Comments concerning the detection of threats based on radioactive material

1. Comments concerning the detection of threats based on radioactive material Dr. Rick B. Spielman ABQ Technologies, Inc. Albuquerque, NM 87123

2. What is wrong with this account?What is very good with this account? • Weapons-Grade Uranium Seized in Bulgaria -- In May 1999, Bulgarian customs officials at the Ruse border crossing in Bulgaria discovered 10 grams of highly enriched uranium (235U) inside a lead "pig" concealed in an air compressor in the trunk of a car. The 35-year-old Turkish driver was transporting his nuclear cargo from Turkey to Moldova. It is believed the smuggler was offering this small shipment of 235U as a "sample" to buyers in Moldova in advance of a larger shipment of the same materials. The Bulgarian customs officer who found the 235U had just received counter-proliferation training from Customs and Border Protection. His supervisor had been trained by Customs and Border Protection at RADACAD. The Bulgarian lab director who examined and identified the materials had also received training from CBP and Border Protection.

3. What are the limitations of existing detector systems? • Testimony to Congress describes… • CBP: Radiation isotope identification device - Handheld tool used to scan vehicles for materials that emit radiation. • CBP: Radiation-detection pagers: Worn by border agents to pick up radiation emissions. • Is Congress getting detailed information?

4. The easy problems • Most radiation detection systems are designed around high activity sources • 60Co at 1.25 MeV (1.32 MeV also) • 137Cs at 611 keV • Even small amounts of these materials are detectable by relatively small, simple detectors. • These radiation sources are difficult to shield.

5. The hard problems • It is important for all agencies, responsible for detecting a potential weapon of mass destruction, to know how hard it is to detect nuclear weapons. • 235U bombs1 emit little detectable gamma rays • 239Pu bombs (shielded) essentially emit no detectable g rays • In these cases SIZE MATTERS and DISTANCE MATTERS for radiation detectors. • Hand-held detectors are totally ineffective. 1Lamarsh, Basic Nuclear Engineering

6. An example is in order • You are a Coast Guard officer responsible for radiation monitoring of in-bound shipping. • What can you see with a hand-held g-ray detector (4 cm2) at 200 m distance? Attenuation by 200 m of air is 81%. • Given a dirty bomb composed of 1 kg (4.2X1016 Bq) of 60Co shielded with 1 inch of lead (81% attenuation) • This source will generate ~106g/sec at the detector, which is an easily detectable amount. (Ignoring self absorption.) • On the ship this source is a serious health hazard. • Given a nuclear weapon shielded with 1 cm of lead…. • NOTHING!

7. A comment about dirty bombs • For a dirty bomb to have physical impact it must have significant quantities of high activity material. • We typically are talking about 137Cs and 60Co. • These are hard to shield and easy to detect with almost any g-ray detector. • If the amount of radioactive material is small enough or well-shielded it probably can slip through any detection system but it will have little threat by itself.

8. It is a mistake to assume that terrorists are stupid • No terrorist is going to spend the time and money needed to acquire or build a nuclear weapon and… • then ship it into the US without any precautions. • No terrorist is going to waste time shipping in easy-to-detect radioactive material when... • such material is readily available in the US. • Today, Customs and Coast Guard personnel will only detect radioactive material from stupid terrorists.

9. A thorough cargo inspection system must use multiple detectors • Large, passive x-ray/g-ray detectors are a good start. • X-ray imaging systems are a big help in detecting shielded radiation sources or nuclear weapons. • Pulsed neutron sources will easily detect near critical masses of fissile material. • Personnel hazards are always a concern.

10. Bottom line: There is no single silver bullet for detecting radioactive material • You need to have good equipment • Again SIZE and DISTANCE matter. • Training, training, more training • If you don’t understand the problem you will not use your equipment effectively. • You must understand the fundamental limitations of your equipment. • Know what you can detect and what you cannot and why!

11. Why is a uranium-based nuclear weapon hard to detect? • 235U weakly emits 185-keV x rays • Easy to shield (0.5” lead transmits 5X10-8) • 232U is hard to detect because of its low concentration in 235U • Low levels of 2614 keV g rays • 10 kg of 235U contains 10-6 g of 232U • This is 2.5X1015 232U atoms or 8X105 Bq. • A U weapon emits 2.9X105 2614-keV g rays per second. • Even the best detector with the best software still needs ~ 1  g -ray per second to overcome background. • Ignoring any shielding - a 150-cm2 detector would have to be less than 18 m away to detect a weapon

12. Why is a plutonium-based nuclear weapon hard to detect? • 239Pu emits lower energy x rays. • 51.6, 56.8, 98.7, 129, 375, & 414 keV. • Low energy x rays are much easier to shield. • Deliberate shielding can make Pu invisible. • With 3 inches of shielding - a 150 cm2 detector would have to be less than 7.5 cm away to detect Pu 414-keV x rays.