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.
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.
Types of Scintillation Detectors
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.
Comparison of Detector Technologies
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
Choosing a Scintillation Detector
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.
Wish to speak to an expert about your application? Contact Hilger Crystals.