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Strategies and Sensors for Detection of Nuclear Weapons

Strategies and Sensors for Detection of Nuclear Weapons

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Strategies and Sensors for Detection of Nuclear Weapons

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

  2. 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 Paper Number 13

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

  4. 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

  5. Aftermath of Nuclear Bombing of Hiroshima Joseph Papalia Collection

  6. 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

  7. 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)

  8. 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

  9. 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

  10. 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

  11. Neutron Induced Nuclear Fission The Oxford Encyclopedia /

  12. How to Build a Nuclear Weapon Glasstone and Dolan, “The Effects of Nuclear Weapons,” 3rd edition US DoD and ERDA, 1977

  13. 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

  14. 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

  15. Implosion Critical MassesWith and Without a Tamper

  16. Models of Little Boy and Fat Man National Atomic Museum, Albuquerque, NM

  17. Little Boy Bomb Dropped on Hiroshima Joseph Papalia Collection

  18. Fat Man Bomb Dropped on Nagasaki Joseph Papalia Collection

  19. Mushroom Cloud over Hiroshima Joseph Papalia Collection

  20. Structural Damage at Hiroshima • On closer inspection even concrete reinforced buildings suffered significant damage Glasstone and Nolan, “Effects of Nuclear Weapons”, 3rd edition (1977)

  21. Aftermath of Nagasaki Joseph Papalia Collection

  22. Energy Released by Fission

  23. 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

  24. 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

  25. Common Gamma-Ray Detectors

  26. 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%

  27. Ge Detector Spectrum WGU

  28. Depleted Uranium Spectrum

  29. WGPu Spectrum

  30. 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

  31. Gamma Ray Background Spectrum 212Pb e+e- 40K 208Tl 214Bi 228Ac 214Bi 208Tl 214Bi

  32. 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)

  33. 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

  34. Case Study: Hypothetical Weapon Design Steve Fetter et al. “Detecting Nuclear Warheads”

  35. Gamma-Ray Emissions

  36. One 100% Relative Efficiency Ge Detector1000 Second Counting Time

  37. Ten 100% Relative Efficiency Ge Detectors 1000 Second Counting Time

  38. Neutron Emissions

  39. 1 Square Meter Neutron Detector1000 Second Counting Time

  40. 10 Square Meter Neutron Detector 1000 Second Counting Time

  41. 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 > twice electron/positron mass (1.022 MeV) • Probability of detection increases with • Thickness of detector, area of detector, density of detector

  42. Gamma Ray Interactions with Lead

  43. 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

  44. 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

  45. 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

  46. Resolution Matters Multiplet peaks unresolved in NaI spectrum (top) are easily seen in Ge spectrum at bottom

  47. Effect of Resolution on Signal to Noise The peak is lost in the statistical noise as the resolution worsens (top to bottom)

  48. 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

  49. 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