The particle accelerators I know of are much too narrow to go inside. The beam pipes are typically smaller than 10 cm in diameter. You cannot stick your head in them.

If you were to put a rat inside, it would experience the following:

1. When the air is sucked out of the accelerator (beam pipes and cavities) to create a vacuum, the rat will suffocate to death. This is where the story ends for the rat. Perhaps it'll experience going through a tunnel with light at the other end where he sees green fields and a beautiful light with his dead rat friends welcoming him.

2. To get rid of the rest of the air, the beam pipe and cavities are heated up to some 300 degrees celsius. The remains of the rat will slowly cremate until only solids are left. There is a good chance that the vacuum technicians will notice a virtual leak (source of gas) at the place where the rat is and open up the accelerator to have a look.

3. When vacuum is established, then - when the accelerator gets started up, the operators will notice an obstruction that affects the beam. They will stop the accelerator, investigate and remove the rat.

Another scenario is where you are standing next to the accelerator while it is running. This could happen if you were hiding somewhere while a patrol is clearing the area or if they somehow agreed to leave you there. If it is a low energy accelerator like a TV set, you will experience nothing. If it is a high energy accelerator you will be irradiated and sooner or later die. If you survive, you will suffer from radiation sickness.

Lastly, if you somehow manage to put yourself in front of a particle beam like Anatoli Bugorski then a high energy beam will slice through you like a very fast and thin bullet. Depending where it hits you it may do a little damage or none.