Citation: Meyer, A.; Lambert, D.; Morana, A.; Paillet, P.; Boukenter, A.; Girard, S. Simulation and Optimization of Optical Fiber Irradiation with X-rays at Different Energies. Radiation 2023, 3, 58–74. https://doi.org/10.3390/ radiation3010006 Academic Editor: Leonardo Abbene Received: 14 February 2023 Revised: 6 March 2023 Accepted: 15 March 2023 Published: 20 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Simulation and Optimization of Optical Fiber Irradiation with X-rays at Different Energies Arnaud Meyer 1 , Damien Lambert 2 , Adriana Morana 1 , Philippe Paillet 2 , Aziz Boukenter 1 and Sylvain Girard 1, * 1 Laboratoire Hubert Curien, UMR-CNRS 5516, Université Jean Monnet, F-42000 Saint-Etienne, France; arnaud.meyer@univ-st-etienne.fr (A.M.); adriana.morana@univ-st-etienne.fr (A.M.); aziz.boukenter@univ-st-etienne.fr (A.B.) 2 CEA, DAM, DIF, 91297 Arpajon, France; damien.lambert@cea.fr (D.L.); philippe.paillet@cea.fr (P.P.) * Correspondence: sylvain.girard@univ-st-etienne.fr Simple Summary: We investigated the influence of modifying the voltage of an X-ray tube, and therefore its photon energy spectrum, on the Total Ionizing Dose deposited in a single-mode, radiation sensitive, optical fiber. Simulation data, obtained using a toolchain combining SpekPy and Geant4 software, are compared to experimental results and demonstrate an increase of the deposited dose with operating voltage, which is mainly caused by low-energy photons below 30 keV. Abstract: We investigated the influence of modifying the voltage of an X-ray tube with a tungsten anode between 30 kV and 225 kV, and therefore its photon energy spectrum (up to 225 keV), on the Total Ionizing Dose deposited in a single-mode, phosphorus-doped optical fiber, already identified as a promising dosimeter. Simulation data, obtained using a toolchain combining SpekPy and Geant4 software, are compared to experimental results obtained on this radiosensitive optical fiber and demonstrate an increase of the deposited dose with operating voltage, at a factor of 4.5 between 30 kV and 225 kV, while keeping the same operating current of 20 mA. Analysis of simulation results shows that dose deposition in such optical fibers is mainly caused by the low-energy part of the spectrum, with 90% of the deposited energy originating from photons with an energy below 30 keV. Comparison between simulation and various experimental measurements indicates that phosphosilicate fibers are adapted for performing X-ray dosimetry at different voltages. Keywords: optical fibers; X-ray tubes; Geant4; radiation effects; dosimetry 1. Introduction 1.1. Interest of X-rays for Radiation Testing Radiation testing can involve a variety of ionizing radiation sources, such as photons, protons, electrons, neutrons, or heavy ions. The choice of a certain type of radiation source depends on multiple factors, including conformity to a target environment, emphasis on certain physical processes, and observation of standard practices. Availability and ease of use are other factors that play a role in the actual planning of such radiation testing. In this regard, X-ray tests have significant advantages over other kinds of radiation sources. X-ray tubes, in particular, have been used for over a century for various applications, ranging from medical imaging [1] to material characterization [2]. These sources of high-energy photons, typically up to several hundreds of keV, are avail- able commercially and therefore relatively easy to procure, install and manipulate safely, compared, for instance, to radioisotope sources. A typical X-ray tube contains a cathode and an anode, both sealed in a vacuum. The cathode is typically a filament through which a very small electrical current circulates, on the order of several mA. A very high voltage, on the order of tens to hundreds of kV, is Radiation 2023, 3, 58–74. https://doi.org/10.3390/radiation3010006 https://www.mdpi.com/journal/radiation