Final answer:
The Positive Beam Limiting device is used in medical imaging to minimize patient exposure to radiation, while a beam deflector uses magnetic fields to manipulate the direction of charged particles, such as alpha particles, in research applications.
Step-by-step explanation:
The Positive Beam Limiting (PBL) device is a concept related to radiation therapy in the field of medicine. While the provided question concerns a beam deflector system for the manipulation of alpha particles in a research context, PBL is not directly related to this topic. The PBL is a feature in x-ray imaging systems, that automatically adjusts the size and shape of the primary x-ray beam to match the size and shape of the imaging device sensor or film, minimizing patient exposure to unnecessary radiation.
In the scenario given, to transport and direct alpha particles, a magnetic field is used to bend the beam through a 90-degree region. Therefore, the direction of the applied magnetic field should be perpendicular to the velocity of the alpha particles for them to experience a centripetal force. The time taken for the alpha particles to traverse the uniform magnetic field region can be determined by calculating the velocity of the particles and the arc length of the 90-degree bend.