 28 (may 2021), 285901.
[4] Robledo, J., Dawidowski, J., Damián, J. M., Skoro, G., Bovo, C., and Romanelli, G. Measurement of neutron total cross sections at the vesuvio spectrometer. Nucl. Instrum. Methods Phys. Res., Sect. A 969 (2020), 164096.