Pyrolysis-Dependent Uncollided Neutron Transmission in Date-Palm Biochar/Epoxy Composites: A GEANT4 Study with 2 MeV and Cf-252 Neutrons

Pyrolysis changes both the composition and physical structure of biochar, making it necessary to separate chemistry-driven neutron-transport effects from density and geometry effects. This computational study examines date-palm biochars produced at 300, 500, and 800 °C as fillers in an idealized epoxy matrix. The principal 2 MeV comparison used ten independent 106-history GEANT4 runs per material at 20 wt% filler, 5 cm thickness, and equal bulk density (1.25 g cm−3). From BC300 to BC800, total forward track-length fluence increased by only 0.30%, whereas the fluence-weighted H∗(10) response increased by 2.86%. A separate ten-replicate component campaign confirmed that the 2 MeV BC800–BC300 H∗(10) difference (+0.07986 pSv primary−1) was driven by increased uncollided-primary transmission (+0.08398 pSv primary−1), while the collided-primary contribution decreased slightly (−0.00422 pSv primary−1). For a representative 252Cf Watt spectrum, H∗(10) increased by 2.98%, with 91.1% of the net increase arising from uncollided primaries. A 30×30 cm2 full-face geometry increased scattered buildup, yet 84.0% of the BC800–BC300 H∗(10) increase remained attributable to uncollided primaries. Additional density, thickness, and loading sensitivities gave positive BC800–BC300 H∗(10) contrasts under all tested conditions (1.18–8.24%), although the integral-fluence contrast was not universally positive. The model therefore supports a robust chemistry-related increase in high-energy primary survival while showing that the scattered-field partition and engineering performance remain geometry- and processing-dependent.