Technologies to Detect Materials for Nuclear/Radiological Weapons Gerald L. Epstein Senior Fellow, Center for Strategic and International Studies and Adjunct Professor, Georgetown Security Studies Program November 10, 2004 November 10, 2004

Outline Detection principles System considerations Nuclear radiation and radioactivity Technological approaches and limits Can address chemical and biological detection in discussion November 10, 2004

Detector Principles Detectors are physical systems measuring noisy phenomena amidst backgrounds Sensitivity and selectivity must be considered together It’s easy to make a detector with a 100% detection probability (perfect sensitivity) It’s also easy to make one with a 0% false alarm rate (perfect selectivity) The trick is doing them at the same time November 10, 2004

What’s Measured vs. What’s Real (+) Reported (-) What’s Real In fact Correct detection: p(D) False negative: 1 – p(D) False positive: p(FA) True negative: 1 – p(FA) November 10, 2004

Three Useless Detectors and an Impossible One One that never misses One that never falsely detects One that’s somewhere in between One that’s perfect November 10, 2004

Useless Detector 1: Always Reports Detection Detector Report (+) Reported (-) Reality In fact 1.00 p(D) 0.00 p(FA) November 10, 2004

Useless Detector 2: Never Reports False Alarms Detector Report (+) Reported (-) Reality In fact 0.00 p(D) 1.00 p(FA) November 10, 2004

Useless Detector 3: Randomly Reports Detection Detector Report (+) Reported (-) Reality In fact X p(D) 1-X p(FA) November 10, 2004

Unattainable Detector: Perfect Sensitivity and Selectivity Detector Report (+) Reported (-) Reality In fact 1.00 p(D) 0.00 p(FA) November 10, 2004

Actual Detectors Trade Off Selectivity and Sensitivity As threshold T decreases from T1 to T2, more signal peaks are detected (PD increases) but more noise peaks are detected as well (PFA increases too). Source: Robert J. Urick, Principles of Underwater Sound (New York: McGraw Hill, 1983), p. 381 November 10, 2004

“Receiver Operating Characteristic” Obtained by plotting PD vs. PFA as detection threshold varies Curves force PD and PFA to be examined simultaneously The better the detector, the more that PD exceeds PFA Name derives from early days of radar / sonar Source: same as previous November 10, 2004

“Receiver Operating Characteristic” (2) Any one curve represents a single detector with different thresholds Different curves represent different detectors Parameter “d” here describes how close to ideal a given detector is Source: same, p.382 November 10, 2004

Significance of Detection Depends on Number of Expected Positives Case 1: Medical condition expected 5% of the time N=10,000 patients; p(D) = 0.9; p(FA) = 0.01 (+) Reported fraction | events (-) Reported (+) In fact 0.90 | 450 83% (+)’s correct 0.10 | 50 0.5% (-)’s wrong 500 actual positives (-) 0.01 | 95 17% (+)’s wrong 0.99 | 9,405 99.5% (-)’s correct 9,500 actual negatives 505 positive reports 9,455 negative reports 10,000 patients November 10, 2004

Significance of Detection Depends on Number of Expected Positives (2) Case 2: Medical condition expected 0.1% of the time N=10,000 patients; p(D) = 0.9; p(FA) = 0.01 (+) Reported fraction / events (-) Reported (+) In fact 0.90 | 9 8.3% (+)’s correct 0.10 | 1 0.01% (-)’s wrong 10 actual positives (-) 0.01 | 100 91.7% (+)’s wrong 0.99 | 9,890 99.99% (-)’s correct 9,990 actual negatives 109 positive reports 9,891 negative reports 10,000 patients November 10, 2004

Detector Systems Context; expected threat; suite of potential response options; operational protocols and doctrine; all affect choice of detector technology. If you can’t act on the information, do you want it? Must consider how system will be used, by whom; for what; and at what cost; answers will force tradeoffs Real world environment and operations are quite different from laboratory conditions Testing and verification are necessary November 10, 2004

Nuclear Radiation Alpha particles Beta particles Gamma rays Neutrons Energetic helium-4 nuclei emitted from certain radioactive elements Cannot penetrate sheet of paper or much air; cannot remotely detect Beta particles Energetic electrons emitted from certain radioactive elements More penetrative but still do not extend very far through air; cannot remotely detect directly Gamma rays Electromagnetic radiation (like light, but much higher frequency); can be considered to come in packets (photons) Highly penetrating; range depends on energy. Neutrons Produced spontaneously by plutonium but very rarely by other radioactive materials, natural or man-made Penetrative, including through materials that shield gamma rays November 10, 2004

Intensity vs. Energy Energy (of a particle or photon) Determines how far it can penetrate and how much damage it individually can do Measured in “electron-volts” – the amount of energy one electron can get from a one-volt battery. Typical values for radioactive decay are thousands to millions of electron volts (keV to MeV). That’s a lot for an electron but tiny for us. Dropping a paperclip (~500 mg) a distance of 1 cm releases 3 x 1014 ev = 3 x 108 MeV Intensity (of a radiation source) Determines how dangerous the source is or how easily it can be detected Depends on energy of each particle times numbers of particles per second A low-intensity source can produce high-energy radiation, and vice versa November 10, 2004

Nuclear Materials of Concern Nuclear weapon materials Highly enriched uranium (U-235); emits relatively low-energy gamma rays Weapons-grade plutonium (Pu-239 with some mixture Pu-240 and others); emits gamma rays and neutrons Radioactive dispersal device (“dirty bomb”) materials, with key threats including Co-60, Cs-137(primarily gamma emitters) Ir-192, Sr-90 (primarily beta emitters) Pu-238, Am-241, Cf-252 (primarily alpha emitters) However, these materials or their decay products often also emit gamma rays November 10, 2004

Radiation Spectrum Each radioactive substance emits particles or gamma rays with characteristic energies Graph of the intensity of the radiation of a given source as a function of the emitted energy is the source’s energy spectrum The energy spectrum of a source generating gamma rays at 400 keV would show a single peak centered at 400 keV. Detectors do not measure the energy of a radiation source precisely; even for sources at precise energies, they show energies over some range. The narrower the range, the better the energy resolution The better the resolution, the better the source identification November 10, 2004

Gamma Ray Spectrum at Different Resolutions HPGe NaI HPGe: High Purity Germanium detector (high resolution) NaI: Sodium Iodide detector (medium resolution) Source: ORTEC Corp.: http://www.ortec-online.com/pdf/detective.pdf November 10, 2004

Shielding Gamma radiation and neutrons are attenuated by surrounding material Gammas or x-rays of different energies attenuated by different processes, some depending essentially on the mass of the shielding and some depending on the composition (atomic number) Possibility of shielding strongly influences detector system design Things that shield gammas well shield neutrons poorly, and vice versa High-Z (atomic number) materials absorb gammas but only deflect neutrons Low-Z materials slow down and absorb neutrons (possibly below detection thresholds) but affect gammas less There is very little legitimate neutron background; any neutron sources is of high interest November 10, 2004

Backgrounds Individuals treated with medical isotopes Naturally occurring radioactive materials Potassium nitrate fertilizers (40K) Granite or marble (Ra, U, Th) Vegetable produce (40K or 137Cs from Ukraine) Old camera lenses (Th coatings) Thoriated tungsten welding rods or lantern mantles (Th) Certain glasses or ceramic glazes (U, Th) Porcelain bathroom fixtures (concentration of backgrounds) Individuals treated with medical isotopes Legal shipments of radioisotopes November 10, 2004

Detection Process: Ionization Ionizing radiation produces ions along its direction of travel that can be collected and measured by: Geiger-Muller counters Each photon or ionizing particle registers as a single count or click. Measures rough estimate of intensity of radiation but provides no information about type or energy of radiation or source Proportional counters Chamber – usually gas-filled tube – measures the amount of ionization formed by incident particle or photon, which is proportional to incident radiation’s energy. Collecting many such measurements produces source spectrum Solid-state crystals (e.g., germanium) Measure energy spectrum with much higher resolution. The highest-resolution detectors need to be cryogenically cooled November 10, 2004

Detection Process: Scintillation Ionizing radiation passing through certain substances produces flashes of light whose brightness is proportional to the energy of the radiation Flashes of light amplified by photomultipliers Energy resolution is modest at best Different types of scintillator Sodium-iodide or other scintillating crystal Liquid scintillator Plastic scintillator November 10, 2004

Scintillator Detector Examples Radiation “Pagers” November 10, 2004

Scintillator Detector Examples Portal radiation detectors (yellow) at Blaine, WA Port of Entry Source: Physics Today 11/2004 November 10, 2004

Detection Process: Dosimetry Dosimeters measure total dose over some period of time; not real-term measurements. Types include Photographic film Thermoluminescent dosimeters November 10, 2004

Detection Process: Active Neutron Interrogation Neutrons can induce reactions in materials that produce secondary neutrons and gamma rays, which can be detected. This approach can be used to search for explosives or other distinctive materials Nuclear weapon materials are particularly sensitive to this approach, since they react strongly with neutrons Technique not effective for other radiological materials November 10, 2004

Active Neutron Interrogation Lawrence Livermore National Laboratory concept now being prototyped Neutrons irradiate cargo from below Liquid scintillator used in side detector arrays: cheap and responsive November 10, 2004

Futuristic Concept: Muon Deflection Cosmic ray muons (charged particles produced in the atmosphere by incoming protons) constantly bathe the earth and are highly penetrating They are deflected when they pass through matter – more by high-“Z” (atomic number) materials such as uranium, plutonium, or lead used for shielding, than by low-Z materials Measuring incoming and outgoing muon directions can locate high-Z materials November 10, 2004

Muon Deflection Source: http://www.lanl.gov/quarterly/q_spring03/muon_deflections.shtml November 10, 2004

Muon Deflection Source: Borozdin, K.N. et al. “Radiographic imaging with cosmic-ray muons,” Nature, 422, 277, (2003) November 10, 2004

Conclusion Technologies exist to detect radioactive materials remotely from modest distances (several meters) Particularly if shielded, signals from these materials are weaker than materials from legitimate background sources. Therefore, discriminating threatening materials from backgrounds is essential Issues for mass deployment include background rejection; cost; and system design November 10, 2004