Differential cross section for neutron-proton bremsstrahlung

Abstract

The neutron-proton bremsstrahlung process (np → npγ) is known to be sensitive to meson exchange currents in the nucleon-nucleon interaction. The triply differential cross section for this reaction has been measured for the first time at the Los Alamos Neutron Science Center, using an intense, pulsed beam of up to 700 MeV neutrons to bombard a liquid hydrogen target. Scattered neutrons were observed at six angles between 12 • and 32 • , and the recoil protons were observed in coincidence at 12 • , 20 • , and 28 • on the opposite side of the beam. Measurement of the neutron and proton energies at known angles allows full kinematic reconstruction of each event. The data are compared with predictions of two theoretical calculations, based on relativistic soft-photon and non-relativistic potential models.PACS numbers: 13.75. Cs, 25.10.+s A quantitative description of the nucleon-nucleon interaction is one of the primary goals of nuclear physics. Elastic proton-proton and neutron-proton scattering have been studied in detail. There is long-standing theoretical interest in nucleon-nucleon bremsstrahlung (N N → N N γ) as the simplest inelastic process in nucleon-nucleon scattering. The final state in this reaction contains only one additional particle, which interacts only electromagnetically. Bremsstrahlung probes the nucleon-nucleon interaction in a kinematic regime intermediate between elastic scattering (angle between scattered nucleons θ N N ≃ 90 • ; E γ = 0) and neutron-proton ra- The lowest-order Feynman diagrams for N N bremsstrahlung are shown in Fig. 1: (a) the "external" diagram, in which the photon is emitted by one of the nucleons 1 , (b) the "internal" diagram in which the photon is emitted during the N N interaction, and (c) the rescattering diagram. The two N N bremsstrahlung reactions that are accessible to experimental study are proton-proton bremsstrahlung (ppγ) and neutron-proton bremsstrahlung (npγ). The physics of npγ differs from that of ppγ. Since the neutron and proton can interact via the exchange of a charged meson, the internal diagram contributes in first order. Moreover, in npγ electric dipole (E1) radiation is allowed, whereas in ppγ the lowest allowed multipolarities are E2 and M1. As a consequence, ppγ cross sections are approximately an order of magnitude smaller than npγ cross sections. Nonetheless, many fewer experiments on npγ than on ppγ have been attempted, owing to the difficulty of producing intense neutron beams and of detecting at least one uncharged particle.Prior to this work, no kinematically complete measurements of npγ cross sections had been per-1 The photon may be emitted either before or after (as shown) the N N interaction, and by either nucleon if both are charged.