Dear Cecil,
Just found your web page and read your answer to the “suitcase bomb” question on 24 June 2005.
Having been a plutonium worker for 30 years I can say the idea that plutonium doesn't emit significant amounts of ionizing radiation is untrue. A plutonium button gives off triple digit millirem gamma readings. A button is roughly two kilo grams in size If you were to take this same button and machine it into a thinner shape say like a cereal bowl the amount of radiation would increase. (because plutonium is self shielding) Pu also gives off neutrons that are very difficult to stop and those are very easy to detect. Gamma radiation from the amount of Pu needed to make a SADM work is also detectable. There are more intricate ways to find that gamma signature using higher energy gamma rays that are very hard to stop. Most folks think that Pu only gives off alpha radiation that is stopped by paper. Not true! Some of the highest exposure received by plutonium workers are those who handled Plutonium metal.
Years ago I did some engineering on assay device based on neutron detection.
Final calibration was done using a Pu source. This was done at a secure facility, and lacking a security clearance, I had to be escorted and was not allowed in the area where the Pu source was stored. One of the facility workers had to fetch it while I waited with my equipment and escort.
I could determine when the source approached the door to the room the detector was in due to the substatial increase in neutron counts over background. That source was about the size of my thumb, and surrounded by lead bricks.
To Kevbo,
The neutron detectors must have been pretty efficient to see a source that small. However if you are trying to locate material that is what you want. The lead bricks were for the gamma radiation. Neutrons sail right through that stuff. The size of the source could have been as small as a 1/2 gram or ~ 15 grams. It also makes a difference if its weapons or fuels grade. Fuels grade has a higher Pu240 content hence more neutrons.
PU239, does U-235 (and for that matter, U-238) also have neutron and/or gamma radiation signatures which can be detected without too much effort even if its shielded?
I bring this up now, because last week I was exited the Dulles Tool road going north onto Rt. 28 (Sully Road) in the Reston area. I noticed two sets of yellow and black “poles” as approached the toll booth. One set was at the front entrance and the other at the rear exit. I asked the toll guy if they were radiation detectors–he responded in the affirmative. I assume that this more for test and propaganda, then for public safety, as a good cheapskate terrorist would probably take the back roads anyway to avoid detection.