In an earlier blog post, we took a closer look at the scintillation mechanism – from the absorption of ionizing radiation to the production of light pulses within inorganic crystal materials since that’s Hilger Crystals’ area of expertise. In this post on detecting scintillated light, we discuss how that scintillated light is detected and transformed to a format in which it can be usefully managed.
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When a scintillation crystal is excited by an external charged particle (e.g. alpha, beta, neutron, gamma radiation) it subsequently decays emitting light in all directions. It is important to direct that light towards an attached external detector to maximize its, well…detection. We do that by undertaking a variety of surface treatments such polishing, etching, and grinding the scintillator, as well as applying various optical reflectors. Naturally, these treatments are specific to the material in question because its geometry, emission wavelength, refractive index, and other characteristics all influence how the photons travel through the material.
Assortment of Scintillation Detectors
Getting as much scintillated light (as possible) out of a scintillator so it can be detected is no trivial task. You may recall that denser materials absorb more radiation. However, dense materials also tend to have a high optical refractive index which frequently “traps” light within the scintillator due to its multiple surface reflections. Here, too, we can employ surface treatments to reduce this effect, but there can still be a major loss of overall efficiency. Plus, not all crystals are capable of detecting all types of radiation. For example, Caesium Iodide is used to detect protons, and alpha particles and gamma radiation. Sodium Iodide (NaI) is used to detect gamma radiation, zinc sulfide is used to detect alpha particles but not protons.
Once the light particle (aka photon) leaves the crystal it needs to be converted to an electronic signal that can be manipulated and analyzed. There are several types of optical photon light detectors that exist, including photomultiplier tubes (PMT), photodiodes (PD), avalanche photodiodes (APD), and silicon photomultipliers (SiPM) among others. Each technology converts optical photons to an electrical signal that can be manipulated as an analog or digital signal and used to determine the type and intensity of incident radiation.
How a PMT Works Source: https://commons.wikimedia.org/wiki/File:PhotoMultiplierTubeAndScintillator.jpg
Photomultiplier tubes are the oldest type of such detectors based on photosensitive materials and vacuum technology inside a glass envelope. Photodiodes, avalanche photodiodes, and silicon photomultipliers are silicon-based technologies that operate by producing electrons and holes from absorbed photons within the silicon, which is then detected with the application of low voltage.
PMTs sensitivity to light is unrivaled, even today, but its relatively large size and fragility, along with its operational requirement of high voltage impedes its flexibility and portability. On the other hand, silicon-based detectors operate at lower voltage compared to PMTs but they suffer from lower inherent gain (except APDs). They also lend themselves to mass production, and therefore lower per unit costs.
PMT PD APD SiPM
Wavelength Sensitivity Range (nm) 150-1,700 190-13,000 190-1,700 300-900
Gain High Low High Medium
Radiation Flux Low Low Medium Low
Voltage High Low Med Low
Response Timing Fast Fast Slow Fast
Magnetic Field Sensitivity Yes No No No
Temperature Sensitivity Low High Medium High
Size Large Small Small Small
Cost High Low Med Low
Ultimately, the choice of scintillator and its associated photodetector would be determined by your application. No one type of detector is better than the other. Each offers advantages, and disadvantages, so it’s upon the researcher to carefully evaluate key parameters of the scintillator and the related application. The photodetector’s range of wavelength sensitivity, speed of response, signal to noise ratio and more should be matched to the emission wavelength of the scintillator, otherwise the combination of the scintillator and photodetector will not be optimized for the intended application. Additionally, physical and environmental variables such as size, sensitivity to magnetic fields, temperature, and cost factor into the decision-making process.
To help get you started, we’ve developed “Crystal Compass” – an easy 4-step tool to help you determine the best scintillator material for your application.
As an example, pixelated Lutetium Yttrium Silicate (LYSO) in the form of an array can be coupled to SiPMs to produce a high density fast detector suitable for PET imaging or any other application that requires high speed detection. Another example of a SiPM-based detector is Thallium-doped Caesium Iodide (CsI(Tl)), which offers a spectroscopic quality detector for the identification of radio isotopes. It can also be pixelated to provide positional sensitivity.
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.
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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 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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