By Prof. Pozzi
Nevada Nuclear Security Site
(source: National Nuclear Security Administration: nnsa.energy.gov)
Welcome to the website of the Detection for Nuclear Nonproliferation Group (DNNG). As we wrap up the academic year 2012/2013 there are many accomplishments in the group and many plans for the summer! In April 2013, a measurement system was delivered at the Nevada National Security Site for a two-month testing by the Department of Homeland Security . The system uses a 2-inch diameter by 2-inch thick stilbene crystal grown at Lawrence Livermore National Laboratory (LLNL) by Natalia Zaitseva and colleagues with photomultiplier coupling and digital pulse shape discrimination algorithms developed by our group. One of the issues that homeland security is facing is the detection of special nuclear material (i.e., uranium-235 and plutonium-239, the isotopes that can be used in a nuclear explosive) in the presence of a high gamma ray background. In this case, it is paramount for the detection system to be able to detect the neutrons with high fidelity. For a system using organic scintillators, this means being able to keep the gamma ray misclassification ratio (number of gamma rays misclassified as neutrons) very low. A ambitious target value is one gamma ray misclassified as neutron every one million gamma rays detected. The UM-LLNL system is capable of reaching and exceeding this target value while maintaining a reasonable neutron detection efficiency.
As I write this column I am traveling to Italy with two DNNG members for our yearly tests with the plutonium-bearing materials available at the Joint Research Centre (JRC) in Ispra, Italy. These tests will determine how well the passive counter developed at UM by DNNG in collaboration with Idaho National Laboratory is able to determine the plutonium mass in nuclear materials of well-known composition. The objective of the measurement system is to determine the plutonium mass in samples of unknown size: a pressing problem in international safeguards. The instruments that DNNG is developing do not rely on the use of He-3 as the detection medium. He-3 is a decay product of tritium, which in turn is a by-product of weapons production. Because we are not fabricating new nuclear weapons, and because the demand for He-3 has increased in recent years, the world’s supply is not sufficient to keep up with demand. For this reason, the development of new nuclear safeguards systems that do not rely on this type of detector is an area of active research.
At the JRC Ispra, we will also test a new plastic scintillator that is capable of pulse shape discrimination. This material is based on a recipe developed at LLNL and commercialized by ELJEN technologies (EJ299-33). DNNG recently completed a study to compare the capabilities of this detector to those of the liquid scintillator EJ-309. The study is accepted for publication on Nuclear Instruments and Methods in Physics Research A.
In winter 2013, the imaging team within DNNG provided proof of concept for our dual particle imager (DPI). This imager is based on the use of scintillators for neutron and gamma ray detection. The first plane of the imager consists of liquid scintillators and the second plane consists of a combination of liquid and sodium iodide scintillators.
The current system uses back projection to determine the position of a source. An image is produced from neutron correlated counts and a second image is produced by gamma ray correlated counts. These images can be combined to get an image that pinpoints the location of a radiation source. The system is designed to be robust to various shielding configurations placed around a fissile source. The figure below shows a measured result for a Cf-252 source, which is used as a surrogate for plutonium-240. The neutron energy spectrum for Cf-252 is similar to that of Pu-240.
You can read more information on the group’s activities under the Research tab. Our research portfolio is allowing us to contribute to the solution of current pressing and complex issues in the areas of nuclear nonproliferation and safeguards while training the next generation of experts in this field. We are fully committed to the education and professional development of the students that choose to join us or to take our courses (NERS 590-1 Nuclear Safeguards, NERS 535 Detection Techniques for Nuclear Nonproliferation and NERS 554 Radiation Shielding).
I am looking forward to the semester. If you are interested in working with us please drop us a line. Download our flyer for additional information.