Determining the NQR Signal and Spin-Echo Train of Potassium Chlorate

Author(s): Kyle Nixon

Mentor(s): Karen Sauer, Physics

Abstract

Nuclear Quadrupole Resonance (NQR), is a phenomenon that occurs in elements that do not have a spherical symmetry of their nucleus. The non-spherical shape gives the nucleus of said atoms a preferred orientation in an electrical field gradient, which can be exploited to generate a signal. This is done by applying RF pulse to energize the nuclei of a sample out of its preferred orientation. The 39K line of potassium chlorate was chosen as the target for investigation as it has never been detected via direct methods before, it is a member of a class of atoms that are relatively unknown for NQR applications, and there is a need to develop detection technologies for 39K line of potassium chlorate as it is used in improvised explosives. The project focused on the discovery of the optimal pulse sequence for the excitation of the 39K line of potassium chlorate, its related time constants and temperature dependent frequencies, and evaluating the specific behavior them in comparison to the theoretical predictions of the wider class of nuclei that 39K is a member of. This project is the culmination of a previous project wherein a probe, capable of directly exciting and receiving signal from a 1 kg sample of 39K potassium chlorate, was constructed.

Video Transcript

Title Page:
Hello, everyone. My name is Kyle Nixon and my project focused on determining the NQR signal and the optimal excitation signal of the k-39 line of Potassium chlorate and comparing the experimentally found behavior of potassium chlorate to theoretical predictions of the wider class of elements it’s a member of.
Explanation of NQR:
Now the first question that probably comes to mind is: What is NQR? And NQR is short for Nuclear Quadrupole Resonance, which is a phenomenon that occurs in elements whose nuclei are non-spherically symmetrical. The technical term for this is that the material possesses a spin quantum number greater than ¬Ω (Miller) and this non-spherical shape gives the nucleus of these atoms a preferred orientation in an electrical field gradient, which is supplied by the neighboring atoms of a given sample [Miller, 2005]. By applying radio frequency pulses to the sample, it is possible to energize the sample to a higher energy state and out of this preferred orientation. And short time interval later, the atom sends out a signal of the same frequency and returns to its previous, unenergized state. And if you can properly time the pulses you can cause the atoms send out additional echo signals for a given number of excitation pulses. Now NQR is particularly effective because you wide range of elements that have the non-symmetrical nucleus and each of them possess a unique temperature dependent frequency. So when you send out a signal, and get a signal back, you know exactly what you have detected.
Why 39K of Potassium Chlorate?
The 39K line of potassium chlorate was chosen as the subject as my experiment for several reasons. First, the 39K line has never been detected directly detected before, making the work, in and of itself, as we used direct detection methods, inherently novel. Second, potassium chlorate of the 39K line possesses a spin of 3/2, which is a class of atom that has remained experimentally unobserved, relatively speaking for NQR methodologies. This provides us with a unique opportunity to take our found experimental data and compare it to the overarching behavior predictions that we would expect based off of the theoretical calculations for the class of spin 3/2 atoms. Finally, NQR technology and methodology is undergoing a review to see if it can be applicable in bomb detection technology. And unfortunately, potassium chlorate does see usage within improvised explosive devices, making whether or not it can applicable in these time constrained environments for NQR methodologies of interest.

Previous Work

My work this summer was primarily dependent on the URSP work I had completed last summer, where I had constructed a probe capable of directly detecting the NQR signal of potassium chlorate, however, by the time the project had ended, I had not yet gotten an NQR signal from our sample. This was due to a failure to properly account for the temperature dependent nature of our NQR frequency and, in addition, at the time, we did not know precisely what the frequency formula of potassium chlorate was anywhere near well enough to compensate. By further insolating our sample, and incorporating a temperature control system, we were able to ascertain what that temperature dependency on our frequency was, and we were able to overcome it. Over the past year since then, I have conducted experiments in conjunction with my mentor, Professor Karen Sauer, to determine the various time constants, decays and behaviors associated with our NQR signal of potassium chlorate.

Project Conclusions:
The Overarching goal for my URSP project this summer was to take my raw experimental data and turn it into a professional level journal article, capable of being published in the wider fields of physics. Now in order to do so I had to be exposed to, familiarize myself, and utilize several different software to bring this journal article to fruition. The first of which is LaTeX, the standard in text editors for professional level physics publications. I also had to take our raw experimental data and transform it into readable charts and graphs utilizing the Origin software. And I had to use things like Mathematica and MATLAB to evaluate some of these high-level theoretical expectations, or to take these theoretical expectations and to transform them into a more easily read result or evaluate our own experimental data itself. And on occasion, I had to perform some deeper literature searches than previously looking for some relatively niche paper only journal articles for these theoretical expectations in order to make a proper comparison with our experimental results. All of these steps and processes exposed me to, sort of, the next step in producing a contribution to the field of physics and taking your experimental results and turning then into a professional level contribution to the wider field of science.
Asymmetry Effect Calculation:
During the course of the summer, I needed to calculate the effect the that the asymmetry parameter, given on this graph as eta, has on the expected average pulse response over the spherical coordinates of phi and theta. Now, I’d like to say that this work isn’t novel. This is in fact a recreation of work that has already been done in published literature, however the graphs and charts found in published literature take a less sinusoidal approach to displaying the data and show it as series of deformed spheres. Which I found problematic to read and unideal for interpreting. The main components here, really, are that your asymmetry parameter, eta goes from 0 to 1, from symmetrical at 0 to not symmetrical at 1. And your calculations become difficult, really impossible, as eta is not 0. And the key assessments that your can make here is that your average pulse response, your mean of lambda here, does not change significantly as you move from and eta of 0 that is not 0 that isn’t 1. This is the basis for a lot of your assumptions, especially as we use a powder, which makes this assumption even more accurate. As you move forward, it (the assumption) makes your calculations possible, and yet still accurate.
Project conclusions:
In regards to the 39K line of potassium chlorate itself, we found that based off of its decay rates and time constants that it was on par with other materials being discussed for NQR technology applications in bomb detection, such as ammonium nitrate. Now if you’re interested in furthering this research, there are two main avenues. The one of which is furthering the development of NQR detection systems, which are currently unable to detect more than one material. as detecting each of these materials places great design restraints on your detection system. If you’re interested in detecting, and researching the 39K line of potassium chlorate itself, we did find that for specific decay constants, given specific input parameters, that they could be relatively long or relatively short. And the long and short decay domains for this time constants had shifts to them, indicating that different forces were at play in the long and short domain respectively. What forces these are, and how much they are at play is of interest, and as of yet, unknown. Finally, I have produced a journal article in conjunction with my mentor, Professor Karen Sauer with which I am working towards publication on.
And lastly I would like to thank my mentor, Professor Karen Sauer, and all the members of the Magnetic Resonance Lab at George Mason University for their help in this project And I would like to thank URSP for making this project possible.

2 replies on “Determining the NQR Signal and Spin-Echo Train of Potassium Chlorate”

Great job, Kyle. Nice description of some relatively complicated physics as background. Are you planning to work on this more, perhaps investigating the still unknown physics of the molecule? Dr. Lee

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