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

["H.B. Knowles" <hbknowles@hbknowles.com>]

  While it is certainly true that cruising in a jet at 30,000 feet or so 
exposes the flyer to about 0.5 mrem/hr (nominal: higher rate to the north 
and lower rate to the south) the lead-lined bag won't do a great deal for 
protecting film up there. Some of the dose is 
from  x-rays  (brehmsstrahung) from showers, but much of the rest is from 
high-energy charged particles, which can be quite penetrating (pions, 
muons, some fast protons) and these may be more likely to fog film if they 
pass thru a few mm of lead, because of  x-rat production in the shield. 
Also, about 40% of the dose-equivalent at 30,000 feet is from fast 
neutrons, which are unlikely-by themselves- to cause much film fogging, 
although secondary effects ( like (n,p) reactions) may contribute. It still 
seems to me that most of the hazard is in the terminal.



>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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H.B. Knowles, PhD, Physics Consulting
4030 Hillcrest Rd, El Sobrante, CA 94803
Phone (510)758-5449
Fax (510) 758-5508
hbknowls@ix.netcom.com (until 1/31/00)
hbknowles@hbknowles.com (new)
<www.hbknowles.com>

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