Answer to Question #11239 Submitted to "Ask the Experts"
Category: Security Screening
The following question was answered by an expert in the appropriate field:
I read your articles related to backscatter technologies in airports and the dose received by the skin. Can you clarify something regarding vehicle scanners that use backscatter technology? You mentioned that backscatter x rays do not penetrate the skin like medical x rays. How do backscatter x rays used in vehicle scanners penetrate the metal of the vehicles and bounce back? I still don't understand how this radiation can penetrate the metallic vehicle body yet can't penetrate the human body. I would appreciate an explanation. Would you know what type of radiation/x ray is being used?
I am also trying to figure out whether I can purchase a device that can measure the dosage of the radiation/x-ray exposure. I read about devices called x-ray detectors or dosimeters that detect beta and gamma. These devices vary from a few tens of dollars to a couple of hundred. I would like to be able to check the exposure from these vehicle scanners. I drive frequently through the backscatter vehicle scanner and cannot find any answer to my questions on the Internet.
Most types of vehicle scanners, like those used at border crossings, aren't backscatter systems. They use penetrating radiation to see through and into the vehicle. These machines emit radiation that does penetrate the vehicle and your body.
Because of this, they do expose the occupants of the vehicle to radiation. The average radiation dose per scan to a person sitting in a vehicle that passes through one of the vehicle scanners would be, maximally, about 0.05 microsievert (µSv) per screening and probably closer to about 0.02 µSv because of the speed at which a vehicle is screened (µSv, or microsievert, is a unit of effective radiation dose). On one side, the devices have the x-ray device and on the other a detector. The dose received is far below the level of radiation exposure that is known to cause cancer or any other health effect.
Ordinary background radiation received each year from cosmic radiation and radioactivity in soils and building materials is about 3,000 µSv. Another comparison is a chest x ray, from which a person would receive about 100 µSv.
The American National Standards Institute has studied the issue of vehicle scanners (ANSI/HPS N43.17 published in 2009) and has recommended that vehicle screening expose an individual inside the vehicle to no more than 0.25 µSv per screening, which would allow someone to pass through a screening unit 1,000 times before reaching regulatory limits on exposure for the public. As I mentioned, though, the actual doses received are much less than the recommended limit.
According to American Science & Engineering, a maker of a vehicle-screening unit, if a car goes below a specified speed, the x-ray unit is shut off to minimize the possibility that a person could receive extra radiation dose. Most screening units are set for a car to go about 5 kilometers per hour (km h-1).
For most passenger vehicle screenings, a 160–450 kilovolt (kV) system is used. For screenings of larger vehicles—tankers, semitrucks—the energy can go up to 9 megavolts (MV). The 450 kV system x rays will pass through about 10 centimeters (cm) of steel; the 9 MV x rays will go through up to about 20 cm of steel.
As for the justification for this use, the Health Physics Society has a position statement on the topic of using ionizing radiation on humans for security screening and believes that it is a justified practice if certain criteria are met.
There are a couple of different devices you can use to measure the radiation exposure/radiation dose. One is a calibrated Geiger-Mueller detector. This is not as accurate as an ion chamber, but it is likely close enough for your purposes.
These usually read in milliroentgen per hour (mR h-1). For what we're doing here, we can consider an mR equivalent to a millirem (mrem, a unit of radiation dose), which is equal to 0.01 millisievert (mSv) or 10 µSv. So 1 mR roughly equals 1 mrem roughly equals 10 µSv. If you get a reading of 10 mR h-1, that equals about 100 µSv h-1 or 0.028 µSv s-1. If the scan (with you in the beam) takes three seconds, you'd get about 0.08 µSv in three seconds.
Kelly Classic
Certified Medical Health Physicist