Development of a compact Gamma-RAy SPectrometer (GRASP) for inertial confinement fusion and basic science experiments at OMEGA

γ-rays are ubiquitous products of fusion reactions that are useful for diagnosing the performance of inertial confinement fusion (ICF) and basic science experiments. In ICF experiments, γ-rays from primary DT reactions allow for the branching ratio to be diagnosed, while γ-rays from secondary reactions between the DTn and ablator provide insight about the ablator areal density [1]. Despite the utility of spectral γ-ray measurements, no spectrometers currently exist that can simultaneously resolve an ICF- and basic science-relevant γ-ray spectrum and be fielded in existing diagnostic ports on OMEGA. To address this diagnostic capability gap, the Gamma-RAy SPectrometer (GRASP) system is being designed for OMEGA. GRASP consists of a Compton conversion foil in front of a compact magnetic spectrometer composed of permanent magnets that disperse forward-scattered electrons onto a detector. GRASP will resolve γ-rays of energy 12 – 18 MeV with an energy resolution of 5% and an efficiency of 3.8e-7 e^-/γ. Image plates and radiochromic film are being investigated as potential detectors, while MCNP simulations are being used to develop a shielding design that ensures sufficient signal-to-background. Once demonstrated at OMEGA, future GRASP systems will be developed for the NIF and for portable use on other photon facilities.

[1] Kim and Herrmann, RSI 94, 041101 (2023)