(Science: radiobiology) The increase in absorbed dose with depth below the surface in a material irradiated by a beam of x-rays or gamma rays. Build-up may be two kinds:
1. Electron Build-up: this is due to an increasing electron fluence in the material.
2. Photon Build-up: this is due to scattering in broad beams of radiation.
For high energy beams, process 1. Is the more important.