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Tiny Wafer Can Detect Neutron Signals From Fissile Materials

Dr. Douglas McGregor is pictured here holding a single "self-biased" GaAs neutron detector fastened in a test box. On the computer screen is a neutron reaction product spectrum taken with the device.
Manhattan - Dec 18, 2002
Does he or doesn't he? Only a weapons inspector knows for sure if Iraqi president Saddam Hussein has nuclear weapons or not. And he refuses to admit United Nations weapon inspectors.

A year after the September 11 attacks, the United States has become a country changed forever, the constant threat of another terrorist attack looming on the horizon.

As tensions between the United States and Iraq move ever closer to the brink of war, research by a Kansas State University professor may help fight the war on terrorism by making it easier to detect weapons of mass destruction -- in particular nuclear weapons.

When fully developed these neutron detectors could assist international weapons inspectors detect the presence of unauthorized nuclear weapons and materials, such as those alleged to be in possession of Iraq.

The essence of the small, portable detectors is a small wafer developed by Prof. Douglas McGregor, a K-State associate professor within the Mechanical & Nuclear Engineering Department.

McGregor, who has been working on producing semiconductor neutron detectors for approximately nine years, has been at Kansas State University since April where he has moved his entire detector fabrication laboratory from its previous location at the University of Michigan.

The U.S. Department of Energy's (DOE) Argonne National Laboratory, where McGregor has developed a long-standing collaboration with co-inventor Dr. Raymond Klann, funds much the detector research. Lawrence Livermore National Laboratory and other DOE programs, such as a Nuclear Engineering Educational Research (NEER) grant and DOE SBIR awards, have contributed additional funding to the detector research.

The neutron detectors are designed and fabricated by McGregor, which undergo a variety of tests and characterization procedures at the Kansas State University Nuclear Reactor Facility.

Afterwards, the devices are shipped to various DOE agencies for additional testing, which include Argonne National Laboratories (ANL) and Lawrence Livermore National Laboratory (LLNL).

Klann's research group at ANL has studied the performance of the detectors under various conditions, which has helped to provide information for improved devices. Collectively, McGregor and Klann have published over twenty papers on the detectors and their development.

The working part of the wafer, which is about the size of a collar button, operates by fabricating a diode out of semiconductor material such as gallium arsenide (GaAs), a semiconductor material similar to silicon. McGregor said GaAs was selected because of its low-noise characteristics at room temperature.

The devices are coated with combinations of boron-10 or lithium fluoride to make them neutron sensitive. When neutrons strike and are absorbed by the coatings, McGregor said they undergo an "immediate" reaction, ejecting charged particles and leaving a trail of ionization within the detector, a signature easily discernable by the detector as a neutron induced event.

One major improvement introduced by Klann and McGregor is the incorporation of tiny holes that cover the entire wafer surface. The holes assist in adhesion of the neutron sensitive films and also increase the neutron detection efficiency.

At present, the devices are capable of 13.5% thermal neutron detection efficiency, meaning that approximately 14 out of every 100 thermal neutrons that pass through the device will be detected, all with a device no thicker than a dime. Newer designs promise even higher efficiency. Several patents on the device structures and fabrication processes are pending.

According to McGregor, the wafers can be tailored according to the need of the user. The detector designs utilize a minimal amount of voltage, with some designs actually operating on their own internally generated voltage without the need for an external power supply. Some designs are tailored to be sensitive only to fast neutrons, whereas others are designed to be mostly sensitive to slow (or thermal) neutrons.

"We can batch-produce these detectors and make hundreds at a time with one process run, whereas some alternative methods of applying boron, as used by other research laboratories, allow for fabrication of only one or two detectors at a time," McGregor said. "When batch produced, we can actually make many versions of these detectors for a cost of no more than $10 to $20 apiece, so that's pretty low."

McGregor said the process of detecting weapons is basically a two-fold problem: weapons monitoring in other countries and weapons monitoring in the United States.

"We have stockpiles of weapons that we don't want anyone tampering with," McGregor said. "These detectors can be placed within the vicinity of these stockpiles and can monitor any change in the amount of neutrons that are being emitted by those weapons. Any alteration in the expected signal would set off an alarm and draw attention towards that facility."

McGregor said that although the tiny neutron detectors are still in the research phase and there are other neutron detectors that are more efficient. McGregor said those other detectors are much larger and require far more voltage to operate.

Previous neutron detectors have been made of large tubes of gas. The gas in the tubes is ionized when neutrons pass through the tube. McGregor said that not only are the tubes bulky in size, but also require more power to operate than the novel GaAs detectors.

By comparison, typical gas-filled neutron detectors require between 2000 - 5000 volts to operate, whereas the various GaAs neutron detectors designed and fabricated by McGregor require between 1 - 50 volts to operate.

"Feasibly we could make our devices so that they could compete with what is out there right now," McGregor said. "The advantage is that our devices are thinner, smaller, and far more rugged than what is produced now."

McGregor said that although he has been conducting his detector research for over nine years, it has generated a lot more interest since the 9/11 attacks. "The increased interest in this research has a lot to do with the fear of weapons of mass destruction being moved into this country," McGregor said. "These detectors would add another option to our ability to detect neutron radiation emissions from controlled nuclear materials."

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The ABC of Science and Security in an Age of Terrorism
 Washington - Oct 18, 2002
After the September 11, 2001, assaults on the World Trade Center and the Pentagon, and the subsequent anthrax attacks via the postal system, the scientific, engineering, and health research community was quick to respond at many levels, from initiating new research to analyzing needs for improved security.


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