Introduction to Scintillation
In 1895, German mechanical engineer and physicist Wilhelm Röntgen painted a screen with barium platinocyanide, an inorganic scintillator. At the time, Röntgen was investigating the existence of a new kind of ray and incidentally discovered that barium platinocyanide luminesced when exposed to them. These new rays became known as X-rays, and barium platinocyanide became the first radiation detector.
Early scintillators used a simple “yes or no” indicator, answering the question “Is radiation present?” Since then, new scintillators have been discovered and developed to provide quantitative information about ionising radiation, including differentiating alpha, beta, gamma and x-ray radiation.
Subatomic particles with enough energy to displace electrons from atoms and molecules – thus ionising them – is what makes up ionising radiation. There are two types of radiation: direct ionising radiation and indirect ionising radiation. Direct ionising radiation are charged particles with mass—alpha particles and beta particles. Indirect ionising radiation, on the other hand, is uncharged particles that do not directly ionise matter.
- 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.
- Gamma Rays are electrically neutral photons produced in the nucleus through nuclear reaction, subatomic particle decay or radioactive decay. Through indirect ionization — the photoelectric effect or the Comptom effect — gamma rays eject electrons turning them into beta particles. Gamma rays are used in spectrometry, non-contact sensors, gamma ray detectors, equipment sterilization, cancer treatment and diagnostics.
- X-rays are electrically neutral photons produced outside the nucleus. X-rays are used in medical diagnostics, imaging, radiography, computed tomography (CT) scanning, fluoroscopy, and radiotherapy for cancer treatment.
The unique luminescence ability of scintillators was instrumental in developing radiation detection equipment and, ultimately, the field of nuclear physics. Our recent blog article, What are Scintillators? How do they work? offers a primer on the physical mechanisms that cause an inorganic scintillator’s radioluminescence. When paired with the appropriate photodetectors, inorganic scintillators are an essential component in medical imaging, nuclear physics, oil and gas exploration, dark matter research, space exploration, geophysics, environmental monitoring, non-destructive testing, and homeland security.
Properties of Scintillators
Inorganic scintillators come in crystalline, polycrystalline, and microcrystalline forms. According to Radiation Detection: Concepts, Methods, and Devices by Douglas McGregor and J. Kenneth Shultis, there are five qualities of a “useful” inorganic and organic scintillation material. These are:
- Capable of absorbing radiation
- A significant portion of absorbed radiation be released as photons
- Must luminesce (spontaneous emission) rather than phosphoresce (continuous emission)
- Be transparent to its own emissions
- Emissions must be detectible with conventional light detecting systems
These five qualities encompass a large number of inorganic scintillators. When it comes to designing a radiation detecting instrument, there are other important properties to consider.
- Decay Constant: 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.
- Atomic Number: The atomic number is often designated as Z-number. Efficient gamma radiation absorbers have high Z values. The alpha-particle and beta-particle backscatter also increase with Z which can cause reflection at the detector.
- Density: 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: 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.
- Peak Scintillation Wavelength: The point of maximum light yield is the peak scintillation wavelength expressed in nanometers (nm). It is important when pairing with light detectors such as photomultiplier tubes (PMT), photodiodes (PD), avalanche photodiodes (APD), and silicon photomultipliers (SiPM).
- Emission Spectral Range: Scintillator materials do not emit light discreetly at only one wavelength. The emission curve varies over the visible spectrum, and each scintillator has a characteristic emission curve. Along with the peak scintillation wavelength, the emission spectral range (nm) is essential when selecting matching light detectors.
- Refractive Index at Peak Emission: 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: 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.
- Radiation Length: The mean path length in the scintillator required to reduce the energy of charged particles by the factor 1/e. Measured in centimeters (cm), the radiation length is affected by atomic number and mass density.
- Afterglow: 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.
- Solubility: For scintillators, solubility refers to the scintillator’s ability to dissolve in water, usually given as the number of grams dissolved per 100 grams of water at 300K.
- Stability: 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.
- Cleavage Planes: 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.
Scintillator Crystals: Characteristics and Applications
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.
Zinc tungstate is very similar to cadmium tungstate in that it is a transition metal scintillator with high-density, high Z, and good stopping power. It is an effective gamma-ray absorber useful for x-ray applications.
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.
Cerium-doped yttrium aluminium garnet (YAG:Ce) is a garnet scintillator that emits yellow light. It is useful for beta and X-ray counting applications. It has relatively low light output but a reasonably fast response time and high electron conversion efficiency.
YAG(Ce) is rugged, non-hygroscopic, and chemically resistant making it ideal for thin scintillation screens used in electron and X-ray imaging. YAG(Ce) is also used in low energy particle detection.
Cerium-doped lutetium yttrium Oxyorthosilicate (LYSO) is a garnet with high density, good anti-radiation hardness and efficient gamma radiation absorption. Its combination of high light output, quick decay time, and good energy resolution make it a nearly ideal scintillator.
With high light output and short decay times, LYSO offers excellent resolution for fast timing applications. It is rugged, non-hygroscopic, and has good chemical stability. The output spectrum also pairs well with PMTs.
Radiation detectors, radiation monitoring systems, positron emission tomography (PET) applications, and time-of-flight positron emission tomography (ToF PET) often use LYSO. LYSO(Ce) properties makes it an attractive alternative in XCT and other nuclear medicine applications. Electromagnetic calorimeters used in particle physics and gamma pulse spectroscopy commonly use LYSO as well.
GLuGAG(Ce) is a garnet scintillator supplied as a sintered ceramic. It has high stopping power combined with a high light yield and fast decay time making it a suitable candidate for radiography applications.
GLuGAG(Ce) is stable and non-hygroscopic with a cubic crystal structure. Unlike its undoped counterpart, it can be fabricated into large, uniform shapes at lower temperatures. GLuGAG has low afterglow and when paired with PMTs it is suitable for high-resolution applications and is an excellent alternative to thallium-doped caesium iodide, cadmium tungstate and lead tungstate.