A magnet pole for an isochronous sector-focused cyclotron comprising hill and valley sectors alternatively distributed around a central axis, Z, each hill sector comprises an upper surface bounded by four edges: an upper peripheral edge, an upper central edge, a first and a second upper lateral edges, and a peripheral surface extending from the upper peripheral edge to a lower peripheral line. The upper peripheral edge of at least one hill sector further comprises a concave portion with respect to the central axis defining a recess extending at least partially over a portion of the peripheral surface of the corresponding hill sector
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