1. Geiger Counter
When it comes to radiation detection, many people think of the Geiger counter, as it is often introduced in school textbooks. The Geiger tube is a simple yet effective radiation sensor sensitive to alpha, beta, gamma rays, and other charged particles (e.g., protons). It also has a slight sensitivity to neutrons.
Geiger tubes come in various shapes and materials. Larger tubes can detect more radiation, reducing detection time and improving the capture of high-energy rays. The cheapest versions use glass tubes, but alpha and low-energy beta rays cannot penetrate glass. Thin metal tubes are better for detecting low-energy rays. If alpha rays need to be measured, a Geiger tube with a mica window is required.
Inside the instrument, the Geiger tube is powered by several hundred volts of direct current. When no radiation is present, the tube acts as an insulator. When radiation enters, it generates free electrons or ions, which under the electric field create an “avalanche” of charged particles. This is equivalent to a short circuit in the tube, which results in a brief drop in voltage. The number of these drops over time is counted to determine the radiation level.
However, the Geiger tube cannot differentiate between types of radiation, and its efficiency in detecting gamma rays is low. Background radiation usually results in only about one count per second (1cps). To detect small variations in radiation, longer measurement times are necessary. For example, detecting a difference of 0.02 μGy/h might require a 5 to 10-minute accumulation of data.
Many commercial Geiger counters are not designed to measure subtle radiation changes, as they do not allow adjustments in accumulation time or data export. However, DIY improvements can increase sensitivity, such as extending averaging time when the radiation is weak or shortening it when high radiation levels are detected.
The simplicity and robustness of the Geiger counter, coupled with its resistance to interference, make it an affordable and widely accessible radiation detection tool. It remains the most common product available for personal use, with prices as low as 200 RMB ($30 USD). For more precise detection of surface contamination, a Geiger counter with a mica window, typically priced above 2000 RMB ($300 USD), is recommended.
2. Semiconductor Detectors
Semiconductor detectors, which measure weak electrical discharges caused by radiation, are another common radiation detection tool. There are two basic types: those with a PN junction (like a PIN diode) and those without. The former generates electrical pulses when exposed to radiation, while the latter, using materials like germanium or cadmium zinc telluride crystals, produces a tiny electrical pulse when radiation induces charge migration.
Semiconductor detectors are less sensitive than Geiger counters, requiring an amplifier to enhance their output signals, which makes them more prone to interference. The most common form uses PIN diodes, and while these detectors can be small and inexpensive, they are less effective than Geiger counters at detecting radiation.
Despite their limitations, semiconductor detectors can be compact, with some models as small as a coin. Although they may not be as sensitive, they can still serve as practical warning devices for hazardous radiation levels.
One of the key advantages of semiconductor detectors is that the pulse size correlates with the energy of the detected radiation, allowing for energy compensation and, to some extent, the identification of the radiation type. However, these features are usually absent in low-cost products.
3. Scintillation Detectors
Scintillation detectors, which use light to indirectly measure radiation, are the third most common type of radiation detector. When radiation enters certain materials, it causes them to scintillate (emit flashes of light). Scintillation detectors typically consist of three parts: a scintillating material, a photosensor, and a pulse processing and data readout circuit.
The most common scintillating materials are sodium iodide (NaI) and cesium iodide (CsI). These materials are relatively affordable, though the size of the scintillating material is critical for sensitivity and response. Larger scintillators allow for faster detection of weak radiation changes.
To measure the faint flashes of light, sensitive photosensors such as photomultiplier tubes (PMTs), silicon photomultiplier tubes (SiPMs), or avalanche photodiodes (APDs) are used. These components are not cheap, and combined with the cost of scintillating materials, scintillation detectors usually cost several thousand RMB. However, entry-level models below 2000 RMB are now available due to advancements in technology.
Compared to Geiger counters, scintillation detectors are more sensitive and can detect radiation much faster. They can also measure the energy of radiation, allowing for more accurate energy compensation and the identification of specific radioactive isotopes.
4. Radiation Meter Accuracy
The raw data from any radiation meter, regardless of its operating principle, is a count rate—either the number of short circuits in a Geiger tube or the number of flashes in a scintillator. This count rate, measured in counts per second (Cps) or counts per minute (Cpm), is then calibrated against known radiation levels.
However, calibration introduces inaccuracies. For example, the relationship between count rate and radiation intensity is not perfectly linear, and different energy levels of radiation can result in significant measurement deviations. Additionally, environmental factors such as temperature and power supply variations can cause further drift in measurements.
For these reasons, the accuracy of consumer-grade radiation meters is often not very high, and the relative change in radiation levels (e.g., kitchen vs. bedroom) is often more reliable than the absolute value of the measurement itself.
5. Measuring Pulsed Radiation
Many everyday radiation meters have limited measurement ranges. For instance, a Geiger counter exposed to high radiation might not recover from one event before the next one arrives, leading to an underestimation of radiation levels. Some detectors, like scintillators, may also be affected by afterglow following intense radiation exposure.
Pulsed radiation, such as that from medical X-ray machines or airport security scanners, can far exceed the detection range of common devices. For instance, a medical X-ray machine may emit radiation at over 10 Gy/h, but for a very short duration (e.g., 0.1 seconds). In these cases, specialized pulse radiation meters are required to accurately measure the dose.
Conclusion
For everyday use, a Geiger counter is often sufficient to detect dangerous radiation levels. For more precise detection or measurement of subtle radiation changes, a scintillation detector or a more sophisticated DIY solution may be necessary. Understanding the limitations of each device is crucial to making informed decisions about radiation safety.
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