Quick Shot

Neutron spectroscopy is a primary diagnostic technique for inertial confinement fusion research on the OMEGA Laser System. Neutron time-of-flight (nTOF) spectrometers having a high signal dynamic range are used routinely on OMEGA to infer the stagnated core conditions of laser direct-drive DT cryogenic implosions, such as the primary neutron yield, the average ion temperature, the compressed areal density, and hot-spot flow velocity. Three orthogonal lines-of-sight (LOS’s) are necessary to observe and diagnose the multidimensional effects on hot-spot formation. The second of three shielded and collimated LOS’s was commissioned in November 2018. It penetrates the OMEGA and OMEGA EP shield walls and is further collimated by a 1-m-long polyethylene tube. The end of the white collimator tube can be seen in the background of the photograph above. The collimation and shielding were designed with a neutron transport code to minimize neutron scattering from the OMEGA target chamber and surrounding support structure. The two nTOF spectrometers, shown in the foreground, are positioned about 22 m from the OMEGA target chamber center (TCC). The collimated neutron beam propagates through the first nTOF spectrometer (see the narrow black strip in front of the stainless-steel housing in the photograph), consisting of a small plastic scintillator (5.0 cm × 5.0 cm × 0.5 cm) coupled to a single photomultiplier tube (PMT), and then propagates through the second nTOF spectrometer (see the stainless-steel housing in the photograph), consisting of a liquid crystal scintillator (oxygenated xylene) coupled to four PMT’s. The collimated neutron beam does not interact directly with any of the PMT’s. These nTOF spectrometers are similar to those in the first LOS, which has been operating on OMEGA 13.4 m from TCC along the TIM-6 axis in LaCave since 2012. A third LOS is planned for 2020.