|
Sandy is right on both counts. A dosimeter is issued to an individual to monitor occupational dose received at a specific facility. Taking it to a different facility can lead to attributing dose to the wrong facility. For NRC-regulated operations, this would probably be regarded as a CFR violation.
As for exposure in transit, things are changing slowly for the worse. All airports in the US now use pulsed x-ray equipment (rather than the continuous beam type), mainly to control exposure of security staff without forcing the machine to heavy with shielding. In normal use, these devices won't yield a measurable dose unless the dosimeter goes thru quite a few of them. The operator can adjust the intensity, but only a little - giving the user much adjustment would defeat the ALARA benefits of the design. However, the machines used for CHECKED luggage are dual purpose devices. They do the same kind of snapshot as the ones at gate security, but they also can do a full CT scan. If a bag with a dosimeter is selected for a scan (some are chosen at random, some are selected based on what is seen in the snapshot examination, and almost all last minute reservation customers are chosen), our experience is that a slice making a direct hit on the dosimeter will yield 2-3 rem, and a near miss 50-100 mrem depending on the scattering characteristics of the material actually hit. The machine that does this is made by Inovision and is their model CTX 5000.
A new machine is gradually being installed around the country. Anyone who traveled to the Olympics in Salt Lake City saw them - the Inovision CTX 5500. This is a smaller scale 5000, sized to allow only bags permitted as carry-ons. These machines are placarded with warnings to remove your film from your bag before putting the bag thru the machine, and they aren't kidding.
We have people who travel with dosimetry for a reason, and we stress to them that they should never put dosimetry in checked luggage, and hand-carry the dosimeter any place that warns travelers to remove their film. It's working so far.
There is an uneven implementation of the screening available with these machines; some airports use them much more aggressively than others. The Seattle-Tacoma (SEA-TAC) and Baltimore (BWI) airports have been the ones to cause the most anomalous dosimeter readings for us.
BTW, if you travel outside the US, these same machines are in use throughout the industrialized world - Canada, Europe, Japan, but the rest of the world is using our old discarded continuous beam x-ray machines (we gave the machines to them when we wanted them to improve security screening). Anything that goes thru one of those machines can receive a measurable dose.
Bob Flood Nevada Test Site
|