A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillating material, and detecting the resultant light pulses.
It consists of a scintillator which generates photons in response to incident radiation, a sensitive photodetector (usually a photomultiplier tube (PMT), a charge-coupled device (CCD) camera, or a photodiode), which converts the light to an electrical signal and electronics to process this signal.
Scintillation counters are widely used in radiation protection, assay of radioactive materials and physics research because they can be made inexpensively yet with good quantum efficiency, and can measure both the intensity and the energy of incident radiation.
The first electronic scintillation counter was invented in by Sir Samuel Curran[1][2] whilst he was working on the Manhattan Project at the University of California at Berkeley. There was a requirement to measure the radiation from small quantities of uranium, and his innovation was to use one of the newly available highly sensitive photomultiplier tubes made by the Radio Corporation of America to accurately count the flashes of light from a scintillator subjected to radiation.
This built upon the work of earlier researchers such as Antoine Henri Becquerel, who discovered radioactivity whilst working on the phosphorescence of uranium salts in . Previously, scintillation events had to be laboriously detected by eye, using a spinthariscope (a simple microscope) to observe light flashes in the scintillator. The first commercial liquid scintillation counter was made by Lyle E. Packard and sold to Argonne Cancer Research Hospital at the University of Chicago in . The production model was designed especially for tritium and carbon-14 which were used in metabolic studies in vivo and in vitro.[3]
When an ionizing particle passes into the scintillator material, atoms are excited along a track. For charged particles the track is the path of the particle itself. For gamma rays (uncharged), their energy is converted to an energetic electron via either the photoelectric effect, Compton scattering or pair production.
The chemistry of atomic de-excitation in the scintillator produces a multitude of low-energy photons, typically near the blue end of the visible spectrum. The quantity is proportional to the energy deposited by the ionizing particle. These can be directed to the photocathode of a photomultiplier tube which emits at most one electron for each arriving photon due to the photoelectric effect. This group of primary electrons is electrostatically accelerated and focused by an electrical potential so that they strike the first dynode of the tube. The impact of a single electron on the dynode releases a number of secondary electrons which are in turn accelerated to strike the second dynode. Each subsequent dynode impact releases further electrons, and so there is a current amplifying effect at each dynode stage. Each stage is at a higher potential than the previous to provide the accelerating field.
The resultant output signal at the anode is a measurable pulse for each group of photons from an original ionizing event in the scintillator that arrived at the photocathode and carries information about the energy of the original incident radiation. When it is fed to a charge amplifier which integrates the energy information, an output pulse is obtained which is proportional to the energy of the particle exciting the scintillator.
The number of such pulses per unit time also gives information about the intensity of the radiation. In some applications individual pulses are not counted, but rather only the average current at the anode is used as a measure of radiation intensity.
The scintillator must be shielded from all ambient light so that external photons do not swamp the ionization events caused by incident radiation. To achieve this a thin opaque foil, such as aluminized mylar, is often used, though it must have a low enough mass to minimize undue attenuation of the incident radiation being measured.
The article on the photomultiplier tube carries a detailed description of the tube's operation.
The scintillator consists of a transparent crystal, usually a phosphor, plastic (usually containing anthracene) or organic liquid (see liquid scintillation counting) that fluoresces when struck by ionizing radiation.
Cesium iodide (CsI) in crystalline form is used as the scintillator for the detection of protons and alpha particles. Sodium iodide (NaI) containing a small amount of thallium is used as a scintillator for the detection of gamma waves and zinc sulfide (ZnS) is widely used as a detector of alpha particles. Zinc sulfide is the material Rutherford used to perform his scattering experiment. Lithium iodide (LiI) is used in neutron detectors.
The quantum efficiency of a gamma-ray detector (per unit volume) depends upon the density of electrons in the detector, and certain scintillating materials, such as sodium iodide and bismuth germanate, achieve high electron densities as a result of the high atomic numbers of some of the elements of which they are composed. However, detectors based on semiconductors, notably hyperpure germanium, have better intrinsic energy resolution than scintillators, and are preferred where feasible for gamma-ray spectrometry.
In the case of neutron detectors, high efficiency is gained through the use of scintillating materials rich in hydrogen that scatter neutrons efficiently. Liquid scintillation counters are an efficient and practical means of quantifying beta radiation.
Scintillation counters are used to measure radiation in a variety of applications including hand held radiation survey meters, personnel and environmental monitoring for radioactive contamination, medical imaging, radiometric assay, nuclear security and nuclear plant safety.
Several products have been introduced in the market utilising scintillation counters for detection of potentially dangerous gamma-emitting materials during transport. These include scintillation counters designed for freight terminals, border security, ports, weigh bridge applications, scrap metal yards and contamination monitoring of nuclear waste. There are variants of scintillation counters mounted on pick-up trucks and helicopters for rapid response in case of a security situation due to dirty bombs or radioactive waste.[4][failed verification][5][failed verification] Hand-held units are also commonly used.[6]
In the United Kingdom, the Health and Safety Executive, or HSE, has issued a user guidance note on selecting the correct radiation measurement instrument for the application concerned. This covers all radiation instrument technologies, and is a useful comparative guide to the use of scintillation detectors.[7]
Radioactive contamination monitors, for area or personal surveys require a large detection area to ensure efficient and rapid coverage of monitored surfaces. For this a thin scintillator with a large area window and an integrated photomultiplier tube is ideally suited. They find wide application in the field of radioactive contamination monitoring of personnel and the environment. Detectors are designed to have one or two scintillation materials, depending on the application. "Single phosphor" detectors are used for either alpha or beta, and "Dual phosphor" detectors are used to detect both.[8]
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A scintillator such as zinc sulphide is used for alpha particle detection, whilst plastic scintillators are used for beta detection. The resultant scintillation energies can be discriminated so that alpha and beta counts can be measured separately with the same detector,[8] This technique is used in both hand-held and fixed monitoring equipment, and such instruments are relatively inexpensive compared with the gas proportional detector.
Scintillation materials are used for ambient gamma dose measurement, though a different construction is used to detect contamination, as no thin window is required.
Scintillators often convert a single photon of high energy radiation into a high number of lower-energy photons, where the number of photons per megaelectronvolt of input energy is fairly constant. By measuring the intensity of the flash (the number of the photons produced by the x-ray or gamma photon) it is therefore possible to discern the original photon's energy.
The spectrometer consists of a suitable scintillator crystal, a photomultiplier tube, and a circuit for measuring the height of the pulses produced by the photomultiplier. The pulses are counted and sorted by their height, producing a x-y plot of scintillator flash brightness vs number of the flashes, which approximates the energy spectrum of the incident radiation, with some additional artifacts. A monochromatic gamma radiation produces a photopeak at its energy. The detector also shows response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak for the creation of electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously (pile-up, within the time resolution of the data acquisition chain), appearing as sum peaks with energies up to the value of two or more photopeaks added[8]
Scintillators are a group of materials that luminesce when exposed to ionizing radiation. In layman’s terms that means these materials emit light when they absorb particles or electromagnetic waves that create “free” electrons in the material. While this may be viewed as a seemingly dry and uninteresting definition, scintillators enable a plethora of exciting and very necessary applications ranging from personal radiation detection to the detection of dirty weapons, and medical applications such as treating cancer.
Scintillation Crystals, Arrays and Detectors
In this series of posts, we’ll explore the world of scintillation, starting at the most basic – how scintillation materials respond to radiation. While scintillators come in many forms – gas, liquid, solid, and both organic and inorganic – we will focus on inorganic crystal forms, a sweet spot for us since Hilger Crystals has grown crystals since the mid ’s. Then, in successive posts, we’ll discuss how they’re produced, specific applications, popular materials types, and more. Let’s get started!
Ionizing radiation is caused by atoms shedding energy in the form of particles or rays (photons) to become molecularly stable. Typical particle radiation types are alpha, beta, or neutron while electromagnetic radiation types include x- and gamma-rays. Particles and rays interact with matter differently, but a common trait of both is that in order to have optimised sensitivity (and therefore luminesce) they need to absorb as many particles or photons as possible. In general, high density materials with high atomic numbers (Z) meet this requirement.
The University of Florida’s Radiation Safety Short Course, Chapter 2, Section VII offers excellent deeper information on the interaction of particles and ionizing radiation.
Put more succinctly, the absorbed energy produces electron-hole pairs that migrate to the activator sites in scintillators in order to luminesce.
Stages of Scintillation. Source: https://web.stanford.edu/group/scintillators/scintillators.html
One quick caveat: when ionized radiation in any form is absorbed by the host optical material, it does not always follow that the material will scintillate. Consider tungsten or lead for a moment – both deliver high density and high Z, but both are use as radiation shielding materials. While they absorb ionized radiation, this energy is lost within the material by a number of non-radiative processes. Scintillating materials must contain luminescent centers that convert ionized radiation into a form that can be detected.
As we are focusing on solid scintillators, one obvious property is that the inorganic crystal needs to be transparent to the scintillating light colour (wavelength). If not transparent, then that energy is lost to non-radiative processes.
Luminescence from scintillators vary by material, but generally ranges from 0.3 – 0.8 microns, spanning the range of human eye sensitivity so it is possible we can see it with the naked eye – if there is high enough excitation flux or dose.
Once light has been generated in a scintillator crystal it needs to be detected and this is done by attaching a photosensor along with suitable optics to reflect and scatter the light to the photosensor. Photosensors can range from photomultiplier tubes (PMTs), PIN diodes, SiPMs, and more, depending on emission wavelength of the scintillator and operating environment. We’ll review different types of photodetectors and key factors in choosing the right one in a future post – stay tuned.
If you have questions about a specific scintillation principle or crystal, please reach out our experts at Hilger Crystals.
We also invite you to try our newly-developed “Crystal Compass” – an easy 4-step tool to help you determine the best scintillator material for your application.
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