**4. Measurement techniques**

The measurement techniques for radon can be classified based on a) whether the technique measures radon gas <sup>222</sup>*Rn* or its daughter products, b) time resolution, and c) radioactive detection of the type of emission resulting from radioactive decay—alpha, beta, gamma radiation. The commonly used methods for measuring radon and its daughter products are shown in **Figure 2**.

Active methods require electric power for measurements, whereas passive methods require no power. Measurements can be performed at specified intervals and data can be stored and read directly with active methods. In contrast, in the case of passive methods, integrated exposure concentrations can be measured, and data analysis requires special equipment. Time resolution techniques can be classified into three types, as shown in **Figure 3**.

**Grab sample technique**: This technique involves measurement of <sup>222</sup>*Rn* in a discrete sample of air collected over a very short period of time (compared with the mean life of <sup>222</sup>*Rn* at a single point). Radon measuring instruments such as RAD7 can be used to measure <sup>222</sup>*Rn* and its daughters. When it is used in "sniffer" mode, in which radon is typically present with minimal growth of its progeny, a large number of measurements can be taken in a relatively less period of time [8].

*Radon gas and daughter (progeny) product measurement methods [8].*

### **Figure 3.**

*Time resolution techniques for measuring radon and its daughter products [8].*

Grab sampling technique for radon progeny involves drawing a known air volume through a filter and counting the alpha activity during or following the sampling. Usually, a known volume of air is drawn through a filter using an air sampling pump for very short sampling periods usually 5 minutes. Filters are counted for alpha particle emissions during mathematically determined periods after the sample is collected. There are three main methods available for counting these particles, namely the Kusnetz method, where the filter is counted once, and the modified Tsivoglou method, where the filter is counted three times to measure the decay. Another method, named the Rolle method, is quite popular in Canadian mines. It is similar to Kusnetz method but is more rapid, and the procedure differs only in the timing of filter counting after sample collection. **Figure 4** shows one of the MSHA recommended instruments for sampling radon progeny that works based on the Kusnetz method.

**Continuous technique**: This technique provides time series concentrations of <sup>222</sup>*Rn* in air samples; in this method, sampling and counting are performed simultaneously. Most of the continuous monitors are portable, and nearly all of those are designed to detect alpha radiation (by an ionization chamber, gross alpha counting, or alpha spectrometry). Specific ionization and scintillation chambers are shown in **Figures 5** and **6**, respectively.

**Integrating technique**: This technique provides the integrated concentration over a certain period of time. Such measurements are used to determine monthly or average <sup>222</sup>*Rn* . The passive detectors, which are expensive, are examples of integrating techniques [8].

Apart from measuring alpha particles during the decay of <sup>222</sup>*Rn* , radon concentrations are determined by measuring the beta activity during the decay of <sup>214</sup>*Pb* and <sup>214</sup>*Bi* by assuming secular equilibrium between <sup>222</sup>*Rn* and its progeny in the air. The beta activity is assayed with plastic scintillators mounted on photomultiplier tubes or the filter paper can be counted in a beta counter with the appropriate use of absorber film. Gamma spectrometry can also be used to determine radon concentrations by measuring the gamma activity during the decay of <sup>214</sup>*Pb* and <sup>214</sup>*Bi* .

### **Figure 4.** *Ludlum 2000 with accessories.*

**Figure 5.**

*Schematic diagram of an ionization chamber [9].*

**Figure 6.** *A typical scintillation detector [10].*
