Preliminary Results

The collaboration between the CNRS-CRHEA and the IMPT-CAL started in 2018. Schottky and p-i-n diode structures based on GaN have been grown by metalorganic chemical vapor deposition (MOCVD) at CNRS-CRHEA. Square diodes with different size (i.e. 800, 300, 200, and 100µm) have been processed (top left picture), cut into 1x1 cm², and then mounted and wires bounded into 32 pins package (middle left picture). At the end, this device unit is plugged into a printed circuit board allowing single detector characterization (bottom left picture) but also horizontal translation in front of the 65 MeV proton beam MEDICYC line (top right picture).

The tested detectors clearly show very promising performances:

• Good sensitivity with detection down to proton current of 10 pA corresponding to 5x10⁵ protons per diode. This detection level is comparable to the one of current ionization chambers>
• Perfect characteristic linearity over 3 decades is obtained. This clearly demonstrates one of the fundamental prerequisites for detection use.
• Time response short enough for the targeted application (less than 1 sec) has been estimated. This is very important to reduce the duration of the proton beam calibration before patient irradiation, but it would also open the way to continuous real-time beam position measurements in the future
• No device performance degradation was observed up to a fluence of 0.5x10¹⁴cm^-2. As a comparison, the degradation fluence in silicon (Si) is found to be in the 10¹⁰-10¹² cm^-2 for proton energies of 50 MeV. This highlights the robustness of GaN and it superiority against Si for high-energy particles detection.

All these very encouraging preliminary proofs (detection limit, linearity, time response, material robustness, imaging capacity) motivated us to layout this project. Large efforts remain however necessary before delivering a complete imaging system.