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November 10, 200 4 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, 20041 Technologies to Detect Materials for Nuclear/Radiological Weapons Gerald L. Epstein Senior Fellow, Center for Strategic and International

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November 10, 2004 1

Technologies to Detect Materials for Nuclear/Radiological Weapons

Gerald L. Epstein

Senior Fellow, Center for Strategic and International Studiesand

Adjunct Professor, Georgetown Security Studies Program

November 10, 2004

November 10, 2004 2

• Detection principles

• System considerations

• Nuclear radiation and radioactivity

• Technological approaches and limits

• Can address chemical and biological detection in discussion

Outline

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

What’s Measured

(+)

Reported

(-)

Reported

What’s R

eal

(+)

In fact

Correct detection:

p(D)

False negative:

1 – p(D)

(-)

In fact

False positive:

p(FA)

True negative:

1 – p(FA)

November 10, 2004 5

Three Useless Detectors and anImpossible One

• One that never misses

• One that never falsely detects

• One that’s somewhere in between

• One that’s perfect

November 10, 2004 6

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

November 10, 2004 7

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

November 10, 2004 8

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

November 10, 2004 9

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

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)

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

November 10, 2004 13

Significance of Detection Depends on Number of Expected Positives

(+) Reportedfraction | events

(-) Reportedfraction | events

(+)In fact

0.90 | 45083% (+)’s correct

0.10 | 500.5% (-)’s wrong

500 actual positives

(-)In fact

0.01 | 9517% (+)’s wrong

0.99 | 9,40599.5% (-)’s correct

9,500 actual negatives

505 positive reports

9,455 negative reports

10,000 patients

Case 1: Medical condition expected 5% of the timeN=10,000 patients; p(D) = 0.9; p(FA) = 0.01

November 10, 2004 14

Significance of Detection Depends on Number of Expected Positives (2)

(+) Reportedfraction / events

(-) Reportedfraction / events

(+)In fact

0.90 | 98.3% (+)’s correct

0.10 | 10.01% (-)’s wrong

10 actual positives

(-)In fact

0.01 | 10091.7% (+)’s wrong

0.99 | 9,89099.99% (-)’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 timeN=10,000 patients; p(D) = 0.9; p(FA) = 0.01

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– 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: High Purity Germanium detector (high resolution)

NaI: Sodium Iodide detector (medium resolution)Source: ORTEC Corp.: http://www.ortec-online.com/pdf/detective.pdf

HPGeNaI

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

• 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

November 10, 2004 25

Scintillator Detector Examples

Radiation“Pagers”

November 10, 2004 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

November 10, 2004 31

Muon Deflection

Source: http://www.lanl.gov/quarterly/q_spring03/muon_deflections.shtml

November 10, 2004 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