Treating cancer with a proton beam is a bit like archery: you need to hit the bull's-eye without hitting anything else. Protons release their energy at the end of their path, so healthy tissues are largely spared. But to make the shot safe, every treatment plan is checked in advance — measuring the dose the tumor will get and what the surrounding organs will receive.

Typical detectors for this check, ionization chambers, are bulky and slow. They struggle with angled beams and don't show how the dose changes with depth. Scientists proposed a different tool: a detector made of gallium oxide, a semiconductor crystal that changes its resistance when exposed to radiation. The crystal is as small as a fingernail and responds to protons instantly.

Tests at the MEVION S250i accelerator showed that the gallium oxide detector produces a signal directly proportional to the proton dose. At different beam energies, its readings matched those of an ionization chamber and the calculated dose distributions. This means the crystal can be used to verify proton therapy plans: it's accurate, fast, and doesn't mind angled beams.

Such a detector is like a miniature sentinel at the accelerator's gate. It catches every proton pulse and reports how much energy went into the target. The more accurate this check, the more confidently doctors adjust the treatment and the lower the risk for the patient.