The discovery and development of scintillators have progressed alongside the understanding and applications of ionizing radiation such as x-rays, alpha particles and gamma rays. In the late nineteenth century, physicists found scintillators would illuminate in the presence of ionizing radiation; that is, they gave off visible light when bombarded with x-rays.
The unique luminescence ability of scintillators was instrumental in developing radiation detection equipment and, ultimately, the field of nuclear physics. “What are Scintillators? How do they work?” offers a primer on the physical mechanisms that cause a scintillator’s radioluminescence.
Early scintillators were used as simple detectors for visibly counting radioactive particles—when a particle was present, they lit up. It wasn’t until the advent of photodetectors in the 1940s when scintillators became a component in functional systems ushering in a scintillator development period that has been ongoing ever since.
Now, scintillators are a crucial component in nuclear medicine, homeland security, non-destructive testing, nuclear detection, flash radiography and digital radiography. They also play a role in such advanced technology as dark matter detection and astrophysics.
Scintillator arrays are finding a home in many of these applications.
An array is made of advanced manufacturing techniques where scintillators are precisely processed into rectilinear “pixels.” They are arranged in a matrix and assembled with a reflective filler between each element. the aforementioned pixels. Linear and 2-dimensional scintillator arrays are coupled with discreet photodetectors to improve detection accuracy, imaging and spatial resolution, and instrument efficiency.
Gamma cameras may be designated as γ-cameras, scintillation cameras, or Anger cameras (named after Hal Anger, the inventor of the first gamma cameras). Gamma camera is a generic catchall term for scintigraphy imaging cameras. Scintigraphy is an imaging method that uses scintillators to capture gamma radiation emitting radioisotopes.
This section refers to single-headed gamma cameras used in Single Photon Emission Computed Tomography (SPECT) for nuclear medical imaging. SPECT images radionuclide tracer particles injected or inhaled by a patient to model cardiovascular function such as nuclear cardiac stress testing.
Commonly, SPECT cameras use a single flat scintillator crystal with an array of photodetectors to create two-dimensional images of tracer particles in the body. For special resolution, the gamma camera requires a lead collimator to be placed over the crystal.
Positron Emission Tomography
Positron Emission Tomography (PET) is similar to SPECT except that it uses multiheaded gamma cameras for faster and higher resolution imaging. The mechanics of the imaging are identical to that of SPECT.
Increasingly, scintillator arrays with a photodetector for each scintillator element are replacing the single crystal-photodetector arrays configuration. Scintillator arrays offer better resolution and reduce blurring in the image that is common with collimators.
Computed Tomography (CT) Imaging
Unlike gamma cameras that rely on radioactive pharmaceuticals to image functions in the body or object, Computed Tomography (CT) imaging can image without such tracers. In CT imaging, an X-ray source is used to map the density of materials in the scan plane. These materials would be human tissue in medical CT scanners or the contents of a suitcase in baggage scanners.
Medical CT scanners are mounted on a cylindrical gantry that rotates around the patient’s body. The X-ray source is opposite the detector, which consists of scintillators and detectors. The detector captures the X-rays exiting the body. In simplified terms, the energy of the exiting radiation is used to compute the density of the tissue for identification. Each pass creates a 2D image or slice of the patient’s tissue. Stacking these images creates a 3D image doctors can use as a medical diagnostic tool.
Similar techniques are used to scan baggage at airports to check for weapons, explosives, and suspicious liquids. Engineers use CT scanners to look inside assemblies to examine components and tolerances and create 3D models of mechanisms for finite element analysis. In metrology, they are employed for flaw and crack detection and failure analysis.
CT detectors are arranged in a variety of configurations depending on the application and construction. As the technology matures, there is a push for faster, higher resolution images. Since scintillator arrays can be constructed with multiple crystals in a compact space, they are useful for addressing both speed and resolution demands.
Computed radiography (CR) is a time- and labor-intensive medical imaging technique. X-ray images are recorded on phosphorescent film cassettes that are manually fed into a cassette reader. Inside the cassette reader, the image is laser-scanned to create a digital image.
Digital radiography (DR) and direct digital radiography (DDR) eliminate many intermediate steps between taking an X-ray and viewing the image. It allows practitioners to view images moments after X-rays are taken.
Scintillator arrays are used for quickly creating high-resolution DDR images. Scintillator array-photodetector assemblies are more sensitive to X-rays than the phosphorescent film. This means DDR reduces the amount of x-ray exposure to the patient.
DDR is significantly more expensive than CR, but the systems are gaining popularity for dental and endodontic imaging. They are especially useful for installing dental implants as images can be taken and reviewed during the procedure to guide the surgeon. Dedicated imaging facilities and hospitals with high workloads are increasingly implementing DDR systems to increase throughput and shorten the diagnostic time.
Although the primary application of DDR is in medical imaging, DDR has also been used for industrial and security applications. Nondestructive X-ray inspection using DDR is typical for vehicle, aircraft and customs inspections. In electronics and aerospace applications requiring high purity alloys, DDR imaging is used to nondestructively inspect materials.
Flash radiography is a unique kind of imaging technology in that it captures objects moving at extremely high speeds. Initially, flash radiography was invented by researchers in the middle of the 20th century in conjunction with the development of nuclear weapons. It was a critical element of the Manhattan Project which produced the first atomic weapons for the United States.
It is similar to traditional flash photography in that a light source is used to illuminate the object of interest, and the camera captures the reflected light. In this case, the light source is an x-ray source and the camera is a quantum efficiency detection system. These high-resolution systems record the radiographic image while computational systems analyze it to create a useful image. Using reference data from other explosions, high-speed projectiles or blasts, these systems can generate measurements of the position, speed, shape and internal density of the objects in the image.
Due to the better resolution of scintillator arrays over collimator assemblies, they are ideal for flash radiography. The resolution of the array is based on the dimensions of each pixel. The size of a pixel is limited by a variety of parameters including scintillator material, crystalline cleave planes, and manufacturing technique. For example, Thallium-doped Caesium Iodide, a scintillator crystal often used in scintillator array assemblies, can be made with pixel sizes down to 0.5-mm square with a 5-mm depth for each element.
Flash radiography is primarily used for military and intelligence applications, but there are a few civilian applications such as crash reconstruction, failure analysis, geological monitoring, and oil and gas exploration.
The most prominent applications for scintillator arrays are high-tech medical imaging, positron emission tomography (PET), computed tomography (CT), and direct digital radiography (DDR). However, as the scintillator materials and array manufacturing techniques continue to improve, scintillator arrays find their way into more imaging applications. As demand for higher resolution and faster imaging speeds continues, we’ve seen an increased need and demand for scintillator arrays in medical imaging.
In addition, industrial and security applications are implementing scintillator arrays for imaging needs. One notable application is imaging high-speed projectiles using flash radiography. However, in everyday applications, scintillators are critical for scanning baggage, packages, and even passengers. They are useful for nondestructive testing, failure analysis and advanced analytics.
While scintillator arrays are useful for nearly any nuclear imaging application, their crystal properties and manufacturing techniques impose practical limitations on pixel sizes and array dimensions. In the next article on scintillators, we will discuss some of the design parameters and considerations for designing scintillator arrays.