Errors in radiometric measurements. Sensitivity and background of
Radiation Detector. The main source of errors in
radiation detector measurements are:
1. Instability of equipment parameters.
2. Inaccuracy of standardization.
3. The statistical nature of radioactive decay and radiation interaction.
The errors of the first two types are similar to the errors of other measurements. With careful work, they can be less than 1-2% of the measured value.
Let us consider the third type of errors caused by random fluxes of particles (quanta). The variance σ and the relative measurement error δ of the radiation intensity I during the time t is determined by the formula:
Hence, it can be seen that a decrease in the relative measurement error can be achieved by increasing the measurement time t and increasing the counting rate I. The latter is achieved by increasing the sensitivity of detectors (their size and efficiency), and in nuclear geophysics methods using artificial radiation sources, also by safety precautions).
One of the reasons for the decrease in the measurement accuracy, especially in the case of low-intensity anomalies, is the presence of a radioactive background. The background of the instruments includes three components: the cosmic background (CF), the radiation of the surrounding rocks, and its own (residual) background, caused by radioactive contamination of the materials of the counter and other parts of the radiometer.
The sum of the space and residual background is the natural background (NF) of the device. The residual background of scintillation counters does not exceed 1 μR / h, the natural background is 2 μR / h. Multiple measurements with and without screens can also be used to determine NF. When the value of the natural background is not determined, readings in areas with weakly active rocks (pure limestone, rock salt, etc.) are taken as its value.
When standardizing field
Radiation Detector and during field measurements, the concept of a normal background is also introduced, due to the radioactivity of the surrounding rocks with a normal (clarke) content of radioactive elements.
Let us calculate the error in determining the radiation intensity of the sample in the presence of background radiation Iph. Through I 'we denote the intensity of radiation from the sample, including the background, through t and tf - respectively the measurement time with the sample and without the sample (background measurement). The true radiation intensity of the sample (without background) is determined by the formula: I = I '- Iph. In this case, the relative measurement error δI of the intensity will be:
Hence it can be seen that an increase in the background significantly reduces the measurement accuracy if the values of I and Iph are comparable. Therefore, when studying weakly active samples, great attention is paid to reducing the background. To do this, select indicators with a small intrinsic background and place them in lead houses to shield them from cosmic radiation and radiation from surrounding objects.
The presence of the background also limits the so-called threshold of sensitivity - the minimum radiation intensity that can be reliably measured with a radiometer. The threshold for the sensitivity of radiometers is usually taken as three to four times the standard deviation of the background, which for various radiometers ranges from 0.5 to 2 μR / h.
