Characterization and optimization of signal and background for the time-resolving magnetic recoil spectrometer on the National Ignition Facility

Abstract

The evolution of fuel assembly, hot-spot formation, and nuclear burn in an Inertial Confinement Fusion (ICF) implosion at the National Ignition Facility (NIF) can be quantified through time-resolved measurements of the neutron spectrum. This information will be obtained with the next-generation Magnetic Recoil Spectrometer (MRSt) that will measure the neutron spectrum (12-16 MeV) with high accuracy (~5%), unprecedented energy resolution (~100 keV) and, for the first time ever, time resolution (~20 ps). To successfully implement the MRSt on the NIF for this measurement, the signal and background distributions at the MRSt detector must be characterized; the detector response to the signal and background must be determined; and the shielding enclosing MRSt must be designed and implemented to reduce the background to the required level. These things have been done, which constitute the main results of this thesis. First, an MCNP model of the MRSt in the NIF target bay was implemented to assess the neutron- and gamma-background fluxes at an unshielded MRSt. Second, models of the MRSt-detector response to the signal protons (or deuterons), and neutron and gamma background were implemented to assess the signal-to-background (S/B) for the unshielded MRSt case. Using these models, it is discussed in this thesis that the combined neutron and gamma background in the MRSt data needs to be reduced 100-400 times. Third, a shielding design, consisting of polyethylene, tungsten, and stainless steel, fully enclosing the MRSt, was developed to reduce the background to the required level. This design reduces the background 100-200 times, and meets the requirement of S/B > 5 for the down-scattered-neutron measurement. Obviously, this design depends on the MRSt-detector response to the signal and background, and some minor adjustments to the design might be applied depending on the results from the upcoming measurements of the MRSt-detector response to signal and background. As the shielding design depends on the engineering design of the MRSt system, which has not been fully defined yet, some adjustments to the design will most likely be made when the MRSt engineering design is finalized.