For some composites like glass or concrete doped with heavy elements you can find high Exposure Build-up Factors "peaks" at low photon energies however, this region of energies is dominated by the Photoelectric absorption mechanism and I tried to connect the reason to the K or L shell absorption peaks for these heavy elements but no I didn't find any logic explanation to do that unlike the case with those peaks you usually find for these heavy composites in the MAC and LAC curves that are really can be connected to the K and/or L absorption peaks for their Incorporated heavy elements. Please guys, I do need your help to find a logic explanation for this observation. Many thanks.
I found this reference which may help you answer this question. It may be due to the contribution of characteristic X-rays produced after absorption.
From p.1362
Shultis, J. K., & Faw, R. E. (2010). Radiation Shielding and Radiological Protection. In Handbook of Nuclear Engineering (pp. 1313–1448). Springer US. https://doi.org/10.1007/978-0-387-98149-9_11
"As may also be seen from the figure, the attenuation factor increases greatly at photon energies just above the 0.088-MeV K-edge for photoelectric absorption, each absorption resulting in a cascade of X-rays. For these reasons, buildup factors may be extraordinarily large, as evidenced by the line for 0.089-MeV photons in Fig. 21. At energies below the K-edge, the buildup factors are very small."
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