Browsing by Subject "Photodisintegration"

Measurements of the three-body photodisintegration of 3He were performed at the High Intensity &gamma-ray Source (HI&gammaS). Neutrons emitted in this reaction inside a 3He gas target were detected with seven 12.7 cm diameter liquid scintillator detectors. Time-of-flight (TOF) and pulse-shape discrimination (PSD) techniques were used to identify neutron events. The absolute differential cross sections for the 3He(&gamma, n)pp reaction as a function of outgoing neutron scattering angle and energy were determined from the measurements at the incident &gamma-ray energies of 11.4, 12.8, 13.5, and 14.7 MeV to within a precision better than +/- 6 %.

The absolute cross sections at each incident energy are compared to the results of Gorbunov [Gor74], phase space calculations, and state-of-the-art three-body calculations. The inclusion of the Coulomb interaction in the three-body problem has been a long-standing challenge in theoretical nuclear physics. The present experimental data were found to be in good agreement with the state-of-the-art theory, which includes a full treatment of the Coulomb interaction between

the protons in the final state [Del05].

Experiment to measure the differential cross-section for photodisintegration of triton is being developed at Triangle Universities Nuclear Laboratory (TUNL). The goal of this experiment is to provide data for assessing the theoretical treatment of meson-exchange currents in photodisintegration of nuclei and for investigating long-range features of three-nucleon interactions. Measurements will be performed using a linearly polarized gamma-ray beam at the High Intensity Gamma-ray Source (HIγS) at TUNL. This thesis describes the Monte-Carlo simulation developed as a tool for aiding in the optimization of the experimental design and the data analysis software. The Monte-Carlo simulation is important for guiding the data acquisition strategy and interpretation of data. The simulation is based on the GEANT4 toolkit and theoretical cross-section predictions for current experimental setup.

The first measurements of the two- and three-body photodisintegration of longitudinally

polarized 3He with a circularly-polarized gamma-ray beam were carried out at the High Intensity gamma-ray Source facility located at Triangle Universities Nuclear Laboratory (TUNL). A high pressure 3He target, polarized via spin exchange optical pumping with alkali metals, was used in the experiments. The protons from the two-body photodisintegration experiment were detected using seventy two silicon surface barrier detectors of various thicknesses while the neutrons from the three-body photodisintegration were detected with sixteen 12.7 cm diameter liquid scintillator detectors. The spin-dependent cross sections and the contributions from the two- and three-body photodisintegration to the 3He Gerasimov-Drell-Hearn sum rule integrand were extracted and compared with state-of-the-art three-body calculations at the incident photon energies of 29.0 MeV (two-body) and 12.8, 14.7, and 16.5 MeV (three-body).

These are the first measurements of the contributions of the two- and three-body photodisintegration of 3He to the GDH integrand. These measurements were found to be in good agreement with the theoretical calculations which include the Coulomb interaction between protons in the final state. They also reveal-for the first time-the importance of the three-nucleon forces and the relativistic single-nucleon charge corrections which are responsible in the calculations for the observed difference

between the spin-dependent cross sections.

This experiment consisted of the planning of the photodisintegration of the triton at the High Intesity γ-ray Source and supporting experiments. The primary supporting experiment was photofission of 18O. The 18O photofission experiment verified the experimental techniques, analysis methods, and computational approaches that will be used for the photodisintegration of the triton. Neutrons were detected using an array of 30 liquid organic scintillators positioned at 3 angles in θ. Neutrons were detected both individually and in coincidence to map differential cross section as a function of angle. Measurements were made using circularly polarized γ-rays at 23.7 and 32.0 Mev. These measurements are the first anuglar differential cross section measurements performed for the 18O(γ,n) and 18O(γ,nn) reactions.

Also in support of photodisintegration of the triton, background neutron spectra were taken for the 27 Al(γ,n) reaction using the planned detector array. This provides in-situ feedback on expected signals and informed the proposed experimental setup for the photodisintegration of the triton. Theoretical predictions for tritium breakup were used to produce events processed by a GEANT4 simulation to assess the true coincidence versus interference coincidence rates. The results for these measurements and predictions are presented in this thesis.