Re: Exposure to Phograhic Film in Airport X-ray Scanners

[Bob Flood <bflood@SLAC.Stanford.EDU>]

I may be the cause of this thread - I answered a post in a photography
newsgroup about 10 days ago and made a statement that the exposure of the
film in US and Eurpoean airports would be a fraction of a millirem per pass
through the gate security xray machines. I also said that the dose at
cruising altitude for a transcontinental flight would be a few millirem each
way, and an intercontinental flight (the subject of the newsgroup
discussion) would be correspondingly higher.

The original post to the newsgroup wondered if there was any value in
placing film in lead-lined bags. Such bags are made for this purpose and
sold in major camera stores and mail-order houses. The discussion had raised
the question of the xray machnine operator turning up the intensity and
thereby undoing the protection the bag offered.

I replied to that group that the operator doesn't have much adjustment
available, mainly so that the shielding of the xrays isn't "undone"
resulting in higher than desired exposure of the staff that work the machine
40 hours a week. There would also be some benefit in xray tube life
expectancy to limiting the operator's latitude in adjustment. But the real
benefit of the lead-lined bag in reducing radiation exposure to your film
would be at cruising altitude, since that's where the large majority of dose
would be received, especially for intercontinental travel. I deliberately
ran 3 of our Panasonic UD-802 dosimeters through the gate xray at SFO
International a few years ago and the processing results show evidence of
xrays but the dose was too small to quantify. This result is consistent with
claims of a fraction of a millirem per pass.

One fact that may not be common knowledge on this list is that US FAA rules
grant to the passenger the right to have photographic film hand-inspected at
the gate security post instead of passing it thru the xray machine. This
right does not exist outside the US, however. The newgroup discussion
included quite a few useful "war stories" from those traveling outside the
US on how cooperative the security people are or aren't in various places
around the world, and how best to help them be cooperative. In general, it
seems that removing each roll of film from its box and plastic capsule (if
it has one) and placing it all in clear plastic bags for easy inspection
dramatically increases your chances of getting a hand-inspection overseas.
It stands to reason - doing some preparation to make it easier for the
security guard do what you want may well bear some fruit. But not always -
apparently some airports are getting famous for being uncooperative.

Another issue raised had to do with checked baggage. US airports now use the
CTX machines to inspect all checked bags. The machine can do a single pass
xray just like the gate machine, although exposure levels are undoubted
higher (SFO wouldn't say, this is my estimation) - the machine will
encounter larger, thicker bags, so it will have a need to, and it won't have
a crowd of workers and civilians around it, so there's no reason not to.
According to SFO security people, if the operator of the CTX sees anything
unidentifiable or suspicious looking, they back up the belt and a full CT
scan is done, allowing them to examine the contents of the bag in detail
without opening the bag. This is also done of very infrequent random
samples, too. It will, however, thoroughly ruin your film, whether it has
been exposed yet or not. NEVER put your film in your check-in luggage.
============================
Bob Flood
Dosimetry Group Leader
Stanford Linear Accelerator Center
bflood@slac.stanford.edu


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