Here's another reference on using backscatter x-ray technology for
screening passengers.
http://www.rapiscansystems.com/sec1000faqs.html
This one claims typical doses per passenger-screening amount to 3 µrem.
The TSA site claimed 5 µrem to 10 µrem. There is even more information
on this page regarding dosages. For example, frequently scanned
personnel (such as flight crew members) can safely receive at least 5000
scans per year. It's obvious to me that they pick up much, much more
radiation from flying at altitude than they do from these scans.
Nicely, this source cites ANSI N43.17 "Radiation Safety for Personnel
Security Screening Systems Using X-rays". If you have access to that
standard, it should be able to tell you more.
What frustrates me is that these devices are called "low energy" x-ray
sources, but the plate voltage in use are not given. The phrase "low
energy" can refer to the flux level ("brightness") of the device, but
more often it refers to the energy of the highest energy photon
generated, which is determined by plate voltage (and to a small extent,
material).
Yet another source of information is at
http://www.public.asu.edu/~atppr/images/RPD-Manuscript.pdf
This is the pre-print for the article published by Oxford Journals at
http://rpd.oxfordjournals.org/content/early/2010/11/09/rpd.ncq358.abstract
but that site charges $32 for the download.
The paper cites an ANSI 43.17 limit of 0.25 µSv [25 µrem] per screening.
It states that the equipment manufacturers are claiming doses per scan
of less than 0.1 µSv. For comparison, it provides the figure of 6.2 mSv
[6200 µSv] that members of the US population receive every year from all
sources [which are mostly natural] of radiation.
I would like to point out that the cosmic radiation we all receive
naturally includes particles of vastly higher energy than those from
x-ray sources, especially those with low plate voltages. That makes
cosmic radiation particles much more likely to cause ionization damage.
According to this second source, typical scanning machines use plate
voltages in the order of 100 kV. Thus, such a machine generates x-ray
photons (particles) with energies of 100 kV or less. (Backscattering
machines for vehicles use higher voltages than do those for personnel,
such as 200 kV.) In comparison, cosmic radiation particles are
frequently in the megaelectronvolt region and go even into the
gigaelectronvolt region.
Note that this second source concludes that frequently scanned personnel
could reach their annual limits in fewer scans than the first source
above claims. The second paper used "ab initio" calculations to reach
that conclusion. It also uses the NRC's annual limit of 0.25 mSv for the
general public.
Jim
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
