This has actually happened:
Anatoli Bugorski was a Russian physicist working at the  Institute for High Energy Physics on an accelerator called the  U-70 synchrotron. In 1978, he stuck his head inside the tube in order to diagnose a problem, when a safety mechanism failed, and the particle beam hit him in the head.https://www.quora.com/What-would-happen-if-a-person-was-inside-a-particle-accelerator-while-it-was-running