Transcript Strategies and Sensors for Detection of Nuclear Weapons Gary W. Phillips Georgetown University February 23, 2006

Strategies and Sensors for
Detection of Nuclear Weapons
Gary W. Phillips
Georgetown University
February 23, 2006
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
Paper Number 13
Outline
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Nuclear Weapons
Detection at a distance
Gamma-Ray Detectors
Neutron Detectors
Portals, Search Systems, Active Imaging Systems
Summary and Conclusions
Nuclear Weapons
The 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
Hiroshima
Joseph Papalia Collection
http://www.childrenofthemanhattanproject.org/index.htm
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
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)
Ranges of Nuclear Particles
alpha particles
beta particles
x-rays (mfp)
gamma rays
(mfp)
neutrons (mfp)
energy
(keV)
5000
1000
air
0.04
4
10
1.9
30
100
400
1000
1000
30
50
80
120
200
range (m)
water aluminum
-5
-5
4x10
2x10
0.004
0.002
lead
-5
1x10
-4
7x10
0.002
1.4x10-4
7x10-6
0.03
0.06
0.09
0.14
0.1
0.004
0.02
0.04
0.06
0.1
3x10
-4
1.7x10
0.004
0.013
0.08
-5
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 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 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
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
Neutron Induced Nuclear Fission
The Oxford Encyclopedia
http://www.oup.co.uk/oxed/children/oise/pictures/atoms/fission
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/publications/effects/effects.shtml
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
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/Hbomb.gif
Implosion Critical Masses
With and Without a Tamper
Critical Masses (kg)
Bare Sphere
Thick Tamper
Uranium Sphere
56
15
http://www.fas.org/nuke/intro/nuke/design.htm
Plutonium Sphere
11
5
Models of Little Boy and Fat Man
National Atomic Museum, Albuquerque, NM
http://www.atomicmuseum.com/
Little Boy Bomb Dropped on Hiroshima
Joseph Papalia Collection
http://www.childrenofthemanhattanproject.org/index.htm
Fat Man Bomb Dropped on Nagasaki
Joseph Papalia Collection
http://www.childrenofthemanhattanproject.org/index.htm
Mushroom Cloud over Hiroshima
Joseph Papalia Collection
http://www.childrenofthemanhattanproject.org/index.htm
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
Aftermath of Nagasaki
Joseph Papalia Collection
http://www.childrenofthemanhattanproject.org/index.htm
Energy Released by Fission
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
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
Common Gamma-Ray Detectors
Characteristics of Gamma-Ray Detectors
detector
type
atomic
size
number
peak
room temp
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 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 WGU
Depleted Uranium Spectrum
WGPu Spectrum
Gamma-Ray Background
Natural gamma-ray backgrounds can be divided into three sources
1.
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%)
2.
Atmospheric background from radon gas
–
member of 238U decay chain
–
released from decay of radium in soil
3.
Cosmic-ray background
–
Primarily from muon interactions with environment
–
Increases rapidly with altitude
Gamma Ray Background Spectrum
212Pb
e+e40K
208Tl
214Bi
228Ac
214Bi
214Bi
208Tl
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)
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
Case Study: Hypothetical Weapon Design
Steve Fetter et al. “Detecting Nuclear Warheads”
http://www.princeton.edu/~globsec/publications/pdf/1_3-4FetterB.pdf
Gamma-Ray Emissions
One 100% Relative Efficiency Ge Detector
1000 Second Counting Time
Peak Gamma-Ray Counts from a
Hypothetical Nuclear Weapon
counts/1000 s (100% Ge)
100000
peak counts
background
3 sigma
10000
1000
100
10
1
0
10
20
distance (m)
30
40
Ten 100% Relative Efficiency Ge Detectors
1000 Second Counting Time
counts/1000 s (ten 100% Ge)
Peak Gamma-Ray Counts from a
Hypothetical Nuclear Weapon
1000000
peak counts
background
3 sigma
100000
10000
1000
100
10
0
10
20
distance (m)
30
40
Neutron Emissions
1 Square Meter Neutron Detector
1000 Second Counting Time
counts/1000 s (1 m2 detector)
Neutron Counts from a
Hypothetical Plutonium Weapon
10000
source counts
background
3 sigma
1000
100
10
1
0.1
0
10
20
range (m)
30
40
10 Square Meter Neutron Detector
1000 Second Counting Time
counts/1000 s (10 m2 detector)
Neutron Counts from a
Hypothetical Plutonium Weapon
100000
source counts
background
3 sigma
10000
1000
100
10
1
0
10
20
distance (m)
30
40
Principles of Gamma-Ray Detection
Size 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 Lead
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
New Halide
Scintillator
Crystals
• 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
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
Resolution
Matters
Multiplet peaks
unresolved in NaI
spectrum (top) are
easily seen in Ge
spectrum at bottom
Effect of
Resolution on
Signal to Noise
The peak is lost in the
statistical noise as the
resolution worsens (top
to bottom)
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
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
Cross Section versus Neutron Energy
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
Fast Neutron Reaction Cross Sections
Lithium
Doped
Glass Fiber
Scintillators
NUCSAFE Inc.
Oak Ridge, TN
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
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
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
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 Images
Rapiscan Corporation
Backscatter Images
AS&E Corporation
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
AS&E Corporation
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