Strategies and Sensors for Detection of Nuclear Weapons

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Strategies and Sensors for Detection of Nuclear
Weapons

• Gary W. Phillips
• Georgetown University
• February 23, 2006

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Based On
A Primer on the Detection of Nuclear and
Radiological Weapons Authors Gary W. Phillips,
Georgetown University David J. Nagel, George
Washington University and Timothy Coffey,
National Defense University Published by Center
for Technology and National Security Policy
National Defense University
http//www.ndu.edu/ctnsp/Defense_Tech_Papers.htm P
aper Number 13
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Outline

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

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Nuclear WeaponsThe 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

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Aftermath of Nuclear Bombing of Hiroshima
Joseph Papalia Collection http//www.childrenofthe
manhattanproject.org/index.htm
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Terrorist 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

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The primary observables from nuclear weapons 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)

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

• 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 mfps 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

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Radiation from Nuclear Materials

• Natural uranium
• Primarily gamma emitter
• 99.3 238U, not fissionable by low energy
  neutrons
• 0.7 235U, fissionable isotope, need gt20
  enrichment to make a usable fission weapon
• Weapons grade uranium typically gt 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

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Criticality

• 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

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Neutron Induced Nuclear Fission
The Oxford Encyclopedia http//www.oup.co.uk/oxed/
children/oise/pictures/atoms/fission /
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How to Build a Nuclear Weapon
Glasstone and Dolan, The Effects of Nuclear
Weapons, 3rd edition US DoD and ERDA,
1977 http//www.princeton.edu/globsec/publication
s/effects/effects.shtml
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Gun 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
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Schematic 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/Hbom
b.gif
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Implosion Critical MassesWith and Without a
Tamper
http//www.fas.org/nuke/intro/nuke/design.htm
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Models of Little Boy and Fat Man
National Atomic Museum, Albuquerque,
NM http//www.atomicmuseum.com/
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Little Boy Bomb Dropped on Hiroshima
Joseph Papalia Collection http//www.childrenofthe
manhattanproject.org/index.htm
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Fat Man Bomb Dropped on Nagasaki
Joseph Papalia Collection http//www.childrenofthe
manhattanproject.org/index.htm
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Mushroom Cloud over Hiroshima
Joseph Papalia Collection http//www.childrenofthe
manhattanproject.org/index.htm
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Structural 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
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Aftermath of Nagasaki
Joseph Papalia Collection http//www.childrenofthe
manhattanproject.org/index.htm
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Energy Released by Fission
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Effects 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

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Requirements 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

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Common Gamma-Ray Detectors
Characteristics of Gamma-Ray Detectors Characteristics of Gamma-Ray Detectors Characteristics of Gamma-Ray Detectors Characteristics of 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
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Requirements 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 lt 0.1

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Ge Detector Spectrum WGU
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Depleted Uranium Spectrum
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WGPu Spectrum
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Gamma-Ray Background

• Natural gamma-ray backgrounds can be divided
  into three sources
• 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

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Gamma Ray Background Spectrum
212Pb
ee-
40K
208Tl
214Bi
228Ac
214Bi
208Tl
214Bi
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Neutron 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)

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Factors 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

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Case Study Hypothetical Weapon Design
Steve Fetter et al. Detecting Nuclear Warheads
http//www.princeton.edu/globsec/publications/pdf
/1_3-4FetterB.pdf
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Gamma-Ray Emissions
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One 100 Relative Efficiency Ge Detector1000
Second Counting Time
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Ten 100 Relative Efficiency Ge Detectors 1000
Second Counting Time
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Neutron Emissions
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1 Square Meter Neutron Detector1000 Second
Counting Time
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10 Square Meter Neutron Detector 1000 Second
Counting Time
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Principles of Gamma-Ray DetectionSize 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 gt twice electron/positron mass
  (1.022 MeV)
• Probability of detection increases with
• Thickness of detector, area of detector, density
  of detector

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Gamma Ray Interactions with Lead
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NaI(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

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New Halide Scintillator Crystals

• Resolution better than half that of NaI
• LaBr3Ce (top) lt 3 at 662 keV
• LaCl3Ce (bottom) lt 4 at 662 keV

Bicron St. Gobain
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Germanium is the Gold Standard for Gamma-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

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Resolution Matters
Multiplet peaks unresolved in NaI spectrum (top)
are easily seen in Ge spectrum at bottom
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Effect of Resolution on Signal to Noise
The peak is lost in the statistical noise as the
resolution worsens (top to bottom)
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Neutron 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

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Thermal 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,a)3H, 10B(n,a)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

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Cross Section versus Neutron Energy
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Fast 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,a)3H

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Fast Neutron Reaction Cross Sections
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Lithium Doped Glass Fiber Scintillators
NUCSAFE Inc. Oak Ridge, TN
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Portals

• 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

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Search 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

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Active 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

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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

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Transmission Images
Rapiscan Corporation
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Backscatter Images
ASE Corporation
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Combination Imaging

• 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

ASE Corporation
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Summary 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

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