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Accelerator Based Techniques for Detection of Explosives and SNM
Richard LanzaMIT
Department of Nuclear Science and Engineering
IAEA RCM“Neutron Based Techniques for the Detection of Illicit Materials and Explosives”Johannesburg, South Africa
16 – 20 November 2009
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Outline• Overview• Explosives
– Neutron Resonance Radiography• Monoenergetic Neutron Radiography
• SNM– Materials of concern– Monoenergetic Gamma Radiography
• New Accelerator developments
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Neutron Resonance Radiography
• Make transmission images of objects using high energy (MeV) neutrons• Attenuation by a particular element varies with energy in a manner unique to a given element• Utilizes element specific resonances in energy to enhance the contrast• Especially for imaging low Z elements such as carbon, oxygen and nitrogen, but can be extended to others• Utilize this contrast enhancement mechanism to produce elementally resolved images of objects under inspection
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Advantages Of Using Neutron Attenuation
• Multiple Element (H,C,N,O +) Discrimination For 2-6 MeV Neutrons
• Proof Of Concept Application– Separation Of Diamonds
(Carbon) From Kimberlite Rock.
• Sensitivity scales as 1/R2 rather than 1/R4– TNA or PFNA detect excited
gammas which adds another factor of 1/R2 for overall scaling of 1/R4
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NRR concept• Like dual energy x-ray machines, NRR uses multiple neutron energies to extract information.
• Each energy produces its own equation.Consider a simple set of linear equations:2H + 3C + N + 2O = 85H + 4C + 3N + O = 133H + C + 2N +3O = 9H + C + 2N + 3O = 7Solving the set of equations will tell how much of each element is present. A set of neutron attenuation images can be decomposed into contributions from each basis element.
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NRR prototype concept: Accelerator rotates to vary neutron energy
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NRR Prototype System – Side View
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System components• Deuteron RFQ accelerator generates incoming d+ beam• Beam transport system focuses beam onto deuterium gas target to make neutrons.• Rotation of accelerator and adjustable collimator allows
variation of neutron energies• Neutron detection via plastic scintillator and photomultiplier tube. • Custom readout electronics / data acquisition provides
experimental control and data recording.• Prototype built in 2006 at Bates Linear Accelerator Center in warehouse with minimal climate control (airport
conditions).
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Elemental Cross Sections-Theory vs. Measurement
Nitrogen
Oxygen Silicon
Carbon
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Elemental Calibrations- Measurement of Melamine chemical composition
(constrained by angle selection)
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Elemental Calibrations: Measurement of Ammonium Nitrate chemical composition
(unconstrained angle selection)
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Application: Detection of explosives in air cargo
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LD3 Container
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LD3 Container
Explosive Simulant,Oxygen Detection1.0 m cargo
Carbon block, Carbon detection 1.5 m cargo
Carbon block, Carbon detection, 0.5 m cargo
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LD3 Container
Carbon Image with detection
Detection targets100cm Hydrangeas
Total elemental map (H+C+N+O+Si)
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Carbon in cut flowers.
Target correctly and automatically detected in typical air cargo background
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Detection of SNM Using NRR Facility
• Basic Approach– Use fast neutrons from NRR to induce
fission in SNM– Detect fission neutrons and/or fission
gammas– Large area detectors located out of
neutron beam
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Detection of SNM Using NRR Facility
• What radiation to detect?– Fission neutrons during RFQ pulse
• Requires a “gamma-blind” detector• Possible candidate is large liquid scintillator
– Delayed fission neutrons– Delayed (10’s of s) high energy gammas > 3MeV
• May be a unique signature for fission products of SNM• Great penetration of cargo for easier detection• Does not require good energy resolution• May be able to use liquid scintillator for both neutrons and γ’s
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Advantages of this approach• Container is imaged with existing NRR neutron beam• Require relatively small add-on to current NRR system• Present design allows increased interrogation time for suspicious regions
• Pulsed nature of RFQ allows for both prompt and delayed γand neutron detection
• Keeping neutron energy below 10 MeV avoids production of interfering 6.1 MeV γ from 16O(n,p)16N and 7.1 s decay of 16N
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Catastrophic Event Matrix
High Medium LowLikelihood
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What is the SNM problem?• Active detection and localization of small quantities of fissile materials in containers
• Minimize dose• Reduce size and cost• Separate Hi-Z (> 74) and low-Z materials• Separate W, Pb, U• Separate 235U and 238U
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Area of Destruction from 2 kTon Nuclear Weapon
(from T. Postel, MIT)
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Small quantities of Fissile Material are Involved
• Nuclear weapons need only about 1-2 critical masses of weaponizable material
• Critical assembly must be supercritical to explode• Chart shows the critical masses for some weapon isotopes• Bare refers to the critical mass of a sphere of material without anything surrounding it• Reflected refers to the critical mass when surrounded by a neutron reflector such as Iron or Tungsten• Using a neutron reflector always
reduces the critical mass• Generally a few tens of kilograms or less of material will be involved.
15
4354
10
36
12
75
8718
32
5
20
5
31
3
120
0
20
40
60
80
100
120
92-U
-233
92-U
-235
93-N
p-237
94-P
u-239
94-P
u-240
94-P
u-241
95-A
m-24
1
94-P
u-242
96-C
m-24
3
Isotope
Critic
al Ma
ss (k
g)
Bare Reflected
M.V. Hynes 2009
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Critical Mass and U-235 Enrichment
•Critical mass of U-235 depends on enrichment level.•HEU is formally any enrichment above 20%•WGU is formally any enrichment above 80%•Any enrichment above about 20% can be weaponized•Smaller masses are preferred because it is a lot of metal to move around with a propellant
M.V. Hynes 2009
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Background Neutrons are a Problem Because of Predetonation Probability
• Predetonation happens when the chain reaction is triggered before the device is fully assembled– Due to spontaneous fission neutrons
– Due to (alpha,n) reactions with Oxygen– This is called a fizzle
• The rate of neutron emissions is plotted in adjoining graphs for various isotopes (top) and oxides (bottom)• For U-235 this rate is very small• For Pu-240 and Pu-238 the rate is very high• Keeping the Plutonium metal free of oxides suppresses the (alpha,n) reactions
FOUO
FOUO
1.0E-041.0E-031.0E-021.0E-011.0E+001.0E+011.0E+021.0E+031.0E+04
U-235 Pu-238 Pu-239 Pu-240
Isotope
Neutr
ons/s
-gr1.0E+00
1.0E+01
1.0E+02
1.0E+03
1.0E+04
1.0E+05
(238)PuO2 (239)PuO2 (240)PuO2Isotope Oxide
Neutr
ons/s
-gr
M.V. Hynes 2009
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Fizzle Yields can be Substantial
• Fizzles happen when the neutron background triggers the chain reaction before the assembly is complete– Only a problem for Plutonium– All Pu weapons have a chance for a Fizzle
• For a Plutonium implosion weapon can use LANL model to estimate probabilities and yields– 45 generations of neutrons, time for supercriticality, and time between generations are assumed values– Graph shows probability of full yield versus background
• Every Pu implosion weapon has a minimum yield regardless of Pu grade.– What changes with grade is the likelihood it will perform at the minimum yield– In LANL model Ymin = 0.027Y0 where Y0 is the design yield– For a 20Kt design Ymin = 540 tons– This is 54 times bigger than a MOAB
0.00
0.20
0.40
0.60
0.80
1.00
1.0E+04 1.0E+05 1.0E+06 1.0E+07
Neutron Background (per sec)Pr
obab
ility o
f Full
Yield
Weapon grade
Reactor grade
FOUO
FOUO
M.V. Hynes 2009
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Nuclear Effects of 500 Ton Burst
• 500 Ton burst in Manhattan -– GZ at 42rd and Fifth Avenue– Three prompt nuclear effects– Radiation – 500 rads is LD50– Overpressure – 5psi is edge of survivability– Thermal – 3rd degree burns is edge of survivability
• All effects increase getting closer to GZ• Deaths for daytime burst
– Blast – 27,500 – Radiation (LD50) – 44,700
• About 70,000 people dead from a fizzle• Not corrected for urban environment
500 rad dose5 psi blast
3rd degree burns
Unclassified
Unclassified
M.V. Hynes 2009
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Basic Approach• The fundamental approach is to generate both neutrons and monoenergetic gamma rays by means of low energy nuclear reactions.
• This multi-particle method images the SNM through shielding and also identifies it by inducing fission in the suspect material.
• Use transmission imaging to locate high-Z materials
• Use photofission and/or neutron induced fission as final check
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Multi-particle Probing
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Why use Monoenergetic Gamma Imaging?
• orders of magnitude reduction in dose• enhanced sensitivity in detection and verification• lower requirements for shielding• ability to reduce the size of the source part of the system by an order of magnitude or more while still meeting requirements for detection and identification of SNM
• flexibility in detection through radiography and active probing with the same device
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Transmission Issues
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Dose Matters!• ANSI 43.14 standard “Radiation Safety Guidelines for
Active Interrogation Systems for Security Screening of Cargo” (now in draft) – Allows as much as 5 mSv (500 mrem) to potential stowaways– But, principle of ALARA remains in place for all such systems
• requires making every reasonable effort to maintain exposures toradiation as far below the regulatory dose as practical, taking into consideration the state of technology, the economics of improvements in relation to benefits to health and safety, and other societal and socioeconomic considerations
• Monoenergetic approach reduces dose to less than a mrem
• Reduced dose meets ALARA and allows for portable systems and reduced amounts of shielding
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Some Candidate Reactions
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Use Reaction d(11B,n)12C at 4 MeV• Produces fast neutrons peaking at around 12 MeV (with a broad distribution at lower energies)
• Intense gamma rays at 4.4 and 15.1 MeV– others at 10.7 and 12.7 MeV
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11B(d,γ) Reaction Gammas
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Neutron Yields for Various Targets
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Transmission Statistics100 cc U cube 4.64 thickness 40 Fe thickness cm
1/cm SNR = 5Energy Fe trans ratio Counts Out Counts In counts U U trans ratio
4.4 0.2551163 0.3060633 0.948493 30.6 8.28E+05 0.63 0.8380145 0.0204633 3.79820310.7 0.2366538 0.3334438 1.033345 27.2 3.52E+05 0.26 1.005596 0.0094019 1.74508512.7 0.2395002 0.329068 1.019785 26.7 3.87E+05 0.19 1.062475 0.0072206 1.34022115.1 0.2437217 0.3226838 1 26.3 4.50E+05 0.14 1.125574 0.0053876 1
Object solid angle(sr) at200 cm 5.38E-04Source strengthphotons/sec/sr at 100 microA
dose comversion photons time doserem/hr/photon/cm2/s on cube (sec) mrem photons/cm2/s dose rem/hr
4.4 6.00E+09 6.00E-06 3.23E+06 2.6E-01 6.4E-02 1.50E+05 9.00E-0110.7 2.00E+07 1.10E-05 1.08E+04 3.3E+01 5.0E-02 5.00E+02 5.50E-0312.7 4.00E+07 1.20E-05 2.15E+04 1.8E+01 6.0E-02 1.00E+03 1.20E-0215.1 6.66E+08 1.50E-05 3.59E+05 1.3E+00 8.7E-02 1.67E+04 2.50E-01
Total Dose Rate 1.17E+00
"Counts out" are the number of detected photons required to distinguish between material when U is substituted for Fe and is the number that pass through the iron without U. Counts U is the number that pass through the region with Fe and U.The SNR is shown above. The total attenuation must still be such that sufficient number of events pass through the bulk of material with a non-U thickness given above. "Counts in" is the total input to a non-U block. A "pixel" in this case is just the area of the face of the block.
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15.1 MeV Transmission Image
100 cc U cube in 40 cm Fe block imaged with 15.1 MeV gammas (MCNPX simulation)
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Identifying SNM• Transmission measurement alone only identifies high-Z material
• Use two approaches to distinguish 235U by detection of fission products– Utilize photofission– Probe with low energy neutrons (< 1MeV)
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Probing With Multiple Energies
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Photonuclear Cross-sections
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Potential Reactions at Higher Energies
Parameters:Produce γ’s >10MeVUse 9 MeV (p) cyclotron
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More Reactions
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Accelerators for this task• Low energy (<500 keV) nuclear reactions• High energy 5 -10 MeV or greater reactions– RFQ
• Difficult to vary particle type and energy– Compact Superconducting Cyclotrons
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The MIT Nanotron: An ultracompactsuperconducting 10 MeV classical cyclotron
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Some Conclusions• Monoenergetic gamma transmission imaging appears practical• Dose is orders of magnitude lower than alternative approaches• Compact accelerator designs are possible
– Smaller, lighter and lower power than alternatives– Flexible design in form factor and reaction choice– Can also be intense neutron sources
• (p,Be) at 12 MeV gives 4.2 x 1012 n/s @ 100 microA)– Shield the target not the machine
• Multi-particle approach gives both position and isotopic identification of SNM • Measurements conducted on full scale NRR prototype conclusively show multi-element discrimination possible with NRR.• Capability to measure rough chemical formulas demonstrated.• Automated detection based on elemental maps in air cargo sized objects demonstrated.• Dual use applications include special nuclear material detection, liquid explosives detection.
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Acknowledgements• Timothy Antaya MIT PSFC• Curtis Bell DHS• Gordon Kohse MIT NRL• Dave Perticone L-3• Rich Sheffield LANL• Vitaliy Ziskin L-3• Bates Accelerator Lab MIT
