Everything You Need To Know To Find The Best Light Guide For Scintillator Array

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.

If you want to learn more, please visit our website.

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

Goto EBO to know more.

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.

We may request cookies to be set on your device. We use cookies to let us know when you visit our websites, how you interact with us, to enrich your user experience, and to customize your relationship with our website.

Click on the different category headings to find out more. You can also change some of your preferences. Note that blocking some types of cookies may impact your experience on our websites and the services we are able to offer.

One button to accept all cookies and services with a single click.

These cookies are strictly necessary to provide you with services available through our website and to use some of its features.

Because these cookies are strictly necessary to deliver the website, you cannot refuse them without impacting how our site functions. You can block or delete them by changing your browser settings and force blocking all cookies on this website.

One button to accept all cookies and services with a single click.

Refuse cookies and hides message bar.

Opt out from all the cookies (except 2 from av_privacy_allow_cookies)

Disable Google tracking

Disable Google webfonts

Disable Google reCaptcha

Disable Google Maps

Disable video embeds

Adds an accept cookies button

Adds a do not accept cookies button

These cookies collect information that is used either in aggregate form to help us understand how our website is being used or how effective our marketing campaigns are, or to help us customize our website and application for you in order to enhance your experience.

If you do not want that we track your visist to our site you can disable tracking in your browser here:

We also use different external services like Google Webfonts, Google Maps and external Video providers. Since these providers may collect personal data like your IP address we allow you to block them here. Please be aware that this might heavily reduce the functionality and appearance of our site. Changes will take effect once you reload the page.

Google Webfont Settings:

Google Map Settings:

Vimeo and Youtube video embeds:

If you are looking for more details, kindly visit Light Guide For Scintillator Array.