Coincident Neutron/proton Pairs from LITHIUM-6 Photodisintegration with 40-100 Mev Photons

Bremsstrahlung photons with endpoint energies from 55 to 100 MeV were used to photodisintegrate ^6Li nuclei, and emitted neutron/proton pairs were observed in coincidence. Measurement of the energies and angles of the emitted pair allowed reconstruction of the photon energy, as well as the recoil angle and energy of the residual ^4He nucleus. Cross sections for the ^6Li(gamma, np) reaction were computed by normalizing the data to measured yields and published cross sections for the ^2H(gamma,np) reaction. The ^6Li target was in the form of solid metal foil. Protons were detected with a DeltaE-E scintillator telescope, and their energy measured by the amount of energy deposited in the "E" element (a NaI crystal). The neutron detector was a large solid angle array of long plastic scintillator elements. Neutron trajectories were resolved within the coverage of the array through a combination of detector element identity and the difference in arrival times of scintillation photons at either end of that element. Neutron energy was determined by measuring the time of flight of the neutron over a distance of ~4 m. Cross sections were measured with the proton detector fixed at 81^circ (in the laboratory frame) for neutron angles of 50-120^circ. In addition, measurements were made with detectors at equal angles of 71-101^circ on opposite sides of the photon beam. The singly differential cross section (dsigma/dOmega_ {p}) at theta_{p } = 81^circ was calculated by integrating over the neutron angle; its ratio to the deuterium cross section varied from 0.26 +/- 0.03 at 45 MeV to 0.55 +/- 0.12 at 95 MeV. Cross sections were also evaluated in terms of the recoil momentum of the residual ^4He nucleus, which should reflect the combined momentum of the neutron-proton pair prior to the photoabsorption. The distribution of recoil momentum transverse to the photon beam had a central peak of width 96.8 +/- 3.5 MeV/c (FWHM); this is consistent with a harmonic oscillator 2s wave function description of the pair- ^4He relative motion with width parameter q_0 = 91.5 +/- 3.3 MeV/c.