How to Choose 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.

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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.

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• 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.

Alpha Particles are high energy, positively-charged particles. Identical to the nucleus of a helium atom, alpha particles are made up of two protons and two neutrons, which makes them relatively heavy. Despite their high energy, they do not travel far in air nor penetrate solids deeply. They are produced in particle accelerators like cyclotrons and synchrotrons and they are used in smoke detectors, some power sources, static eliminators and some cancer treatments.

Beta Particles are emitted during beta decay of an atomic nucleus. They are high-energy, fast-moving positrons and/or electrons. They are lighter and more penetrating than alpha particles. As beta particles decelerate, they produce secondary gamma radiation. Beta particles are used in medical applications for eye and bone cancer treatment and as tracer particles for positron emission tomography (PET) scans. They are also useful for paper inspection and illumination.

The decay constant is the rate at which a fraction of electrons falls into luminescent centers. As discussed in What are Scintillators? How do they work? when a scintillator absorbs radiation, an electron-hole pair (exciton) is created. It takes a period for the energy of the exciton to decay. This is often measured in nanoseconds (ns). Short decay times are essential for fast imaging applications. Some scintillators have multiple decay times.

Along with atomic number, a scintillator’s mass density, often given in grams per cubic centimeter (g/cm3), is a strong indicator of its radiation absorption efficiency. High atomic number and high mass density indicate the scintillator is effective at stopping incoming radiation and is characterised as a highly efficient absorber.

Light yield is the number of photons produced for a given amount of absorbed energy. The absolute light yield is the total number of fluorescent photons released per unit of absorbed energy. It is specified in photons/MeV and reported at a specific energy (keV). High light yield means a brighter scintillator. Generally, brighter emissions lead to better detection performance.

Alternatively, relative light yield is a measure of the linearity of a scintillator’s fluorescent response, which is ideally constant. Relative light yield normalises emission over different specified energies.

The index of refraction is the ratio of the speed of light in a vacuum to the speed of light in the medium. Ideally, this matches the index of refraction for the light-sensing instrument (ideally ɳ ≤ 1.5). Also, scintillator materials should be transparent to their emitted photons or the photons are reabsorbed and not detected.

Photofraction is the ratio between the number of photons that are recorded under a certain peak and the number of photons that are recorded in the spectrum at the same energy.

Also known as phosphorescence, afterglow is the term used for fluorescent photons that delay the transition to a metastable state. These photons take more than multiple decay times to complete the transition. Afterglow is the percentage of light after a given time (% after X ms). The delay time is not correlated to incoming radiation, so afterglow is a source of background noise in the detector. A thermoluminescent detector (TLD) actually uses this phosphorescent decay to its benefit.

This refers to the chemical stability in the presence of water, humidity, or hygroscopicity. Generally, the family of inorganic compounds a scintillator belongs to indicates its hygroscopicity. Hygroscopic scintillators required dry rooms or hermetic sealing. Alkali-metal halides and lanthanide halides are chemically unstable and tend to be slightly hygroscopic or hygroscopic. Oxide-based and alkali-earth halides tend to be non-hygroscopic.

Some crystalline materials have definite crystallographic structural planes where the bonds are weaker. These crystalline materials tend to easily split along these planes. In scintillators, these cleavage planes affect the fabrication of the crystal, limiting the sizes and shapes available. In some cases, the cleavage plane is a manufacturing advantage because the cleaved surface can be polished for clarity. The cleave plane is typically identified according to its Miller Index.

Sodium iodide is a high-density and high-Z scintillator sensitive to low- and intermediate-energy gamma radiation with mild sensitivity to high-energy beta radiation. Many gamma-ray spectrometry applications use Sodium Iodide, and, due to its high radiopurity, it is attractive for dark matter research applications.

Sodium iodide can be grown in various forms and sizes which makes it less costly to produce. It also exhibits high light output at short wavelengths, which means it is easily matched with various photomultiplier tubes. Since it can be grown in larger formats, it also offers good resolution and efficiency.

Undoped sodium iodide has a smaller decay constant compared to doped sodium iodide, which makes it attractive for fast imaging applications. It is an alkali-metal halide that is hygroscopic and must be hermetically sealed to prevent deterioration. It is also susceptible to radiation and ultraviolet damage.

Sodium iodide is available in both single-crystalline and polycrystalline formats.

Like undoped sodium iodide, thallium-doped sodium iodide detects low- and intermediate-energy gamma radiation. It also has the highest light output of any available scintillator and is well-matched to photomultipliers. Thallium-doped sodium iodide is the most widely used scintillator because of its performance, low cost, and availability.

Thallium-doped sodium iodide crystals are available in a wide range of standard sizes and configurations, either as separate crystals or as complete assemblies. The maximum light transfer is achieved by employing a high-efficiency reflector chosen to suit the application. Materials used are selected to ensure a low background count.

Like undoped NaI, NaI(Tl) is widely available at a lower cost compared to other scintillators. It is used in a variety of applications including medical imaging, nuclear physics, oil and gas exploration, geophysics, and environmental monitoring. Like its undoped counterpart, it is hygroscopic and susceptible to radiation and ultraviolet damage.

Sodium-doped caesium iodide is a high density, high Z alkali-metal halide scintillator sensitive to gamma radiation. It has high light output with slight hygroscopicity and requires hermetic sealing to prevent degradation. However, its mechanical and thermal shock resistance makes it an attractive scintillator for rugged applications such as oil and gas logging, space research and industrial monitoring.

CsI(Na) is much less hygroscopic. It has good resistance to thermal and mechanical shock as well as radiation damage. The trade-off is slightly lower light output, approximately 85% that of thallium-doped sodium iodide. Still, it is an attractive alternative due to its high gamma radiation stopping power. Its peak emission is in the blue spectral region, which makes it a good match for many photomultipliers and silicon photodiodes.

Thallium-doped Caesium Iodide, CsI(Tl) and Europium-doped Caesium Iodide, CsI(Eu)

Doped caesium iodides are an alkali earth halide with low-density used for detecting beta radiation and some low-energy gamma radiation (up to several hundred keV). It has low photofraction which makes it unsuitable for high-energy gamma radiation applications. Both, thallium-doped and europium-doped caesium iodide, are chemically inert with virtually no solubility in water.

Although CsI(Tl) and CsI(Eu) have moderate light output (~50% of NaI:Tl) they are suitable for beta radiation applications due to their low backscattering which is a characteristic of low Z crystals. Since their refractive index is 1.47, doped caesium iodide is optically transparent and couples easily with many photodetectors. CsI(Tl) and CsI(Eu) are used in particle detection and medical diagnostic applications.

Cadmium tungstate is a transition metal scintillator with high-density and high Z, which gives it exceptional stopping power. It is an effective gamma-ray absorber and is useful for x-ray applications.

Cadmium tungstate has moderate light output (~30-50% of NaI:Tl) and a portion of its emission spectra is above 500nm, which makes it less effective when paired with PMTs, although it pairs well with silicon photodiodes. It also has virtually no afterglow making it ideal for use in CT scanners.

CdWO4 has high radiopurity, low background, is non-hygroscopic and mechanically robust. The scintillator has a wolframite-type crystalline structure and cleaves on the <110> plane. Often the cleavage is used in manufacturing to produce polished surfaces.

Cadmium tungstate has low level of intrinsic radioactivity and is commonly used in nuclear medicine imaging, security systems, oil and gas logging, and CT scanners. It has been instrumental in developing industrial X-ray CT (XCT) scanners used to scan containers and cargo.

Although zinc tungstate has low afterglow, its decay time is longer than that of cadmium tungstate so it is not ideal for medical imaging applications. It is mainly used for applications in particle physics and dark matter research.

Bismuth germanate is a post-transition metal scintillator with high density and high Z which gives it exceptional stopping power. It is a highly efficient gamma-ray absorber used in applications requiring high detection efficiency.

Like cadmium tungstate, part of BGO’s emission spectra is above 500nm, reducing its useful emission spectrum when paired with PMTs or photodiodes.

Bismuth germanate (BGO) is a non-hygroscopic, relatively hard crystal which has good gamma radiation absorption. However, BGO is intrinsically radioactive which makes it unsuitable for certain applications. Specialised manufacturing techniques can reduce the intrinsic radioactivity of BGO which allows it to be used widely in PET medical imaging and security scanning applications. It is also useful for high-energy physics applications like Compton suppression spectrometers.

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