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.

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.

We have measured cross sections for the (pi (+), pi (+/-)p) reactions on H-3, He-4, Li-6 and Li-7 in quasi-free kinematics at incident pion beam energy 500 MeV. An enhancement of the (pi (+), pi (-)p) cross section in this kinematics is observed. If this is interpreted as due to quasi-free scattering from preexisting Delta components of the nuclear wave function, the extracted probabilities are in agreement with theoretical expectations. (C) 2001 Elsevier Science B.V. All rights reserved.

Inclusive measurements of pion double-charge--exchange in 3He[l] and 4He[2] in the A(1232) resonance region suggest the dominance of a two-step sequentim single-charge-exchange mechanism involving quasi-free nucleons. To investigate this reaction mechanism, we have observed protons in coincidence with the outgoing pion in r + + 4He --* ~r+ 4p at T~+ = 240 MeV. Pions were detected in a magnetic spectrometer at 0~= 32 ~ while protons were detected in a close--packed array of plastic scintillator telescopes covering 0 v = 67.5* 157.5 ~ on the same side of the beam as the spectrometer, and 8p = 22.5* 157.5 ~ on the opposite side. We will present preliminary results for the distributions in energy and angle of the coincident protons. Two-nucleon correlations have long been a subject of study in nuclear physics, from the point of view of medium modifications to the NN interaction as well as nuclear structure issues. Two-nucleon processes play a large role in the absorption of photons[3] and pions[4, 5, 6] by nuclei. The contributions of these multi-nucleon mechanisms are in many cases difficult to study directly, since these mechanisms are by nature second order and may be obscured by first order processes. Pion-induced double charge exchange (DCX), however, must proceed via interactions with at least two nucleons (since two nucleons must change charge during the reaction) and thus affords the study of two nucleon processes directly. Over the last decade, the cross section for inclusive DCX has been systematically studied over a range of target mass and beam energy, with emphasis on the energy region near the A(1232) resonance[l, 2, 7, 8, 9]. Several distinct features of these data are worthy of mention. The total cross section for DCX as a function of A obeys a scaling law (a function only of A and Z) based on competition between DCX and other channels[9]. Only the helium isotopes differ significantly from this scaling behavior. Despite this apparent universality in the total cross section, there is a distinct change in the appearance of 542 9 1995 American Institute of Physics S. F. Pate et al. 543