Determination of Traces of Fissionable Materials Using Delayed Neutron Activation Analysis

Detecting clandestine fissile material like ²³⁵U and ²³⁹Pu is technically challenging in the lab or in the field because it emits little radiation, and that radiation is energetically weak. Shipments are easily concealed and very difficult to detect by conventional methods. However, when these materials are handled or transported, traces of uranium or plutonium in the form of particles smaller than grains of pollen can be left behind. Inspectors for the International Atomic Energy Agency (IAEA) have used wipecloth collection techniques to search for such evidence of handling and transport of clandestine treaty-monitored fissile materials. One of the most sensitive forensic techniques used to detect and measure the minute remnants of fissionable uranium and plutonium on these wipes is delayed neutron activation analysis (DNAA). The Nuclear Methods Team in the Chemical Sciences Division recently installed a DNAA system at the NIST Center for Neutron Research (NCNR).

DNAA works with excellent speed, sensitivity, and specificity, but few such systems exist worldwide. NCNR is an excellent facility to take advantage of this analytical method because of the intense thermal flux available with a minimal proportion of fast neutrons. These highly desirable characteristics mean that interference from fast-neutron reactions produced from ubiquitous thorium and oxygen are negligible. Overall, DNAA is well-tested, rapid, specific, matrix-independent, nondestructive, and sensitive. Consequently, the analysis process is greatly simplified and detection limits enhanced. The technique is sensitive solely to nuclides that fission with delayed emission of neutrons to give an unambiguous indication of controlled material. At the NIST DNAA facility, less than one nanogram of fissile species (e.g., ²³³U, ²³⁵U, ²³⁹Pu, ²⁴¹Pu, ²⁴¹Am, ²⁴³Cm, ²⁴⁴Cm, ²⁴⁵Cm, ²⁵²Cf) can be detected, and the analysis requires less than 3 min. Furthermore the system can process a great variety of sample types that might host the fissionable particles hidden in volumes as large as 40 mL. In the last year researchers from Oak Ridge National Laboratory (ORNL) and our team successfully demonstrated that the newly installed DNAA system response is strictly linear over at least a 25-fold concentration range of ²³⁵U (see plot above). This demonstrated ability enabled the system to immediately be used for the analysis of samples from ORNL. Recently, the sample transfer system was automated, leading to increased sample throughput and improved sensitivity. As the technique is further developed it will be used to certify the trace uranium in Standard Reference Materials. With the completion and verification of the DNAA system, NIST has a readily accessible, rapid means of measuring traces of fissionable U and Pu in samples of forensic interest for homeland security and for characterizing Standard Reference Materials.

• Modified pneumatic transfer system to achieve rapid delivery of samples to the neutron detectors.
• Successfully verified analytical accuracy using external standards and real-world samples.