## 2 The LHC Mixed Radiation Field  
The operation of particle accelerators at CERN generates a mixed radiation field that contains different types of particles with broad energy ranges [19]. The composition of the radiation field varies significantly depending on the accelerator under exam, on the position within the accelerator, and on the amount of shielding between the radiation source and the location of interest. When considering the LHC [33], the source of the radiation showers can be the inelastic collisions in the interaction points (IPs), the interactions of the beam with residual gas molecules in the beam pipes, or the interaction of the beam with LHC beam intercepting devices (typically collimators or absorbers).  
Figure 1 illustrates an example of the radiation field composition in the LHC tunnel in the proximity of the final focusing magnets at 50 m from IP1, hosting the ATLAS experiment [34], in the form of an energy spectrum in lethargy units simulated with the FLUKA code. In this position, the radiation field originates from the secondary products of inelastic proton-proton collisions in the IP, and the spectrum is obtained in a 20 \(\times\) 20 \(\times\) 20 cm\({}^{3}\) volume at floor level around 1 m below the accelerator beam line. The radiation field contains charged hadrons (mostly pions and protons) with energies up to several GeV, neutrons with energy extending from the meV range (thermal) to the GeV scale, electromagnetic particles (photons, electrons, positrons) from below the MeV scale to the GeV scale, and muons up to tens of GeV. Instead, the environment is characterised by the absence of heavy ions and by a scarce presence of alpha particles, while the relative contribution of the different particle types and the respective energy distributions can vary depending on the position, the spectrum given in Figure 1 is used as the reference radiation field for which the Timepix3 Radiation Monitor should be optimised.  
The existing radiation monitors used for machine protection (BLMs) and for R2E purposes (BLMs, RadMons, DOFRS, and high-level dosimeters) are all capable of measuring the total ionising dose (TID) via different technical solutions [3; 4; 5; 6; 7; 8]. The RadMon system can also measure single event effects (SEEs) induced by high energy hadrons (HEH) and thermal neutrons in SRAM memories, calibrated to measure the respective flux, and it also provides measurements of the silicon 1-MeV neutron equivalent fluence via p-in-n  
Figure 1: Energy spectra (in lethargy units) simulated with FLUKA at floor level below the beam in the LHC accelerator tunnel near the IP hosting the ATLAS experiment for the 2018 operational conditions, right downstream of the inner triplet magnets. The vertical continuous (dotted) grey lines indicate the lower energy threshold set for hadrons and gammas (electrons and positrons) in the simulation, i.e., 10\({}^{-4}\) GeV (10\({}^{-3}\) GeV) [19].  
diodes. The Timepix3 Radiation Monitor presented in this work is also designed to be used to measure the TID by summing the energy deposited in its pixels, but it can also be used for charged particle flux measurements. Its excellent time- (1.25 ns) and spatial-resolution (provided by the pixel array with a pixel pitch of 55 \(\upmu\)m) make it an optimal instrument to promptly detect radiation showers caused by localised beam losses and, in some circumstances, to provide information about their origin.