The surface analysis of ash residue using ToF-SIMS showed 98.7% Ni-62 enrichment, while the bulk ash contents were found to have 99.3% enriched Ni-62. Similarly, the surface-layer enrichment of Li-6 is 92.1%, while the bulk contains 57.5% Li-6.
If these figures are accurate, then there exists a gradient between surface and bulk for both of the elements displaying isotope shifts. This could be indicative of the difference between steady-state operation of the reactor vs. the conditions present when the reactor was shutting down. If a cyclic reaction is occurring between lower-numbered Ni and Ni62, and between Li-7 and Li-6, then a condition of reaction starvation could leave excess Ni-58 and Ni-60 on the surface (0.8% surface vs 0.3% bulk for Ni-58, 0.5% surface vs. 0.3% bulk for Ni-60) of the nickel-dominated grain, and significant depletion of Li-7 on the surface (7.9% surface vs. 42.5% bulk) of a lithium-dominated grain. I believe this inverse isotope fraction gradient between nickel and lithium is in fact indicative of a cyclic reaction between these elements. I am not familiar with the details of ToF-SIMS, and am having difficulty interpreting the significance of the mass 69 signal. Is this an instrument artifact? This seems implied by the text, since Ga-69 is listed as being enriched in the ion source. However, the abundance of 69 varies widely between various spectrums on pages 46-52. Would somebody with familiarity with this instrument be so kind as to explain the significance of this peak? Is there a chance that mass 69 is appearing as valid signal, as opposed to an artifact from Ga-69 in the ion source? I am struck by the coincidence of 62+7=69 here, and wonder if this peak could indicate a reaction product, intermediate or final, of Li-7 and Ni-62. Thanks for any comments folks would care to offer. -Bob Ellefson
