Re: x-ray units and dosimeters

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

The modern security xray machines in use at US airports use a pulsed xray
beam that greatly reduces the exposure of operators (and the amount of
shielding required) and, as a convenient side effect, reduces exposure of
items passed through it. Several photography magazines have experimented
with film of various speeds and manufacture and found no observable effect
after numerous passes through such machines. Outside the US, Canada,
western Europe, and Japan, one is likely to encounter the older continuous
beam machines (most of the US hand-me-downs) in which the doses will be
higher. We've had many dosimeters reported to us as having gone through
these machines, and there is never a measurable dose (Panasonic UD-802
dosimeter).

Checked-in luggage is now subject to a CT scan if an initial baggage xray
identifies anything suspicious or indeterminate. The CT scan is done in
slices, and if a slice hits your roll of film, it will be fogged - a dose
of a few rads is typical. I have read recently that some airports have a
new version of this machine in place at gate security stations and use it
to examine selected items.

Some (perhaps many) US airports also now use a neutron activation exam
intended to activate nitrogen and detect the short-lived isotope. Since
explosives are rich in nitrogen, an alarm on this machine is generally
regarded as a reason to evacuate the area and let the bomb squad deal with
the bag. Exposure in this device is rather small, but I don't have any dose
figures to offer. The field isn't strong enough to cause a noticeable
effect on photographic film (unless, perhaps, you enlarge a 35 mm negative
to barn door size, in which case neutron tracks won't be the major feature
of the photograph).

The bottom line (to coin a cliche): never put your film in your check-in
luggage - take it on the plane with you. There's no danger if it passes
through a routine x-ray machine, but if they want to put it through one of
the CT scanners, insist on hand inspection of the film. Under FAA
regulations, your have the right to have film hand inspected instead of
xrayed, and the security staff have to comply with your request. Now that
doesn't mean they have to be quick about it, and there's definately an
advantage in it for you to make it easier for them to do what you want.
Most photographers these days remove their film from the manufacturer's
packaging and carry the film in clear plastic Zip-Loc-type bags that are
easy to inspect.

===================================
Bob Flood
Dosimetry Group Leader
Stanford Linear Accelerator Center
(650) 926-3793
bflood@slac.stanford.edu
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