The New ANSTO Radon Detector at MLO

S. WHITTLESTONE

ANSTO, PMB 1, Menai, NSW 2234, Australia

INTRODUCTION

After 6 years, many of the questions posed at the commencement of the Australian Nuclear Science and Technology Organization (ANSTO) CMDL radon program at the Mauna Loa Observatory, Hawaii (MLO) have been answered. By 1994 the radon detector was obsolete. It is, therefore, timely to review the output from the program and describe a new radon detector designed to meet the needs of the future.

The program commenced in June 1989. Its aim was to assess the value of radon as a tracer for air masses which have been subject to recent contact with land and, therefore, not representative of the global baseline. By the end of 1991 it was evident that radon is indeed the best readily-measured indicator of perturbation of air masses by contact with land beyond Hawaii [Whittlestone et al., 1992]. Because it is chemically inert, radon concentration is unaffected by the complex reactions affecting most other atmospheric species during transport. Samples of trace gases sampled in the baseline wind sector are strongly correlated with radon concentrations (Figure 1). For the first time there was a quantitative continuous measure of the influence of the Asian continent on air samples at MLO when jet streams transport air from Asia in only a few days [Kritz, 1990].

Fig. 1. Dependence of trace gas concentrations on radon concentration in the baseline wind sector at MLO.

Radon has been useful in the study of dry deposition because its decay products are very small and highly reactive.  Radon acts as a uniform source of ultrafine reactive particles that have a short half life and so indicates deposition only over a limited area.  Measurements at sea level at Cape Grim, Australia, indicated very strong deposition in the lower marine boundary layer.  Studies at MLO showed higher concentrations of ultrafine radon decay products were present at the altitude of MLO but that they were subject to strong deposition close to MLO in certain weather conditions [Schery and Whittlestone, 1995; Schery et al., 1992]. 

Several other studies have made use of radon.  For example it was valuable in validation of use of air mass trajectory cluster analysis of variations of methane at MLO [Harris et al., 1992].  Data were provided for MLOPEX-11 and were part of an evaluation of Pb-212 as a tracer for local ground contact [Whittlestone et al., 1996a, b]. 

Radon has proved to be useful to other programs at MLO.  However, the detector required a degree of skilled technical supervision that was not sustainable in the long term.  It was desirable to be able to obtain radon measurements with no more effort than wind speed and direction.  To meet this need, a new radon detector design was incorporated into the MLO detector.  This design, although it was markedly simpler and more rugged than the previous one, provided improved time response, better sensitivity, lower power consumption, and freedom from routine maintenance.

Because of the simplicity of the new instrument, it is possible to obtain high-quality preliminary data in real time.  Steps have been taken to make the radon data readily available from the CMDL computer. 

The Radon Detector

Figure 2 is a schematic diagram of the detector.  It is an implementation of the design described in Whittlestone et al. [1996a] for MLO.  Table 1 gives the basic specifications.  As with most high sensitivity radon detectors, this design uses the two-filter principle.  The first filter removes all radon decay products from the inlet air.  This air moves steadily through the delay chamber and while there, a portion of the radon decays.  The second filter, in this case a wire screen, collects as many of the decay products as possible.  These decay products in turn decay, and the alpha radiation is detected by the zinc sulfide screen and photomultiplier.  Two key features of the new design are use of a wire screen for the second filter and separation of the external flow which changes the air sample from the internal flow which acts to draw radon decay products produced inside the chamber onto the screen.

Fig. 2. Schematic diagram of the ANSTO radon detector at MLO.

TABLE 1. Specifications of the ANSTO Radon Detector at MLO

It is possible to use a wire screen as the second filter because the decay products form ion clusters about 1 nm in diameter which diffuse so rapidly that they are trapped very easily. In this detector 70% of the decay products are collected on the screen even though the flow rate is 800 L min^-1. The high flow rate is necessary to minimize loss of decay products through the walls of the chamber and to allow measurement of the first decay product, Po-218, whose half life is only 3 minutes.

The major benefit of using the wire screen is a reduction in power compared to that required by a conventional filter. Only 20 W are needed, compared to about 500 W for the filter. This translates to a reduction in power costs over a year from $440 to $18 at 10 cents per kW hour.

As with the use of a wire screen, the second design feature, separation of the sampling from the collection flows, results in economies in design. An 800-L min^-1 flow would need 100 mm air lines and a large inlet filter. It is desirable to include a delay of about 4 minutes to allow any thoron gas in the inlet to decay. At 800 L min^-1 this would need a capacity of at least 3200 L. Since the only requirement of the external flow is to achieve an average residence time of air in the chamber of 20 minutes, 100 L min^-1 is sufficient, and 20 mm air lines are adequate.

It can be seen from Figure 2 that the only moving parts are the blowers. There are no components that require regular maintenance or that are sensitive to temperatures over the range -10 to 40°C. Only a few components would need to be adapted for arctic use.

DATA ACQUISITION

Because of the simplicity of the detector and its insensitivity to environmental conditions, passive data logging is all that is required. A few parameters should be monitored, such as supply volts and flow rates. Since installation in February 1996, data are averaged over 30-minute intervals by a data logger. A computer downloads data automatically at preset intervals. At the time of preparation of this paper, it was not possible to operate a DOS-based download program from the pseudo-DOS accessible from Microsoft windows. The serial port was unreliable. As a result, data transfer to the computer network has to be done manually. It is hoped this incompatibility will be solved so that preliminary data can be viewed on the Internet in close to real time. Parameters needed for data quality assessment are included in the data set permitting largely automatic data editing and prompt review for general release.

CONCLUSION

Radon has been established as a useful tracer for tagging air recently in contact with land. In the case of air from Asia, it is the only species transported without loss by wash-out or chemical change which can indicate such contact. New instrumentation at MLO is inherently robust and simple to operate. Up-to-date preliminary data are available and a path established which should see processed data on the CMDL computer system within a few months of acquisition.

Acknowledgment. This work has been made possible by the active participation of the staff of MLO.

REFERENCES

Harris, J.M., P.P., Tans, E.J., Dlugokencky, K.A. Masarie, P.M. Lang, L.P. Steele, and S. Whittlestone, Variations in methane at Mauna Loa Observatory related to long range transport, J. Geophys Res., 97(D5), 6003-6010, 1992.

Kritz M. A., The China Clipper-fast advective transport of radon-rich air from the Asian boundary layer to the upper troposphere near California, Tellus, 42B, 46-61, 1990.

Schery, S.D., and S. Whittlestone, Evidence of high deposition of ultrafine particles at Mauna Loa Observatory, Atmos. Environ., 29(22), 3319-3324, 1995

Schery, S.D., R. Wang, K. Eak, and S. Whittlestone, New models for radon progeny near the earth's surface, J. Radiat. Prot. Dosim., 45, 343-347, 1992

Whittlestone, S., E. Robinson, and S. Ryan, Radon at the Mauna Loa Observatory: Transport from distant continents, Atmos. Environ., 26A(2), 251-260, 1992.

Whittlestone S., W. Zahorowski, and P. Wasiolek, High sensitivity two filter radon/thoron detectors deploying a wire or nylon screen as the second filter, ANSTO E718, 1994

Whittlestone, S., S.D. Schery, and Y. Li, Thoron and radon fluxes from the island of Hawaii, J. Geophys Res., 101(D9), 14,787-14,794, 1996a.

Whittlestone, S., S. D. Schery, and Y. Li, Pb-212 as a tracer for local influence on air samples at Mauna Loa Observatory, Hawaii, J. Geophys Res., in press, 1996b.

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