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November 10, 20041 Technologies to Detect Materials for Nuclear/Radiological Weapons Gerald L. Epstein Senior Fellow, Center for Strategic and International.

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Presentation on theme: "November 10, 20041 Technologies to Detect Materials for Nuclear/Radiological Weapons Gerald L. Epstein Senior Fellow, Center for Strategic and International."— Presentation transcript:

1 November 10, 20041 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

2 2 Detection principles System considerations Nuclear radiation and radioactivity Technological approaches and limits Can address chemical and biological detection in discussion Outline

3 November 10, 20043 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

4 November 10, 20044 What’s Measured vs. What’s Real What’s Measured (+) Reported (-) Reported What’s Real (+) In fact Correct detection: p(D) False negative: 1 – p(D) (-) In fact False positive: p(FA) True negative: 1 – p(FA)

5 November 10, 20045 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

6 November 10, 20046 Useless Detector 1: Always Reports Detection Detector Report (+) Reported (-) Reported Reality (+) In fact 1.00 p(D) 0.00 (-) In fact 1.00 p(FA) 0.00

7 November 10, 20047 Useless Detector 2: Never Reports False Alarms Detector Report (+) Reported (-) Reported Reality (+) In fact 0.00 p(D) 1.00 (-) In fact 0.00 p(FA) 1.00

8 November 10, 20048 Useless Detector 3: Randomly Reports Detection Detector Report (+) Reported (-) Reported Reality (+) In fact X p(D) 1-X (-) In fact X p(FA) 1-X

9 November 10, 20049 Unattainable Detector: Perfect Sensitivity and Selectivity Detector Report (+) Reported (-) Reported Reality (+) In fact 1.00 p(D) 0.00 (-) In fact 0.00 p(FA) 1.00

10 November 10, 200410 Actual Detectors Trade Off Selectivity and Sensitivity As threshold T decreases from T 1 to T 2, more signal peaks are detected (P D increases) but more noise peaks are detected as well (P FA increases too). Source: Robert J. Urick, Principles of Underwater Sound (New York: McGraw Hill, 1983), p. 381

11 November 10, 200411 “Receiver Operating Characteristic” Obtained by plotting P D vs. P FA as detection threshold varies Curves force P D and P FA to be examined simultaneously The better the detector, the more that P D exceeds P FA Name derives from early days of radar / sonar Source: same as previous

12 November 10, 200412 “Receiver Operating Characteristic” (2) Source: same, p.382 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

13 November 10, 200413 Significance of Detection Depends on Number of Expected Positives (+) Reported fraction | events (-) Reported fraction | events (+) In fact 0.90 | 450 83% (+)’s correct 0.10 | 50 0.5% (-)’s wrong 500 actual positives (-) In fact 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 Case 1: Medical condition expected 5% of the time N=10,000 patients; p(D) = 0.9; p(FA) = 0.01

14 November 10, 200414 Significance of Detection Depends on Number of Expected Positives (2) (+) Reported fraction / events (-) Reported fraction / events (+) In fact 0.90 | 9 8.3% (+)’s correct 0.10 | 1 0.01% (-)’s wrong 10 actual positives (-) In fact 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 Case 2: Medical condition expected 0.1% of the time N=10,000 patients; p(D) = 0.9; p(FA) = 0.01

15 November 10, 200415 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

16 November 10, 200416 Nuclear Radiation Alpha particles –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

17 November 10, 200417 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 10 14 ev = 3 x 10 8 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

18 November 10, 200418 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

19 November 10, 200419 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

20 November 10, 200420 Gamma Ray Spectrum at Different Resolutions HPGe:High Purity Germanium detector (high resolution) NaI:Sodium Iodide detector (medium resolution) Source: ORTEC Corp.: HPGe NaI

21 November 10, 200421 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

22 November 10, 200422 Backgrounds Naturally occurring radioactive materials –Potassium nitrate fertilizers ( 40 K) –Granite or marble (Ra, U, Th) –Vegetable produce ( 40 K or 137 Cs 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

23 November 10, 200423 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

24 November 10, 200424 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

25 November 10, 200425 Scintillator Detector Examples Radiation “Pagers”

26 November 10, 200426 Scintillator Detector Examples Portal radiation detectors (yellow) at Blaine, WA Port of Entry Source: Physics Today 11/2004

27 November 10, 200427 Detection Process: Dosimetry Dosimeters measure total dose over some period of time; not real-term measurements. Types include Photographic film Thermoluminescent dosimeters

28 November 10, 200428 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

29 November 10, 200429 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

30 November 10, 200430 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

31 November 10, 200431 Muon Deflection Source:

32 November 10, 200432 Muon Deflection Source: Borozdin, K.N. et al. “Radiographic imaging with cosmic-ray muons,” Nature, 422, 277, (2003)

33 November 10, 200433 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

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