Some Geiger counter manufacturers claim their devices can be used to test things like food, yet they provide no instructions on how to do so. Additionally, people on social media can be seen measuring objects with dosimeters or Geiger counters using the Sievert unit, with some even calling the results an “emitted dose rate.” This guide explains the proper method and reasoning for measuring radioactive objects, such as clocks or rocks, with a Geiger counter or dosimeter to minimize the potential for transmitting faulty or meaningless information, which could result in embarrassment.
Definitions and Limitations
Activity
Activity refers to the rate at which a radioactive material decays, measured as the number of radioactive disintegrations or decays occurring per unit of time. The internationally recognized unit for activity is the becquerel (Bq), expressed as decays per second.
A Geiger counter does not measure or quantify the activity of a source. Instead, it detects ionizing radiation by registering a count for each particle detected, without distinguishing the energy or origin of the radiation. This means it identifies the presence of radiation but does not measure the emission rate or differentiate between energy levels in a sample. Therefore, it is not possible for a Geiger counter to measure activity.
Dose Rate
Dose rates are used to quantify the health risk of radiation exposure to the human body. The internationally recognized unit for dose rate is the sievert (Sv), which can also be expressed as µSv (microsieverts) or mSv (millisieverts). Using Sv implies that ionizing radiation was being absorbed by the entire human body at a specific rate during the measurement
Count Rate (CPM)
Counts per minute (CPM) or counts per second (CPS) represent the number of particles detected by a device in one minute or second. While the count rate provides a measure of the number of occurrences per unit of time, it does not provide context about the quality or type of radiation activity. Without additional information—such as the device’s make and model—CPM-only readings are essentially meaningless.
Measuring the Dose Rate of an Object
Unless explicitly stated in the dosimeter manufacturer’s instructions for calibration purposes, measuring the dose rate of an object is not done professionally. The Sievert unit is used to quantify radiation exposure to human tissue from the surrounding environment. If someone reports a dose rate by measuring the front of a clock, they have not actually measured a human dose rate as no flesh was being exposed during the measurement. A true dose rate measurement implies that the human body is being exposed at a certain rate.
In reality, there is no such thing as an “emitted dose rate,” and suggesting or demonstrating otherwise reflects a fundamental misunderstanding of radiation dosimetry.
Measuring an Object Using a Geiger Counter
1. Power on the Geiger counter and switch the display to show CPM or CPS.
2. Position the object to be measured on the Geiger tube side of the detector, as close as possible to
the Geiger-Müller tube.
3. Wait until the device reaches its maximum CPM or CPS reading.
4. Document the maximum count rate displayed by the Geiger counter.
Since the count rate alone is not very meaningful, always report the count rate along with the make and model of the Geiger counter used. This ensures that others with the same device can understand and even replicate your measurements when taken in a similar manner with the same source. Providing as much relevant information about the source as possible (mineral name, size, weight and locale) is also a good idea when sharing a measurement.
Conclusion
Understanding the limitations and proper use of Geiger counters, dosimeters, and SI units is critical to avoiding the conveyance of inaccurate or meaningless radiation measurements. While Geiger counters are excellent for detecting ionizing radiation and providing relative count rates, they are not suitable for scientifically measuring the activity or dose rates of objects.
When using a Geiger counter to measure items like clocks or rocks, focus on reporting count rates (CPM or CPS) along with the make and model of the instrument. Avoid terms like “emitted dose rate,” which misrepresent the capabilities of these tools. By following these guidelines, you can ensure your measurements are accurate, repeatable, and responsibly shared.
This guide is by no means exhaustive. For more information regarding the differences between Geiger counters and count rate measurements, feel free to check out this article titled “How Many CPM Should My Sample Read”.