Strategies and Sensors for Strategies and Sensors for Detection of Nuclear WeaponsDetection of Nuclear Weapons

Gary W. PhillipsGary W. PhillipsGeorgetown UniversityGeorgetown University

February 23, 2006February 23, 2006

A Primer on the Detection ofA Primer on the Detection ofNuclear and RadiologicalNuclear and Radiological

WeaponsWeapons

AuthorsGary W. Phillips, Georgetown University

David J. Nagel, George Washington Universityand Timothy Coffey, National Defense University

Published byCenter for Technology and National Security Policy

National Defense University

Based OnBased On

http://www.ndu.edu/ctnsp/Defense_Tech_Papers.htmPaper Number 13

OutlineOutline• Nuclear Weapons• Detection at a distance• Gamma-Ray Detectors• Neutron Detectors• Portals, Search Systems, Active Imaging Systems• Summary and Conclusions

Nuclear WeaponsNuclear WeaponsThe True WMDThe True WMD

• “Nuclear weapons are the only weapons that could kill millions of people almost instantly and destroy the infrastructure and social fabric of the United States. – Frederick Lamb, in APS News, Aug/Sep 2005

Aftermath of Nuclear Bombing of Aftermath of Nuclear Bombing of HiroshimaHiroshima

Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm

Terrorist WeaponsTerrorist Weapons

• To date have used conventional or improvised weapons– 9/11 most destructive single act

• Nuclear weapons have not been used– Nuclear weapons difficult to steal– Nuclear materials difficult to obtain

• Radiological weapons – could contaminate many city blocks, no immediate casualties– material highly radioactive, difficult to handle and transport

safely• Chemical weapons have been used in conventional warfare

– Terrorist attack could kill thousands• Biological weapons – dangerous to make and handle,

anthrax not contagious, smallpox could start a worldwide epidemic, kill friends as well enemies

The primary observables from nuclear The primary observables from nuclear weapons are gamma rays and neutronsweapons are gamma rays and neutrons

• Emissions from nuclear materials– Charge particles (alphas and betas)

• Short range, easily shielded will not get out of weapon

– Neutral particles – Neutrons and high energy photons (x-rays and gamma rays)

• More difficult to shield, no fixed range, continuously attenuated by matter

• Mean free path: distance attenuated by factor of e (2.7)

energy (keV) air water aluminum lead

alpha particles 5000 0.04 4x10-5 2x10-5 1x10-5

beta particles 1000 4 0.004 0.002 7x10-4

x-rays (mfp) 10 1.9 0.002 1.4x10-4 7x10-6

30 30 0.03 0.004 3x10-5

gamma rays 100 50 0.06 0.02 1.7x10-4

(mfp) 400 80 0.09 0.04 0.0041000 120 0.14 0.06 0.013

neutrons (mfp) 1000 200 0.1 0.1 0.08

range (m)

Ranges of Nuclear Particles

Radiation from nuclear weapons cannot Radiation from nuclear weapons cannot be detected by satellite or high flying be detected by satellite or high flying

aircraftaircraft

• Factors which limit the distance at which nuclear weapons and materials can be detected– Inverse mean square law

• Intensity decreases as the square of the distance

– Air attenuation• Gamma and neutron mfp’s in air are ~ 100-200 m

– Shielding• Can greatly reduce emissions

– Interference from natural and manmade background– Counting errors due to random statistical noise in the relatively weak

signals

Radiation from Nuclear MaterialsRadiation from Nuclear Materials

• Natural uranium– Primarily gamma emitter

– 99.3% 238U, not fissionable by low energy neutrons

– 0.7% 235U, fissionable isotope, need >20% enrichment to make a usable fission weapon

• Weapons grade uranium – typically > 90% 235U– Emits very few neutrons

– Primary observables – gammas, mostly low energy

• Weapons grade plutonium – 239Pu– Primary observables – both gammas and neutrons

– WGPu contains about 6% 240Pu• 240Pu has a relatively high neutron activity

CriticalityCriticality

• Subcritical masses of 235U and 239Pu have a small probability of decay by spontaneous fission emitting 2 to 3 energetic neutrons– These can be captured by neighboring nuclei inducing

additional fissions, leading to a chain reaction• A critical mass is that just necessary for a self-sustaining

nuclear chain reaction– Nuclear reactors adjust the neutron flux using control rods to

sustain criticality• Rapid assembly of a supercritical mass can result in a

nuclear explosion– Rapid release of energy in the form of radiation, heat and blast

Neutron Induced Nuclear FissionNeutron Induced Nuclear Fission

The Oxford Encyclopediahttp://www.oup.co.uk/oxed/children/oise/pictures/atoms/fission/

How to Build a Nuclear WeaponHow to Build a Nuclear Weapon

Glasstone and Dolan, “The Effects of Nuclear Weapons,” 3rd edition US DoD and ERDA, 1977http://www.princeton.edu/~globsec/publications/effects/effects.shtml

Gun AssemblyGun Assembly

• A (probably) more realistic design is shown here

• The target is a subcritical sphere with a cylindrical hole

• The projectile is a cylindrical plug that is propelled into the hole to create a supercritical mass

• The fuel is WGU– WGPu has too high a neutron activity– Weapon would pre-ignite

From: “The Los Alamos Primer”, Robert Serber, Univ. of California Press

Schematic of Implosion Weapon DesignSchematic of Implosion Weapon Design

• The fuel can be WGU, WGPu or a combination

• Ignition of the explosive lens compresses the spherical core increasing the density to a supercritical state

• The tritium gas serves as a source of additional neutrons

• The 238U tamper serves to contain the blast and reflect neutrons back into the core

• The Beryllium serves as an additional reflector

http://nuclearweaponarchive.org/Library/Brown/Hbomb.gif

Implosion Critical MassesImplosion Critical MassesWith and Without a TamperWith and Without a Tamper

Uranium Sphere Plutonium SphereBare Sphere 56 11Thick Tamper 15 5

Critical Masses (kg)

http://www.fas.org/nuke/intro/nuke/design.htm

Models of Little Boy and Fat ManModels of Little Boy and Fat Man

National Atomic Museum, Albuquerque, NMhttp://www.atomicmuseum.com/

Little Boy Bomb Dropped on HiroshimaLittle Boy Bomb Dropped on Hiroshima

Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm

Fat Man Bomb Dropped on NagasakiFat Man Bomb Dropped on Nagasaki

Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm

Mushroom Cloud over HiroshimaMushroom Cloud over Hiroshima

Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm

Structural Damage at HiroshimaStructural Damage at Hiroshima

• On closer inspection even concrete reinforced buildings suffered significant damage

Glasstone and Nolan, “Effects of Nuclear Weapons”, 3rd edition (1977)http://www.princeton.edu/~globsec/publications/effects/effects.shtml

Aftermath of NagasakiAftermath of Nagasaki

Joseph Papalia Collectionhttp://www.childrenofthemanhattanproject.org/index.htm

Energy Released by FissionEnergy Released by Fission

Effects of Nuclear WeaponsEffects of Nuclear Weapons

• Most of destruction comes from the blast or shock wave– Due to rapid conversion of materials in the weapon to hot

compressed gases– Followed by rapid expansion generating shock wave

• High temperatures result in intense thermal radiation– Capable of starting fires at considerable distances

• Radioactivity– Initial radiation is highly penetrating gamma-rays and neutrons

• Fallout comes from slowly decaying fission products – Mostly delayed beta particles and gamma rays

• The greatest fallout from a ground level terrorist explosion would come from activation of debris sucked into the fireball

Requirements for Gamma-Ray DetectorsRequirements for Gamma-Ray Detectors

• High atomic number (Z)– For good peak efficiency

• Reasonable Size– Depth for stopping the gamma rays

– Area for solid angle

• High Resolution – For detection of gamma ray peaks above background

– For separation of close-lying peaks

• Ease of operation– Room temperature preferred

– Simple electronics

Common Gamma-Ray DetectorsCommon Gamma-Ray Detectors

Characteristics of Gamma-Ray Detectors

detector atomic size peak room temp

type number resolution operation

plastic scintillators low sq. m. none yes

crystal scintillators high 1000 cm3 moderate yes

Ge semiconductor high 250 cm3 very high no (77 K)

CdZnTe semiconductor high 1 cm3 good yes

Requirements for Neutron DetectorsRequirements for Neutron Detectors• Thermal (low energy) neutrons

– Gas filled cylindrical proportional counters– Plastic or glass scintillator– Require moderator to reduce fast neutron energies– Characteristic requirements

• Low atomic number• Reasonable Size• High thermal neutron reaction efficiency

– Maximum a few percent• Ease of operation

• Fast neutron detectors– Plastic or glass scintillator– No moderator needed– Similar requirements

• Efficiencies < 0.1%

Ge Detector Spectrum WGUGe Detector Spectrum WGU

Depleted Uranium SpectrumDepleted Uranium Spectrum

WGPu SpectrumWGPu Spectrum

Gamma-Ray BackgroundGamma-Ray Background

Natural gamma-ray backgrounds can be divided into three sources1. Terrestrial background

– Natural radioactivity primarily due to decay of 232Th, 238U and 40K– Known collectively as KUT gamma rays– 232Th and 238U have long decay chains ending in lead– 40K decays by one of two branches either to

40Ar (10.7%) or 40Ca (89.3%)• Atmospheric background from radon gas

– member of 238U decay chain– released from decay of radium in soil

• Cosmic-ray background – Primarily from muon interactions with environment– Increases rapidly with altitude

Gamma Ray Background SpectrumGamma Ray Background Spectrum

212Pb

e+e-208Tl

214Bi228Ac

214Bi

214Bi208Tl

40K

Neutron BackgroundNeutron Background

• Primarily from cosmic rays– At ground level, cosmic rays consist primarily of high

energy muons– Interactions with matter produces neutrons

• Ground, buildings, ships, any massive object• Broad spectrum (no characteristic peaks)

Factors Influencing Detection CapabilitiesFactors Influencing Detection Capabilities• Configuration of the weapon or material

– Outer layers shield the inner layers• Depends on material and thickness of outer layers

– Self-shielding• Thick layers shield radiation from inside the layer

• Characteristics of the emitted gamma-ray spectrum– Low energy gamma rays are attenuated more than high– Continuum from higher energy gamma rays obscures lower energy

gamma rays

• Interaction with the environment– Attenuation and scattering by intervening materials

• Interference from the environmental background• Interaction with the detector

– Detector may not be thick enough to completely absorb the gamma ray– Detector resolution may not be high enough

Case Study: Hypothetical Weapon DesignCase Study: Hypothetical Weapon Design

Steve Fetter et al. “Detecting Nuclear Warheads” http://www.princeton.edu/~globsec/publications/pdf/1_3-4FetterB.pdf

Gamma-Ray EmissionsGamma-Ray Emissions

One 100% Relative Efficiency Ge DetectorOne 100% Relative Efficiency Ge Detector1000 Second Counting Time1000 Second Counting Time

Peak Gamma-Ray Counts from a Hypothetical Nuclear Weapon

1

10

100

1000

10000

100000

0 10 20 30 40

distance (m)

counts/1000 s (100% Ge)

peak countsbackground3 sigma

Peak Gamma-Ray Counts from a Hypothetical Nuclear Weapon

10

100

1000

10000

100000

1000000

0 10 20 30 40

distance (m)

counts/1000 s (ten 100% Ge)

peak countsbackground3 sigma

Ten 100% Relative Efficiency Ge DetectorsTen 100% Relative Efficiency Ge Detectors 1000 Second Counting Time 1000 Second Counting Time

Neutron Emissions Neutron Emissions

1 Square Meter Neutron Detector1 Square Meter Neutron Detector1000 Second Counting Time1000 Second Counting Time

Neutron Counts from a Hypothetical Plutonium Weapon

0.1

1

10

100

1000

10000

0 10 20 30 40

range (m)

counts/1000 s (1 m

2 detector)

source countsbackground3 sigma

Neutron Counts from a Hypothetical Plutonium Weapon

1

10

100

1000

10000

100000

0 10 20 30 40

distance (m)

counts/1000 s (10 m2 detector)

source countsbackground3 sigma

10 Square Meter Neutron Detector10 Square Meter Neutron Detector 1000 Second Counting Time 1000 Second Counting Time

Principles of Gamma-Ray DetectionPrinciples of Gamma-Ray DetectionSize MattersSize Matters

• Gamma rays are long range neutral particles– Do not produce an electrical signal when they pass through a

detector

– For detection, energy must be transferred to a short range charged particle (typically an electron)

• Gamma rays interact with detector in one of three ways– Photoabsorption – full energy transfer to atomic electron

– Compton scattering – partial energy transfer to atomic electron

– Pair production – electron/positron pair creation• Requires energy > twice electron/positron mass (1.022 MeV)

• Probability of detection increases with– Thickness of detector, area of detector, density of detector

Gamma Ray Interactions with LeadGamma Ray Interactions with Lead

NaI(Tl) ScintillatorsNaI(Tl) Scintillators

• Thallium activated sodium iodide has become the standard crystal scintillator for gamma-ray spectroscopy– Common configuration of 3” diameter cylinder by 3” deep – Often used as standard of comparison for efficiency of

gamma-ray detectors• High fluorescent output compared to plastic scintillators• Moderate photopeak resolution

– Typically ~ 8% at 662 keV• Large ingots can be grown from high purity materials• Polycrystalline detectors can be made in almost any size

and shape– By pressing together small crystal fragments

New Halide New Halide Scintillator Scintillator CrystalsCrystals

• Resolution better than half that of NaI

– LaBr3:Ce (top) < 3% at 662 keV

– LaCl3:Ce (bottom) < 4% at 662 keV

Bicron – St. Gobain

Germanium is the Gold Standard for Germanium is the Gold Standard for Gamma-Ray DetectorsGamma-Ray Detectors

• Germanium semiconductor detectors were developed to overcome limitations of low resolution scintillator detectors– Resolutions typically 0.2% or less at 662 keV

• Roughly a factor of 40 better than NaI

– Easily separate peaks close in energy

– Easily observe small peaks on high background

Resolution Resolution MattersMatters

Multiplet peaks unresolved in NaI spectrum (top) are easily seen in Ge spectrum at bottom

Effect of Effect of Resolution on Resolution on

Signal to NoiseSignal to Noise

The peak is lost in the statistical noise as the resolution worsens (top to bottom)

Neutron DetectorsNeutron Detectors• Neutron Detectors rely on neutron scattering or nuclear

reactions to produce an energetic charged particle

• Typical reaction cross sections are much greater at thermal energies– This requires moderating the fast neutrons by multiple

elastic scattering– All spectral information is lost by moderation

• The physics of moderation and detection means useful detectors cannot be too small or lightweight– Several cm of moderator required to slow neutrons to

thermal energies– Detection at a distance requires large enough areas to

give reasonable solid angles

Thermal Neutron DetectorsThermal Neutron Detectors

• Thermal neutrons usually defined as energies less than 0.025 eV– Approximate kinetic energy of gas molecules at room

temperature

• Thermal neutron detectors make use of neutron reactions which produce one or more heavy charged particles (HCP) – e.g. 3He(n,p)3H, 6Li(n,)3H, 10B(n,)7Li

– HCP reaction products highlighted in green

– One or both reaction products are detected

• The most common neutron detectors are gas proportional counters

• Others include lithium doped plastic or glass scintillators

Cross Section versus Neutron EnergyCross Section versus Neutron Energy

Fast Neutron DetectorsFast Neutron Detectors

• Use fast neutron reactions which produce charged particles that can be measured directly– Efficiencies relatively small

– No moderation so some spectral information possible

• Fast detectors typically make use of one of two reactions– 3He(n,p)3H and 6LI(n,)3H

Fast Neutron Reaction Cross Fast Neutron Reaction Cross SectionsSections

Lithium Lithium Doped Doped

Glass Fiber Glass Fiber ScintillatorsScintillators

NUCSAFE Inc.

Oak Ridge, TN

PortalsPortals• Portals are used to detect gamma-rays or

neutron sources on pedestrians or vehicles• Pedestrian portals similar in concept to

airport metal detectors– Except use nuclear detectors instead of

ferromagnetic • Contain plastic or NaI gamma ray detectors• May be combined with 3He neutron

detectors

Search SystemsSearch Systems

• Vehicle or helicopter mounted arrays of gamma ray and/or neutron detectors– Usually contain large NaI(Tl) scintillator

crystals and large 3He or BF3 neutron proportional counters

– May be combined with GPS and mapping software

Active ImagingActive Imaging• Active imaging

– Not limited by natural emissions from the target

– Can give a much improved signal to background ratio

– Useful for finding a weapon hidden inside other cargo

• Transmission imaging – Takes an “x-ray” image of the target

– However uses much higher energy x-rays or gammas than traditional medical x-ray machines

– Most sensitive to high Z materials

– Can penetrate low density materials and image high density uranium or plutonium

Other Active Imaging Technologies Other Active Imaging Technologies

• Backscatter imaging – Complementary to transmission imaging

– Looks at backscattered gamma rays from the source

– Most sensitive to low Z materials such as explosives

• Stimulated emission imaging– Source of high energy x-rays, gammas or neutrons can be

used to induce emissions from the target

– Can look for induced gammas or neutrons or both

– Source can be pulsed to look for delayed emissions

Transmission ImagesTransmission Images

Rapiscan Corporation

Backscatter ImagesBackscatter Images

AS&E Corporation

Combination Combination ImagingImaging

• Transmission image at top reveals heavy shielding

• Bar shows approximate location of radioactivity detected by passive array

• Backscatter image at bottom shows organic explosive material in bright white

AS&E Corporation

Summary and ConclusionsSummary and Conclusions• Gammas and neutrons are the only detectable emissions from nuclear

weapons– Both have limited penetration in air or solids– Cannot be detected from satellites or high flying airplanes

• Emissions from weapons are weak and difficult to detect– Size Matters– Resolution Matters – Background Matters

• Germanium is the Gold Standard for gamma-ray detectors– Has very high resolution, good efficiency, requires cooling

• Thermal neutron gas proportional counters are the standard for neutrons– Moderate efficiency, requires moderation

• Active imaging has the best chance of detecting a weapon hidden inside a container – Systems are large and complex– Require experienced operator to interpret