Around the Lab

To measure neutron spectra with a dynamic range of up 10⁶ [which, for a deuterium–tritium (DT) implosion on OMEGA, spans an energy range from 1 MeV to 34 MeV], required a highly collimated line of sight to be developed over the last year at LLE. Identifying and constructing a suitable line of sight was challenging because of the presence of existing detectors, a scarcity of available locations around the target chamber, and unwanted background neutrons from scattering in the target chamber walls and surrounding concrete structures.

Two new neutron time-of-flight (nTOF) spectrometers, modeled after an earlier iteration designed at LLE in which the detectors are positioned much closer to the target chamber center at 13.4 m, were designed and constructed along a clear 22-m line of sight to the OMEGA target chamber. These diagnostics have recently begun to measure several key experimental parameters that are essential to diagnosing the performance of cryogenic DT implosions. The first spectrometer uses a four-microchannel-plate–photomultiplier-tube (MCP-PMT) detector design. It consists of a 20-cm-diam, 10-cm-thick stainless-steel cylindrical housing that contains a scintillation fluid as described below. This primary detector uses thin-wall (2-mm) construction to minimize neutron scattering within the scintillator housing. Thin (<0.3-cm) stainless-steel end plates seal the cylindrical housing to minimize neutron attenuation. The ports for the MCP-PMT's are 40 mm in diameter using fused-silica windows mounted on the cylindrical housing and sealed with Viton O rings. This diagnostic uses oxygenated xylene doped with diphenyl oxazole C₁₅H₁₁N_O + p-bis-(o-methylstyryl)-benzene (PPO + bis-MSB) wavelength-shifting dyes, which generates light emission in the visible to near-ultraviolet wavelength range (i.e., from 380 nm to 420 nm). This oxygenated liquid scintillator has a fast temporal response with a low-light afterglow required to measure neutrons in the low-energy region.

The second spectrometer employs a much simpler design using a thin, square plastic scintillator (BC-422Q) 5-cm × 5-cm × 0.5-cm, which is coupled to a single PMT. This design enables a fast response time required to measure the spectral moments of the primary peak distribution. To increase the signal-to-background ratio of the measurements, the spectrometers are located in a shielded, highly collimated line of sight. Three 10-in.-diam holes were cored through the thick concrete shield walls on OMEGA to allow for the insertion of two collimators. The first collimator is appropriately called the defining aperture and is used to “define” the neutron beam to reduce the detector’s field of view from the target chamber. The second collimator is located close to the time-of-flight diagnostic and acts as a cleanup collimator to fine tune the incoming neutron flux.

Two current University of Rochester Physics and Astronomy graduate students, Owen Mannion and Zaarah Mohamed, are using these new diagnostic capabilities in concert with additional spectrometers located around the target chamber. Mannion has developed a technique using the new 22-m collimated line of sight to measure both the magnitude and direction of collective motion of the hot spot. Such collective motion represents residual kinetic energy present in the fusing hot spot. Residual kinetic energy results from an asymmetric implosion not coalescing perfectly at the center of the target. It is an experimental signature indicating a colder and less dense plasma being formed at stagnation. The new 22-m line of sight enables the first measurements of collective motion of the hot spot in spherical direct-drive implosions. These data, along with ion temperature measurements made along five separate lines of sight, are now used to diagnose and characterize the extent of the asymmetric compression of hot spots in OMEGA cryogenic implosions. Understanding asymmetric hot-spot formation will be crucial in optimizing and improving future experiments.

Another important experimental observable is the angularly resolved energy of elastically scattered neutrons that scatter in the dense DT shell. The neutron energy spectrum contains valuable information concerning the areal density and the cold fuel distribution. Detectors placed along multiple lines of sight can produce a mapping of areal density within different regions of the target. Analysis from some detectors, such as the xylene detectors located along the 13-m and 22-m lines of sight, can also extend to cover a wide range of energies (corresponding to the neutron scattering angle) for the purpose of inferring variations in areal density. Mohamed has combined this analysis with information from other detectors that are located along different lines of sight to provide additional information concerning the symmetry of compressed targets. An understanding of existing asymmetries is necessary in order to design future experiments that will have improved target performance as a result of increasingly symmetric implosions.