|Fig. 1: Radiation survey meter. (Source: Wikimedia Commons)|
A wide range of detectors encompassing several transduction principles exist for the measurement and quantification of ionizing radiation. [1,2] The selection of a radiation detector is guided by measurement requirements as well as practical considerations related to the complexity and cost of each detector type. For example, some detectors can quantify radiation energy, whereas others only count ionizing events. The design and shielding of a radiation detector also affects its sensitivity to different ionizing sources, including alpha, beta, and gamma radiation.  An example of a radiation survey meter housing a detector is shown in Fig. 1. This article explores three common types of radiation detectors, highlights their major differences, and provides insight into when and why certain detectors may be chosen to perform a radiation survey.
Geiger-Müller (GM) detectors are frequently used for radiation surveys and rely upon avalanche multiplication of ionizing events within a gas-filled detection chamber. [1,4] However, the proportionality between the amplified signal and ionizing event is lost in the avalanche process. Therefore, while GM detectors count the number of ionizing events per unit time, they do not provide direct information about the radiation energy or source. Another notable feature of GM detectors is their inability to detect many ionizing events occurring over a short period of time. This limitation is often quantified by a metric called dead time.  While accurate count rates may be achieved for background and low radiation levels, significant inaccuracies can result at moderate and high count rates due to an inability to resolve ionizing events in rapid succession without dead time correction. 
The design of a GM detector strongly affects its sensitivity to alpha, beta, and gamma radiation sources which have different penetrating characteristics. Geiger-Müller detectors for alpha and beta radiation commonly use tube geometries with ultrathin and highly transmitting windows to increase the likelihood that less penetrating radiation will reach the inside of the ionization chamber. In contrast, the detection of gamma radiation results primarily from the interaction of gamma rays with the wall of the ionization chamber.  Therefore, the chamber geometry, gas, wall material, and thickness are all important design factors in determining the sensitivity limits of GM detectors.
When quantification of ionizing energy is of interest during a survey, scintillation detectors are commonly used.  Scintillation detectors rely upon the conversion of ionizing radiation energy into light through luminescence.  A calibration transfer function allows the intensity of captured light to be related to the energy of the incident radiation, yielding quantitative energy distribution information.  Ideally, the majority of light conversion in scintillation materials occurs via fluorescence, which is a rapid transition, as opposed to phosphorescence. Fluorescence allows for fast detector response times and quantification of moderate- to high-level radiation where the dead time of GM detectors may result in erroneous count rates.
In practice, many organic and inorganic scintillation materials are available and chosen based on the type of radiation to be measured. Trade-offs include light yield, speed, long-term material stability, and ease of integration with light-collection optics.  Therefore, a detailed understanding of the detector construction, constituent materials, and any window or filter accessories is necessary to make claims about the sensitivity of a scintillator to different radiation sources. While scintillation detectors allow for spectroscopic measurements resolving the energy of ionizing radiation, the energy resolution of scintillation detectors is limited, and may prevent the identification of radionuclides with closely spaced energy signatures. 
Solid-state detectors based on semiconductor diode structures are commonly used when improved energy resolution and radionuclide identification capabilities are required.  These detectors rely upon the production of electron-hole pairs within a diode depletion region resulting from incident ionizing radiation. In a solid-state device under a reverse bias, these charge carriers can be directly swept to electrodes producing a current signal before recombining. This removes additional transduction steps, for example, such as the conversion to optical energy in scintillation and avalanche multiplication in GM detectors. Therefore, solid-state detectors can offer greater energy resolution than scintillation detectors. 
However, there are several practical limitations associated with the use of solid-state detectors. [1,10] In order to increase the diode depletion region which forms the interaction volume of the detector, production of high-purity semiconductor materials, such as silicon or germanium, is required. Additionally, leakage current in the diode due to room-temperature excitation of charge carriers can significantly degrade the noise performance of solid- state detectors. Therefore, cooling to cryogenic temperatures is generally required for optimal resolution and operation. As a result, solid-state detectors and spectrometer instruments are typically more expensive than alternative technologies.
Additional classes of detectors exist beyond those discussed here. However, each detector type possesses unique advantages and disadvantages depending on survey requirements. Unfortunately, the details of measurement instruments, including detector type, calibration, and survey environment, are rarely provided when radiation measurements are reported in the media following a nuclear incident. However, an understanding of the differences between detector designs and capabilities can help facilitate informed interpretation and discussion of radiation measurements.
© Tim English. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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