NQR mine detector

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NQR mine detector

• Anton Gradišek
• Supervising officer doc. dr. Tomaž Apih
• Jožef Stefan Institute, F-5

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Overview

• Land mines and where how to find them
• Nuclear quadrupole resonance
• How a detector is supposed to work
• What tricks we use
• Wait, Ill show you samples later

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Land mines
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• Used to prevent the enemy crossing an important
  terrain
• Cheap and easy to plant
• Land mines are bad
• especially if you are a civilian!
• Searching metal detector, ground penetrating
  radar We need something better!

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Nuclear magnetic resonance

• A powerful analytical tool
• Energy levels split in magnetic field
• Molecular environment determines transition
  frequencies

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Nuclear quadrupole resonance

• For a quadrupole nucleus, the energy depends on
  the orientation in the electric field
• (get ready for some theory now)

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Energy
Taylor
(no dipole here)
quadrupole
monopole
We introduce this quantity
And the quadrupole gets simple
And this is what Wigner-Eckart theorem tells us
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This is the Hamiltonian
using the principal axis
or if we introduce the asymmetry parameter ?
Coming up next what this term causes at strong
and weak external magnetic field
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Strong magnetic field
Zeeman (B in z direction)
Quadrupole (principal axes in z direction)
I3/2
I1
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Weak magnetic field
Only the quadrupole term remains
Degeneracy remains in this case
I3/2
I1
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NQR detector

• Different molecules have significantly different
• NQR fingerprints

TNT (trinitrotoluene)
If we get a signal at the specific frequency, we
can be sure that there is some TNT around!
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The principal method
Two basic NMR pulse sequences its almost the
same for NQR
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Advanced methods

• TNT has low NQR frequencies (800 kHz)
• Difference between energy levels small gt low
  signal -gt we have to average many measurements
  (1000)
• Long relaxation time T1 (10 s) gt to make many
  measurements takes lots of time -(
• We use some tricks!

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Enhanced polarization

• We use an external magnet to split Zeeman levels
  of hydrogen
• When TNT is moved out of the magnet, polarizarion
  is transfered to quadrupole levels of nitrogen
• We get much
• stronger signal !

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Multiple pulse sequences

• Using the spin-lock spin-echo sequence we can get
  thousands of echoes in one shot -gt much faster!

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Where are we now?

• This method works just fine for RDX
• TNT is a bit more tricky
• Advanced methods are promising
• Still lots of work left (optimization,
  miniaturization, field testing)
• NQR will probably be a supplementary technique to
  others
• NQR has many many many more interesting
  applications!