Nimrod is a constant-gradient or weak-focusing proton synchrotron, designed for a maximum energy of 7 GeV, at the Rutherford High Energy Laboratory of the National Institute for Research in Nuclear Science near the Atomic Energy Research Establishment, Harwell. The final decisions on the design of the most important features of the machine were made early in 1957, and construction was begun in July of that year. A general view of Nimrod is shown in figure 95, plate 35. Protons are accelerated to an energy of 15 MeV in a linear accelerator (figure 99, plate 37), and are then injected into the magnet ring of the main accelerator with the help of an achromatic system of bending magnets and deflecting electrodes. This injection system also contains matching lenses, consisting of quadrapole magnets, to assist in transferring the maximum number of protons from the injector into the magnet ring. The magnet ring has a diameter of 150 ft., and is divided into eight sectors separated by field-free sections for convenience in inj ecting, accelerating and extracting the proton beam (figure 97, plate 36). The magnet weighs 7000 tons and is laminated in the usual way to reduce effects due to eddycurrents as the magnetic field rises during acceleration. It is a feature of weakfocusing synchrotrons that a large magnet aperture is required to contain the proton beam, and in Nimrod the useful aperture is 36 in. radially by 9 in. vertically. The large aperture has the advantage that injection can take place over a relatively long time as the injected protons spiral inwards in a slowly rising magnetic field, thus giving a high beam intensity. In Nimrod, the injection time can be adjusted between limits of 200 p,s and 2 ms; there will be an optimum time, to be determined by operational experience.