Examination of the maximum signal amplitude as a function of polarization time, for a fixed polarization field, yields surprising and important data for the student to ponder. For times longer than about ten seconds, the signal does not change with increasing polarization time; saturated magnetization. For shorter times, the signal decreases, but obviously not linearly.
Earth's Field Nuclear Magnetic Resonance
The high Q sample coil surrounds a 125 ml plastic bottle containing a liquid rich in either hydrogen or fluorine nuclei. The sample is placed in a uniform part of the Earth's magnetic field with the coil's axis aligned perpendicular to this field. The sample coil is connected to either a dc current regulated power supply or a high-gain, low-noise, tuned amplifier, with both switching and tuning electronically controlled. The output of the tuned amplifier is presented on a storage oscilloscope for observation and measurement.
At the beginning of an experiment, the sample coil is connected to the dc power supply. This supplies a maximum of 3 amperes for a pre-determined length of time. The electronic circuit then disconnects the supply, quickly dissipates the stored energy, and connects the coil to the tuned amplifier.
The sample nuclei (usually protons), having been polarized in a large magnetic field created by the power supply, find themselves oriented with their net magnetization perpendicular to the Earth's field. With the polarizing field now off, this magnetization precesses in the Earth's field, producing a time varying magnetic flux through the sample coil. The time varying flux creates an emf at the coil's terminals, which is magnified by the tuned amplifier.
Although the fundamental ideas behind the instrument are simple, the actual design of the apparatus has important subtleties. As the photographs show, there are actually two coils around the sample. The outer or "halo" coil is a bucking coil, designed to significantly reduce random noise pick-up from the always present stray electromagnetic fields. The bucking coil has the same turns-area as the inner sample coil. The coils are connected in series but in opposition. The output of the two coils is connected to the receiver. Since the two coils are in opposition, local noise fields produce equal and opposite emf in each coil and the net noise emf at the receiver is theoretically zero. Because the precessing magnetization is a dipole field, it couples primarily to the sample coil. This common mode rejection is essential to the outstanding signal-to-noise ratio of the apparatus. The students can study this system by rearranging the bucking coil connections.
The instrument uses a linear full-wave rectifier and low-pass filter as an amplitude detector. Its output is proportional to the maximum amplitude of the precession signal. This detector is particularly useful for signal averaging weak signals to enhance the signal-to-noise ration.
[Earths Field Nuclear Magnetic Resonance Device]
Turning off the polarization field with different damping configurations has some subtle yet interesting physics for advanced students to consider. The instrument allows for various damping resistors to be added. It is also possible to monitor both the sample coil current and voltage.
The experimenter can also listen to the precessing nuclei. The nuclear precession signals are amplified and fed into an internal loud speaker. If the local magnetic field is reasonable uniform, the "ping" can be heard for several seconds. External speakers can be used for classroom demonstrations.
A Conceptual Introduction to Nuclear Magnetic Resonance (NMR)
* Observe both Proton and Fluorine Free Precession
* Discover both the Curie Law and Spin-Lattice Relaxation
* Measure Spin-Lattice Relaxation as a Function of:
* Paramagnetic Ion Concentration
* Observe and Measure Proton-Fluorine J-Coupling
* Measure Absolute Value of gproton/gfluorine
* Precisely Measure Earth's Magnetic Field
* Hear the Precessions on Built-In Audio System
* Study Bucking Coils for Enhancing Signal-to-Noise
* Examine Effects of Tuning on Signal-to-N
It is hard to imagine a college physics or chemistry major graduating without having performed some kind of magnetic resonance experiment. Nuclear magnetic resonance has been, and clearly will continue to be, an important experimental tool in the arsenal of physicists, chemists, biologists and medical diagnosticians. Recent developments in quantum computing seem to indicate that magnetic resonance might become the basic platform of computer science hardware. There is no doubt that science majors should have a basic understanding of this type of spectroscopy.
TeachSpin, in collaboration with Professor Bill Melton of the University of North Carolina at Charlotte, has developed THE experiment to introduce sophomore, junior, or even senior physics majors to the exciting field of nuclear magnetic resonance (NMR). This new equipment allows students to perform experiments that naturally lead them to an understanding of precession, Curie's Law of paramagnetism, spin-lattice relaxation and even spin-spin coupling. The apparatus lends itself to inquiry based explorations. Its adjustable variables and magnetic resonance signals are all on a human scale. That is, the polarization times are in seconds, and precession frequencies are in the audio range. This significantly enhances the students' connection to the laboratory experience. TeachSpin's Earth's Field Nuclear Magnetic Resonance (NMR) instrument is rapidly becoming a "new classic," replacing the old, enigmatic marginal oscillator.
The student's first job is to find the precession signal by adjusting the coil's tuning capacitor and tuning the amplifier. A typical signal from a water sample doped with CuSO4 is shown in Figure 2. The student has two apparent experimental parameters to play with: (1) the polarizing field, (2) the polarization time.
TeachSpin's apparatus is also capable of detecting fluorine nuclear precession. Various fluorinated liquids are available from TeachSpin with different relaxation times. A particularly interesting sample of fluorobenzene exhibits a pronounced beat signal on the proton's free precession. The beats are due to the proton-fluorine spin-spin coupling. A measure of the beat frequency accurately determines the J-coupling between the proton and fluorine spins.
A simplified block diagram in Figure 1.
Accessible Nuclei: H1 protons, F 19 fluorine
Frequency range: 1.9 - 2.7 kHz
Bandwidth: 33 Hz at 2.1 kHz
Sample: Vol. 125 ml, Diameter 5.1 cm
Polarizing Power Supply: 0.5 - 3.0 A, 40 Volts max
Polarizing times: 0.1 to 99.9 s (in increments of 0.1 s)
Dead Time: 100 ms
Signal-to-Noise for H2 0: Nom. 200:1, Optimum 700:1
For a fixed polarization time, students can quickly discover the linear relation between polarization field and maximum signal amplitude. That's Curie's Law. A graph of student data is shown in Figure 3.