Research Areas

RMD's Material Science Group is on the cutting edge of radiation detection materials and sensor research. We design scintillators, semiconductors, ceramics and Large Area Avalanche Photodiodes (LA-APD) for advanced X-ray, gamma-ray and neutron detectors. These detectors are used to meet the demanding needs of the medical imaging, medical diagnostics, small animal medical research, homeland security, nondestructive testing and nuclear particle physics markets.

RMD Advanced Materials Science Group's goal is to find innovative materials and compounds that improve the detection characteristics of scintillators. Our research has led to design breakthroughs with the superior properties to take nuclear detector development to a new level. Our scintillators exhibit:

• High light output (a high efficiency for converting the energy of incident radiation into scintillation photons).
• Are transparent to their own scintillation light (for good light collection).
• Efficient conversion of the input signal into light.
• High stopping power.
• Short rise time for fast timing applications.
• Short decay time to reduce detector dead-time and accommodate high event rates.

Our ongoing research falls into three categories of materials:

1. Scintillators
2. Semiconductors
3. Ceramics

Scintillators and semiconductors are comprised of either bulk single crystals or thin film crystals. Ceramics are transparent polycrystalline solids. The following figure shows some of the radiation detection materials that have been produced at RMD by material category.

Growing and Processing Techniques:

We use a large variety of growth and processing techniques to develop these materials. For bulk crystal growth we use the following techniques:

• Vertical and horizontal Bridgman
• Czochralski
• Kyropoulos
• Micro-down pull
• Solution growth
• Physical Vapor Transport (PVT)
• Traveling heater method (THM)
• Flux growth, Hydrothermal
• Top seeded solution growth (TSSG)
• Zone refining.

For thin film crystal growth we use the following techniques:

• Screen printing
• Physical Vapor Transport (PVT)
• Hot wire recrystallization.

For ceramics preparation we use:

• Hot pressing/annealing
• Cold pressing/sintering coupled with hot isostating pressing/annealing
• Cold pressing/one step sintering - HIP/annealing.

Application Areas - Scintillator Materials

Medical Imaging
One of the most important markets for our research is medical imaging systems such as Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT) and Computed Tomography (CT). These systems benefit from higher energy and timing resolution and higher count-rate performance offered by our scintillators. These improvements result in better images, improved background reduction, and potentially lower patient radiation dose. Additionally, a sensitive detector with high response speed can be used in other important non-imaging nuclear medicine instruments such as surgical probes. The high speed of the material may make possible new approaches, such as combined CT with PET or SPECT using a single detector for both modalities. Our scintillators research also shows great promise for medical radiology applications such as mammography (with fine pixilation), general snap-shot radiography, and high frame-rates fluoroscopy.

Medical Diagnostic Applications
Procedures such as bone densitometry for diagnosing osteoporosis use x-ray detectors to measure differential absorption of x-rays of differing energy. The availability of better, more sensitive detectors shorten the measurement time needed for diagnosis. High performance x-ray detectors have also been proposed for use for the non-invasive diagnosis of lead and heavy metal poisoning.

Small Animal Medical Research
RMD's detectors can be used in the emerging market for nuclear imaging systems for small animal medical research. These systems are becoming key tools in understanding disease and in drug development studies for evaluating the effectiveness of new pharmaceuticals and treatments. This work involves the use on animals such as mice and rats. Imaging small animals requires exceptionally high spatial resolution to clearly view small structures and accurately identify tumors and other lesions. Additionally, to minimize the stress placed on animals during imaging, the sensitivity of the detector must be very high to minimize scan time. The advantage of the techniques is that the same animal can be tested numerous times so that the evolution of the disease or the progress of cure can be studied in the same animal. This significantly reduces the time, cost and number of animals needed to perform such studies.

Homeland Security and Defense
The proliferation of weapons of mass destruction and special nuclear materials is a serious threat in the world today. Preventing the spread of these materials and weapons has reached is critical to our national security. The military, the Department of Homeland Security, and disaster site first responders require systems capable of detecting radioactive and other hazardous substances at very low concentrations and often at a significant distance.

Gamma ray spectrometers are an increasingly important tool for maintaining control and checking for unauthorized movements of nuclear materials. To address this need, RMD's materials group is developing semiconductor based detectors. The ideal detector for security applications should have good energy resolution, high detection efficiency, compact size, light weight, easy portability, low power requirements and low cost. RMD is exploring a wide bandgap semiconductor material, thallium bromide (TlBr), as a room temperature detector material to fill this role.

Nondestructive Testing (NDT)
Nondestructive testing (NDT) refers to the method of examining material and components in order to identify and quantify defects and degradation in material properties before they result in failure. The aim of NDT is to ensure the safe utilization of engineering structures, as well as to assure product quality and performance on production. The aim is to evaluate defects in objects without having to physically break them up and test them. NDT encompasses a host of non-invasive measurement techniques including radiography.

X-rays are produced by high-voltage X-ray machines whereas gamma rays are produced from radioactive isotopes such as Iridium 192. The X-rays or gamma rays are placed close to the material to be inspected and are passed through it and captured on a medium, such as film. This film is then processed and the image is obtained as a series of grey shades between black and white. The choice of radiation type (X-ray or gamma rays) depends on the thickness of the material to be tested. Gamma sources have the advantage of portability, which makes them ideal for use in construction-site working.

The materials group's research in radiation detection materials can be used in the latest versions of X-ray NDT to increase their sensitivity thereby enhancing their ability to find more minute defects.

Nuclear Physics and other Research
Current and next generation experiments in nuclear and particle physics require sensors with fast response and high signal-to-noise ratio for detection of low intensity optical signal. Such photodetectors can be coupled to scintillators for detection and spectroscopy of gamma-rays, charged particles and neutrons. High performance photodetectors are also required in Cherenkov detectors, liquid xenon and argon detectors for dark matter studies. For several decades photomultiplier tubes (PMT) have been the key technology for sensing light for most particle physics research operations. Particle physics experiments regularly employ thousands of photomultipliers of many different sizes. PMTs are also used widely in calorimetry and scintillation tracking devices. The key advantage of PMTs in all these applications is their large amplification (~106) with low noise which enables them to achieve high sensitivity for single photoelectron detection. Although extraordinarily successful, the PMT technology also has a number of limitations: PMTs cannot operate under pressures exceeding a few atmospheres, their sensitivity is limited over a small wavelength band, their optical quantum efficiency is relatively low, they cannot operate under high magnetic fields, they can have some radioactive background, and they are bulky. As a result, there is a real need for alternative photo sensors with fast response, high gain and high signal to noise ratio in many nuclear physics experiments. To overcome the limitations of PMTs in nuclear experiments including scintillation spectroscopy, we plan to explore a new photodetector technology, silicon photomultiplier (SiPM). A SiPM consists of a large number of micropixels that are joined together on a common silicon substrate and are operated with a common output. Excellent timing resolution (<1 ns-FWHM) has been measured with SiPM at room temperature.

LA-APDs are unique devices that combine the advantages of solid state photodetectors with the high gain of photomultiplier tubes (PMTs). LA-APDs have internal gain that provides a high signal-to-noise ratio and has high quantum efficiency. They are fast, compact and rugged, and combined with scintillation crystals are excellent detectors. They are used in important applications such as medical imaging, homeland security, detection and identification of toxic chemicals and bio-warfare agents, and laser induced breakdown spectroscopy (LIBS).

Application Areas - LA-APD

One of the most important areas we are researching using APDs is the fusion of PET and MRI technologies into one system for medical imaging. An ideal imaging system would provide non-invasive, high-resolution, high-sensitivity, three-dimensional (3D) functional and anatomic images of laboratory test animals and human beings.

Positron Emission Tomography (PET) is a powerful molecular imaging tool with a number of uses in the study of experimental animal models. Several groups have successfully developed dedicated small animal PET and a number of companies are now producing these small animal PET systems. With highly specific radio labeled molecular probes, PET provides exceptionally sensitive assays of a wide range of biological processes. Its principal drawback is relatively poor spatial resolution, making unambiguous localization of signal extremely difficult in many cases. This is particularly true when using highly specific radio labeled probes that do not produce images with significant anatomical information for reference. Furthermore, quantification of the PET signal, especially in small volumes of tissue, is often hampered by its limited spatial resolution.

On the other hand, magnetic resonance imaging (MRI) provides exquisite high-resolution anatomical information, as well as access to volume specific chemical and physical information (ie. metabolite concentrations and water diffusion characteristics). However, recently developed molecular assays using "smart" contrast agents have limited sensitivity compared with traditional nuclear imaging approaches.

Each modality has its advantages and limitations. Merging these two modalities in the study of experimental animal models will allow us to exploit, in a synergistic fashion, the strengths of both techniques. Interest in a combined imaging system is motivated by the possibility of developing imaging probes that contain both PET radionuclide and MR contrast elements, allowing probes to be tracked in both modalities.