A neutron spectrometer for precise measurements of DT neutrons from 10 to 18 MeV at OMEGA and the National Ignition Facility

A model independent method to determine fuel [rhoR] is to measure the energ y spectrum and yield of elastically scattered primary neutrons in deuterium -tritium (DT) plasmas. As is the case for complementary methods to measure fuel [rhoR] (in particular from knock-on deuterons and tritons [S. Skupsky and S. Kacenjar, J. Appl. Phys. 52, 2608 (1981); C. K. Li (unpublished)]), minimizing the background is critical for successful implementation. To ach ieve this objective, a novel spectrometer for measurements of neutrons in t he energy range 10-18 MeV is proposed. From scattered neutrons (10-13 MeV), the DT fuel [rhoR] will be measured; from primary neutrons (similar to 14 MeV), the ion temperature and neutron yield will be determined; and from se condary neutrons, in the energy range 12-18 MeV, the fuel [rhoR] in deuteri um plasmas will be inferred at the National Ignition Facility. The instrume nt is based on a magnetic spectrometer with a neutron-to-deuteron (nd) conv ersion foil for production of deuteron recoils at nearly forward scattered angles. In its initial phase of implementation, CR-39 track detectors will be used in the focal plane to detect the recoil deuterons with extremely hi gh spatial resolution. Besides simplicity, CR-39 track detectors will facil itate a highly accurate energy calibration. However, in a later implementat ion of the spectrometer design, the recoils will also be detected by an arr ay of fast scintillation counters functioning in current mode. In either de tection scheme, the detection efficiency is about 10(-9) for measuring 14 M eV neutrons with an energy resolution of about 3%. Due to its large dynamic range, its relatively high efficiency, and a compliant design that allows for significant background rejection, this spectrometer can be effectively used, with very high resolution, at both OMEGA and the National Ignition Fa cility. (C) 2001 American Institute of Physics.