Two-filter monitor for atmospheric 222Rn

145

110

Abstract. A new design of two-filter radon detector has been developed for measurement of extremely low levels of radon in the harsh environments on board ships and remote islands. These were needed for the First Aerosol Characterization (ACE 1) multiplatform experiment in the Southern Ocean. By employing an internal recirculation system and a wire mesh screen as the second filter it has been possible to reduce the power consumption by as much as a factor of 10 and the weight and cost by a factor of 2 compared to current designs of comparable sensitivity. A very high efficiency of 0.38 count Review of the Two-Filter Radon DetectorThe new radon detectors for shipboard use belong to the class of instrument known as two-filter detectors. The name is derived from the mode of operation: an air sample is drawn through one filter which removes all radon and thoron decay products ("daughters"), then through a delay chamber in which some daughters are produced. Finally, the air passes through a second filter which retains the daughters. The daughters on the second filter have been produced in controlled conditions, so their number is proportional to the radon and thoron concentrations.There are many variations of the two-filter detector. The simplest design has an easily removable second filter. After a prescribed sampling period, the filter is removed and placed in an alpha particle detector, based typically on a zinc sulphide scintillatot [Thomas and LeClare, 1970]. The instrument responds to both radon (222Rn, half-life 3.82 days) and thoron (222Rn, half-life 55.6 s). If thoron is present, it decays to 2•2pb which has a half-life of 10.64 hours and causes an unwanted background count in an instrument intended to detect radon. It is possible to lower the thoron background count to an acceptable level by delaying air in the inlet by a few minutes. Since thoron has a half-life of less than a minute, it decays before entering the main delay chamber.This simplest form of two-filter detector can be automated by adding a filter changing mechanism. Hutter et al. [1990] have done this for an application where there was effectively no thoron in the air and continuous operation was essential. Their detector retains the well-defined time resolution of the simple detector. Its sensitivity per unit delay chamber volume was

Section: Review Of the Two-filter Radon Detectormentioning

confidence: 99%

Section: Review Of the Two-filter Radon Detectormentioning

confidence: 99%

Baseline radon detectors for shipboard use: Development and deployment in the First Aerosol Characterization Experiment (ACE 1)

Whittlestone¹,

Zahorowski²

1998

145

110

“…Soil temperature measurements were made with probes buried beside the flowthrough accumulator. Rain measurements were taken at a station about a kilometer away and a separate continuous monitor [Schery et al, 1980] was available for measurement of airborne ::2Rn. Not all meteorological variables were necessarily meas'ured in any single experiment.…”

Section: Meteorological Measurementmentioning

confidence: 99%

Factors affecting exhalation of radon from a gravelly sandy loam

Schery¹,

Gaeddert²,

Wilkening³

1984

Self Cite

164

105

Measurements of radon exhalation from a gravely sandy loam have been made in a semi‐arid climate by using a combination of closed accumulation, flow‐through accumulation, and 222Rn and 210Pb soil profiles. The meteorological factors that most affected the instantaneous value of exhalation of 222Rn were atmospheric pressure and rain. Effects due to other parameters such as wind or temperature were either comparatively small or undetectable. No evidence was seen for migration of radon from distant (≫10m) sources or for an effect on exhalation due to limited nearby seismic activity. Measurements for 220Rn indicated its exhalation was also sensitive to pressure variation but to a lesser extent than for 222Rn. While instantaneous exhalation of 222Rn could easily vary by a factor of 2 or more, time‐averaged exhalation was found to be close to that expected for pure diffusion. There is thus some indication that the time‐averaged effect of cyclic environmental variables is small for this soil. Comparison with transport equations indicates that it is difficult to explain the observed variation in surface flux density solely in terms of the radon concentration gradient in the top few decimeters of soil. A contribution to transport from direct flow, perhaps through inhomogeneities such as cracks or channels, is one possible explanation.

“…Most radon measurements were carried out with one of two con•tinuous monitors, modified somewhat for the high radon levels of the tunnel section. 'The design of one continuously recording monitor is based on the two-filter principle and has been described by Schery et al [1980]. This monitor is somewhat bulky but is capable of high sensitivity and good time resolution (<15 min).…”

Section: Radon Measurementsmentioning

confidence: 99%

Transport of radon from fractured rock

Schery¹,

Gaeddert²,

Wilkening³

1982

Self Cite

The transport of 222Rn from fractured rock has been studied in an abandoned mine. Pressure‐induced flow (both natural and artificial) is quite important and can easily cause an order of magnitude increase in the instantaneous transport of radon into the tunnel airspace compared to that due to flowfree diffusion. Permeability studies indicate that large‐scale cracks of surface density of the order of several cracks per square meter dominate flow. In first order, effective flux due to natural pressure variation follows an approximate dP/dt dependence. In higher order, there exists an enhancement of time‐averaged flux by typically a factor of 2 due to natural pressure variation. Mathematical modeling indicates that the relation between pressure and the strength and time dependence of the radon transport is difficult to explain solely with conventional models for semi‐infinite homogeneous porous media. A model of cul‐de‐sac chambers is proposed to account for some of this dependence.