NQR mine detector

Anton Gradišek

Supervising officer: doc. dr. Tomaž Apih

Jožef Stefan Institute, F-5

Overview

• Land mines and where how to find them

•Nuclear quadrupole resonance•How a detector is supposed to

work•What tricks we use•Wait, I’ll show you samples later…

Land mines

• 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!

Nuclear magnetic resonance

• A powerful analytical tool • Energy levels split in magnetic

field

• Molecular environment determines transition frequencies

Nuclear quadrupole resonance

• For a quadrupole nucleus, the energy depends on the orientation in the electric field

(get ready for some theory now…)

Taylor

Energy

We introduce this quantity

And the quadrupole gets simple

(no dipole here)

And this is what Wigner-Eckart theorem tells us

monopole quadrupole

This is the Hamiltonian

…using the principal axis

…or if we introducethe asymmetry

parameter η

Coming up next: what this term causes at strong and weak external magnetic field

Strong magnetic field:

Zeeman(B in z’ direction)

Quadrupole(principal axes in z direction)

I=1 I=3/2

Weak magnetic field:

Only the quadrupole term remains

I=1 I=3/2

Degeneracy remains in this case

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!

Two basic NMR pulsesequences – it’s almost the

same for NQR

The principal method

Advanced methods

• TNT has low NQR frequencies (800 kHz)

• Difference between energy levels small –> low signal -> we have to average many measurements (1000+)

• Long relaxation time T1 (10 s) –> to make many measurements takes lots of time :-(

•We use some tricks!

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 !

Multiple pulse sequences

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

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!