Buildup factor

In the narrow beam geometry (see Figure, left) the beam of γ-radiation is strongly collimated, such that only photons that go straight reach the observational point P. Each interaction in the absorber ensures that the photon either disappears or is scattered and thus does not end up in the point P.

In the broad beam geometry (see Figure, right), the collimator is missing. Photons that in the narrow beam geometry were stopped by the collimator will now reach the absorber and will partially be scattered into the direction of point P, so that the radiation level at that point is larger than in the narrow beam geometry.

This increase in radiation level is accounted for by the quantity buildup factor (B).

Related concepts

• attenuation coefficient
• dose conversion coefficient
• half-value thickness
• kerma-rate constant
• radiotoxicity equivalent
• square law
• transmission