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…
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…)
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
This is the Hamiltonian …using the principal axis …or if we introduce the 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=3/2 I=1
Weak magnetic field: Only the quadrupole term remains Degeneracy remains in this case I=3/2 I=1
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!
The principal method Two basic NMR pulse sequences – it’s almost the same for NQR
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!