The exotic ions that are created in nuclear reactions at the NSCL are delivered with a small range of velocities (up to approximately 5%). The larger the range of velocities that the experimenter can use, the larger will be the number of ions available. The range of a moving ion is directly related to its velocity, thus, ions with different kinetic energies will penetrate different distances into the gas cell. Weick and collaborators in Germany have studied ion-optical systems including degraders that can convert the spread of velocities into spreads in other properties of the beam. For example, a monoisotopic (one component) beam that has a distribution in velocity can be converted into a distribution in space and velocity by passing that beam through a simple dipole magnet. Such a beam is said to be dispersed according to it's momentum (in analogy to the way that white light is dispersed as it passes through a prism). If the higher velocity part of a dispersed beam is passed through the thicker part of a wedge and the lower velocity part is passed through the thinner part then the beams can come out of the wedge at the same (lower) velocity. This wedge is called a monoenergetic degrader and the shape of the wedge must be carefully designed. This technique is called energy bunching.
The real secondary beams at the NSCL have a distribution or spread in angles as well as in velocity. The ion-optical system shown in the accompanying figure was designed by Weick and collaborators to transform a non-dispersed and focused beam of exotic nuclei entering from the left into a focused beam that is dispersed in momentum at the (red) monoenergetic wedge. Lower velocity charged particles are bent more in a dipole (grey) magnetic field and arrive at the bottom of the wedge. The ions are recombined after they emerge and focused onto the window of the gas cell.