Adam Wallace and Dr. Lawrence Rees, Physics and Astronomy
Over the last year I have built several neutron detectors with lithium-6 glass and tested their efficiency. As stated in my proposal, this detector is being designed for use by the United States Department of Homeland Security (DHS) at portal monitors. In order to meet their requirements we had to test our detectors for neutron detection efficiency and minimize their sensitivity to gamma radiation.
For these requirements to make sense I will explain why gamma radiation (high energy light) is a problem and why neutron detection is so important. Every minute of every day you are being bombarded by radiation. Some of this comes from the sun or other cosmic sources, but most of it comes from decaying radioactive particles in every-day objects, such as concrete and bananas. It is important to note that almost all of this radiation is due to gamma rays. Such radiation only causes minimal damage to our living tissue, and our bodies are experts at replenishing dead cells. However, neutron radiation does not come from normal sources such as bananas, but only comes from a few certain heavy elements such as uranium, californium and plutonium. This means that if we can detect a significantly increased amount of neutrons there is a dangerous source of radiation nearby.
One worry of DHS is that the detectors would get confused about the type of radiation being detected. They require that only one out of a million gamma rays appear in our detector as a neutron. In order to do this I wrote a program to analyze the output data of our detectors and help distinguish where the signal came from.
My detectors are built out of special materials called scintillators that emit visible light when hit by radiation. I used two different types of scintillators. One is a plastic that emits light when hit by gamma rays and the other is made out of lithium-6 glass that emits light due to neutrons and gamma rays. Thus a single gamma ray will often create light in both the plastic and the glass. Because the light that is emitted from the glass and the plastic is different, I was able to write a program that would differentiate between the two types of light.
In order to test the sensitivity of our detectors to gamma radiation, I irradiated it with cobalt-60, which spontaneously emits gamma rays. We determined that one out of ten thousand gamma rays create a signal in our detector. This is not quite the insensitivity to gamma rays that we hoped, but is still very good. We are currently working on modifications for this detector that initial results show may meet DHS requirement. I cannot write about these modifications because we are still working on a patent.
The test for neutron detection efficiency is more difficult than the test for gamma radiation. Neutrons reflect off of heavier atoms such as carbon or oxygen and often bounce back into the detector. This gives an artificially high detection efficiency because the neutrons that bounce back into the detector should not have been detected. In order to neutralize this effect we took our detector to the Indoor Practice Facility at BYU so we could hoist the detector fifty feet into the air. With the detector so far away from the floor, the neutrons that fly past the detector almost never bounce back and are detected. This allows us to get an accurate measurement of the detector’s efficiency. I tested the neutron detection efficiency to be one and a half times more efficient than the DHS requirements.
This work has important implications in the field and we have been able to do some tests at different facilities across the nation. We tested detectors in Los Alamos National Laboratory in New Mexico and Ohio University. I was able to play a major role in testing these detectors and this experience has helped me immensely in applying to graduate school in physics. I applied and have been accepted to the top graduate school in nuclear physics as well as two other premier schools in other fields. I largely attribute this to my opportunities to do research as an undergraduate. Thank you for your support in this project. If you are interested in a more scientific view on this project my thesis will soon be available on the physics department web page. This is over a 20-page document with graphs and numerical results on all of our tests.