NMR Background Curve FittingCaitlyn Meditz, on behalf of the Nuclear and Particle Physics Group. Advisor K. Slifer, E. Long

Overview

Dynamic Nuclear Polarization (DNP) is a technique used in experiments where the scattering of electrons from a polarized target is used to probe the internal structure of the target. DNP uses low temperature and a high magnetic field to polarize a target and orient spins. Our target is oriented using a 5T magnetic field and is submerged in a vessel of liquid helium held at 1K. This polarizes the electrons to a high degree. Millimeter waves tuned to the Lamour frequency are used to induce spin flips on the electrons and a nearby proton.

NMR Curve

The NMR curve is composed of two signals. The x values of the graph are given by an input triangle wave which shows a reference frequency vs time. The y values are the output of a modulator from which we use the amplitude over time.

My Contribution

It has been my task to eliminate the wings on either side of the NMR curve by assuming they can be described by a polynomial, and finding and subtracting their polynomial equation of fit. In addition to displaying the NMR curve, the program also displays temperature data from various points in the fridge using the current through resistors whose resistance changes depending on the temperature. It also shows a graph of the temperature of the Q Meter over time, and the integral of the calibrated modulator signal over time. These other outputs can be seen in the upper left, lower left, and lower right graphs, respectively.

NMR Background Removal

My code works by first accepting user defined input values for the cutoff points of the left and right wings: the x value at which the left wing ends, and by comparing the arrays of the input triangle wave (x values) and the modulator (y values), creates new arrays with only the values on the wings. A function within LabVIEW then accepts the new arrays as inputs and outputs an equation of fit, which is subtracted. A simplified version of the outcome of this procedure can be seen below in a set of curves taken from James Maxwell's thesis.

Before the magnetic field is applied, the NMR graph shows a curved background wave which peaks or reaches a minimum at the value in MHz where we would expect to find the proton. When the magnetic field is applied, we see the NMR curve, which is displayed below in the graph in the upper right hand corner. This graphic is a screenshot of our program's output. The curve points upwards or downwards depending on the direction of the polarization, and the higher the spike the greater the polarization. On either side of the spike we can see the “wings” of background information.

The polarization is measured using Nuclear Magnetic Resonance (NMR). Our data is viewed using the visual programming language LabVIEW. NMR can be observed with a small pickup coil positioned near our target sample, and is observed as a sharp resonant signal. The integral of this curve is proportional to the polarization of the target. On either side of the spike are "wings" that result from background contributions that are artifacts of the circuit response and are not related to the resonance. These wings must be subtracted in order to isolate the NMR signal.

Graphs to the right from:Maxwell, James D. Probing Proton Spin Structure: A Measurement of G 2 at Four-momentum Transfer of 2 to 6 GeV. Thesis. University of Virginia, 2011. N.p.:. n.d. Print.

The above diagram shows the layout of the four subsystems of our DNP setup