Kinematically complete measurements of three-body photodisintegration of $^3$He were performed at the High Intensity $\gamma$-ray Source (HI$\gamma$S) with nearly monoenergetic 15 MeV photons. The experiment relied on two-nucleon coincidence measurements in which the nucleons are emitted on opposite sides of the incident $\gamma$-ray beam axis. The setup consisted of seven 10 cm long cylindrical gas targets pressurized near 4 atm with thin windows to allow low-energy charged particles to exit with acceptable energy loss. Charged particles were detected in silicon strip detectors with angular acceptance constrained by a collimator system. Neutrons were detected in arrays of liquid organic scintillator cells. Data for neutron-proton (np) coincidences were acquired in configurations which selectively include or exclude the np final state interaction. Measurements of proton-proton (pp) coincidences along the same kinematic locus containing the np final state interaction (FSI) were also taken in-situ. Products from the two-body reaction were used as a luminosity monitor. Theory predictions were propagated through a GEANT4 simulation of the experimental setup. There was good agreement between predictions and measurements in the vicinity of the collinear point in which a proton remains at rest as measured by np coincidences. The measured np FSI peak included additional low-energy neutrons not anticipated by the simulation, which are likely associated with intermediate neutron scattering. The np FSI peak was found to be underpredicted by about 20$\%$. The pp coincidence data were consistently about 39$\%$ above predictions.
Nuclear physics and radiation, Physics, Particle physics
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