|Year : 2011 | Volume
| Issue : 1 | Page : 37-40
Indigenous development and networking of online radon monitors in the underground uranium mine
JJ Gaware, BK Sahoo, BK Sapra, YS Mayya
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai, India
|Date of Web Publication||17-Mar-2012|
Y S Mayya
Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai
Source of Support: None, Conflict of Interest: None
There has been a long standing demand for online monitoring of radon level in various locations of underground Uranium mine for taking care of radiological protection to workers. Nowadays, radon ( 222 Rn) monitors, based on electrostatic collection of charged progeny and subsequent detection by semiconductor detector are increasingly employed for radon monitoring in environment. However, such instruments have some limitations such as (i) requirement of additional dryer since sensitivity is dependent on the humidity, (ii) cannot be connected to a network and (iii) not cost effective etc. Hence use of such instruments in underground uranium mine (humidity level >90), may not be reliable. Towards this end, we have indigenously developed radon monitor based on electrostatic collection and scintillation technology for the online monitoring in uranium mine. This instrument overcomes the above mentioned limitation of commercial radon monitors and based on custom made features. Different tests and measurements were carried out and compared with commercial instruments. It was found to be in an excellent agreement with the commercial instruments. A few such instruments have been installed in different locations of uranium mine at Turamdih and connected to a network system for online monitoring and display.
Keywords: Online monitoring of radon, radiological protection, uranium mine
|How to cite this article:|
Gaware J J, Sahoo B K, Sapra B K, Mayya Y S. Indigenous development and networking of online radon monitors in the underground uranium mine. Radiat Prot Environ 2011;34:37-40
|How to cite this URL:|
Gaware J J, Sahoo B K, Sapra B K, Mayya Y S. Indigenous development and networking of online radon monitors in the underground uranium mine. Radiat Prot Environ [serial online] 2011 [cited 2020 Jan 28];34:37-40. Available from: http://www.rpe.org.in/text.asp?2011/34/1/37/93943
| 1. Introduction|| |
One is often required to carry out continuous monitoring of radon concentrations in the environment for understanding it's real time variations due to either natural processes or due to human activities. Unlike time-integrated monitoring which is mainly limited to dosimetric applications, continuous monitoring yields insight into spatio-temporal correlations, build-up in confined spaces, atmospheric transport, extreme excursions, duration of specific highs and lows etc. all of which provide important inputs to understand radon and environmental physics. While the increased computational capabilities on environmental modeling have given rise to greater needs for real time data, the corresponding developments in networking and data transmissions have made it possible to achieve large scale simultaneous measurements. Such facilities are being increasingly developed as part of systems for earthquake predictions, in uranium mining, environmental monitoring and geophysical research.
An important component in real time monitoring is the availability of relatively inexpensive, yet sufficiently sensitive, continuous radon monitors. Systems such as Alphaguard (User Manual, Genitron Instruments GmbH, Germany, 1998) and RAD7 (User Manual, DURRIDGE Co, USA, 2000), ubiquitously used as reference instruments today, are no doubt quite useful, but are expensive and do not have networking capability from the point of view of large scale deployment. Many radon measuring instruments uses 218 Po, singly charged positive at the end of the recoil path (Porstendorfer and Mercer 1979; Chu and Hopke 1985), collected in an electrostatic field as a measure of the radon content in air (Sapra B.K.et al.1999; Negro Vincent 1990; Keller et al. 1982; Porstendorfer et al. 1980; George, 1977). One uses surface barrier silicon detectors to detect only 218 Po atoms by spectroscopic method (Chambaudet et al. 1997; Voytchev et al. 2001) electro-statically deposited on the collection surface. This has the advantage of being quite prompt for varying radon levels since 218 Po has 3.05 min. half-life. However due to size limitation on silicon detectors to minimize the silicon crystal defects and capacitance, the high electric field of the order of 10 4 V/m can not be achieved through out the detection chamber and in fact the high electric field is confined to a very small volume near the detector (~100 cc for 400 mm 2 detector). As a result the weak electric field is insufficient to overcome the neutralization process of 218 Po by trace gases causing low collection efficiency and effect of humidity on sensitivity. Also, surface barrier detectors are relatively more expensive as compared to photomultiplier tubes and one would like to have less expensive systems for large-scale measurements. On the other hand, this electrostatic collection techniques can provide less expensive continuous radon monitors if used with scintillator based alpha detector instead of Silicon detector. The activity of the decay products collected on the surface is estimated through alpha counting using standard alpha counting technique in accordance with a selected counting period. However, interpreting the gross counts under time varying radon concentrations in the chamber requires correction to the residual counts due to the past concentrations. This in turn requires an analysis of deposition faction of 218 Po and 214 Pb as well as formulae for the growth and decay of alpha activity due to simultaneous depositions.
| 2. Description of the System|| |
The radon monitor is an integrated microprocessor based system consisting of a collection chamber, the conventional ZnS:Ag scintilator detector and an alpha counting set up. [Figure 1] shows the schematic diagram of the radon monitor. The collection of progenies is carried out electrostatically as well as by wall deposition through diffusion process of neutralized unattached fraction of progenies. The scintillations of alpha are detected by photomultiplier tube and counted for each interval.
A microprocessor based board is utilized for counting signal pulses from photomultiplier, processing data and handling communication with host computer or RS485 network. Multiple radon monitors can be connected through RS485 network to a central data monitoring and logging computer. Electrostatic collection and detection is done by a cylindrical Stainless steel chamber of volume 1 L (Height 11 cm and diameter 12 cm) with a semi hemispheric cap made of S.S. mesh. The radon is sampled either by diffusion path or by flow path with blocking the other path of sampling. During diffusion mode only the both radon gas enters through the progeny filter eliminating radon progenies, then through the thoron discriminator which is a diffusion type time delay allowing the short lived thoron 220 Rn (half life-55.6 sec) to decay during diffusion time. The hemispherical mesh allows only radon to diffuse inside the active volume of detector while capturing the unattached radon progenies. For flow mode of sampling, the diffusion path via filter can be covered with a blind plate and having sample flow by external pump. The sample drawn through a progeny filter enters through the thoron progeny filter inside the detection chamber. During flow mode of sampling, the radon response is instantaneous. The outer S.S. cylinder walls and the hemispherical S.S. mesh acts as an anode. Thus the radon progenies inside this chamber are exclusively originated from the radon inside the chamber.
The cathode at the inside of the detector is a perspex cylinder coated with ZnS:Ag scintilator covered by a 15 μm aluminized mylar acting as a cathode. The anode and cathode are insulated from each other by a bakelite plate and a high voltage of -2000 volts is applied across them to generate the electrostatic field for collecting positively charged radon progenies. The electric field inside various detector geometries and dimensions were modeled using commercial software. The best field with simple to fabricate design is obtained with the lowest electric field of 9KV/m and field higher than 20 KV/m in the 75% of the detector volume. The Perspex cylinder utilized inside cathode is an optically transparent block, polished to optical grade and is coupled to the photomultiplier tube using optical silicon grease. This block acts as the light guide for the scintillation light to reach effectively to the photomultiplier tube without much loss of light intensity. These scintillations are detected by the PMT and converted to electrical pulses which in turn are coupled to the pre-amplification circuit. The alpha counts obtained are processed by a microprocessor control unit to displays the concentration of radon at preset intervals through the algorithm developed based on the theoretical growth and decay calculations of radon progenies.
| 3. Response Characteristics|| |
The detector was kept for 3 hrs with electrostatic field voltage off and radon free air inside the detector volume. Then the background counting is performed for several hours of counting duration. Alpha counting efficiency was taken using a standard 239 Pu source at various locations on the surface of alpha detector. The average alpha counting efficiency is observed as 35) and the background of 15 cph is obtained.
The response characteristics of the electrostatic chamber are essentially governed by it's activity collection characteristics. This has been determined by controlled experiments with specific radon concentration, applied plate collection voltage and relative humidity. The results are shown in [Figure 2]. The humidity is varied from 5% to 95% and the field voltage varied from zero to -2500 volts. The sensitivity platue is obtained above -1000 volts but at -2000V and above, the effect of humidity is very nominal on the sensitivity. The overall sensitivity varied from 3.0 cph/bq/m 3 to 2.6 cph/Bq/m 3 for 5% to 95% humidity. Hence, change in sensitivity due to maximum change in humidity is upto 20%. The tempreture and humidity sensors are inbuilt inside the detector volume for compansating the humidity effect by applying the corresponding sensitivity factors.
|Figure 2: Effect of field voltage and humidity on overall sensitivity of detector|
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The response characteristic of the detector is mainly controlled by the detector sensitivity for each 220 Rn, 218 Po and 214 Po. The counting efficiency η for a radionuclide is a fraction (counts / disintegrations) cps/Bq of a radionuclide during any time interval. For the ECAS detector η for 2220 Rn, 218 Po and 214 Po are respectively 11%, 26% and 34% and the corresponding sensitivity factors are (ηxVx3600sec) 0.38, 0.94 and 1.24 cph/(Bq/m3). Assuming the overall average surface gross alpha sensitivity of dectector as 35% these η yields the respective collection efficiencies as 30%, 75% and 99%. Note that 220 Rn is not collected on the cathode hence the 30% collection efficiency means that 60% (2η) alpha reaches directly from volume space radon to the detector surface. During the zero field testing the η for 2220 Rn, 218 Po and 214 Po were obtained respectively as 11%, 8% and 14%.
| 4. Networking in Uranium Mine at Turamdih - A Field Validation|| |
The prototype continuous radon monitor has been calibrated with the RAD-7 continuous radon monitor and Lucas cell measurements in the laboratory. The laboratory experiments gave very good agreement with the conventional Lucas cell measurements and RAD-7 instrument. Since the instrument was designed for uranium mines, a few online monitors have been installed inside the uranium mines for field measurements [Figure 3].
The instrument was used in a uranium mines and compared with the AlphaGuard radon monitor. Instruments were placed in the underground of uranium mines where the concentration of radon is expected to be high because of return air flow of mine ventilation. Results of the measurements are shown in [Figure 4]. The changes in concentration are very well recorded in the instruments. It could also be seen that the response time of the present monitor is better compared to AlphaGuard instrument because of the larger diffusion area for sampling. Results show one to one agreement among the two instruments.
|Figure 4: Radon levels monitored simultanuousely at various locations inside Uranium mines|
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| 5. Conclusions|| |
An electrostatic chamber based real-time radon monitor has been built and characterized. The voltage response clearly demonstrates the collection of 214 Pb atoms in addition to 218 Po atoms and it is most probable by theoretical consideration that a significant fraction of 214 Pb comes from re-suspension due to recoil. Also, a clear evidence for long time survival of 214 Pb ions (against neutralization) is seen as compared to 218 Po ions. These findings are not only important in their own right from a basic point of view, but also are essential to develop algorithms for correcting residual counts from the previous concentrations. The performance evaluations have been carried out using alpha guard instrument. A large number of trials conducted in controlled conditions (in a chamber), in a room and in uranium mine have shown that the monitor can successfully measure concentrations ranging from 10 Bq m -3 to several kBq m -3 within a counting interval of 1 hour. Since this detectors has direct radon sensitivity which is absolutely independent of electric field, humidity and trace gas concentrations, the effect of these parameters to overall sensitivity is reduced. Since the ZnS:Ag is sensitive to the alpha emission from radon and it's progenies in the volume of detector, this detector can be operated without any electrostatic collection. Although the sensitivity of the detector is low in the absence of electric field, this mode with zero field is particularly very useful for online measurements of high radon concentration in the presence of very high humidity and trace gases. Also the detector can be operated without any electric field. Considering that the instrument is relatively inexpensive (essentially PMT cost), while at the same time capable of measuring normal environmental levels, it is quite useful for large-scale networking. The continuous radon monitor is as sensitive as that of AlphaGuard radon monitor. The instrument also could be adapted for continuous radon flux measurement.
| 6. References|| |
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]