Coincident neutron/proton pairs from lithium-6 photodisintegration with 40-100 MeV photons
Ritchie, Buddy Boyce
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https://hdl.handle.net/2142/23105
Description
Title
Coincident neutron/proton pairs from lithium-6 photodisintegration with 40-100 MeV photons
Author(s)
Ritchie, Buddy Boyce
Issue Date
1991
Doctoral Committee Chair(s)
Brussel, M.K.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Nuclear
Language
eng
Abstract
Bremsstrahlung photons with endpoint energies from 55 to 100 MeV were used to photodisintegrate $\sp6$Li 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 $\sp4$He nucleus. Cross sections for the $\sp6$Li($\gamma,$np) reaction were computed by normalizing the data to measured yields and published cross sections for the $\sp2$H($\gamma,$np) reaction.
"The $\sp6$Li target was in the form of solid metal foil. Protons were detected with a $\Delta$E-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 $\approx$4 m."
Cross sections were measured with the proton detector fixed at 81$\sp\circ$ (in the laboratory frame) for neutron angles of 50-120$\sp\circ.$ In addition, measurements were made with detectors at equal angles of 71-101$\sp\circ$ on opposite sides of the photon beam. The singly differential cross section $(d\sigma/d\Omega\sb{p})$ at $\theta\sb{p}$ = 81$\sp\circ$ was calculated by integrating over the neutron angle; its ratio to the deuterium cross section varied from 0.26 $\pm$ 0.03 at 45 MeV to 0.55 $\pm$ 0.12 at 95 MeV. Cross sections were also evaluated in terms of the recoil momentum of the residual $\sp4$He 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 $\pm$ 3.5 MeV/c (FWHM); this is consistent with a harmonic oscillator 2s wave function description of the pair-$\sp4$He relative motion with width parameter $q\sb0$ = 91.5 $\pm$ 3.3 MeV/c.
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