Design Considerations for Imaging Arrays
As discussed in the previous post, “What are Scintillators? How do they work?,” scintillators are essential for various applications due to their ability to emit visible light in the presence of ionizing radiation such as x-rays, alpha particles, and gamma rays. Scintillator imaging arrays are manufactured using advanced techniques coupled to photodetectors such as photomultiplier tubes (PMT), photodiodes (PD), avalanche photodiodes (APD), and silicon photomultipliers (SiPM). Gamma cameras, Positron Emission Tomography (PET), Computed Tomography (CT), Digital Radiography, and Flash Radiography are practical applications of scintillator arrays with scintillation being at the heart of their imaging technologies.
Scintillator imaging arrays are made with inorganic scintillator crystals cut into rectangular pixels with precise dimensions. The crystals are arranged in a grid pattern and encapsulated in composite matrix. The matrix is structural (linear or 2D), optically reflective, and optically isolating. When designing a scintillator array, many factors such as array and pixel size, reflector material and type, and overall form factor of the array, contribute to the crystal selection. The first step in designing a scintillator array is matching the crystal to the application.
This article will discuss commonly used scintillator crystal arrays. It will take a look at reflector and separator material selection and parameters. Finally, we’ll discuss the geometric considerations for designing scintillator arrays.
Properties of Scintillator Arrays
Scintillators are typically arranged in either linear or two-dimensional arrays (see figure below.) The selection of a scintillator crystal is guided first by the scintillator performance parameters then by spatial constraints. For example, crystals for medical imaging applications typically have fast decay times for rapid imaging, high light output for good resolution, and high density to minimize the detector’s size.
Linear and 2D Scintillation Crystal Arrays
While various properties influence scintillator array design, light output, density, scintillation wavelength, decay time, cleavage planes, and uniformity are some important considerations.
Light Output — The amount of light emitted at the scintillation wavelength in photons per megaelectron volts (MEV) of input energy. Light output is often considered the most crucial property for selecting a scintillator because it drives efficiency, resolution and ultimately dictates photodetector selection. Although reflector and separator materials can help increase the photodetector’s light, designers typically select crystal materials with the highest light output they can get for their application.
Density — The higher the density of the crystal, the higher its scintillation energy. Since scintillator arrays aim to get as many elements as possible into a small space while maximizing output, higher density crystals are desired.
Scintillation Wavelength or Wavelength of Maximum Emission — The scintillation wavelength of the crystal designates the wavelength of the light emitted. This guides the type of photodetector (PMTs, PDs, APDs, or SiPMs) that can be used with the array. The choice of the photodetector is further guided by the size and configuration of the system.
Decay Time — Following excitation, a scintillator exhibits afterglow. The light output peaks quickly, then decreases exponentially. The time it takes for the afterglow to diminish to e-1 of its peak value is called the decay time. Rapid imaging applications require fast decay times.
Cleavage Plane — Some scintillators have cleavage planes due to their particular crystalline structure. These planes can dictate the geometry of an array element. For example, Cadmium Tungstate (CdWO4) has a (010) cleavage plane which limits the size of a pixel.
Uniformity — While there is no physical property to describe uniformity, the consistency from pixel to pixel is critical to an array’s performance. Non-uniform crystals can generate imaging artifacts and diminish the resolution of an image.
Scintillator Material Selection — Comparison of various properties for scintillators typically used in scintillator arrays. (see Table below)
Bismuth Germanate (BGO) — A relatively hard, high density, non-hydroscopic crystal with good gamma ray absorption. Often used for PET imaging and high energy physics applications as Compton shields.
Cadmium Tungstate (CdWo4) — A non-hygroscopic scintillator offering good light yield. Often used for CT applications. High radiopurity and low background.
Thallium-doped Caesium Iodide (CsI(TI)) — A high light yield scintillator that emits at a wavelength suitable for silicon photomultipliers (SiPMs). Typical applications include arrays of this material used in security imaging systems, such as baggage scanners.
Lutetium Yttrium Silicate (LYSO) — A non-hydroscopic scintillator that is both bright and fast for applications that demand fast timing such as PET and TOF PET.
Europium-doped Calcium Fluoride (CaF2(Eu)) — Widespread application as a non-hydroscopic crystal for low energy and particle detection.
Properties of Typical Scintillator Array Materials
Reflector and Separator Materials for Imaging Arrays
Scintillator pixels are mounted in a matrix of separator material that is more than just structural. The material in-between the pixels reflects light and isolates. It also optically isolates each pixel, ensuring there is little to no crosstalk between pixels. The back reflector (see Figures 1 and 2 below), which is on the side of the array where the ionizing radiation enters, ensures no light is emitted away from the detectors.
Selecting an array reflector material balances the structural integrity of the material and its reflection and isolation properties. Array designs often aim to maximize resolution by minimizing the thickness between the separator and reflector. However, the thinner the gap between the separator and reflector the more likely reflection, optical leakage (crosstalk), and structural integrity are compromised.
Two common reflector materials are white powder and Teflon sheets. These materials offer excellent reflectivity. Because of the difficulty bonding these materials to scintillators, they also pose limitations in array manufacturing, especially in arrays with very small elements.
In addition, white powder is also mixed with epoxy. This reduces its reflectivity and presents physical and reflectivity limitations for the “thinness” of the separator.
Metal separators are ideal for minimizing crosstalk. They also provide good structural characteristics especially relative to thickness. Polishing metals for sufficient reflectivity in the array assembly is prohibitive. However, in some applications, a metal separator such as one made of lead may be desirable in reducing noise because they absorb radiation.
Other epoxies, plastics, and paints are also available for separators. Relative to white powder and Teflon, these alternatives typically have lower reflectivity, but different structural reasons may dictate their use. In addition, some metal composites may offer some absorption of low-energy radiation.
Geometric Parameters of Imaging Arrays
The following parameters, shown in Figures 1 and 2 below, define the geometry of scintillator arrays. Additional parameters include the thickness of the back reflector and the thickness of the walls.
Element/Pixel Size — Width (“X”) and Height (“Y”) of the scintillator
Thickness — Depth (“Z”) of the scintillator.
Separator Thickness — Gap(X) and Gap(Y). These are typically the same (e.g. Gap (X) = Gap (Y)).
Pitch — Center-to-center distance between pixels. A 2D array will have both an X-direction and a Y-direction pitch.
Schematic of a linear array shown with a portion of separator, reflector and side walls removed.
Schematic of a 2D array shown with a portion of separator, reflector and side walls removed.
To design a scintillator array for a specific application, it’s best to speak with a scintillator design expert. However, there are few rules of thumb for preliminary guidance.
- The width (“X”) and height (“Y”) of a pixel are limited to 0.5mm or larger for a crystal depth (“Z”) of 5mm.
Depending on the separator and reflector materials used, the minimum thickness of Gap(X) and Gap(Y) is 0.2mm.
Material for the separator, sidewalls, and back reflectors is the same. For some constructions, a reflector layer may be added to side walls and back reflector.
The demand for high-performance scintillator arrays in imaging applications is increasing. They are a crucial component in nuclear medicine, homeland security, non-destructive testing, nuclear detection, flash radiography and digital radiography. Along with increased demand, the design and manufacturing techniques of scintillator arrays is continuously evolving and improving. And, Hilger Crystals continuously strives to bring the best crystal technologies to market.
Although crystal selection, construction methods and materials, and other factors influence the design of scintillator arrays, this discussion only offers preliminary guidelines. For more help designing a scintillator array for your specific application, contact a Hilger scintillation expert.