What does “pounds per square foot” mean to me when evaluating e-beam dose penetration?

You may hear the limitation of “up to 8 pounds per square foot” regarding the penetrating capability of a 4.5 MeV accelerator. What does that mean?

Start with the image of your box of medical devices on a conveyor going through a shower of electrons raining down on it.

As the electrons contact the surface of your product they penetrate the box, delivering a dose from top to bottom. If the weight of the box is up to approximately 8 pounds per square foot, then the penetration is probably sufficient to give a satisfactory dose distribution within the box.

Example calculations:

(1) A box is 18 inches long, 12 inches wide, 10 inches high and weighs 12 pounds. Assume that that the shortest dimension (10 inches) faces up in the beam direction. That would make the footprint of the box on the conveyor 18 inches by 12 inches. The calculation is:

18 inches x 12 inches = 216 square inches

216 square inches ÷ 144 square inches per square foot = 1.5 square feet

12 pounds ÷ 1.5 square feet = 8 pounds per square foot…GOOD!

(2) A box 20 x 12 x 9, weighing 15 pounds:

20 x 12 = 240

240 ÷ 144 = 1.67

15 pounds ÷ 1.67 = 9 pounds per square foot…NOT GOOD!

(3) A box 18 x 12 x 9, weighing 9 pounds:

18 x 12 = 216

216 ÷ 144 = 1.5

9 pounds ÷ 1.5 = 6 pounds per square foot…GOOD!

but wait; if we shoot through the 12 inch dimension, then the calculation is:

18 x 9 = 162

162 ÷ 144 = 1.125

9 pounds ÷ 1.125 = 8 pounds per square foot…GOOD!

In example (3), the penetration would probably give a satisfactory dose distribution in either orientation, but the second orientation would be more cost-effective because more boxes would fit on the conveyor.

The actual dose distribution within your box and product is measured by dose mapping. Whether the outcome is satisfactory (i.e. within your maximum – minimum dose range) will depend on various factors, including varying densities of components within the box. The quick calculation of the overall density, as shown above, can answer the initial question of the likely feasibility of using e-beam.

Contact E-BEAM Services to help you with a simple review of your device’s materials, size and packaging configuration to determine if your medical device can be sterilized using electron beam technology.