The PEP-N interaction region

The PEP-N project consists of a small, very low-energy e ^{storage ring (VLER) located in one of the interaction-straight regions of PEP-II. The small ring is brought into collision with the low-energy (3.1 GeV) e+ beam (LER). The center-of-mass energies from this collision are between the Φ and J/ψ resonances. We achieve a head-on collision through the use! of a central magnetic dipole field that generates a large horizontal bending field. This field is also the central field of the detector. The large energy range of the VLER, in order to maximize the center-of-mass energy range, complicates the collision point geometry. In order to maintain the beam orbits near the collision point two techniques are used. The first is to scale the central dipole field up and down with the energy of the VLER and the second is to use passive shielding to decrease the integral B·dl of the dipole field seen by the VLER. Changes in the orbit of the LER are corrected with local bending magnets. Further details of the interaction region geometry as well as design issues that include synchrotron radiation from the high-current positron beam are discussed}