Why Do Different Instruments Show Different Numbers and Which One to Trust?

Often, the first encounter with radiation involves buying a simple Geiger counter. Later, a more advanced scintillation dosimeter or spectrometer might be purchased. At this stage, you may notice discrepancies in the readings of the two devices. Finding a logical answer to why the devices show different values can be challenging. We have prepared a detailed explanation of why this happens and how to avoid it.

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1. Differences in Measuring Natural Radiation Background

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Geiger counters cannot differentiate the energy of gamma radiation, which varies among different radiation sources. Therefore, manufacturers calibrate them to display readings based on a single radiation source, usually the radioactive isotope cesium-137. Other sources show distorted readings.

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The natural radiation background significantly differs from cesium-137 in energy. A Geiger counter, unable to distinguish cesium-137 from the overall background, shows distorted values. Scintillation dosimeters measure dose rate considering the energy of gamma radiation, a parameter not accessible to Geiger counters. The lower the gamma radiation energy, the lower the harm and the readings. Thus, for background measurements, scintillation dosimeters show lower values than Geiger counters.

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2. Intrinsic Background of Geiger Counters

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All Geiger counters have their intrinsic background due to the spontaneous emission of electrons from the counter’s cathode, creating measurement errors. Usually, this intrinsic background is around 0.04 µSv/h. These parameters are easily found on the manufacturers’ pages of Geiger counters. The problem is that manufacturers often do not subtract the intrinsic background when displaying readings, thereby adding about 0.04 µSv/h to all measurements. People used to such readings are surprised by the low values of scintillation dosimeters.

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3. Geiger Counter Displays Dose Rate Correctly Only in the Absence of Beta Radiation

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If beta radiation enters the Geiger counter and the readings are displayed in sieverts, the readings are distorted and do not reflect reality. The interaction efficiency between the Geiger counter and beta radiation is high, but the actual harm is extremely low. The readings are distorted to the point of being unable to assess the real harm. Measuring beta radiation with a Geiger counter requires a special measurement mode that applies corrections for beta radiation characteristics and subtracts gamma radiation from the result. Comparing the readings of a scintillation dosimeter and a Geiger counter is only valid when beta radiation is excluded.

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4. Different Distances to the Sensor

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Measuring a radiation source close to the device is considered bad practice among professionals. At zero distance, the source gives very high numbers, relevant only for local exposure. It is incorrect to equate these numbers to the overall dose.

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Radiation from a point source spreads according to the inverse square law. The closer the distance to the source, the higher the background. A 1 mm shift at a close distance can cause a tenfold change in readings.

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Example:

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Two identical devices can show completely different readings when measured up close if one is a millimeter further from the source.

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Therefore:

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When measuring with the source close to the body, the device with the detector closer to the source will always show higher numbers. Therefore, comparing devices should be done at the same distance from the sensors, not from the body.

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Since the distance to the sensor is not always known, measurements are taken at a small distance, such as 5 centimeters. At this distance, millimeter differences are not critical, and the data can be compared.

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5. Non-uniform Radiation Field

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In metrology, there is a rule: the radiation field during measurement must be uniform, ensuring that the radiation level in all parts of the detector is the same. Otherwise, the readings will be distorted, and they cannot be compared with another device. To eliminate field non-uniformity, you need to know the size of your detector and conduct measurements no closer than the size of your detector.

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6. How to Get Accurate Data from the Source If You Can’t Measure Up Close?

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It is enough to know the distance from the source. Radiation spreads according to the inverse square law. If we know the readings and the distance of one measurement, we can calculate the readings at any distance from the source. It’s easiest to use an online calculator, which will calculate the readings for any distance for you.

Inverse Square Law Calculator.

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By understanding these factors, you can make more accurate and reliable radiation measurements, and choose the right instrument for your needs.