Tritium depth dependence of tritium imaging

A Monte Carlo simulation of tritium β particle motion in a matrix material indicates that the probability of escape through the material surface is greater than 50% for near-surface tritium, but drops quite rapidly with increasing depth. Beyond an areal density depth of 2×10−5 g/cm2, the decline is close to exponential, with an e-folding length of approximately 3×10−5 g/cm2, the exact value varying slightly with the material. The sensitivity of the tritium imaging technique will decrease at a similar rate with increasing depth of tritium. Experimentally, the image intensity of a tritium-implanted graphite specimen was observed to decrease exponentially with coating thickness as an aluminum layer was evaporated onto its surface, with an exponent that was within 30% of the predicted value. The Monte Carlo simulation also indicated that the limit of lateral resolution of the tritium imaging technique will be slightly less than the depth of the tritium, subject always to Recknagel’s limit of resolution due to “chromatic aberration” of the electron optics, which is of the order of 200 nm. If tritium is uniformly distributed through the material, surface tritium so dominates image formation that Recknagel’s limit inevitably applies.