|Year : 2011 | Volume
| Issue : 4 | Page : 257-261
Thoron interference test of different continuous passive radon monitors
CG Sumesh, A Vinod Kumar, RM Tripathi, VD Puranik
Department of Atomic Energy, Environmental Assessment Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India
|Date of Web Publication||17-Jan-2013|
C G Sumesh
Department of Atomic Energy, Environmental Assessment Division, Bhabha Atomic Research Centre, Trombay, Mumbai
Source of Support: None, Conflict of Interest: None
The instruments measuring radon concentration without energy discrimination may have some sensitivity towards thoron concentration. In the present paper, thoron interference tests of a pulse - ionization - chamber - type instrument - Alpha Guard and a semi - conductor - based instrument - Radon Scout Plus (RSP) have been studied in detail. The instruments are compared with a standard active radon - thoron discriminating monitor - RAD7. As a result 9% interference in measured radon concentration for the Alpha Guard monitor and 4% interference for the RSP monitor were observed. The results obtained indicate that the interference of thoron in radon monitors depends on the area of diffusion of gas, volume of detection and sensitivity factor of the monitor.
Keywords: Calibration chamber, National Institute of Science and Technology (NIST), radon progeny, pulse ionization
|How to cite this article:|
Sumesh C G, Kumar A V, Tripathi R M, Puranik V D. Thoron interference test of different continuous passive radon monitors. Radiat Prot Environ 2011;34:257-61
|How to cite this URL:|
Sumesh C G, Kumar A V, Tripathi R M, Puranik V D. Thoron interference test of different continuous passive radon monitors. Radiat Prot Environ [serial online] 2011 [cited 2020 Jun 6];34:257-61. Available from: http://www.rpe.org.in/text.asp?2011/34/4/257/106187
| 1. Introduction|| |
Global average annual effective dose to the members of public from natural radiation is 2.4 mSv (UNSCEAR, 2000).  In this, inhalation of radon ( 222 Rn, T 1/2 = 3.82 days), thoron ( 220 Rn, T 1/2 = 55.6 seconds) and their decay products comprise about 50%. Thoron, which is a member of the decay chain of thorium ( 232 Th) is an isotope of radon and is also an alpha emitter, and thus interferes in the radon measurements using gross alpha measuring techniques. The accurate measurement of radon and thoron in the environment is important from the viewpoint of radiation protection. A problem regarding environmental radon measurements is how to distinguish radon and thoron experimentally. Presence of thoron is negligible in most of the places. However, high thoron concentrations have been found in some areas of the world. Indoor surveys in Europe and Asia have revealed that the dose contribution due to the inhalation of 220 Rn and its progeny can be equal or even higher than that due to 222 Rn and its progeny (Tokonami et al., 2009).  Additionally, thoron exhalation rate from soil is reported to be higher than radon (Porstendorfer, 1994). 
There are many instruments available in the market for measuring radon concentration. The most commonly used instruments are based on detection of alpha particles, like Pulse Ionization Chamber (PIC), electrets ionization chambers, scintillation detectors with Zinc Sulphide [ZnS (Ag)], alpha particle spectrometers using silicon diodes, gamma ray spectrometers using NaI (Tl)/Ge (Li) detectors and nuclear track detectors. The choice of instrument depends on the information needed, sensitivity required, duration of measurement, etc., Radon and thoron emit alpha radiation of different energy, a fact that can be utilized in discriminative measurement techniques. However, instruments based on gross alpha measurements of radon and thoron emission cannot differentiate between them. Like radon, thoron is present everywhere and will thus be detected in the monitor leading to an overestimation of measured radon concentration. In general, the presence of thoron in environment is negligible; however, high concentrations are observed in some areas like high background natural radiation areas (Zhuo et al., 2000; Doi et al., 1994) , of Kerala (Mayya et al., 1999),  India. It is therefore essential to find the interference of thoron in radon monitors. Note that a 10% interference of thoron concentration in passive radon monitor based on PIC is reported by Ishikawa, 2004 (Ishikawa et al., 2004). 
| 2. Materials and Methods|| |
2.1. Chamber description
A leak proof cubical chamber made of stainless steel (SS) with dimensions of 80 cm × 80 cm × 80 cm (L × B × H) has been used in the present study. The chamber includes ports for injecting radon into the chamber and for collecting air samples from the chamber for measurement. There are two fans inside the chamber for mixing and maintaining uniform concentration of gas inside the chamber. The speed of the fan can be regulated from outside the chamber. There are sensors provided inside the chamber for measuring temperature and humidity. A leak test of the calibration chamber was carried out by injecting radon gas from a standard 226 Ra/ 222 Rn source. The decay of radon gas in the calibration chamber is shown in [Figure 1]. The decay rate of radon concentration was measured using the Alpha Guard Professional radon monitor for a period of 9 days. The concentration obtained over this period was fitted to an exponential curve. The fit shows an effective decay constant of 0.00794 h -1 , yielding an effective half-life (T 1/2 ) of 3.637 days. Since the radon half life is 3.825 days, the leak test indicates that the half life for leak alone works out to be 74 days, which is quite long as compared to the time required (4-5 days) to carry out a given set of experiments. This assures that no appreciable air exchanges takes place with the outside air, thereby maintaining a controlled environment in the calibration chamber.
2.2.1. Passive radon monitors
[Table 1] and [Figure 2] gives the details and layout of the instruments compared in this study.  Alpha Guard PQ 2000PRO, from Genitron is based on a PIC. This device has a volume of 0.62 litres and its metallic interior has a potential of +750 V with the ground when the instrument is turned on. Mode of operation is three dimensional alpha spectrometry. The main advantages of the system are its fast response, high sensitivity (1 CPM (Counts Per Minutes) at 20 Bqm -3 ) and a wide dynamic measurement range (2 Bqm -3 to 2 MBqm -3 ). In the diffusion mode, only 222 Rn gas passes through the glass fiber filter (Retention coefficient >99.9%) into the chamber while the filter retains the radon progeny products. The filter also protects the interior of the chamber against contamination from dust particles and aerosols. The radon monitor has an internal quality assurance system, which performs a series of control and plausibility checks in a repeating mode whenever it is switched on. 
|Figure 2: Layout of instruments (a) Alpha guard (b) RAD7 (c) Radon scout plus|
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Radon Scout Plus (RSP), from SARAD is a semiconductor - based continuous radon monitor. It shows a wide range of measurement (0 to 2 MBqm -3 ) operating in a passive diffusion mode. The volume of the detector is 60 cm 3 . The sampling time can be varied from 1 minute to 255 minutes. Gross alpha detection technique is used and the sensitivity of the detector is 2 CPM/kBq/m -3 . Besides the activity concentration of radon, air temperature, relative humidity and barometric pressure can also be simultaneously measured with this instrument. The detector response is insensitive to any environmental changes in the atmosphere. Two of these instruments named as RSP 158 and RSP 161 have been used in this study.
2.2.2. Radon/Thoron discriminating monitor
RAD7, from Durridge Company Inc, is a Silicon - based semi conductor detector and works using alpha spectrometry technique.  Air from the chamber is drawn to the RAD7 inlet first through a small drying tube filled with desiccant and then through a 0.45 μm filter. The air from the outlet of RAD7 is then re -circulated into the chamber. The filter prevents 222 Rn and 220 Rn progeny from getting into RAD7. The pump of the RAD7 starts running when the preset program is executed. The pump is microprocessor controlled and the flow rate is around 0.65 L/min. RAD7 has an internal hemispherical sample cell of 0.7 L, with a solid state ion-implanted planar silicon alpha detector at the centre. The inside of the hemisphere is coated with an electrical conductor which is charged to a potential of 2-2.5 kV relative to the detector, so that positively charged progeny nuclides decayed from 222 Rn and 220 Rn are driven by the electric field towards the detector. The progeny atom reaches the detector and subsequently decays emitting an alpha particle that is detected. An electrical signal is generated with the strength being proportional to the alpha energy. RAD7 amplifies and sorts the signals according to their energies. The RAD7 spectrum is a scale of alpha energies from 0 to 10 MeV, which is divided into 200 channels each of 0.05 MeV width. The alpha energies associated with 222 Rn and 220 Rn are in the range of 6-9 MeV. The 200 channels are grouped into eight energy windows (labelled as A-H). Windows A-D is the functional ones. Window A records 6 MeV alpha particles from 218 Po, window B, records 6.78 MeV alpha particles from 216 Po, window C records 7.69 MeV alpha particles from 214 Po and window D records 8.78 MeV alpha particles from 212 Po. In sniffer mode, radon and thoron concentration is calculated using the counts in the window A and B, respectively. In normal mode, radon concentration is calculated using the counts from both the windows of A and C. This enables RAD7 to measure 222 Rn and 220 Rn gas concentrations discriminately.
2.3. Standard source
A 226 Ra/ 222 Rn emanation standard reference source (SRM 4974), supplied by NIST was used as a radon source. Thorium oxalate powder ( 232 Th) separated from monazite, packed in a porous container was used to generate thoron ( 220 Rn) in the chamber. By varying the thorium oxalate powder quantity, 220 Rn concentration was varied inside the chamber. Concentration in all the monitors was recorded on an hourly basis.
2.4. Thoron interference test
Thoron interference of radon monitors which are operating without energy discrimination was studied. Initially, a known amount of radon gas was injected in-to the chamber and then the thoron gas was continually injected into the chamber using a pump. Radon with a half life of 3.82 days will decay by 6% of its initial value in the following 8 hours. Since the thoron interference study was carried out for 8 hours, the decay of radon during this period is insignificant, and the radon concentration can be considered to be more or less constant in the chamber, for that particular experiment. However, for the calculation, exact values of the radon concentration monitored were used. The thoron interference of the monitors was calculated using the following equation:
Where S (in %) is the relative interference of thoron in the measured radon concentration, R g is the radon concentration (Bqm -3 ) measured by the investigated monitors without energy discrimination, R d is the radon concentration (Bqm -3 ) measured with the radon - thoron discriminative monitor (RAD7) and T is the thoron concentration (Bqm -3 ) measured with RAD7. For quality assurance purposes, a number of samples were collected from the calibration chamber during the thoron interference test using scintillation cell and the concentration of gases has been measured using the simultaneous method.
| 3. Results and Discussion|| |
Thoron interference of radon monitors which operate in gross counting mode (Alpha Guard and RSP)  was measured at different radon and thoron concentrations. The results of the study are given in [Table 2]. In this table S is relative interference of thoron, calculated as per equation 1 and rounded off to nearest integer. The thoron interferences of the Alpha Guard varied from 7 to 17 % with an average value of 9%. This value is somewhat smaller than the value of 10% reported by Ishikawa (Ishikawa et al., 2004).  The thoron interference of the RSP detector varied from –1 to 10% with an average value of 4%. The thoron interference value of zero or a negative value observed for the RSP monitor is likely due to non - equilibrium between the detector volume and the surrounding atmosphere due to a small inlet entry area of the detector that makes the detector to respond in longer time.
|Table 2: Radon and thoron concentration measurement and thoron sensitivity ( ±1 σ )|
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The study indicates that thoron interference of Alpha Guard is larger than that of semi -conductor based RSP. This can be explained as given below. Thoron atoms are exchanged naturally by diffusion through the filter paper medium positioned at the boundary wall of the chamber. The variation of thoron concentration in the chamber can be described by the following differential equation:
Where C in and C out are the concentrations of 220 Rn inside and outside the chamber. λ is the decay constant of 220 Rn in sec -1 and γ is the air exchange rate in sec ?1-1 . The air exchange rate in the chamber is defined as:
Where D is the diffusion coefficient (cm 2 s -1 ) of 220 Rn in the filter, A and δ are the area (cm 2 ) and thickness (cm) of the filter paper, respectively and V is the interior volume of the chamber. The mean 220 Rn concentration inside the chamber can be calculated using the solution of the Eq. (2). If the exposure time is long enough, the ratio of concentrations inside to outside the chamber will be approximated to
Assuming an equal value for in both the detectors, the air exchange rate in the Alpha Guard will be 14 times higher than that in the RSP. Therefore, there will be 3.5 times higher infiltration rate in alpha Guard leading to a higher interference due to thoron in the radon measurements. The larger area of opening for the entry of gas will increase the air exchange between the inside and the outside of the monitor. The increased infiltration rate and higher sensitivity of the Alpha Guard detector increases thoron sensitivity in radon measurements.
In contrast, the small area of opening and volume restrict the thoron entry in the RSP monitor and is thus a good choice for a radon monitoring instrument. However, the very low sensitivity (0.12 CPH/Bqm -3 ) limits the utility of this instrument. A low value of sensitivity also increases the time required to measure a certain concentration with a desired accuracy. From [Table 2], it is clear that the interference of thoron in the RSP monitors is quite consistent.
| 4. Conclusion|| |
Passive radon monitors operating without energy discrimination showed a positive sensitivity towards thoron. Thoron sensitivity of the investigated passive radon monitors depends on the air exchange rate and infiltration rate of the monitor for thoron gas and on the sensitivity factor of the detector. Thoron interference in radon measurements will increase with increasing infiltration rate. The interference of thoron in radon measurements can be used to measure thoron concentration with certain limitation. There may be a higher interference of thoron in these monitors while operating in flow mode. The over estimation of radon (i.e. 9% of thoron concentration in Alpha Guard and 4% in RSP) due to the presence of thoron is negligible for general environments. However, attention should be paid if the detector is used in thoron-enhanced areas.
| References|| |
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|8.||User Manual, Alpha Guard, Genitron make, 2004, Germany. |
|9.||User Manual, RAD7, 2007. Railroad Avenue. Suite D Bedford, MA: Durridge Company Inc; 01730. |
|10.||User Manual, Radon Scout Plus, Sarad Company, Germany. |
[Figure 1], [Figure 2]
[Table 1], [Table 2]