A practical soil radon ( 222 Rn) measurement method

Transcription

1 Nuclear Science and Techniques 21 (2010) A practical soil radon ( 222 Rn) measurement method DING Weicheng 1,2,* WANG Yi 1 LI Yuanjing 1 FANG Fang 2 YANG Liu 2 1 Department of Engineering Physics, Tsinghua University, Key Laboratory of Particle & Radiation Imaging, Ministry of Education, Beijing , China 2 Chengdu University of Technology, Key Laboratory of Applied Nuclear Technology in Geosciences, Chengdu , China Abstract Soil radon measurement of high stability and sensitivity is widely applied, and in some applications, such as in uranium prospecting, 222 Rn should be distinguished from 220 Rn. To meet this requirement, a practical method based on soil radon diffusion and accumulation theory to measure soil radon by Alpha Particle Spectroscopy (α-ps) is discussed in this paper. The α-ps measurement method can effectively overcome the effects of 220 Rn and its daughters ( 216 Po, 212 Bi, 212 Po). The system can eliminate the impact of soil radon field disturbance and non-uniformity through soil radon static diffusion. Radon daughters ( 218 Po, 214 Po) are accumulated under the action of an electrostatic force, which not only enhances the measurement sensitivity, but also increases robustness of the measurement. Simultaneous measurement of multiple points can increase the comparability of measurement data and the measurement efficiency. Experimental data shows that the soil radon measurement method was robust. So it has wide applications such as in geological prospecting, in fissure groundwater exploration and in ground subsidence inspection. Key words Soil radon; α-ps; 222 Rn; diffusion and accumulation theory; Soil radon concentration measurement 1 Introduction 222 Rn is a gaseous radionuclide from uranium decay. Studies on 222 Rn migration have shown that 222 Rn has a very strong upward transfer capability [1 3], and the information about uranium deposits hundreds of meters underground can be obtained by measuring surface soil 222 Rn concentration [4,5]. 222 Rn can diffuse from deep underground through geological fault zones, hence high radon concentration near the fault zones. Measuring soil radon is used for geological prospecting, groundwater exploration, ground subsidence inspection, landslide belt surveys, geothermal exploration [6 8] and earthquake monitoring [9]. Soil radon measurement techniques can be done instantaneously or cumulatively. Cumulative soil radon measurement methods in common use include α-track etching, thermo-luminescence, α aggregating (α-card and α-cup), and activated carbon adsorption. And common instantaneous measurements include the ZnS scintillation chamber, extraction sampling of RaA * Corresponding author. address: sourcenet@sina.com Received date: (daughters of Rn), and α particle spectrometer (α-ps) with pump suction sampling of soil 222 Rn and RaA. Soil radon migrating to earth surface is affected by many factors [1,2]. The information obtained in an instantaneous soil radon measurement can be less robust than a cumulative measurement. For a cumulative soil radon measurement, however, the results can be influenced by 220 Rn. In this study, aimed at solving the above problems, we propose a soil radon measurement technique, i.e. a multi-point detection system in a network for α-ps, based on soil radon diffusion and electrostatic accumulation theory. The technique is applicable to both the cumulative and instantaneous measurements. 2 The instrument and principles 2.1 The α-ps data acquisition unit The α-ps data acquisition unit (α-psdau) for measuring soil radon diffusion and accumulation is shown schematically in Fig.1. The entire α-psdau is buried in soil. The 222 Rn, in t 1/2 = 3.82 d, and as a

2 DING Weicheng et al. / Nuclear Science and Techniques 21 (2010) non-electrically charged noble gas [1], has strong diffusion ability. Soil radon can spread into the radon diffusion cavity (RDC) due to external factors of clusters [3], air convection, and radon concentration gradient difference [1,2]. The soil radon continues to spread into the radon gathering cavity (RGC) under these conditions, reaching a dynamic radioactive equilibrium of the 222 Rn in soil and the RGC. The 220 Rn, however, with a very short diffusion distance due to its short half-life of 55.6 s, is far less likely to reach the RGC. The RGC, made of stainless steel with the goblet mesh at the bottom of cavity, has a Au-Si surface barrier detector (SBD) for detecting α-particles from decay of 222 Rn, which, as a noble gas, diffuses freely through the electric field applied positively from the SBD and the ground. The α-psdau has a measurement module for soil temperature and humidity, using a low-power management solution. According to the decay of 222 Rn and its daughter 218 Po, they will be radioactively balanced in 30 min, i.e. the number of α particle from the parent and daughter nuclides equals. Therefore, by detecting the α-ps of 218 Po, the soil radon concentration can be determined. Most daughters of 222 Rn and 220 Rn are positively charged, and their chances of spreading into the positively biased RGC are small. After one measurement, the Au-Si SBD shall be contaminated by the radon daughters, which would affect the next measurement without proper consideration of this artifact. It is generally agreed, however, that a nuclide having no parent nuclides will fade away after 10 half-lives. The main short-lived α emitter radon daughters are 218 Po (t 1/2 =3.05 min), and 214 Po, which is not to interfere the next measurement with its t 1/2 =164 s. 218 Po is the daughter of 222 Rn of just the first generation, and the residual 218 Po should be eliminated after 30.5 min. Typically, the air radon concentration is two orders of magnitude less at least than soil radon concentration. The RGC can be purged in air after a spot measurement. And if the next measurement is performed after a long enough period of time, the impact of residual radon is negligible. 2.2 The soil radon concentration Fig.1 A schematic view of the α-ps data acquisition unit (α-psdau). Based on characteristics of α-ps and the decay of 222 Rn from radioactive uranium and thorium [10], signature information of 222 Rn can be obtained through α-ps measurement. However, it is difficult to meet the special α-ps measurement conditions that 222 Rn should decay on surface of the Au(Si) detector. As radon daughters are mainly positively charged [11], they can accumulate, which can increase their ability to be adsorbed to the surface of the detector. Then, when the daughters undergo α-decay, the α-rays can be measured by α-ps technique in the system. Radon concentration, C Rn, can be referred as the radon radioactivity measured per unit volume, i.e. radon α- decay of in Bq/m 3. It can be expressed as, C Rn = N/(Vtηδ). (1) where, N is the number of α-particles detected from α- decay of 218 Po; V is the RGC volume; t is the measurement time; η is detection efficiency of the SBD (<50 %); and δ is the collection efficiency of 218 Po, which is defined by δ = f(v, T, H, E, S) (2) where, V, T, H and E are respectively the volume, temperature, humidity and electric field of the RGC; and S is the sensitive area of the SBD. The detection efficiency η and 218 Po-collection efficiency δ should be calibrated with 222 Rn in a standard radon chamber (SRC). For a certain system, the V, E and S are constant, then the δ, and the radon concentration measurement, is mainly affected by temperature and humidity of the RGC.

3 184 DING Weicheng et al. / Nuclear Science and Techniques 21 (2010) From soil radon measurement applications for uranium deposit exploration, the soil temperature varied over a short period of time (about 10 h) and the soil humidity varied from 75% to 95 % RH [10]. Therefore, for applications that do not need accurate the soil radon concentration, the temperature and humidity effect can be neglected. For an accurate analysis, however, an SRC calibration is necessitated to determine the curves corresponding to temperature and humidity ranges, and the field measurement data can be fitted by the curves. 3 Results and discussion The α-psdau measurement system consists of several α-psdaus, an on-site data management unit (DMU) and a computer data analysis system (CDAS), as shown in Fig.2. The α-psdaus are connected to the DMU through a ZigBee wireless communication network. The DMU is integrated with a GPS module, a surface temperature measurement module and an atmospheric pressure measurement module. α-ps was measured in an SRC by an α-psdau, and the energy resolution of 218 Po is better than 2 %. Fig. 4 shows the results of 20 consecutive 1-h measurements with three α-psdaus on March 15, 2009 in an open field in Chengdu, Sichuan province, China. The consistency is good between different α-psdau units, with a good response to the changes in the last six hours (10:00 to 14:00), which might be caused by air temperature changes and an earthquake. The daily temperature varied by over 6 C at several weather stations close to the measurement site (meteorology data from Chengdu Weather Bureau, China), and an aftershock in 4.7 Richter magnitude scale took place on March 12 afternoon, 2009, in Qingchuan, and it could be felt in Chengdu. α-rsdau 1 α-rsdau 2 α-rsdau n ZigBee Fig.2 The measurement system. DMU USB Fig.3 One α-ps was measured in SRC by α-psdau. 3.1 Consistency of the measurements CDAS A consistency comparison experiment was performed after the α-psdaus were calibrated. In Fig.3, a typical Fig.4 Calibration of the three α-psdau units. 3.2 Comparison of different soil radon instruments A piece of bare land without evident geological cracks was chosen as the experimental area. It has a slight slope in the diagonal of the area. The area was divided into four sections, each having eight measurement points that were 5-m apart from one another. Three types of soil radon measurement instrument were used to measure soil radon synchronously at the same measurement site. They were the α-ps of this work, which is referred hereinafter as SRMS, and two emanometers of CD-1 α-cup which was produced by Chengdu University of Technology and FD3017 which was produced by China Nuclear Industry Corporation, Shanghai Electronic Instrument Factory. The latter was based on a static sampling RaA measurement method. Each measurement of SRMS in α-ps was in a 4.5-h timeframe: 30 min for radon accumulation, and 1 4 hour for four sets of cumulative measurements. The FD3017 and CD-1 measurements of the soil radon concentration were performed afterwards.

4 DING Weicheng et al. / Nuclear Science and Techniques 21 (2010) Fig.5 shows the radon concentration measured by the three instruments. The data in the first hour of the SRMS measurement was used for comparison with the CD-1 and FD3017. Large data fluctuation caused by non-linear adsorption and thorium emanation can be seen in the FD3017 and CD-1data, while the SRMS data is stable and robust, with noticeable fluctuations. We note that both the FD3017 and CD-1 methods are based on adsorption sampling, but the radionuclides cannot be replenished in the measurement, and prolonging the measurement time does not produce more data. However, using the SRMS in a stable soil environment, one can increase the cumulative measurement time to improve quality of the data. 3.3 Repeatability of the measurement The measurement repeatability was tested in the same area in two periods in an interval of 10 days. The subsequent measurement points were backfilled with soil. The same timeframe as Section 3.2 was adopted. Results of the two groups of measurement are given in Fig.7, and the trends show good reproducibility. However, data of the second group are slightly lower. This may be due to the soil radon destruction in Group 1 measurements. Fig.7 The repeatability of the two experiments. 3.4 Equivalent diagram analysis for soil radon concentration Fig.5 Comparison of the methods for soil radon measurement. Fig.6 Soil radon concentrations in different accumulation periods. Fig.6 shows the soil radon concentration results in different accumulation periods. The data was measured by the SRMS for 4 consecutive hours, one cumulative measurement per hour. The trend in soil radon concentration is consistent in each accumulative period. So the 218 Po in soil can re-equilibrate 4 h after the soil was backfilled from the first SRMS measurement of the soil radon concentration. Fig.8 shows a radon concentration isopleth map measured on the site in Section 3.2 by SRMS in the first hour of cumulative measurement. The location information came automatically from the GPS positioning module. High soil radon concentration appears at the bottom of the slope. This may be attributed to rain and leaching, which caused deposition of the radio-nuclides. In addition, weeds and plant roots may cause the groundwater to move to the surface, and this stimulates the underground migration of radio-nuclides to the surface Fig.8 Radon isopleth map (Bq/m 3 ).

5 186 DING Weicheng et al. / Nuclear Science and Techniques 21 (2010) Conclusion We measured soil radon concentration based on radon static diffusion and electrostatic accumulation theory. It not only eliminates the impact of soil radon field disturbance and non-uniformity, but also enhances the measurement sensitivity and increase the robustness of the measurements gathered. The α-ps technique measures 222 Rn by measuring the decay of radon daughter 218 Po, and this distinguishes uranium and thorium. Separate data acquisition and management setups are used in this design and ZigBee communication technology is used to form a multi-measuring-point measurement network. This approach allows synchronous completion of soil radon measurements, hence higher efficiency and increased comparability of data. And it also facilitates the applications. The automatic positioning by GPS was introduced in the field which is appropriate for data analysis needed in prospecting for uranium. The DAU brings soil temperature and humidity measurement modules together, and the DMU brings earth surface temperature and pressure measurement modules together. These are convenient for analyzing the data in multi-parameter form. The system can be slightly modified further and then continuous soil radon concentration measurements may be realized (and remotely monitored), so that the technology can be used in earthquake monitoring or other applications which need measure soil radon in continuously in field. References 1 Zhang Y, Hua R Z. Measurement of Radioactive Prospecting, Beijing: Atomic Energy Press, June, Ershaidat N, Al-Bataina B, Al-Shereideh S. Environment Geolog, 2008, 55(1): Le R, Jia W. Radiat Protect, 2002, 22(3): JIANG M. Uranium Geolog, 2006, 22(1): Fasasi, M K, Oyawale, A A, Mokobia, C E et al. Nucl Instr Meth, 2003, A505(1 2): Enu E I. Sedimentary Geol, 1985, 44: Adegoke O S. Niger Min Geosci. 1980, 2: Crocket L O, Westcott B E. West Africa, 1954, 17617: Planinic J, Radolic V, Vukovic B. Nucl Instr Meth, 2004, A530(3), DING W, The study of radon measurement system in α energy spectrum based on diffusion cumulative theory, Doctoral Dissertation, Chengdu University of Technology, April, Lu Z. Aerosol Science Introduction. Beijing: Atomic Energy Press, 2000.