Scintillators

Type of physical science: Elementary particle (high-energy) physics

Field of study: Techniques

The detection of ionizing radiation, which includes quanta of light (photons), electrons, protons, and other charged particles, by the scintillation light that is produced in certain materials is one of the most common techniques in use in modern nuclear and particle physics experiments. The scintillation process is also one of the oldest techniques on record and has been used since the earliest investigations of radioactivity.

Overview

Ernest Rutherford used scintillating zinc sulfide crystals in his α particle scattering experiments that led to the discovery of the nucleus in 1911. Here, a scintillator is defined as any material that produces a pulse of light after the passage of a particle. This is closely related to fluorescence, which is usually defined to be the production of a light pulse shortly following the absorption of a photon. Both inorganic and organic scintillators have been discovered. The scintillation process is different for the two groups. Inorganic crystals are grown with a small admixture of an impurity, which is located at random positions in the crystal lattice. The positive ions and electrons, created from the ionization of atoms by the incoming particle, diffuse through the lattice and are captured by the impurity centers. Recombination produces an excited center that emits light. The total amount of light produced is greater than that from organic materials, but the movement of electrons through the crystal lattice results in a light pulse that lasts much longer. The most widely applied scintillators include the inorganic alkali halide crystals, especially sodium iodide.

The organic scintillators can be further grouped into organic crystals, liquid scintillators, and plastic scintillators. Their advantages include transparency to their own light, short decay times that give a fast pulse of light with little spread in time, light emission spectra that are well matched to photomultiplier tubes, and easy adaptability. In the organic scintillation detectors, the mechanism depends strongly on the molecular structure of the medium. After the passage of a particle, many of the atoms in the scintillating medium will be excited into higher energy levels. Most of the excitation energy is converted into heat and vibrations of the lattice.

Some of the energy is released as light, which cannot be absorbed by the medium.

The scintillation efficiency is defined as the fraction of the deposited energy that appears as light. The efficiency for even the best scintillators is low, 7 percent for sodium iodide crystals doped with thallium, and 3.5 percent for the best organic material, anthracene. All organic scintillators contain at least one benzene ring in their structure. Anthracene consists of three condensed benzene rings. The organic crystals have the highest efficiency but are usually dissolved (solutes) in a solvent to make liquid and plastic scintillators. A typical concentration of solute to solvent is 3 grams per liter. Although most interactions in the detector will occur with the more numerous solvent molecules, it is possible for a large fraction of the deposited energy to be transferred to the scintillating solute. Xylene and toluene as solvents give good light yields, and toluene is used in the scintillator with the best time resolution (less than one ten-billionth of a second). Plastic scintillators are similar in composition to liquids. Polystyrene and polyvinyltoluene are commonly used base plastics. They have a density about 3 percent higher than that of water. They are rugged, easy to cut and machine, and are available in large sizes.

Since organic scintillators are generally aromatic hydrocarbons, and as such are low in nuclear charge, they are inefficient for stopping γ rays. These scintillators are usually used for the direct detection of charged particles or neutrons. The CEBAF Large Acceptance Spectrometer (CLAS) in Virginia, operational in 1995, has several hundred plastic scintillators that cover a total area of 230 square meters around a toroidal magnetic field.

The scintillation process also occurs in pure noble gases, such as helium, neon, argon, krypton, and xenon. Atoms excited by the energy loss of a passing particle may dissipate as much as 20 percent of the excitation energy through the emission of ultraviolet light. The emission spectra consist of a sharp peak near the lowest energy (threshold), together with a broad continuous spectrum. The continuum portion of the spectrum results from transitions from vibrationally excited states to the ground state. The spectrum is nearly independent of the type of incident particle but is strongly affected by the gas pressure.

Another type of scintillating material is glass. Despite the fact that its relative light output is low (20 to 30 percent of anthracene), it is used commonly in applications in which severe environmental conditions prevent the use of more conventional scintillators. Examples include conditions in which the scintillator must be exposed to corrosive chemical environments or operated at high temperatures.

The use of scintillation detectors in nuclear and particle physics would be impossible without the availability of devices to convert the weak output of the light pulse into a corresponding electrical signal. The photomultiplier tube does this remarkably well. It converts the emitted photons into an electronic signal that may be interpreted by modern electronics modules and computers that analyze the event that scientists are attempting to understand. Little random noise is added to the signal in the process. In a photomultiplier tube, the light strikes a photosensitive surface called the photocathode. The photoelectric effect (explained by Albert Einstein in 1905) results in the release of at most one electron (here called a photoelectron) from the photocathode. The photoelectron is accelerated by a potential difference in the tube, where it strikes another metal surface (called a dynode) from which more secondary electrons are emitted.

These secondary electrons are in turn accelerated and multiplied in usually ten stages of dynodes, each at a higher electric potential. This results in a total multiplication of about 1 million. These million or so electrons form the output pulse of the photomultiplier tube. The typical potential difference between adjacent dynodes is about 100 volts; thus, electrons strike the dynodes with about 100 electronvolts of energy. The dynodes are made of materials with a high probability of secondary electron emission. It may take 2 to 3 electronvolts of energy to release an electron; thus, a gain in the number of electrons by a factor of 30 is possible. Because the electrons are released in random directions in the material, however, relatively few will actually be released at the surface, and a gain by a factor of 4 or 5 at each dynode is more typical. Even so, with a ten-dynode tube, the overall gain would be 4¹⁰, which is about 1 million, or 5 to the power of 10, which is about 10 million.

Applications

The most common use of scintillation counters is for triggering. The signals from the scintillators can be used by fast electronics to decide whether to activate other apparatuses, such as wire chambers, and whether to record the information from the event. A scintillator can be used to measure the energy deposited per thickness of the counter, giving a measure of the differential energy loss of particles. Properties of scintillators that are important in decisions regarding their use include efficiency, energy resolution, spatial resolution, and time resolution.

Here, the detector efficiency is a measure of whether it detects the presence of a particle or not; it is determined by the number of photoelectrons emitted from the photocathode of the photomultiplier tube. The inefficiency can be made as small as one part in a million if there are an average of fourteen photoelectrons per incident particle. The ratio of the energy resolution to the energy is proportional to one divided by the square root of the average number of photoelectrons. A large scintillation counter has poorer position resolution than other detectors, such as proportional or drift chambers. Long scintillation counters often use a photomultiplier tube on each end. Measurements of the pulse height and time difference between the ends can be used to measure more accurately the position of the particle in the long scintillator. The best spatial resolution with these types of detectors is obtained by using many narrow counters in parallel. Such an arrangement is called a scintillation hodoscope. Each element must have its own photomultiplier tube.

A common use of a scintillator hodoscope is in "tagging" photon beams. High-energy photon beams are produced from an electron beam that undergoes bremsstrahlung in a thin target. The electron beam is bent by a magnetic field, while the photons that are produced travel in a straight line to a nuclear target. The electrons are detected by a scintillator hodoscope. The particular element that measures the electron gives the energy, and the photon energy is given by the difference in the energy of the initial electron beam and this measured value. A tagged photon beam has been used for the Phoenix, Arizona, experiment at ELSA in Bonn, Germany. A hodoscope covering up to 11 meters is planned for the CLAS in Virginia. A 0.5-meter hodoscope with two rows of sixty-four 8-millimeter scintillators is used to tag the photon beam for the Laser Electron Gamma Source (LEGS) experiment at the National Synchrotron Light Source of Brookhaven National Laboratory in New York. Here, polarized laser light is Compton scattered from an electron beam that is in a storage ring. The resulting photon travels backward along the line of the electron, is polarized, and has gained a factor of up to 100 million in energy over the original photon from the laser. The hodoscope measures the energy of the electron, giving the energy of the high-energy photon.

One of the important applications of scintillation counters is the measurement of time intervals at the nanosecond level. The resolution is limited by variations in the time at which the detector responds to the particle that has entered it. Time variations include those in the energy transfer from the radiation to the optical levels in the scintillator and those in the finite decay time of the light-emitting state. There are variations in the transit time between the scintillator, the light collector, and the photocathode of the photomultiplier tube. Effects resulting from the photomultiplier tube include variations in the transit time from the photocathode to the first dynode because of the spread in initial photoelectron velocities and the location of the photoelectron emission, and variations in the electron amplification in the tube.

An important application of these counters is in time of flight systems. The time it takes for a particle to travel from the target to the detector is a function of its velocity. This can be used with other information to determine the mass of the particle.

The discovery of thallium-activated sodium iodide in the early 1950's allowed the scintillation spectroscopy of the decay γ rays emitted from nuclear excited states. A scintillator can be used to detect electromagnetic radiations because these radiations can liberate electrons by photoelectric, Compton scattering, or pair production processes. These electrons in turn traverse the detector, causing excitations that result in the scintillation events. The energy of the γ rays can be measured if they undergo a photoelectric process, and it can be used to deduce the energy of the nuclear levels. This led to an understanding of the knowledge of the excited nuclear levels, much as the observation of the light from heated gases and metals led to the knowledge of atomic levels. Thallium-activated sodium iodide is still the favorite crystal, and large ingots can be grown from high-purity material to which about one part in a thousand of thallium has been added as an activator. It is hygroscopic and will deteriorate as a result of water absorption if exposed to the atmosphere for any length of time. Crystals are stored in airtight metal containers for normal use. They can be used at normal room temperature but are somewhat fragile and can be damaged easily by mechanical or thermal shock. The light output is slow, with a nearly 230-nanosecond decay constant and a delayed emission of 0.2 second that contains nearly 10 percent of the light. One detector, called the Crystal Ball, constructed at the Stanford Linear Accelerator Center (SLAC), had complete coverage around a target using hundreds of thallium-activated sodium iodide crystals. The size of commercially available crystals is as large as a cylindrical ingot of equal diameter and length of more than 48 centimeters. The LEGS experiment in New York has a cylindrical thallium-activated sodium iodide detector of this size.

Good resolution of γ rays of energies up to 300 million electronvolts is achieved with such a large single crystal.

Cesium iodide is another alkali halide that is commercially available with either thallium or sodium as the activator material. Europium-activated lithium iodide enriched in the isotope lithium 6 is of special interest in neutron detection. Silver-activated zinc sulfide is one of the older inorganic scintillators. It is available only as a polycrystalline powder. Rutherford used scintillation screens made from this, and the light was viewed directly with a low-power microscope by scientists who recorded each observed pulse of light. Europium-activated calcium fluoride is notable as a nonhygroscopic and inert inorganic scintillator that often can be used when severe environmental conditions preclude other choices. In the 1980's, bismuth germanate became popular because the high electric charge of the bismuth nucleus leads to a high probability of the photoelectric effect occurring from incident γ rays. It has only about 8 percent of the light output of thallium-activated sodium iodide, but other considerations lead to its use in certain applications.

Context

Modern nuclear and particle physics experiments employ scintillators for particle identification or rejection following scattering reactions. The number of particles detected varies from one to several hundred. The detector may consist of magnetic devices, Cherenkov counters, wire chambers, calorimeters, and hodoscopes. Signals are entered into fast electronics that enable scientists to determine that various signals in the detector came from one common event.

Information may include position and angles, energy deposited in a counter, and the time difference from one position to another along the particle's path. In high-energy physics, the amount of data for one event can be 100,000 computer bits. Some experiments also generate large event rates, more than a million per second. The data handling is clearly a serious problem.

If the event rate is high, the selection of events for complete processing must proceed in stages.

Signals from scintillation counters are used extensively in these decisions.

Scintillators may be used in particle calorimeters. The calorimetric method is based on the approximation that the total amount of ionization released by particles in a thick absorber is proportional to the energy of the particle and independent of particle type. This is only a good approximation for very high energies and for certain absorbers such as lead or depleted uranium.

Most of the absorber is in the form of a stack of plates of a dense inert material such as lead, interleaved with an active detector medium. A second approximation is made that the ionization measured in the active medium is proportional to the total. Layers of plastic or liquid scintillator several millimeters thick are ideal in many respects. The energy resolution can be good and the response relatively fast. In the late 1970's, techniques were developed for bringing out the light without creating gaps in the calorimeter. One method is called wavelength shifting. The light from an individual scintillator passes through an air gap and into a plastic sheet doped with a dye that absorbs the scintillation light and reemits it isotropically at a longer wavelength. About one-fourth of the light is trapped by total internal reflection in the plastic and proceeds to the end of this wavelength shifter bar, where it enters a photomultiplier tube. One hundred or more scintillator sheets may be coupled to a wavelength shifter bar only a few millimeters thick. There is a loss of about 80 percent of the light, which may be acceptable in this application. The position of the particle in the calorimeter is usually obtained by three stereoscopic projections similar to the use of three radio signals in the navigation of airplanes.

Scintillators may be used to monitor low-intensity charged particle beams or photon beams directly. Rates of 10 million particles per second are possible with many photomultiplier tubes.

Principal terms

BREMSSTRAHLUNG: the energy loss of charged particles (usually electrons) resulting from the electromagnetic radiation emitted in the violent accelerations that occur during collisions with atomic electrons

IONIZATION: the process by which the passage of radiation through materials disturbs the electrons in the atoms and sometimes ejects electrons from the atoms, leaving behind charged atoms or ions; as a result, α, β, and γ rays are sometimes called ionizing radiation

PAIR PRODUCTION: the process by which a photon encounters a virtual photon in the electromagnetic field of the atom and creates an electron-positron pair that has the total energy of the original photon (except for the small recoil momentum of the nucleus)

PHOTOMULTIPLIER TUBE: a light-sensitive vacuum tube used to convert the extremely weak light output of a scintillation pulse or of Cherenkov radiation into a corresponding electrical signal that is proportional to light intensity

Bibliography

Birks, J. B. SCINTILLATION COUNTERS. New York: McGraw-Hill, 1953. Provides a good discussion on the usage of scintillation counters. Includes a description of the photomultiplier tube.

Curran, Samuel C. LUMINESCENCE AND THE SCINTILLATION COUNTER. New York: Academic Press, 1953. The author was the coinventor of the first scintillation counter. The importance of photomultiplier tubes is discussed.

Schwarzchild, Bertram. "Solar Neutrino Update: Three Detectors Tell Three Different Stories." PHYSICS TODAY 43 (October, 1990): 17. Discusses the results of solar neutrino measurements using different detectors.

Schwarzchild, Bertram. "Underground Experiments Will Look for Proton Decay." PHYSICS TODAY 33 (January, 1980): 17. Discusses various detectors that were constructed to search for proton decay.

Willis, William J. "The Large Spectrometers." PHYSICS TODAY 31 (October, 1978): 32. Willis discusses large detectors used in high-energy physics.

Detectors on High-Energy Accelerators

Electron Emission from Surfaces

Electrons and Atoms

Radiation: Interactions with Matter