Radon alpha counts

Gross alpha and gross beta activity can be determined by various radioactive counters, such as internal proportional, alpha scintillation, and Geiger counters. Radium in water can be measured by co-precipitating with barium sulfate followed by counting alpha particles. Radium-226 can be measured from alpha counting of radon-222. Various methods are well documented (APHA, AWWA, and WEF 1998. Standard Methods for the Examination of Water and Wastewater, 20 ed. Washington DC American Public Health Association). 

Zikovsky, L. and N. Roireau. 1990. Determination of radon in water by argon purging and alpha counting with a proportional counter. Appl. Radiat. Isot. 41 679-681. 

Several techniques to measure air concentrations are outlined by Breslin (1980). Most of the techniques for measuring radon use the fact that both radon-222 and the short-lived daughters are alpha- emitting nuclides. The sample is collected and taken back to the laboratory for "alpha-counting" or an alpha-detector is taken to the field for on-site measurement. There are several ways to measure alpha decay. A scintillation flask is one of the oldest and most commonly used methods. The flask is equipped with valves which are lined with a phosphor (silver-activated zinc sulfide) and emit light flashes when bombarded with alpha particles. Other methods draw the air through a filter (or filters) for a variety of time intervals and then count the number of alpha-decays occurring on the filter. EPA (1986) and NCRP (1988) reports provide more in-depth discussions of these methods. 

The variations in the background, the sensitivity to moisture, the alpha activity of the chamber itself and the influence of recombination were discussed by Hultqvist. The standard deviation due to counting statistics was estimated to be about 3 % (in a few measurements 6 %). The calibration was made by counting each alpha particle by a proportional counter specially designed at the Department for this purpose. The statistical uncertainty of the calibration of the equivalent radon concentration was estimated to be 12 %. 

During the transport in the chamber, radon atoms decay to form RaA atoms. The RaA atoms except those diffused to the wall of the chamber are collected on the exit filter. The sample collected on the exit filter is removed and counted. The present method is able to simultaneously measure radon and thoron concentrations by alpha spectroscopic technique (Ikebe et al., 1979). 

The radon in the air i/as measured continuously by electroprecipitation of the positively charged Po-218 ions in an electric field (10 kV) on a surface barrier detector (Porstendorfer, et al., 1980). For this purpose the air i/as dried, filtered and sucked into an aluminium sphere ( 2 1) with a flowrate of 0.5 lmin-1. The counts due to Po-218 and Po-214 were proportional to the radon activity concentration. Their disintegrations were directly detected by alpha spectroscopy with an energy resolution of about 80 keV. The monitor could detect down to 5 Bq m 3 with a two hour counting time and 30 % statistical accuracy. 

Each serie of measurements consisted of two parallel samples with counting during and after sampling, one with the screen diffusion battery and the second as the reference sample, so that the fractional free radon daughters could be calculated. The radon daughters are collected on a membrane filter (filter diameter 25 mm, pore diameter 1.2 ym) and the decays of Po-218 and Po-214 are counted by means of alpha spectrometry with a surface barrier detector (area 300 mn ). 

Nazaroff, W.W., Optimizing the total-alpha three-count technique for measuring concentrations of radon progeny in residences, Health Phvs. 46 395-405 (1984)... 

Radon-222 may be transported with a carrier gas into an ionization chamber and its alpha particles counted. Short-lived isotopes in a carrier gas stream are measured this way using a flow-type ionization chamber. 

An alternative, for low activities of 222Rn, is to count individual alpha pulses in the ionisation chamber. Kraner et al. (1964) used this method to measure exhalation of 222Rn from the soil. To obtain maximum sensitivity, radon from a large volume of air is adsorbed in activated charcoal, and transferred to an alpha-pulse ionisation chamber in a flow of inert gas. 

Air with radon is passed into a vessel coated internally with zinc sulphide. Alpha particles from radon in the chamber, and from decay products deposited on the walls, give scintillations which are counted by photomultiplier tubes viewing the chamber through windows (Lucas, 1957). With a chamber of volume 0.11, and a counting time of 1 h the detection limit of 222Rn in air was about 10 Bq m-3, but by concentrat-... 

In a passive detector developed by the National Radiological Protection Board (Wrixon et al., 1988), the etched pits in the detectors are filled with scintillator fluid. After exposure to radon, the detector is irradiated with an alpha source, and the resulting scintillations counted with a photo-multiplier tube. In this way, track density over 1 cm2 of detector can be measured in a few seconds. Passive detectors used in the UK National Survey were sensitive down to 20 kBq m-3 h of accumulated exposure, equivalent to a radon concentration of 5 Bq m-3 measured over 4000 h exposure. 

Lucas, H.F. (1957) Improved low-level alpha-scintillation counts for radon. Review of Scientific Instruments, 28, 680-3. 

A typical scintillation cell is shown schematically in Fig. 9.23. It consists of a hemispherical or right circular cylinder (George, 1976 Lucas, 1957). The walls of the chamber are optically clear, and are coated with ZnS (Ag) scintillator powder. Air is introduced into the chamber passively (after evacuation of the chamber with a pump, and opening to atmosphere) or by means of a pump. Inside the chamber, the Rn decays into the RnD. The alpha particles from the decay causes the scintillation powder on the walls to scintillate. Four hours after sampling, the radon is in equilibrium with its daughters. The container is then placed on a photomultiplier tube and the scintillation pulses (light pulses) from the ZnS layer are counted with associated electronics. With calibration, the number of light pulses can be related to the concentration of Rn in the sampled air. 

Details of the analytical techniques used to determine the activities of the radionuclides employed are given by Turekian et al. (1973). Briefly, 5 gm samples of peat ash were leached in hot 6 N HCl and the leachate analyzed for Ra and Pb. Radium-226 was determined by recovery of Rn gas produced by decay of parent Ra in solution within a closed vessel for at least ten days, followed by alpha scintillation counting of the separated radon. The procedure was repeated at least twice. Blanks, estimated by extraction without sample in the recycle loop, were small relative to sample activity. 

Another widely used method is solid state nuclear track detection. In the case of radon, an alpha track detector is used. It consists of a small piece of plastic enclosed in a container with a filter-covered opening. Alpha particles in the air strike the plastic and produce submicroscopic damage tracks. At the end of the measurement period the plastic is placed in a caustic solution that accentuates the damage tracks. The tracks are then counted using a microscope or automated counting system. 

Various methods may be used to determine the concentration of radon in the air. The choice depends on the available measurement, direct or subsequent reading and of the sampling time of the equipment performing the measurements. Generally, measuring the concentration of radon in a particular environment is based on the count of alpha particles emitted by radon and its short life descendants. 

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