An example system includes a particle accelerator to produce a particle beam to treat a patient and a carrier having openings including a first opening and a second opening. The carrier is made of a material that inhibits transmission of the particle beam and the carrier is located between the particle accelerator and the patient. A control system is configured to control movement of the particle beam to the first opening to enable at least part of the particle beam to reach the patient, to change an energy of the particle beam while the particle beam remains stationary at the first opening, and to control movement of the particle beam from the first opening to the second opening. The example system also includes an energy degrader that includes at least some boron carbide.