First test beam of the DMAPS-based proton tracker at the pMBRT facility at the Curie Institute.

Abstract

\textbf{Objective.} Proton radiotherapy's efficacy relies on an accurate Relative Stopping Power (RSP) map of the patient to optimise the treatment plan and minimize uncertainties. Currently, a conversion of a Hounsfield Units (HU) map obtained by a common X-ray Computed Tomography (CT) is used to compute the RSP. This conversion is one of the main limiting factors for proton radiotherapy. To bypass this conversion a direct RSP map could be obtained by performing a proton CT (pCT). The focal point of this article is to present a proof of concept of the potential of fast pixel technologies for proton tracking at clinical facilities.

\textbf{Approach.} A two-layer tracker based on the TJ-Monopix1, a Depleted Monolithic Active Pixel Sensor (DMAPS) chip initially designed for the ATLAS, was tested at the proton MiniBeam RadioTherapy (pMBRT) beamline at the Curie Institute. The chips were subjected to 100 MeV protons passing through the single slit collimator (0.4$\times$20 mm$^2$) with fluxes up to $1.3 \times 10^7$ p/s/cm$^2$. The performance of the proton tracker was evaluated with GEANT4 simulations.



\textbf{Main Results:} 
The beam profile and dispersion in air were characterized by an opening of 0.031~mm/cm, and a $\sigma_x=0.172$~mm at the position of the slit. The results of the proton tracking show how the TJ-Monopix1 chip can effectively track protons in a clinical environment, achieving a tracking purity close to~70~$\%$, and a position resolution below 0.5 mm; confirming the chip's ability to handle high proton fluxes with a competitive performance.

\textbf{Significance:} This work suggests that DMAPS technologies can be a cost-effective alternative for proton imaging. Additionally, the study identifies areas where further optimization of chip design is required to fully leverage these technologies for clinical ion imaging applications.