The buildup factor is defined as follows: in the passage of radiation through a medium, the ratio of the total value of a specified radiation quantity at any point to the contribution to that value from radiation reaching the point without having undergone a collision.
The ratio of the total photons at a point to the number arriving there without being scattered.
Y. Harima, AN HISTORICAL REVIEW AND CURRENT STATUS OF BUILDUP FACTOR CALCULATIONS AND APPLICATIONS, Radiat. Phys. Chem. Vol. 41, No.4/5, pp. 631-472, 1993.