Detection and Identification of Radiological Sources Materials Challenges
Raytheon is developing a suite of tools to inspect cargo and freight for weapons of mass destruction or disruption. This began with the development of the Advanced Spectroscopic Portal (ASP) inspection system, which was sponsored by the Department of Homeland Security (DHS) Domestic Nuclear Detection Office (DNDO). Originally designed as a stationary nuclear materials portal, ASP was intended to be located at border crossings, bridges and other sites where vehicles and cargo containers pass through. The system contains several gamma ray- and neutron-sensitive detectors that can identify the location and type of radioactive isotopes in a moving vehicle. An ASP variant, shown in Figure 1, which contains a smaller number of radiation detectors, was next developed as a mobile detection system to search for and identify types of radiological materials as it drives by suspect objects.
In contrast to the ASP systems, the DNDO-sponsored Stand Off Radiation Detection System (SORDS) (Figure 2) has been developed to seek out radiological materials being transported on a moving platform at larger stand-off distances (greater than 100 meters). SORDS is designed as an imaging system, and it is more adept at identifying low levels of radiation such as those generated by lightly shielded point sources. Its objective is to detect small radiation threat-generated gamma ray signatures embedded in a sea of confusing background radiation.
At the heart of these radiation inspection systems is a network of specialized ionizing radiation detectors called scintillation detectors that have unique material properties, enabling them to discriminate and identify the type of radioactive isotopes being detected. These radiation detectors consist of a scintillating material that generates a burst of light when excited by radiation and an appropriately attached light detector such as a photomultiplier tube to quantify generated light levels.
Identifying the isotope type is critical since a host of naturally occurring radioisotopes is found in foods (such as potassium-40 in bananas) and in other common materials such as kitty litter. The system must differentiate these from potential threat materials. Isotope typing is best addressed by identifying uniquely emitted gamma ray spectral “fingerprints.” To detect and identify these spectral fingerprints, the radiation detector must have good energy resolution.
Good energy resolution enables the identification of a particular radioactive isotope gamma emission among closely spaced characteristic spectral emission lines produced by a large number of other radioactive isotopes. Under some circumstances high energy resolution is needed. The gamma ray detector behaves as a spectrometer, discriminating closely spaced spectral lines from a composite spectrum. Fundamentally, energy resolution is directly related to light output that is ideally proportional to incoming photon energy.
Scintillation detectors are constructed from either organic or inorganic materials, with inorganic-based scintillators being preferred for spectroscopic applications. Recent efforts in the search for new inorganic scintillation crystals have led to the discovery of several crystals with light output approaching 100,000 photons/MeV. This is comparable to the light output of commonly used thallium-doped sodium iodide (NaI:Tl) (roughly 40,000 photons/MeV). Figure 3 shows spectrometer resolution from several different types of commercial radiation detectors. Among the group is the NaI:Tl scintillator. Its resolution, though significantly better than that shown for organic plastic scintillators, is inferior to semiconductors such as the costly, high-purity germanium detector.
High-resolution detectors are required to find the elusive needle in a haystack: the presence of a real threat imbedded in a complex spectra derived from a mixture of innocuous radioactive isotopes. As shown in Figure 3, germanium identifies fine-line structures, differentiating between closely spaced spectral lines with unmatched resolution and high efficiency. The germanium detector is recognized as the gold standard for all gamma ray detectors. However, these same detectors are operationally difficult to handle since they must be cooled to liquid nitrogen temperatures and are very costly for large-area imaging applications.
In general, semiconductors inherently outperform scintillators, in terms of resolution, because their narrow bandgaps create greater numbers of electron-hole pairs from absorbed gamma ray energy. Semiconductor-based detectors are limited in size and for a variety of reasons, including cost: this makes them impractical for the large-area applications previously discussed. The challenge is to develop low-cost scintillator materials that closely match the resolving power of germanium without the large cost and complexity.
Specialized Scintillating Materials
There have been two recent developments in scintillation detectors. The first is a Raytheon development geared toward fabricating large-area gamma ray detectors. This has the potential to fill the need for improved passive- and active-based inspection systems to detect and identify concealed nuclear materials. The second development is an innovation in the evolution of a new material, Cs2LiYCl6 (CLYC), which has the unique ability to detect both gamma rays and neutrons. This material has the potential to impact many applications, such as the detection of fissile material where neutrons may be the dominant telltale signature as opposed to the more easily detectable shielded gamma rays.
Large Area Yttrium Aluminum Garnet (YAG)
Raytheon is developing a large-area scintillation detector by utilizing two of our key technologies: (1) world-class ceramic YAG laser gain material fabrication and (2) edge bonding, used to manufacture large-scale, high-performance scintillator panels. Cerium-activated yttrium aluminum garnet (YAG:Ce) scintillators offer moderate luminosity (>15,000 photons/MeV), fast decay time (<70ns), good (relative to NaI:Tl) energy resolution (<7 percent at 662 keV), and are non-toxic and non-hygroscopic. Ceramic YAG:Ce has excellent chemical resistance and environmental stability, as well as the mechanical strength necessary to produce large-scale scintillator panels (~1m x 1m).
Ceramic YAG:Ce scintillator panels made from bonded tiles are an enabling technology for nuclear radiation detection. Ceramic YAG:Ce tiles can be produced as substantially larger tiles (e.g., 25 cm x 25 cm) than is possible with single-crystal boules due to the inherent size limitations of typical single-crystal YAG:Ce growth (limited to ~10 cm dia). Ceramic YAG:Ce panels 25 cm x 25 cm having performance equivalent to or better than a single crystal are currently within Raytheon’s processing capability. Furthermore, unlike single crystals, ceramic YAG:Ce has negligible stress and excellent dopant uniformity. Large YAG:Ce scintillation panels lower manufacturing cost and improve the nuclear detection capability through their large surface area. This material promises to meet the demanding needs of the DHS and the Defense Threat Reduction Agency (DTRA) requirements for large-area radiation detection applications.
To meet the demands for a material that can detect and quantify both gamma rays and neutrons, one of Raytheon’s collaborating partners, Radiation Monitoring Devices (RMD), is developing a new material, CLYC, which belongs to the elpasolite crystal family. Due to their outstanding overall properties, CLYC scintillators can be used as gamma ray detectors, neutron detectors, or both. The energy resolution of CLYC crystals has been found to be better than 5 percent full width, half maximum (FWHM) at 662 keV, representing an improvement over the typical value of 6 to 7 percent FWHM measured for NaI:Tl. A comparison of 137Cs energy spectra measured for both scintillators is shown in Figure 4.
This energy resolution is a result of excellent proportionality; i.e., linear response of a scintillator to particles of various energies. The fact that Cs2LiYCl6 also incorporates Li (6Li) ions enables it to detect thermal neutrons. Li, interacting with thermal neutrons, produces charged particles that in turn create a scintillation pulse. Generated in this way, gamma ray energy is equivalent to about 3.3 MeV. This is higher than the energy of gamma rays emitted by the most common radioisotopes, which in turn gives CLYC the ability to automatically discriminate all gamma ray energies below ~3 MeV. CLYC can also discriminate between gamma rays and neutrons based on the shape of its temporal response. The level of discrimination using this method has been recently estimated to be at least 10-6. In terms of stopping power, a 1 cm thick crystal of 6Li enriched CLYC can capture a majority of thermal neutrons, providing 80 percent detection efficiency.
The discovery of new materials, as well as the optimization of existing, established scintillating materials is proceeding at a rapid pace, being driven by the demands of various applications such as medical imaging, nuclear physics, nuclear non-proliferation monitoring, materials research, well-logging, non-destructive evaluation and other related fields. While requirements for these applications may be different, many desired features are common. These include high light output, high proportionality, high-energy resolution, reasonably fast response and low cost. Raytheon, through the development of enabling material technologies and its experience in applying these technologies to radiation inspection systems, is playing a leading role in advancing the state of the art and improving our nation’s level of security in response to nuclear threats.
Bernard Harris, Raytheon and Kanai Shah
Radiation Monitoring Devices, Inc.
Scintillation is a form of luminescence that occurs when ionizing radiation travels into a material and loses some or all of its energy, promoting the emission of a short flash of light that emerges from the absorbing material. The scintillation process begins when ionizing radiation, such as gamma radiation, enters the material and interacts with it through photo absorption, Compton scattering or the production of electron-positron pairs.
Each of these processes generates quantities of secondary electrons proportional to the energy deposited from the ionizing radiation. These secondary electrons continually lose energy as they collide with atoms in the material. They interact with individual atoms exciting electrons into the conduction band, thereby creating electron-hole pairs. Electrons excited to the conduction band eventually return to the valence band with some emitting light.
The most commonly found gamma ray detector in use today is the thallium activator doped sodium iodide scintillation detector (NaI:Tl). Optically coupled photomultiplier tubes are typically used to convert the emitted light into electrical pulses. NaI:Tl is a very efficient gamma ray detector, due to its large crystal sizes. However, it is hygroscopic and must be packaged in a hermetically sealed container. Its spectral resolution is also relatively poor, opening the field for the development of more suitable scintillating materials for applications requiring higher resolution.