By the time you read this, I will have been to Europe and back on an airplane. I will have passed through several security checkpoints, along with millions of fellow travelers, suitcases, cargo, supplies and pretzel snacks, looking to keep out any bad guys or their stuff. It is a search for a needle in the haystack, and it has to be done accurately and quickly.
There are many other situations where radiation needs to be detected, for example, for security, monitoring nuclear materials, remote inspection (gas well logging) or medical diagnosis. Department of Homeland Security requires, for example, detecting certain kinds of radiation. Detection of radiation such as neutron and gamma rays requires instruments with materials that are sensitive to particular radiation types.
Scintillators are such materials. They are comprised of single crystal host materials doped with rare earth “activator” atoms. The host matrix is very dense, a characteristic that allows it to absorb the incident radiation and transfer it to the activator, which emits light. In the case of Eu2+ activator ions, for example, the luminescence comes from the 4d-5f transition. The emitted photons are captured by a photomultiplier tube, completing the radiation detection process. For some applications, the signal from the PMT is reconstructed to create an image of the original source of the radiation, and this is the principle behind medical computed tomography scans, for example.
Increasingly sophisticated applications, especially for homeland security and medicine, are driving the development of new scintillator materials with improved energy resolution and higher light outputs, according to Mariya Zhuravleva, a crystal grower and research assistant professor in the Scintillation Materials Research Center at the University of Tennessee, Knoxville. In a phone interview she said, “Especially after 9-11 there has been lots of research. People are desperate to find new materials.”
“There are several properties we are looking for depending on the application,” she says. “For medical applications we are interested in a fast scintillation decay time and high light yield.” For homeland security applications, however, the most important parameter is energy resolution because that gives the ability to detect which isotope is emitting radiation. We are looking for only traces of radiation.”
The state-of-the-art energy resolution of scintillators in use today is 3-5 percent (the lower the number, the better the resolution).
The host matrix is key, she says. “The properties are dependent on the host material. For example, if the activator is within the band gap of the host material, efficient luminescence is possible,” according to Zhuravleva. Her research program focuses on ternary alkali–metal–halide single crystals with general formulas of AM2X5, AMX3 and A4MX6, where A is a an alkali metal, M is an alkaline earth metal and X represents the halide, either chlorine, iodine or bromine.
“By nature [halides] are much more efficient scintillators [than oxides]. The light they emit is very bright and easy to collect, which has the advantage of constructing smaller devices.” Also, these crystals are denser than silicates, which means they can stop radiation more effectively and give a better signal. There are commercially available alkali–halide scintillators, such as NaI:Tl and CsI:Na. However, Eu2+ is an attractive activator because it has a very efficient luminescence, which, with the right host matrix, could lead to scintillator crystals that have high light output and high-energy resolution.
Zhuravleva explained that she is studying crystal structures with a divalent cation because they have a built-in site for incorporating Eu2+, and her new paper published the Journal of Crystal Growth reports on the properties of two new single crystal scintillators: CsCaCl3:Eu and CsCaI3:Eu. Other materials she has investigated over the last several years include Ce3+ activated scintillator crystals, such as KGd2Cl7:Ce, Cs3CeCl6, Cs3CeBr6, CsCe2Cl7 and CsCe2Br7.
There are significant property trade-offs between iodides and chlorides. Zhuravleva explained that iodides have the smallest band gap and are therefore more efficient. Chlorides have a wider band gap and can trap energy, which reduces efficiency. Iodides are denser and better able to stop incident gamma-ray radiation, which is important for medical applications, for example. However, chlorides are less air-sensitive, which could make them easier to work with and assemble into deployable devices.
As she considers which compounds to test as host crystals, Zhuravleva says, “I want congruent melting compounds. They are very stable and do not decompose, and we can avoid cracks that come with phase transitions during crystal growth.”
The halide family of compounds have low melting points, too, which is an advantage. She grows these crystals via the Bridgman process (most oxides are grown by the Czochralski method), however, the process in blind: She cannot see the crystal during the process. The compounds are very sensitive to growth conditions, she says, and require a lot of thermal insulation. More importantly, halides are extremely hygroscopic and are grown in sealed quartz ampoules and handled in dry boxes.
In the recent work on cesium-calcium halides, the optimal Eu dopant concentration (determined on light yield) was 10 atomic percent for the chloride and 3 atomic percent for the iodide. Interestingly, the new scintillators exhibited energy resolution that is comparable to the benchmark scintillator, NaI:Tl, which is widely used in medical and security applications.
Zhuravleva’s next goals, supported by a five-year grant from the Department of Homeland Security, are to optimize on crystal growth technology, scale up the process to grow two-inch diameter crystals and to find a matrix-activator combination that will drive the energy resolution toward one percent.
Editor’s note: Zhuravleva will provide an update on scintillator developments for homeland security and defense applications in the March 2013 issue of ACerS’ Bulletin magazine.