Polarized Target (POLTAR) Group
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Polarized Target Physics:
A polarized target is a piece of material in which the spins or magnetic moments of the nuclei are mostly pointing in one direction. When this is achieved we can then shoot neutrons at the target material to investigate the effect the spin of the nucleus has on its interaction with the neutron.
It is pretty simple to polarize nuclei. First you put them in a large magnetic field to line up the magnetic nuclei and then cool them to a low temperature to remove the thermal motion of the nuclei.
The first step is to start with a sample of material at room temperature (300 Kelvin). The polarization P is about zero, because the temperature is high and the Earth's magnetic field is only 0.5 Gauss. The nuclear spins (arrows) point in all different directions.
Then we put a large magnetic field (about 150,000 times the Earth's field) on the material, using a superconducting magnet. The polarization is still very small and the nuclear spins point in random directions.
Then we lower the temperature in a "dilution refrigerator" to near Absolute Zero. Now most of the spins point in the same direction and we have a reasonable nuclear polarization to use for experiments.