Simulation—such as this Monte Carlo analysis of two interactions from a single gamma ray in a crystal—are aiding the development of advanced detection systems.
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Sandia is pursuing advanced research to create enhanced detection capabilities to meet anticipated future needs.
No fielded detector can differentiate between naturally occurring radioactive material (NORM) and potentially dangerous SNM hidden in cargo and vehicles. Sandia is hoping to overcome this problem by using the capture gated neutron spectrometry technique to develop a portable solid-state detector that is highly sensitivity to fission neutrons, can distinguish gamma rays from neutrons, and can measure the energy of interacting neutrons.
This will require creating a new crystal by adding lithium to the solid-state scintillator trans-stilbene. This crystal will help the detector capture all of the neutron energy in the cargo being inspected—allowing identification of neutrons and rejection of NORM. The parallel development of a readout system using modular laboratory electronics will let us rapidly test the crystals that we produce. If both these efforts are successful, we will then incorporate the crystals and the read-out system in a prototype detector system.
Sandia and Lawrence Berkeley National Laboratory are teaming to develop FIND (fissmat inspection for nuclear detection)—a mobile system capable of detecting and identifying heavily shielded SNM. The FIND system will be based on:
FIND offers several key advantages over other detection systems:
Sandia is shrinking photomultiplier tubes (PMTs), a component that amplifies signals to allow detection. Miniaturizing detector components could in turn lead to smaller, less costly, and more easily portable detection systems. The project leverages Sandia’s capability to produce microparts using the LIGA process—a capability shared by only a handful of organizations worldwide.
The most common gamma ray spectrometers in use today are based on NaI(Tl) (thallium doped sodium-iodide) scintillator technology because this material is inexpensive and methods exist for growing the large crystals used by these spectrometers. The medical imaging market for NaI(Tl) detectors alone is greater than $1 billion, and it is anticipated that NaI(Tl) detectors will form the backbone of future DHS systems.
Unfortunately, NaI(Tl) scintillators have only mediocre energy resolution, limiting the ability to discriminate radioisotopes and separate natural backgrounds from emissions of dangerous materials. A main contribution to this mediocre energy resolution is the inherent non-proportionality of light yield to electron energy. Since incident gamma rays may have multiple interactions in the crystal—with each interaction producing an electron of different energy—this non-proportionality results in different amounts of observable light from each gamma ray, even when the gamma rays have the same initial energy. While alternative high resolution detectors exist, these tend to be bulky, inefficient, and expensive.
The Enhanced NaI Detector Project is seeking to construct reliable and affordable NaI(Tl) scintillation detector systems capable of distinguishing individual electron interactions. To this end, the project team is exploring the possibility of incorporating recent advances in multichannel PMTs and readout electronics to measure individual electron interactions in NaI(Tl) crystals—thereby overcoming the key limitation of today’s NaI detectors.
Initial work will focus on constructing a segmented prototype detector explicitly capable of distinguishing individual electron interactions to verify the validity of the resolution enhancement concept. In parallel, we will conduct simulations to determine the feasibility of distinguishing individual interactions in a single crystal—a much more difficult task that will require sophisticated reconstruction algorithms. If successful, this technique could be applied to existing NaI detectors—improving energy resolution of systems already deployed.