An in-situ technique to measure erosion and deposition in fusion devices

Erosion or deposition of sub-micron layers of graphite or other materials can be measured by bombarding a sub-surface layer of ¹⁰B or ⁶Li with thermal neutrons and observing with a surface-barrier detector the energy loss of the prompt alphas or tritons produced. To demonstrate the feasibility of this technique, a (5250 ± 250) Å layer of boron and a (1.25 ± 0.05) μm layer of Li₂B₄O₇ were electron-beam evaporated onto graphite substrates and exposed to a thermal neutron flux of (8.0 ± 0.5) × 10⁵ cm^-2 s^-1. The (n,α) reactions of the ¹⁰B produce a 1.78 MeV α, a 1.48 MeV α, and a 0.848 MeV ⁷Li. The reactions of ⁶Li produce a 2.73 MeV ³H and a 2.05 MeV α. Carbon coatings of (600 ± 25) Å, (8250 ± 500) Å, (2.0 ± 0.2) μm, and (4.0 ± 0.4) μm were placed between the active layers and a surface barrier detector in vacuuo. The thinner layers shifted the 1.48 MeV α peak by (31.7 ± 4.5) keV and (431 ± 43) keV respectively. The thicker layers shifted the 2.73 MeV ³H peak by (206 ± 15) keV and (346 ± 20) keV respectfully. Therefore, utilizing boron implants, 100 Å to 1 μm of graphite erosion or redeposition can be determined. Utilizing lithium implants, thicknesses in the range of 1 μm to 10 μm can be determined. Theoretical energy shifts, thermal diffusion, and the feasibility of this technique as a between shot diagnostic for limiters, divertor plates, and/or first-wall armor are discussed.