1. 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

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

3. 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

4. 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

5. 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
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6. Useless Detector 1: Always Reports Detection
Detector Report
(+)
Reported
(-)
Reality
In fact
1.00
p(D)
0.00
p(FA)
November 10, 2004

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

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

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

10. 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

11. “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

12. “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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. Gamma Ray Spectrum at Different Resolutions
HPGe
NaI
HPGe: High Purity Germanium detector (high resolution)
NaI: Sodium Iodide detector (medium resolution)
Source: ORTEC Corp.:
November 10, 2004

21. 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

22. 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

23. 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

24. 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
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25. Scintillator Detector Examples
Radiation “Pagers”
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26. Scintillator Detector Examples
Portal radiation detectors (yellow) at Blaine, WA Port of Entry
Source: Physics Today 11/2004
November 10, 2004

27. 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

28. 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

29. 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

30. 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
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31. Muon Deflection
Source:
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32. Muon Deflection
Source: Borozdin, K.N. et al. “Radiographic imaging with cosmic-ray muons,” Nature, 422, 277, (2003)
November 10, 2004

33. 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