Final answer:
A crossed or orthogonal grid in radiography does not allow for beam angulation and requires precise positioning. The particle accelerator known as a synchrotron uses oscillating electric fields to accelerate particles around a fixed radius track.
Step-by-step explanation:
The type of stationary grid that does not allow for any angulation of the beam and requires precise positioning is known as a crossed grid or orthogonal grid. In the context of radiography, this type of grid has strips of lead that are parallel to each other and at a right angle to the direction of the primary beam, prohibiting angulation. This necessitates very accurate placement and alignment of the x-ray tube to the grid to avoid grid cut-off, which can result in the loss of image quality.
Now, turning to the question about particle accelerators, the type of particle accelerator that uses oscillating electric fields to accelerate particles around a fixed radius track is called a synchrotron. A synchrotron accelerates particles by synchronizing the magnetic field strength with the increasing particle speeds, keeping particles on a constant circular path while allowing them to achieve very high energies.