Body and Vehicle Scanners


Figure 1                                                                                                       Figure 2

Machines based on Compton Backscattering are now being used for screening people at airports (it started in Phoenix!) (Fig 1) and vehicles at border checkpoints (Fig 2). The manufacturers claim that the dose from one of these screenings is less than 0.25 mSv. Examination of the images that they publish and some simple Physics based on well known scattering cross sections shows that it is VERY unlikely that the dose is this low.

From TSA

What is the difference between backscatter x-rays and medical x-rays?
A: Backscatter x-rays only penetrate approximately ¼ of an inch before the rays are scattered,
whereas medical X-rays transmit completely through the body.

Rapiscan 50 kVp 28kV average, AS & E 125 kVp 60 kV average, Z Portal 450 kVp 200 kV average

They use an effective dose averages through the body. Clearly this is going to be different for small children and large adults!

Doses based on 2mm feature size for Body Scanners (Personnel Screening) and 2cm feature size for the Vehicle Portal.

Assumes 2 scans per screening (Body Scanner), 3 scans (vehicle scanner) Dose depends on image quality (pixel size and number of grey levels)

Probability of fatal cancer.
1 in 20 million per microSv

Probabiltity of being blown up by Terrorist in Airplane.
Also about 1 in 20 million per flight.

In Comparsion

Probability of being struck by lightening in the US 1 in 500,000 and being killed 1 in 5 million!

Scary thing, what happens if scan jams and fail-safe mechanism fails -- local dose very high of order a few Sv

Finally, Compton Backscatter good for detecting edges and “Protrusions”, not at all good at detecting explosives

For more information see paper to appear in Radiation Protection and Dosimetry

New York Times article

USA Today article