Friday, June 22, 2012

Neutron Detection Using the Neutron’s Magnetic Properties

My name is Matt Conway and I am getting a dual degree in electrical engineering and physics. My mentor is Dr. Daneshvar who also has degrees in electrical engineering and physics. Our work crosses into both fields and is a very current and interesting project.

Our main goal is to design an inexpensive neutron detector, using the magnetic properties of the neutron. In particle physics and engineering, most particles have a charge, making it easier to detect, manipulate, and perform experiments on them. The neutron has no charge, which brings up the question of how a neutron can be detected.

There are several ways that are currently being used, but they rely on measuring secondary phenomenon from reactions to incoming neutrons.  These methods include things like neutron capture and proton recoil methods and involve measuring the emitted gamma rays or energy, the energy of recoiling protons, emitted alpha particles, or nuclear reactions that take place when a neutron enters the detector.

There are several other ways to detect neutrons, including quantum dots and giant magnetoresistance techniques. Quantum dots have properties that can cause them to fluoresce when struck by a neutron, a topic of current research. There is also research into a technique called giant magnetoresistance, or GMR, which uses the spin and magnetic moment of the neutron to reverse the magnetic polarization in the top layer of the GMR. This causes the resistance to increase, hence the name, giant magnetoresistance. This resistance change can be sensed directly. GMRs can be made in semiconductor wafers, so that there are many in a small area, allowing 2-D and 3-D location sensing, another area of current research.

These methods all work well, but ones that are developed and currently in use are expensive and cannot measure every neutron. In nuclear physics there is something called a cross section, which is the area that a neutron can hit and react with. Because of the structure of an atom, there is a small relative cross section to most materials, making it very hard for a neutron to be detected. In fact, many neutrons will go through the sensor completely undetected. In addition to the cross section, Helium-3, one of the main gasses used in neutron detection, is becoming scarce, with commercial prices in the range of $2,000 per liter. The reason for the scarcity of Helium-3 is due to several factors, including the way it is produced (in the generation of nuclear weapons) and the unforeseen need of large quantities of it. Helium-3 was, at one point, being emitted into the atmosphere because it was considered practically useless.

The scarcity of Helium-3 caused the price to rise from roughly $100 per liter to $2,000 per liter. The USA currently produces 8,000 -10,000 liters per year, but is selling roughly 14,000 liters per year, with a few spikes based on absolute need and the lack of alternatives. However, the USA’s stockpile of Helium-3 is declining rapidly and in need of alternatives. There are many current research projects to find alternatives to Helium-3 or to find altogether different methods of accomplishing the same goals.

Our research is based upon the idea of using the magnetic moment of the neutron to detect their presence. When magnetic fields are used, a magnetized object will always affect the field, making every neutron sensable. The challenge is to create a sensor that is accurate enough to sense the small magnetic field of the neutron. There are many challenges with this, especially noting that the neutron magnetic moment is -9.66 x10-27 J/T. We hope to design and begin research into detecting neutrons that are sensitive enough to detect the amount of neutrons that would be emitted by a nuclear weapon, for border control use and look into more sensitive sensing options. During this program, our goal is to design a small, inexpensive neutron detector that can be used at border crossings and other places to check for abnormally high neutron concentrations.
-- Matthew D. Conway under Dr. Kasra Daneshvar

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