## 1 Introduction  
The characterization of the radiation field generated by the accelerators at the European Organization for Nuclear Research (CERN) is an essential task to ensure their smooth and reliable operation, preventing radiation-induced failures of critical equipment and electronics. For this purpose, the Radiation to Electronics (R2E) [1, 2] effort at CERN employs different types of radiation monitors distributed throughout the accelerator complex, including the beam loss monitor (BLM) system [3], the RadMon system [4, 5] and its battery-powered version (BatMon) [6], the distributed optical fibre system (DOFRS) [7], and passive high-level dosimeters [8]. As a possible addition, this work presents a recently developed Timepix3 Radiation Monitor setup, of which the suitability to be used in the scope of R2E radiation level monitoring activities at CERN is being assessed. Timepix detectors have been successfully applied for the following wide range of activities: medical radiotherapy [9], high-resolution photon counting [10], radiation monitoring on the International Space Station [11], 3D colour X-ray [12], radiation imaging [13], luminosity measurements and radiation field characterization in the ATLAS detector [14]. The Timepix technology presents several capabilities that are of interest for R2E monitoring activities at CERN accelerators, among which are the following: (i) the measurement of single-particle hits with good timing resolution, (ii) the possibility of obtaining information about the direction of incoming particles thanks to the pixel granularity, and (iii) the pixel-by-pixel measurement of the deposited energy with a low minimum threshold, which can be used to obtain particle LET measurements, or combined to obtain the total energy deposition measurements in the full module. By exploiting these features, the foreseen usage at CERN accelerators includes the monitoring of the Total Ionizing Dose (TID) via the total energy deposition measurements, as well as the flux measurement of charged particles. In both cases, the good timing resolution of the Timepix can enable the prompt detection of beam losses, and the directionality information can be used to identify their origin. This work aims at providing a first characterisation of the Timepix3 Radiation Monitor for CERN accelerator applications. In Section 2, the discussion begins by presenting the key features of the radiation environment in the tunnel of the Large Hadron Collider (LHC) [15], simulated using the FLUKA Monte Carlo code [16; 17; 18], CERN distributed, version 4.3.3. The radiation field includes different kinds of particles with broad energy spectra, all originating from the loss of TeV-scale beam particles or secondary collision products [19]. After this, a detailed description of the Timepix3 Radiation Monitor is presented in Section 3, starting from the hardware setup and the Timepix detection principle based on the measurement of the time-over-threshold (ToT) and time-of-arrival (ToA). As described in the section, the charge released by the interaction of ionizing particles within the pixel matrix is typically spread over multiple pixels, leading to the need for a cluster reconstruction algorithm to identify the individual particle hits. The Timepix calibration procedure that converts the measured ToT per pixel into deposited energy is a key subject of this paper. Several calibration techniques have been studied in the literature, either using internal test pulses [20; 21] or, alternatively, using test beams as follows: protons at 8 [22] or 5 [23] MeV, alpha sources below 2 MeV [24; 25; 26], low energy gamma ray sources [27], pions at 120 GeV [28], etc. In Section 3, the non-trivial relation between the ToT and the deposited energy per pixel in different energy ranges is presented, involving threshold effects at low energy, a linear regime at intermediate energies, and complex high-energy effects widely discussed in the literature, e.g., in Refs. [22; 23; 24].