Nonintrusive Nondestructive method to Detect Fissile Material

1
Non-intrusive Non-destructive method to Detect
Fissile Material
B. R. Tittmann, S. Jayaraman, G. Maillis, P. J.
Shull, P. M. Lenahan The Pennsylvania State
University, University Park, PA 16802 F.
Ze Lawrence Livermore National Labs, Livermore,
CA

• March 16 2004
• 9th Annual SPIE International Conference on
• NDE for Health Monitoring and Diagnostics
• Homeland Security and Measurement Technologies I
• San Diego, CA

2
Outline

• Objective
• Approach
• Theory
• Electron and Nuclear spin resonance
• Overhauser Effect
• Experiments
• Conclusions

3
Objective

• Need for an effective tool to detect and identify
  smuggled nuclear materials and other materials of
  interest to Nonproliferation and Homeland
  Security
• Nuclear acoustic resonance (NAR) based technique

4
Approach

• Unique nuclear acoustic resonance signatures
  generated when materials are driven by a high
  intensity resonant acoustic waves in the presence
  of a constant magnetic field
• This would cause shifts in the nuclear and
  electronic spin energy levels of the material
• Nuclear energy level shifts induce changes in the
  unique nuclear magnetic properties of the
  material which can then be quantified using
  sensitive instruments

Conceptual Layout
5
Advantages of NAR-based Technique

• It will not rely on nuclear radiation signatures
• defeat effective hostile nuclear shielding
• will not expose the operator to radiation
• It will not be intrusive
• will not hit the target materials with particles
  or radiation beams that can violate the chemical
  integrity of the target substance
• It has the potential to identify individual
  components of composite substances, including
  the fractional isotope composition of the
  material
• The technique will require no specialized
  training
• Wide range of applications for easy use by both
  state and federal government agencies, local
  police, civic, educational as well as commercial
  establishments

6
Disadvantages

• Very small effect DM10-12 Tesla
• Shielding from External Magnetic field
• Design and build Helmholtz Coils for Bias Field
• Introduce high intensity ultrasound into specimen

7
Theory of Spin Resonance

• The energy (Hamiltonian H) of a magnetic moment µ
  in a magnetic field H is given by, H -µ.H
• The angular momentum and magnetic moments can be
  taken as parallel. Therefore, µ ?J, where ? is
  called the gyromagnetic ratio and J is the
  angular momentum operator
• The gyromagnetic ratio is sometimes written as
  gß/h, where ß is the Bohr magneton g is
  typically about 2 for an electron and varies
  considerably among nuclear species.
• The angular momentum operator is defined by, J
  hI, where I is the nuclear spin quantum
  number. Therefore H -?hIH
• If the applied magnetic field Ho is oriented in
  the z-direction, then, H -? h Ho Iz

8
Theory of Spin Resonance (contd.)

• The Eigen values of this Hamiltonian is then
  given by,
• E -?hHom where, m I, I-1, I-2-I-1,-I.
• In conventional nuclear magnetic resonance and
  electron spin resonance an oscillating magnetic
  field held perpendicular to the applied field
  induces transitions between the levels
• The angular frequency is
• ? ?E/h ?Ho gßHo/h
• The perturbing Hamiltonian could then be given
  as
• H pert - ? h Hx Ix cos(?t)
• Fermis golden rule enunciates that the
  probability of inducing a transition from state m
  ? state m is exactly the same as the probability
  of inducing transitions from m ? m
• Thus, for sufficiently large H pert , the
  perturbation will completely randomize the spin
  system destroying the magnetization

9
Acoustic Interaction with Spins

• Electromagnetic waves cant penetrate deeply into
  uranium
• Periodically saturate the 235U nuclear spin
  system with acoustic waves satisfying the
  resonant condition
• The system will come back to thermodynamic
  equilibrium in a time T1, the spin lattice
  relaxation time
• Measure the periodic annihilation and recovery of
  the field near the sample (B field near sample
  4?M)
• The acoustic waves can interact with the electron
  and nuclear magnetic moment and with the nuclear
  quadrupole moment.

10
Overhauser Effect

• Overhauser Effect will enable us to
• Substantially boost the response to the nuclear
  spin system
• Detect a change in M which is still small
• The application of a perturbing Hamiltonian for
  both electron and nuclear spin systems can
  enhance the nuclear spin sensitivity by a factor
  of
• Detection in commercial (off the shelf)
  Overhauser systems

(for 235U nuclei)
11
Energy Levels (Nearby Nucleus)
Red - electron spin Purple - nuclear spin
Large D E
External Magnetic Field
12
Allowed Energy Level Transitions (Nearby Nucleus)
13
Energy Level Population Transitions (Steady State)
14
Energy Level Population Transitions (Steady State)
15
Note in principle, we could utilize this
approach without knowing the magnetic field in
the uranium and possibly with a fairly
non-uniform field in the uranium.
Satisfies both resonance conditions
If we periodically swept though and we
maintain the ratio, we could in principle
excite both spin systems.
16
Detection Limits of Commerical OHM

• signal averaging sensitivity improves with
  (time)1/2
• low frequency lock-in amplifier
• sensitivity of the detector will scale
  (magnetic field)2
• size of the resonance induced change in field
  will scale (magnetic field)1
• Ideally, the system would operate in the earths
  field no applied field required. However,
  application of a field would enhance sensitivity
  substantially
• Sensitivity would scale with (applied field)3
• Ideally, the system would operate in the earths
  field no applied field required
• However, application of a field would enhance
  sensitivity substantially (applied field)3

17
Conceptual Layout
18
Expected Correspondence between Magnetometer
signal and Acoustic Application
19
AM Signal generation

• Combination of 1 MHz and 200 Hz (32 kHz)
• Wavetek Signal generator
• Ultran -1 MHz transducer
• Panametrics - 50 kHz
• ENI Amplifier

20
Resonance Set-up showing the Air-Gap between Al
Cylinder and the Non-Contact Transducer
21
Resonance Condition in Al Crystal - 1 Air-gap
Max Pk-Pk (_at_resonance) 1.3V
Max Pk-Pk (_at_resonance) 1.3 V
22
CONCLUSIONS

• The combination method of nuclear and electron
  spin resonance and Overhauser effect is being
  employed for the first time
• Preliminary experiments show that the technique
  is possible
• Efforts are underway to acquire magnetic
  shielding equipment
• Preliminary tests are to be conducted on Li as
  well
• For further tests, combination of 200 Hz 32 KHz
  will be used for the AM signal
• Attempts are being made to improve OHM signals by
  using neural networks.