Silicon Pin Diode Based Continuous Radon Monitor

Development of a PIN diode-based on-line measurement system for radon ( ²²² Rn) and thoron ( ²²⁰ Rn) in the environment

¹ Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
² Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
³ Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
⁴ Department of Chemical Engineering, Indian Institute of Technology, Powai, Mumbai, Maharashtra, India

Date of Web Publication

18-Dec-2014

Correspondence Address:
P Ashok Kumar
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, Maharashtra
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0972-0464.147281

A silicon PIN diode-based electrostatic collection type online real-time instrument has been developed for simultaneous measurement of radon ( ²²² Rn) and thoron ( ²²⁰ Rn). The system, discussed in this paper, utilizes a hemispherical metal chamber (volume 1 L) for active air sampling. Estimation of ²²² Rn/ ²²⁰ Rn concentration is carried out through alpha spectroscopy of electro-deposited polonium atoms on the detector surface. The system description and the characterization studies carried out with this instrument are presented here. Its performance has been tested with reference equipments. The instrument showed sensitivity of 0.408 counts per hour (CPH)/(Bq/m ³ ) and 0.169 CPH/(Bq/m ³ ) for radon and thoron measurements, respectively, at an optimized collection voltage of + 1.6 kV and relative humidity <10%.

Keywords: Electrostatic collection, PIN diode, polonium

How to cite this article:
Kumar P A, Sumesh C G, Sahoo B K, Gaware J J, Chaudhury P, Mayya Y S. Development of a PIN diode-based on-line measurement system for radon ( ²²² Rn) and thoron ( ²²⁰ Rn) in the environment. Radiat Prot Environ 2014;37:80-8

How to cite this URL:
Kumar P A, Sumesh C G, Sahoo B K, Gaware J J, Chaudhury P, Mayya Y S. Development of a PIN diode-based on-line measurement system for radon ( ²²² Rn) and thoron ( ²²⁰ Rn) in the environment. Radiat Prot Environ [serial online] 2014 [cited 2022 Oct 18];37:80-8. Available from: https://www.rpe.org.in/text.asp?2014/37/2/80/147281

Introduction

Radon, thoron, and their progenies are universally present in the environment and can reach higher levels in indoor air due to poor ventilation. They are the major contributors to internal exposures from natural sources of radiation. ^[1] The human exposures to these nuclides are more prominent in the indoor environment in particular underground uranium mines, thorium mining and extraction facilities, and other confined areas where the natural radionuclides are present in increased concentrations.

As per the requirement of the new international basic safety standard, ^[2] it is the responsibility of the national regulatory body to establish a strategy to control exposures to radon for existing and planned exposure situations. The reference level shall be set at a value that does not exceed an annual average activity concentration of ²²² Rn of 1000 Bq/m ³ . The concept of radon prone areas ^[3],[4] and United States Environmental Protection Agency's recommendations for home buyers ^[5],[6] would point to the need of accurate and reliable measurement of indoor radon concentrations. Therefore, accurate measurement of indoor radon concentration is essential for radon remediation, mitigation as well as occupational radiological surveillance programs.

An increase in awareness of the impact of environmental radon, on public health, has led to the development of more advanced, reliable, and easy to use radon measurement systems. In view of the expansion programs in uranium mining/milling and monazite recovery processes, it is pertinent that comprehensive ²²² Rn and ²²⁰ Rn measurement systems are made available for radiation dose assessment. Hence, a need was felt to develop real-time radon and thoron monitors for use in the mining and dwelling environment.

In the past, several instruments have been developed for the measurement of radon and thoron in the environment. One of the most studied classes of continuous radon monitoring instruments is based on the electrostatic collection of polonium ions. In such systems, sample is made to enter a collection chamber by active methods like pumping where an alpha particle detector is placed at the center of a collection chamber. The positive charged polonium ions are electrostatically collected and subjected to alpha counting employing appropriate particle detectors. Yamamoto (1998) ^[7] describes a real-time radon monitoring system that can simultaneously measure radon concentrations in multiple sites where the radon detector uses a plastic scintillation detector that collects radon daughters in a chamber electro-statically. Gaware et al. ^[8] used this technique to collect ²¹⁸ Po and ²¹⁴ Pb on a foil placed over ZnS (Ag) scintillator for their ECAS continuous radon monitor. Semiconductor detector-based systems employing spectroscopic method have been proved to be the best among electrostatic collection type monitors. Several studies related to radon and its decay products and development of radon monitors utilizing PIN diodes as detectors have been reported earlier. ^{[9],[10],[11],[12],[13]} The development of such a system using indigenously available silicon PIN diode also has been reported. ^[14],[15] In order to enhance the detection sensitivity, an array of PIN diodes was utilized by Ashokkumar et al. ^[16] In the present study, a PIN diode-based electrostatic collection type online real-time instrument has been developed for the measurement of ²²² Rn and ²²⁰ Rn in an environment while both are present. The improved design of the collection cell has been implemented in the system. Here, a single PIN detector is used to get a better sensitivity and alpha energy resolution to enable identification of all alpha-emitting isotopes of radon and thoron progenies.

Materials and methods

The system description

In the present system, the collection chamber has been modified to get an improved sensitivity at a lower applied collection high voltage (HV) as compared to the earlier reported study. ^[15] The mesh inside the detector chamber was replaced by a field focusing ring to enhance the sensitivity. The ring is fixed axially at an elevation of 15 mm from the base plate, and arrangement is made for HV contact. The single PIN detector fixture along with the detector chamber having the field focusing ring fixed axially is given in [Figure 1]. The ring and chamber are kept at the same potential.

A PIN diode ^{[14],[15],[16]} [size-20 mm × 20 mm, [Figure 2]] is fixed at the center of a bakelite plate [fabricated at RSSD work shop, BARC] which is mechanically fitted to a hemispherical (capacity-1 L) aluminum (Al) chamber. The inner surface of bakelite plate is covered with Al plate with opening at the center to expose the detector to the curved surface of the chamber. The stainless steel ring of 40 mm diameter was fitted centrally inside the chamber. [Figure 3] gives the construction of this system.

Figure 1: The single PIN detector fixture along with the detector chamber having the field focusing ring fixed axially Click here to view

Figure 3: Construction of the PIN diode-based radon (222Rn) and thoron (220Rn) measurement system Click here to view

Air is sampled through a dehumidifier using a software controlled DC pump [KNF NEUBERGER make, Germany, model NMP830KNDC 12V]. A rotameter in the sampling stream gives the flow rate. HV is applied to the collection cell with respect to the P side of the detector to facilitate the electro-deposition of charged radon and thoron progenies. The charged progenies are electrostatically deposited over the detector surface.

Two HV supplies are housed inside the system. Both are DC to DC converter modules, one utilized to reverse bias the PIN diode at 40 V and the other is used to apply the HV required for electrostatic collection of the charged progenies inside the collection cell. This can be adjusted to the required level up to 2.5 kV. A charge sensitive preamplifier processes the output signal provided by the detector. The output from the preamplifier is fed to a spectroscopy amplifier, the output of which is simultaneously fed to four individual single channel analyzers (SCA). The spectroscopy amplifier facilitates accurate pulse amplitudes measurements with the accompanying single channel pulse height analyzers. The SCA windows are set such that the pulses due to alpha radiation of 6 MeV from ²¹⁸ Po, 7.687 MeV from ²¹⁴ Po, 6.78 MeV from ²¹⁶ Po and 8.78 MeV from ²¹² Po would fall within the respective voltage threshold levels. The temperature and humidity sensor is provided at the sample exit point. A microcontroller module was utilized to count and analyze the data. The measured concentration is displayed on a liquid-crystal display (LCD). The instrument has a facility to store/recall 170 data sets from memory along with real-time tag and can be interfaced serially with a computer.

The instrument operates on 230 V AC, 50 Hz mains supply or with a rechargeable battery housed inside, when mains not connected. The necessary DC voltages (viz. +12 V, −12 V and + 5 V) for pulse processing electronics are generated inside the instrument. When AC main supply is on, the 12 V, 7.5 Ah capacity, rechargeable battery will get charged. The voltage level of the battery can be viewed on LCD through software menu. Block diagram of the Instrument developed for signal amplification and processing is shown in [Figure 4].

Figure 5: Experimental setup used for the characterization studies during thoron measurements Click here to view

Results and discussion

Subsequent to the development of the system, various experiments were carried out using standard sources of radon and thoron in a calibration chamber available at Bhabha Atomic Research Centre, Mumbai, in order to characterize the system. These experiments mainly included optimization of collection voltage, alpha spectrum generation, and isotopic channel identification, linearity calibration of the system, humidity interference test, and investigation of radon/thoron cross talk. An inter-comparison study of response of the system with commercially available standard radon and thoron reference detectors was also carried out. The experimental procedures and results of these studies are described below.

Optimization of collection voltage

The variation of system sensitivity with the applied collection HV has been studied for both thoron and radon separately. In the case of thoron, HV for the collection of ²¹⁶ Po was determined experimentally as follows. Thoron gas was simultaneously sampled through the PIN diode-based system and RAD7. ^[17] The output pulse from the system amplifier was fed to a USB-based multi-channel analyzer (MCA) for alpha spectrometry, and the spectrum was analyzed using a personal computer. Alpha spectra were acquired for various stable applied collection voltages, and the corresponding sensitivity was calculated using ²¹⁶ Po channel counts and known thoron concentration. ²²⁰ Rn gas was sampled from a 512 L calibration chamber simultaneously through the present system and the standard thoron monitor. The experimental setup is shown in [Figure 5]. To increase the sensitivity for the measurement of ²²⁰ Rn concentration a dehumidifier has been incorporated in the sampling circuit. The relative humidity (RH) and temperature were monitored and logged continuously. The sample flow velocity was maintained at 1.2 L/min. Thoron gas was generated using thorium oxalate powder ( ²³² Th) separated from monazite, packed in a porous container kept in the calibration chamber. The concentration of sampled air was varied by adjusting the speed of mixing fan kept inside the chamber and also by allowing dilution by regulating needle valves V1 and V2. The sensitivity of the system for thoron varied from 0.203 CPM/(kBq/m ³ ) at 0 V to 2.682 CPM/(kBq/m ³ ) at 1700 V.

The optimized collection voltage has been obtained for radon and RAD7 has been utilized to measure the ²²² Rn concentration. The RAD7 was connected in series with the present system through a dehumidifier and radon gas was sampled from a calibration chamber. A USB-based MCA was employed to acquire the alpha spectrum at various collection voltages after 3 h of continuous sampling for an acquisition period of 15 min. The sensitivity value was calculated from counts in ²¹⁸ Po channel and the corresponding radon concentration for each applied collection voltage. The sensitivity varied from 0.093 counts per hour (CPH)/(Bq/m ³ ) at 0V to 0.44 CPH/(Bq/m ³ ) at 1700V.{Figure 5}

[Figure 6] shows the radon and thoron sensitivity variation with applied collection voltages. From the figure, it can be observed that an applied electrostatic collection HV of 1,600 V would be adequate to get optimum sensitivities for thoron and radon to utilize this system as a radon thoron discriminative monitor. Hence, the electrostatic collection HV was set to be 1.6 kV for all studies and measurements associated with both ²²² Rn and ²²⁰ Rn.

Figure 6: Radon and thoron sensitivity variation with applied collection voltages Click here to view

Linearity calibration

The spectroscopy amplifier provides distinct pulse height levels for four of the alpha emitting progenies of radon and thoron namely ²¹⁸ Po, ²¹⁴ Po, ²¹⁶ Po, and ²¹² Po. These signals were fed simultaneously to four SCAs. The collection voltage was initially set at 1.6 kV. The SCA windows were set separately for alpha energies of radon and thoron progeny atoms. The baseline (E) and window (∆E) voltages of each SCA were adjusted in such a way that the alpha pulses of these isotopes were separated to achieve energy discrimination and thereby achieve the Rn/Tn activity separation. The SCA outputs are counted by a microcontroller module. The counts from channels A, B, C, and D represent the total alpha particles emitted by ²¹⁸ Po, ²¹⁶ Po, ²¹⁴ Po, and ²¹² Po respectively during the counting period. The counts from channels A and C represent radon and that from B and D represent thoron activity. Calibration of the system needs to be performed with pure radon and thoron gases to avoid any error due to the presence of other gases. All the measurements were done at low RH (<10%) condition, room temperature and a preset collection voltage of 1.6 kV.

For radon calibration, dehumidified sample was filtered and passed through the collection cell of the system and the radon concentration of the stream was monitored using a Scintillation cell-based Radon Monitor (SRM) ^[8] before pumping back into the radon chamber. The radon gas of known concentration was generated inside the calibration chamber using a standard radon source (PYLON, Canada). The SRM has been calibrated by using standard radon monitor, Alpha Guard (AG) [Genitron Instruments GmbH, Germany]. During the experiment, the collection of ²¹⁸ Po activity was allowed to saturate for 1 h at each stable radon concentration. The equilibrium counts in channel A ( ²¹⁸ Po) were recorded for 15 min, and the corresponding sensitivity was calculated by utilizing the radon concentration value from SRM in the range of 9.2 kBq/m ³ -60 kBq/m ³ .

Experimental arrangement similar to that given in [Figure 5] was used for the calibration of the system with thoron. Activity from a standard thoron source (Pylon Electronics Inc., Ottawa, Canada, Model TH-1025, Source activity of 117.1 kBq as on 8 November 1996) was simultaneously passed through the present system and a standard thoron monitor. A humidity independent Scintillation cell-based Thoron Monitor, ^[18] which was initially cross calibrated against standard thoron detector; RAD7 was used for the purpose. The ²¹⁶ Po counts in the preset SCA channel (channel B) was recorded for 5 min time interval for various thoron concentration of air. The RH level was maintained <10% and temperature around 27°C throughout the experiment.

The SCA count rate in counts per hour from channels A and B were plotted against the corresponding ²²² Rn and ²²⁰ Rn concentration. The calibration plots of the system for radon, as well as thoron, are shown in [Figure 7]. The system showed sensitivity of 0.408 CPH/(Bq/m ³ ) and 0.169 CPH/Bq/m ³ for radon thoron, respectively. The linear dependence of the PIN diode response as a function of the activity concentration has been observed in the regions of ²¹⁸ Po and ²¹⁶ Po peaks. This linearity allows the use of this system for ²²² Rn and ²²⁰ Rn concentration measurements. For one hour counting time, the minimum detection level (MDL) of radon concentration is calculated as 3.5 Bq/m ³ and for thoron the corresponding value is found to be 8.4 Bq/m ³ .

Dependence of sensitivity on relative humidity

The performance and characteristics of electrostatic collection type ²²² Rn/ ²²⁰ Rn detectors are solely dependent on the charge state of the progeny atoms. The charge state and electrostatic collection sensitivity of ^222/220 Rn daughter products are highly dependent on the prevailing RH level inside the collection cell. ^[19] Different sets of experiments were carried out to study and establish the dependence of RH on ²²² Rn/ ²²⁰ Rn detection sensitivity of the system. As a part of these experiments, the radon sensitivity values were recorded for RH levels 9-10%, 30-31% and ~ 79%. The Stable RH level of the sample air stream was maintained by passing the sample through a dryer unit or water bubbler depending upon the requirement. [Table 1] gives the radon detection sensitivity of the system at various RH levels. The sensitivity varied from 0.41 to 0.19 CPH/(Bq/m ³ ) for a humidity variation from 9% to 79%. In a similar manner, the thoron detection sensitivity variations for RH levels 4-8%, 53-54% and 88-90% were recorded. [Table 2] gives the thoron detection sensitivity for various stabilized RH levels. It was observed that this sensitivity varied from 2.86 CPM/(kBq/m ³ ) to 1.62 CPM/(kBq/m ³ ) for a variation in RH in the range 4-90%.

Table 1: The radon detection sensitivity for various stabilized RH levels Click here to view

²²² Rn/ ²²⁰ Rn progeny alpha spectra

The alpha spectra of radon and thoron daughter products were acquired separately with pure radon and thoron sources. The 6 MeV and 7.687 MeV alpha peaks corresponding to ²¹⁸ Po and ²¹⁴ Po are shown in the bottom slide of [Figure 8]. From the figure, it is seen that the alpha energies are well-resolved. The observed full width at half maximum is 150 keV at ²¹⁸ Po alpha line. The middle slide is the spectra acquired during the sampling of thoron gas and the peak corresponds to the 6.78 MeV alpha particles of ²¹⁶ Po whereas the top slide of the figure gives the spectra acquired after some delay of thoron sampling and the peaks are represented by 6.051 MeV and 8.785 MeV alpha energies of ²¹² Bi and ²¹² Po, respectively. For thoron, all the three alpha peaks are well-separated, and collection of these alpha particles in separate SCA windows is possible. For radon also, the peaks corresponding to the two alpha energies are distinct and can be collected in two SCA channels. But the alpha lines corresponding to ²¹⁸ Po (6 MeV) of radon and ²¹² Bi (6.051 MeV) of thoron coincide to form a single peak while both radon and thoron are present. [Figure 9] represents the combined spectra of alpha-emitting radon and thoron progeny atoms. It is observed that there are only four peaks; the ²¹² Bi and ²¹⁸ Po alphas combine to make a single peak and other alpha particles corresponding to ²¹⁶ Po, ²¹⁴ Po, and ²¹² Po are well-separated. These alpha pulses can be counted at different pulse height windows. Therefore, while measuring radon in presence of thoron or vice versa, proper care must be taken to compensate the contribution in counts due to the coexistence of other gas. In the case of radon measurement the ²¹⁸ Po counts were utilized to report the activity. The history of using the instrument for thoron measurement can influence the counts in channel A. It is due the presence of ²¹² Bi (half-life 60.6 min) on the detector surface which can remain for days due to the long half-life (10.6 h) of its parent ²¹² Pb. Proper correction needs to be incorporated for channel A counts to account for the presence of this old thoron.

A closer analysis of [Figure 8] reveals that few counts from lower channels of ²¹⁶ Po can contribute in ²¹⁸ Po window, and a similar contribution from ²¹⁴ Po is also observed in ²¹⁶ Po window. The contribution of ²¹² Po in ²¹⁶ Po channel and ²¹⁴ Po in ²¹⁸ Po channel are minimal due to large alpha energy separation and can be ignored. A detailed study has been conducted to establish the contribution due to this tail counts in the lower energy alpha line of the spectra. The results of this experimental study on radon/thoron cross talk are described in the following section.

Radon/thoron cross-interference

It is necessary to determine the ²²⁰ Rn contribution in ²²² Rn response and vice versa by means of an exposure test, before practical use of the system. This is done to evaluate the reliability of measured values of radon and thoron concentrations. Even though the system is based on alpha spectroscopy method, certain contribution of counts is observed in the channel of interest from higher alpha energies. Therefore, proper quantification of this inter channel crosstalk is a prerequisite for these instruments. A systematic investigation was carried out to introduce proper corrections in the reported radon or thoron concentrations by compensating the effect due to such contributions.

Radon interference in thoron channel

The contribution of alpha counts in thoron window (channel B) due to the coexistence of radon gas during thoron sampling has been investigated. For this, air containing only radon activity (about 56 kBq/m ³ ) was sampled by using the system for about 3 h till equilibrium counts were reached in channels A and C. Then data acquisition was started and after 1 h, the source was removed from the system by disconnecting the stream from the radon source chamber. The counts in channel A, B, C, and D were recorded continuously at 15 min interval over a period of 5 h. [Figure 10] shows the counts profile in various alpha channels during radon sampling. The ²¹⁸ Po counts were reduced to <3% of the original in 1 h after source removal whereas ²¹⁴ Po counts came down to 66% during this period. This can induce an error while monitoring fast varying radon concentration. Due to this reason it was decided not to use the counts in channel C for reporting radon concentration and this instrument will be operated with a cycle time of 15 min using sensitivity factor obtained for counts in ²¹⁸ Po channel. A uniform contribution of counts was observed in channel B due to the lower channel contributions from the alpha particles (7.687 MeV) emitted by ²¹⁴ Po. It is observed that on an average 14% of the channel C counts are spilled into channel B, and it clearly followed after radon source removal also.

Figure 10: Counts profile of channels A, B, C, and D during radon sampling Click here to view

Therefore, a proper correction needs to be incorporated to compensate this crosstalk in the counts of channel B by subtracting 14% of channel C counts. No counts were observed in channel D as expected since alpha particles with higher energy are not present due to the absence of thoron in the sample stream.

The thoron concentration is reported in Bq/m ³ using the formula:

Where T _n is thoron concentration and B, C are counts in channels B and C respectively for 15 min.

Thoron interference in radon channel

Thoron has a half-life of 55.6 s. It emits a 6.29 MeV alpha particle and transforms to ²¹⁶ Po which in turn has a half-life of 150 ms before emitting a 6.78 MeV alpha particle and transforming to ²¹² Pb. Lead-212 has a comparatively long half-life of 10.6 h. It transforms by beta decay to ²¹² Bi, which, in turn, has a half-life of 60.6 min. Bismuth-212 has approximately 2:1 split, with two-thirds transforming by beta decay to ²¹² Po and one-third transforming by 6.05 MeV alpha decay to ²⁰⁸ Tl. The ²¹² Po decays immediately to ²⁰⁸ Pb, emitting an 8.78 MeV alpha particle in the process, while the ²⁰⁸ Tl, with a half-life of 3 min, undergoes beta decay to the same destination, ²⁰⁸ Pb.

The contribution of alpha counts due to the presence of thoron in radon channel has been investigated separately. As mentioned earlier, this contribution in channel A is mainly due to the presence of ²¹² Bi whose alpha energy nearly coincides with that of ²¹⁸ Po. For this study, thoron concentration (about 21.0 kBq/m ³ ) has been sampled through the collection cell for 2 h. Then the source was removed, and the electrostatically collected ²¹⁶ Po atoms were allowed to decay. The counts in channels A, B, C, and D were recorded for a period of 5-day to follow the ²¹² Po counts till it reaches background level. [Figure 11] gives the counts profile of channels A, B, C, and D after sampling high concentration of pure thoron. The profile indicates clearly that the counts in channel A have a specific dependence on counts in channel D. Therefore, the counts due to ²¹² Po can be utilized to compensate the contribution of ²¹² Bi in ²¹⁸ Po channel. After the 2 ^nd h count, the ²¹⁶ Po activity (channel B) instantly came down whereas channel A and channel D counts belonging to ²¹² Bi and ²¹² Po, respectively, initially grew to a maximum in 3 h and then decayed following mainly the half-life of ²¹² Pb. During the decay of ²¹² Bi atoms, a fraction of 0.36 is alpha emission, and 0.64 is beta emission to form ²⁰⁸ Tl and ²¹² Po, respectively. In this way, the ratio of number of ²¹² Bi alpha to ²¹² Po alpha particles is 0.56 which should have been equal to the ratio of counts in channel A to channel D. But the average value was found to be 0.70. This is mainly because of the reduced counts in channel D due to the tail of alpha spectrum and a slight increase in counts of channel A due to ²¹⁶ Po tail counts and ²¹⁸ Po coming from the possible background radon activity present in the air inside calibration chamber. Thus, a correction can be incorporated to the counts in channel A, knowing the counts value in channel D. Therefore, the radon concentration in Bq/m ³ is reported using the formula:

Where R _n is the radon concentration in Bq/m ³ , A and D are counts in channel A and D respectively for 15 min.

Inter-comparison

After development and characterization of the instrument for the continuous monitoring of ²²² Rn and ²²⁰ Rn, it was subjected to a comparison test with standard equipments. This inter-comparison experiment gave confidence in utilizing the presently developed system in various radon/thoron measurement applications. Results of these experiments are given in the following subsections.

Thoron measurement

Subsequent to the calibration of the system, simultaneous measurement of thoron concentration was carried out to get an inter-comparison between standard thoron monitoring instrument, RAD7 and the present system. Time-correlated readings from both the systems in the range 5-65 kBq/m ³ were plotted. [Figure 12] shows the correlation between the measured values of thoron concentration by RAD7 and PIN diode-based radon/thoron monitor. A good one to one correlation was observed with slope 0.97. The variation in observed radon concentration from that of RAD7 was found to be within the statistical limit of acceptability.

Figure 11: Counts profile of channels A, B, C, and D after thoron sampling Click here to view

Figure 12: Comparison of thoron concentration simultaneously measured using the PIN diode-based radon/thoron monitoring system and RAD7 Click here to view

Radon measurement

For the comparison of observations obtained from PIN diode system, the standard radon measurement instrument, AG was utilized in an experiment of simultaneous measurement of radon concentration inside a radon chamber. Radon concentration inside the closed chamber with sampling ports was maintained by keeping a radium source. The AG was kept inside the chamber. Dehumidified air was circulated through the PIN diode system from the chamber. The AG was operated at a sample cycle time of 60 min. Same cycle time was selected for the present PIN diode-based system also.

The measured values of radon concentration from both the instruments were noted for initial 9 h at a concentration of about 1.7 kBq/m ³ and then the concentration was allowed to build up overnight to about 2.8 kBq/m ³ . After noting down four hourly readings at this concentration, the AG was taken out of the chamber, and the sampling for PIN diode system was carried out from the room air. The time correlated radon concentration values from these measurement units were plotted. [Figure 13] shows the results of this inter-comparison experiment performed in our laboratory.

It was observed that the response of PIN diode-based radon monitoring system closely matched with that of AG. The variation in observed radon concentration from that of AG was within ± 10% during normal operation. However, when the source was removed, and sampling was continued from normal atmospheric air, there was a mismatch in response during this transition. Ideally both the systems should have shown atmospheric concentration of ~ 20 Bq/m ³ after source removal. The response of PIN diode-based system was more realistic and responded to steep concentration gradient faster than AG. It is observed that the room radon activity concentrations measured using AG and the present system were matching.

Figure 13: Comparison of radon concentration simultaneously measured using the PIN diode-based radon/thoron monitoring system and Alpha Guard Click here to view

Conclusion

A low-cost, real-time, continuous radon/thoron measurement system has been developed using a silicon PIN photodiode. The response of the system for various electrostatic collection voltages was investigated for both radon and thoron to get an optimized sensitivity with applied collection HV. The instrument showed sensitivity of 0.408 CPH/(Bq/m ³ ) and 0.169 CPH/Bq/m ³ for radon thoron respectively. The RH of the sampled air is to be maintained less than 10%. It can be used for laboratory and field determination of radon and thoron concentrations simultaneously. The methodology of utilizing alpha spectroscopy makes the systems more reliable in comparison with other popular radon monitors. This cost effective system would be most suitable for usage in areas where the number of systems required would be more. Furthermore, this monitor can be used even in high radon/thoron environment, and is, therefore, useful for the investigation of high concentrations in dwellings or fields which were found in some screening surveys.

Acknowledgments

The authors express their gratitude to Dr. D.N. Sharma, Director Health Safety and Environment Group, Bhabha Atomic research Centre, for his encouragement and support in this study. We thank Dr. K.S. Pradeepkumar for his interest in this study. The authors are thankful to Shri D.A.R. Babu and Dr. Anita Topkar for their help in this study. Thanks are also due to Shubhangi Wani, C.M Gaikwad, C Andrews and members of RSSD workshop for their support and participation during the progress of the work.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]
  [Table 1], [Table 2]

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Source: https://rpe.org.in/article.asp?issn=0972-0464;year=2014;volume=37;issue=2;spage=80;epage=88;aulast=Kumar;type=3

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