Better Ways to Detect Radiation
Detecting illicit sources of plutonium and highly enriched uranium is a tricky business for first responders, airport security personnel and port and border inspectors. Both plutonium and highly enriched uranium are typically identified using a combination of devices, working together, to detect their invisible gamma and neutron radiation emissions. For many years, the existing detection technologies were deemed adequate, but the world became a far more dangerous place after September 11, 2001. Since then, concerns of radioactive materials falling into the wrong hands have been rife. Ensuring that the country remains safe from a nuclear or radiological attack is driving the search for more definitive radiation detection and identification technologies.
The Department of Energy has for decades been building the science and engineering basis for the detection of radiological materials; Lawrence Livermore National Laboratory has made a contribution to this ongoing effort by working to improve the efficiency and energy resolution of radiation detectors. It also has developed systems that illuminate objects such as cargo containers and use these radiographic techniques to find hidden nuclear materials. In 2005, the Department of Homeland Security made a bold request for developing even more effective materials to detect gamma and neutron radiation emissions. New materials for smaller, faster and more accurate sensors could improve the nation’s ability to unambiguously identify radiation from illicit sources.
The development of new detector materials used to be a time-consuming, trial-and-error, often decades-long process. Today, greatly improved diagnostic tools for measuring material properties combined with faster, more accurate computer simulations allow researchers to rapidly evaluate materials for performance and ease of fabrication.
Physicist Steve Payne leads several teams in a radiation detector materials campaign at Livermore, in a collaboration that includes Pacific Northwest, Lawrence Berkeley, Oak Ridge, Brookhaven, and Sandia national laboratories; Fisk, Stanford, and Washington State universities; the University of Nebraska; and numerous private firms including, Radiation Monitoring Devices, Inc. “With so many partners working together, we can avoid duplication of facilities, which is more cost-effective,” says Payne.
For many field applications, radiation detectors must be inexpensive and robust, operate at ambient temperature, provide high efficiency and be small enough for use in covert operations. They must also provide unambiguous identification of a material. For example, the tiny amount of thorium in cat litter is radioactive, but thorium is not a security threat. Additionally, detectors must be able to pick up very weak signals, such as from plutonium heavily shielded with lead to avoid detection.
Current detector technology is limited in its ability to meet the requirements of people on the front lines who are responsible for ensuring the nation’s safety. Some of today’s best gamma-ray detectors are those that use germanium as the sensor material. The laboratory has been at the forefront of efforts to make germanium detectors as small and field-portable as possible, and a challenge has been that to achieve the best resolution, the germanium must be cooled to below room temperature. Detectors for low-energy neutrons, known as thermal neutrons, are typically tubes filled with helium gas, but these instruments are large, require high voltage to operate, and are sensitive to vibration. The best material for detecting high-energy, or fast, neutrons is a crystal called stilbene, but the crystal is difficult to grow, expensive, and available from just one company in Ukraine.
The Livermore teams of chemists, physicists and engineers, along with their collaborators, set out to find new, better materials for all three types of detectors—materials that meet the needs of as many users as possible. Payne says, “In our search for novel materials with the necessary properties, we began with the entire periodic table.” Each team used a different process of elimination because the requirements for detecting the various forms of radiation are different.
Earlier work at Livermore on new detector materials for thermal neutrons was funded by the Laboratory Directed Research and Development program. As that project and others demonstrated success, outside funding for new detector materials followed, first from DOE’s National Nuclear Security Administration and later from DHS and the Defense Threat Reduction Agency.
Gamma rays, produced through radioactive decay, have the highest energy in the electromagnetic spectrum and thus can penetrate most materials. Because of this extreme penetrability, gamma rays can be detected even when the radiation source is shielded by concrete, dirt or a few centimeters of lead. However, gamma rays can only be viewed indirectly by observing their interactions with detector materials.
High-purity germanium, a semiconductor, has been the standard for detecting gamma rays for years. An alternative method is scintillation, in which radiation interacts with a material to produce a brief but measurable flash of light. Livermore chemist Nerine Cherepy and her team, which won an R&D 100 award in 2010, are on the hunt for a material that will produce the brightest flash of light when exposed to plutonium or highly enriched uranium. The precision of the scintillator material’s response, or energy resolution, defines the material’s ability to distinguish between gamma rays that have similar energies. Such discrimination is needed because not all gamma rays are indicative of a source that poses a threat.
Scintillators of lanthanum bromide doped with cerium offer the highest energy resolution among commercial devices. However, lanthanum bromide doped with cerium is difficult to grow, is radioactive because of the presence of lanthanum-138, and is expensive. The goal for Cherepy’s team is to find a high-resolution material that is not radioactive, operates at room temperature, is inexpensive, and can be manufactured in large volumes. After much analysis and elimination—with an array of crystals grown from various compounds by national laboratory, industrial, and university partners—Cherepy and her team narrowed their search to two materials: transparent ceramic gadolinium-based garnets and strontium iodide crystals.
Transparent ceramics have already been used in Livermore’s solid-state heat-capacity laser, a system that is setting the stage for “directed energy” laser weapons. “Livermore has established expertise in transparent ceramics,” notes Cherepy. For scintillators, transparent ceramic garnet is relatively inexpensive, strong, and can be fabricated into large, uniform, robust devices. The team has developed a novel fabrication technique for transparent ceramics starting from nanoparticles, for which they earned a Nano 50 Award in 2008.
The best energy resolution for any of the various garnet compounds is about 4 percent, when exposed to gamma energy of 662 kiloelectronvolts. (This energy, from a cesium-137 source, is the standard used in experiments for measuring the resolution of a gamma-ray scintillator.) The energy resolution of lanthanum bromide doped with cerium is 2.6 percent, and for gamma detection, smaller is better. The team’s goal is to find a material with a resolution of 2 percent.
Strontium iodide doped with europium has proved to be the best scintillator material yet. “It is an easily grown crystal with excellent energy resolution,” says Cherepy. It produces more photons—more light—than lanthanum bromide doped with cerium and is not radioactive. Small crystals have demonstrated 2.5-percent energy resolution, slightly better than lanthanum bromide doped with cerium. “The resolution degrades a bit with larger crystals,” says Cherepy. However, we expect to achieve 2-percent resolution by improving the purity of crystals as they grow in size and by optimizing the detector’s optics and digital electronics.”
Identifying High-Energy Neutrons
While working on a Ph.D. at Moscow State University in Russia, Livermore physicist Natalia Zaitseva developed a method for growing extremely large crystals faster than ever before. Zaitseva perfected the process after coming to the Laboratory and won a 1994 R&D 100 Award with her team. At the time, the researchers were focused on producing large crystals of high-quality potassium dihydrogen phosphate (KDP) for Livermore lasers such as the National Ignition Facility (NIF). In 1996, the team produced in just 27 days a KDP crystal measuring 44 centimeters across. Growing a crystal that size under standard growing conditions would have taken 15 months. The NIF laser design required a huge amount of KDP—about 600 large slices—for conversion of the laser’s infrared light to shorter wavelengths as the beams travel to the target chamber. In 1997, the Livermore team produced the world’s largest single-crystal optical element—a pyramid-shaped KDP crystal measuring more than a half-meter tall and weighing about 250 kilograms—in just six weeks. Today, Zaitseva’s team is applying everything learned with KDP—an inorganic substance—to organic substances for neutron detection.
In examining possible compounds, Zaitseva’s aim was to find a material that would most effectively separate a signature for neutrons from the strong background of gamma radiation, a process known as pulse-shape discrimination (PSD). Decades ago, researchers found that the organic crystal stilbene could quickly discriminate neutrons from gamma radiation. Today, liquid organic scintillators are more commonly used for PSD because of stilbene’s limited availability and high cost. However, because of flammability, toxicity, and environmental concerns, NNSA and others are now searching for alternative materials.
“We surveyed 140 organic compounds,” says Zaitseva, “all of which were prepared with characteristics known to be important for fast-neutron detection.” A few such characteristics include the presence of benzene rings for efficient scintillation; high hydrogen content for interactions with neutrons; only low-atomic-number (low-Z) constituents, such as hydrogen or carbon, to avoid excessive interaction with gamma radiation; and a delayed emission to better show PSD.
The LLNL research teams are looking at several more years of research. Cherepy’s team is continuing efforts to improve the performance of both garnets and strontium iodide. They are also exploring the possibility of engineering high-Z nanocomposite polymer scintillators, which could be highly efficient, inexpensive, and provide high resolution. While Zaitseva has shown that large organic crystals can be produced very quickly, her team has yet to determine the final optimal alternative to stilbene. An unexpected discovery was a compound with triple PSD that can discriminate among the three types of radiation—fast neutron versus gamma and thermal neutron versus gamma. This compound may even prove to be an effective material for detecting antineutrinos.
“We have just begun to examine yet another possible technique for detecting neutrons,” says Payne. “This method uses acoustic detection. Boron captures a neutron, which interacts with other materials, ultimately generating an acoustic wave. Livermore has the right mix of experts for meeting this research challenge.”
Katie Walter is a writer at LLNL.