A neutron spectrometer based on recoil proton track imaging for high-yield inertial confinement fusion primary deuterium-tritium neutrons.

The energy spectra of primary deuterium-tritium (DT) neutrons provide essential information about the implosion performance in inertial confinement fusion (ICF) experiments. Recoil proton track imaging is a recently developed technique for measuring neutron energy spectra, which optically records the track image of recoil protons in a gas scintillator using high-performance imaging devices, then derives the neutron spectrum through an unfolding procedure. Here, focusing on the ICF primary DT neutrons with a yield of up to 1019, we design a neutron spectrometer based on this method. Considering the trade-off between energy resolution and detection efficiency, we optimize key system parameters, including recoil angle, recoil proton flight distance, aperture size, polyethylene foil thickness, and gas scintillator pressure through simulation, achieving a recoil proton conversion efficiency of 8.68×10-7 for 14.1 MeV neutrons. In addition, since the high-precision spectrum unfolding requires a high-quality track image, we specially design a large-aperture fixed-focus lens to enhance the efficiency of scintillation photon collection. Furthermore, we propose a realistic track image simulation method that combines Monte Carlo simulation with optical imaging simulation, allowing for a more accurate calculation of the neutron energy response. Based on the designed system, we simulate track images for mono-energetic neutrons, neutrons with spectra from National Ignition Facility ignition experiments, and neutrons with a Gaussian spectrum. The results demonstrate that high-quality track images can be obtained under the designed system. Subsequently, the spectrum unfolding for simulated track images corresponding to energy spectra is performed using MLEM and GRAVEL algorithms. The high quality of the unfolded spectra indicates that the recoil proton track imaging is a promising approach for diagnosing ICF primary DT neutron spectra.