Scintillation counter

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Schematic showing incident particles hitting a scintillating crystal, triggering the release of photons which are then converted into photoelectrons and multiplied in the photomultiplier.

A scintillation counter is an instrument for detecting and measuring ionizing radiation.

It consists of a scintillator which generates photons of light in response to incident radiation, a sensitive photomultiplier tube which converts the light to an electrical signal, and the necessary electronics to process the photomultiplier tube output.

Scintillation counters are widely used because they can be made inexpensively yet with good quantum efficiency and can measure both the intensity and the energy of incident radiation.

Operation[edit]

animation of radiation scintillation counter

When a charged particle strikes the scintillator, its atoms are excited and photons are emitted. These are directed at the photomultiplier tube's photocathode, which emits electrons by the photoelectric effect. These electrons are 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 in the form of a measurable pulse for each photon detected at the photocathode, and is passed to the processing electronics. The pulse carries information about the energy of the original incident radiation on the scintillator. Thus both intensity and energy of the radiation can be measured.

The scintillator must be in complete darkness so that visible light photons do not swamp the individual photon events caused by incident ionising radiation. To achieve this a thin opaque foil, such as aluminised mylar, is often used, though it must have a low enough mass to prevent undue attenuation of the incident radiation being measured.

The article on the photomultiplier tube carries a detailed description of the tube's operation.

Detection materials[edit]

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 Sulphide is widely used as a detector of alpha particles.

Detector efficiencies[edit]

Schematic of a scintillating crystal combined with a photomultiplier.

Gamma[edit]

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.

Neutron[edit]

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.

Applications[edit]

Scintillation counters are used to measure radiation in a variety of applications.

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.[1][2] Hand-held units are also commonly used.[3]

Guidance on application use[edit]

In the United Kingdom the HSE has issued a user guidance note on selecting the correct radiation measurement instrument for the application concerned [1]. This covers all radiation instrument technologies, and is a useful comparative guide to the use of scintillation detectors.

Radiation Protection[edit]

Alpha and Beta contamination[edit]

Hand-held large area alpha scintillation probe under calibration
Hand-held scintillation counter reading ambient gamma dose. The position of the internal detector is shown by the cross

Industrial radioactive contamination monitors, either hand-held for area or personal surveys or installed for personnel monitoring require a large detection area to ensure efficient and rapid coverage of monitored surfaces. For this the scintillation counter with a large area scintillator window and integrated photomultiplier tube is ideally suited and finds 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.

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. This technique is used in both hand-held and fixed monitoring equipment, and such instruments are relatively inexpensive compared with the gas proportional detector.

Gamma[edit]

Scintillation materials are used for ambient gamma dose measurement, though a different construction is used to detect contamination, as no thin window is required.

Scintillation counter as a spectrometer[edit]

The experimental setup for determination of γ-radiation spectrum with a scintillation counter. A high voltage power supply is connected to the scintillation counter. The scintillation counter is connected to the Multichannel Analyser which sends information to the computer.

Scintillators often convert a single photon of high energy radiation into 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.[4]

History[edit]

The modern electronic scintillation counter was invented in 1944 by Sir Samuel Curran[5][6] whilst he was working on the Manhattan Project at the University of California at Berkeley, and it is based on the work of earlier researchers reaching back to Antoine Henri Becquerel, who is generally credited with discovering radioactivity, whilst working on the phosphorescence of certain uranium salts (in 1896). The Spinthariscope was an early method of detecting the scintillation events by eye.

See also[edit]

References[edit]