Performance evaluation of an energy tuning assembly for neutron spectral shaping

Abstract

An energy tuning assembly (ETA) was designed to be fielded at the National Ignition Facility (NIF) to modify the characteristic D-T fusion spectrum to include a prompt fission neutron spectral component. The ETA was characterized at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory to measure the shaped spectrum from an incident deuteron breakup neutron source, test the proposed neutron spectroscopy techniques used to inform the flux measurements at NIF, and validate the ability to predict ETA performance using a Monte Carlo Neutral Particle (MCNP) simulation. Activation foils (i.e., Ni, In, Au, Al) were exposed to a collimated 33-MeV deuteron-breakup beam originating from a tantalum breakup target. The source spectrum absent the ETA was characterized using a set of activation foils and the STAYSL unfolding code. Finally, the ETA-modified spectrum was obtained using activation foil unfolding with a 2 = 0.92. The ETA-modified unfolded spectrum agreed with the MCNP-simulated prediction in the energy range of 0.1-14 MeV, but exhibited disagreements in the 10 eV-100 keV region. This work demonstrates shaping of the NIF neutron spectrum via the ETA to be a viable path forward for tailored neutron beams at NIF.

Nuclear Data Covariance Analysis in Radiation-Transport Simulations Utilizing SCALE Sampler and the IRDFF Nuclear Data Library

“…The ETA, shown in Fig. 1, was previously designed to produce a notional thermonuclear and prompt fission neutron spectrum (TN + PFNS) [26], [27]. The TN + PFNS is an unclassified representation of a boosted nuclear weapon based on the Godiva critical assembly [28].…”

“…The activation foils chosen are natural indium, aluminum, nickel, zirconium, gold, manganese, and tungsten. Each foil has threshold and nonthreshold reactions that span the range of interest for the TN + PFNS energy spectrum, which can be used to unfold the incident neutron energy spectrum to validate the ETA performance [27], [30], [31]. The HEU sample is enriched to 93.15% 235 U.…”