The buildup factor is essentially a multiplier that accounts for the effects of scattered radiation when calculating radiation dose.
According to Shultis and Faw (2000), the buildup factor, B (E, x), is generally defined as the ratio of the total dose to the unscattered dose. This means it tells you how much the radiation dose increases due to particles scattering off the material they pass through, in addition to the direct, unscattered particles.
Understanding the Buildup Factor
When radiation, like gamma rays or neutrons, travels through a material (such as shielding), some particles pass straight through without interacting (unscattered). These contribute directly to the dose on the other side. However, many particles interact with the material's atoms, scattering off in different directions. These scattered particles also contribute to the total dose received.
The buildup factor quantifies this additional dose contribution from scattering.
- Unscattered Dose: The dose from radiation particles that travel in a straight line from the source through the material to the point of interest without any interactions.
- Total Dose: The sum of the unscattered dose and the dose from all scattered particles that reach the point of interest.
Build up Factor (B) = Total Dose / Unscattered Dose
Why is it Important?
Ignoring scattered radiation can lead to a significant underestimation of the actual radiation dose. The buildup factor is a crucial parameter in radiation shielding calculations to ensure safety and effective dose control.
- Shielding Design: It helps engineers determine the correct thickness and material for radiation shielding.
- Dose Assessment: Used in calculating the expected dose in environments where radiation is present.
Factors Affecting Buildup Factor
The value of the buildup factor is not constant; it depends on several variables:
- Radiation Energy (E): The energy of the incident radiation.
- Material Thickness (x): The thickness of the material the radiation is passing through.
- Material Type: The atomic composition of the shielding material (e.g., lead, concrete, water).
- Geometry: The arrangement of the source, shielding, and dose point (e.g., point source, plane source).
Practical Example
Imagine a wall designed to shield against gamma rays.
- Calculate the dose from gamma rays that pass directly through the wall without any interactions (unscattered dose).
- Look up the buildup factor for that specific gamma ray energy, wall material, and wall thickness.
- Multiply the unscattered dose by the buildup factor to get the total dose, which includes the contribution from scattered gamma rays.
This total dose is the more accurate representation of the radiation level behind the shield.
