PURPOSE: To show the torque free precession time of the nuclear magnetic moment for water.
DESCRIPTION:
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/larmor.html#c1
EQUIPMENT: Teachspin NMR with Gradient Field Coils..
SETUP NOTES: 3 hours.
Teach Spin NMR 11/16/05
Most NMR runs at High B. They use a DC B field, and perpendicular to that is a AC B field and the nuclei are all aligned and then topple back.
This is low B field. They are aligned to a DC B field by a polarizing current. This B field is applied by the coils and the power supply. The power supply is turned off and switched, these same coils act as the pick up coils. When the polarized nuclei are relaxed they perform a precession about the earth’s B field direction. The earth’s field is mostly vertical.
It is the vector sum of 10^24 or 10^26 protons. Take out the water bottle and there is no precession signal.
Use a small (1mA) power supply to the Helmholtz coils. This current will either add or subtract to the earth’s field and increase or decrease the precession signal.
The sine wave inside the decay envelope is the free precession frequency.
Pull out the water bottle when you hear the “ping” and the signal dramatically drops to zero; It is the protons that give the signal.
Magnetic Torque Apparatus
Hydrogen nucleus, a proton that has two things:
1. B moment
2. angular momentum.
If spun parallel to B there is no precession.
If spun perpendicular to B it precesses.