Re: Q: dose from airport x-ray?

[kmq@cosmail1.ctd.ornl.gov (Kim McMahan)]

At 01:15 PM 9/27/96 -0500, Melissa Woo wrote:
>I have a question from someone who is concerned about putting food
>through a typical airport x-ray security scanner and would like to give
>her a "real life" frame of reference.  As I have no idea of the typical
>operating parameters of such a unit, I can't do the calcs for
>exposure/dose/whatever to an item going through the scanner.
>
>As numbers tend to be meaningless in a case like this, what would be
>ideal is some sort of comparison relative to say, what dose her food
>would receive from a trans-Atlantic flight or something similar. 
>Anyone have a rough estimate?
>

(Sorry, on the first attempt I began the first paragraph with "From")

FFFrom :-) a brief telephone conversation I had a few years ago with a
technical representative of the major supplier of these units in the U.S.
(EG&G Astrophysics), modern conveyor-type airport x-ray units give a VERY
low integrated dose, about 0.1 mR per pass. (For the gory details, see some
cut & pasted notes from below from that conversation - we were investigating
a "high" TLD reading.) I seem to remember that trans-Atlantic flights
deliver on the order of a few mrem, so that's not a bad comparison if you
like big ballparks.

Older-style x-ray inspection units deliver a much higher dose. I've never
seen one, but was told they look like a pizza oven, with a large pull-down
door on the front. The x-ray beam is continuous, so the integrated dose
depends on how long the inspector wants to examine the contents.


==========================================
Notes from EG&G Astrophysics conversation:

- The FAA, BRH and OSHA all regulate the dose-per-pass that may be delivered
by a tunnel-type x-ray unit.  The limit is 0.5 mR per pass.  Their (EG&G's)
administrative limit is 0.2 mR per pass, and they have a QA requirement of 0.1
mR per pass.  That last number agrees with what we measured here in Knoxville.

- EG&G Astrophysics has about 60-70% of the market in this country for airport
baggage inspection machines. So he was speaking only for his company. (But any
manufacturer must meet the 0.5 mR federal limit.)

- He knows of nothing unusual at National airport.  They have sold units to
National, but other companies may have, as well.

- There are several models to choose from, but in general they are running at
140 kVp and 0.8-0.9 mA, and the conveyor runs at 45 feet per minute. [The
units are rated for 160 kVp and up to 2 mA, but are not operated at these
maxima.]

- Tunnel-type units are used for checked as well as carry-on baggage, and are
therefore subject to the same 0.5 mR per pass limit.  He says the only
difference in their models is the size of the tunnel. The operating voltage
and current are the same.

- The technology used to generate the image is called a linear x-ray diode
array (LXDA). This technology was patented by their company in about 1980.
Prior to that, they did use fluoroscopic techniques in their units; some of
those could generate dose rates of about 1000 mR/hr.  However, current
regulations disallow their use by airports (with the familiar conveyor belts).

- All of their units have safety interlocks, and are only energized when (1) a
photodiode senses something is entering the unit, and (2) the conveyor is
moving.  That agrees with my test at Knoxville where we found that just the
dosimeter alone did not trip the photodiode when entering the tunnel.

- Depending on the model, the x-ray beam may be shot from the top, bottom or
side.

===========================

Hope this helps,

Kim McMahan, CHP
Office of Radiation Protection
External Dosimetry Group
Oak Ridge National Laboratory
e-mail: mcmahankl@ornl.gov