Effects of 3D printed capsule material on activation thin foil irradiation and counting for fusion neutron yield measurements

Abstract

Activation foils are used to independently measure the time integrated neutron yield and total fusion energy produced in both inertial and magnetic confinement fusion, making them crucial in the neutron diagnostic suite. The activated foils must be remotely transported from the neutron source to the detector inside of a small capsule, which can impact both the foil irradiation and the associated activation measurement. The aim of this paper is to evaluate the performance of various activation foils and to characterize the effects of different capsule materials to inform the design choices for future systems, such as the SPARC tokamak. Through a combination of FISPACT simulations and irradiation experiments with a deuterium-tritium neutron generator, we tested several different material choices for foils, capsules, and gamma-ray spectrometers. Aluminum and copper foils are found to be suitable for a multi-foil irradiation configuration. The use of 3D-printed thermoplastic capsules reduces the number of measured decay-photon counts, yet the reduction is smaller than the associated measurement uncertainty. Finally, lanthanum-based detectors are shown to be viable alternatives to the standard high-purity germanium spectrometer, although with poorer energy resolution.

Figures (13)

• Figure 1: Foil samples, both $\sim$0.5 in ($\sim$12.7 mm) diameter, on a grid with 5 mm spacing: (left) Cu with $\sim$0.005 in ($\sim$0.13 mm) thickness, inscribed "I", and (right) Al with $\sim$0.03 in ($\sim$0.76 mm) thickness, inscribed "J".
• Figure 2: \ref{['fig:fispact:decay']} Simulated gamma activity of candidate foils (Si, Cu, Al, Fe) following neutron irradiation. Two neutron irradiation cycles are presented: SPARC irradiation during a PRD and irradiation performed in the Vault Lab at MIT with a DT neutron generator. Parameters of irradiation are presented in \ref{['tab:sim:inputs']}. Blue shaded background shows counting period at the MIT lab. \ref{['fig:fispact:cumulative']} Cumulative gamma emission from four different foil materials (Si, Cu, Al, Fe) simulated with FISPACT. Dashed lines report the gamma emission following the foil irradiation during a PRD on SPARC, while solid lines show gamma emission following irradiation in the Vault Lab at MIT.
• Figure 3: Sample capsules made out of three potential thermoplastic capsule materials: \ref{['fig:rabbit:pla']} Polylactic Acid (PLA), \ref{['fig:rabbit:petg']} Polyethylene Terephthalate Glycol (PETG), \ref{['fig:rabbit:pc']} Polycarbonate (PC).
• Figure 4: Sample experimental setup for gamma source experiments
• Figure 5: \ref{['fig:experimental:setup:source']} Experimental setup during foil irradiation. Foils are pushed against the bottom face inside the capsule (see fig. \ref{['fig:foil:holder']}, which is then taped to the target plane of the DT neutron generator. Neutron rate is recorded by an EJ-301 liquid scintillator placed one meter away. \ref{['fig:experimental:setup:counting']} Experimental setup for measuring gamma spectra of irradiated foils. $\text{LaBr}_{3}$ and $\text{LaCl}_{3}$ detectors are used within a lead “sarcophagus” to minimize the background coming from the surrounding environment. Lead bricks also seal off detectors from the top (not pictured).