Radon Detection

Instantaneous Methods
Continuous Methods
Commercial Radon Detectors

1. Determining Radon Concentrations
2. Determining Radon Daughter Concentrations

Determining Radon Concentrations

• Grab-sampling

  The air to be studied is collected in a 5-15 liter container and sent to a laboratory for study. By passing the sample through a charcoal trap submerged in liquid Nitrogen, it can be concentrated for easier analysis. At this low temperature, radon is retained on the charcoal while the air passes through. By warming the trap in the presence of Helium, the radon can be transferred to a Lucas counting cell or ionization chamber.

• Alpha-particle scintillation counting with liquid scintillators

  In this method, zinc oxide is replaced with a liquid scintillator. Usually the radon-bearing air is passed through an organic solvent in which radon is highly soluble at low temperatures. The mixture is introduced into a vial containing a liquid scintillator, where each radon-222 decay is followed by two alpha particles and two beta particles. Each decay can be detected by light signals.

• Lucas Cells: Alpha-particle scintillation counting with ZnS

  Developed in the late 50's, this technique is one of the oldest methods of determining radon concentration. The gas sample is placed in a counting cell whose walls are covered with zinc sulfide (ZnS). A photomultiplier tube is also included, and when an alpha particle strikes the cell wall, a flash of light is emitted from the ZnS coating. The light is detected by the photomultiplier and converted into an electric signal. For optimal performance, a cell with a capacity of three liters has an accuracy of %11, which is the lowest yet for Lucas cells. 20

  Internal Ionization chamber counters

  In these systems, an electrical signal is produced by individual decay events instead of from scintillation counting. This eliminates the intermediary of scintillation counting, however the higher cost of using ionization chambers has minimized any advantage of this method over Lucas cells.

  Two Filter Method

  For measurement of both radon daughter and radon concentration, the two filter method is used. Air passes through the first filter where radon progeny are removed. The air continues through to the decay chamber, where daughter products are collected on a second filter. The concentration of radon (on the first filter) versus radon progeny (on the second filter) can be counted separately. Variations on this method include the use of alpha spectroscopy, scintillation counting, and a passive detector.

Determining Radon Daughter Concentrations

• Single time period alpha particle scintillation counting

  This simple approach used in Uranium mines counts the alpha particle radioactivity deposited on the filter using a scintillation counter. Using a five minute sampling period and a flow rate of two liters per minute, the sample is allowed to decay for 40 minutes before counting the total number of alpha particles for two minutes.

• Several time period alpha particle scintillation counting

  Theoretically, the three radon daughter concentrations can be individually established if counts are taken during more than one time period. Since we know that the half-lives of the three daughters are different, the relative number of counts in three time periods determines their individual concentrations. Employing a computer can increase the number of short periods counted for greater accuracy.

Continuous Methods

Etched Track Detectors

Perhaps the most common method of radon detection, the etched-track detector operates on the principle that a heavy atomic particle such as an alpha particle leaves a microscopic track of damage when passing through certain plastics. Chemical etching is used to enlarge the tracks, which are counted under a microscope. The radon concentration is proportional to the number of tracks per unit area.

The Electret-Passive Environment Radon Monitor

Recently developed, the E-PERM monitor employs an electrostatically charged plastic disc known as an electret. The positively charged electret is mounted in a canister isolated from particulates by a filter. Radon passes through the filter and induces a negative charge in the air. The negative ions are attracted to the electret surface and a voltage is imparted. This measured voltage is proportional to the radon concentration, and can be corrected for exposure duration and other factors.

Thermoluminescent detectors

Thermoluminescent detectors are based on the principle that atomic or molecular disturbances caused by ionizing radiation can cause a material to emit light when heated. The TLD chip is exposed to the alpha and beta particles and the gamma rays that are emitted by radon and its decaying progeny. In the RPISU (radon progeny integration sampling unit), air passes through a filter, and a TLD chip is placed near the filter surface to integrate radon dose. Over a week-long period at a air flow rate of one liter per minute, levels of radon as low as 0.2 pCi/L can be detected.

Charcoal adsorption detectors.

Radon, like many other gases, can be adsorbed onto charcoal. By counting the gamma ray activity of a bed of charcoal that has been exposed to radon gas, the radon concentrations can be measured over a maximum of one week. Usually, a 1.5 cm deep bed of charcoal is placed on a commercial metal ointment can, which is one inch high and 1.5 inches in radius. Radon enters through a 3/4 inch diameter hole in the top of the can, and a dessicant is included to eliminate water vapor. After sample collection, a NaI gamma ray detection system is used to calculate the 295 keV and 352 keV gamma rays that decay from lead-214. The 609 keV gamma ray from bismuth-214 is also counted.

Measuring Radon in Water

Liquid Scintillation Counters

Radon concentrations in water are typically higher than in air, making liquid scintillation a practical method of radon detection if radon concentrations are high enough (>1000pCi/L). The water sample can be mixed with the counting material and counted using conventional liquid scintillation counters used for air samples.

Gas extraction

For measuring smaller concentrations, the radon is extracted as a gas and the emitted alpha particles counted in a ZnS scintillation cell. Helium is bubbled through the water sample, and the resulting Helium-radon gas mixture is passed through a cold trap, and a Lucas counting cell is employed to calculate concentration.

Direct gamma ray counting

With relatively high concentrations (>500 pCi/L), the gamma rays can be counted using gamma ray spectroscopy techniques with a Ge(Li) detector. The original radon concentration can be distinguished from the radium-226 concentration by repeating the count after 30 days. During the measurement lag time, all of the original radon will have decayed, leaving only the remaining radon that is in equilibrium with radium-226.

Commercially Available Radon Detectors

adapted from Consumer Reports, July 1995, p. 465.

Product	Price	Test Time	Results	Phone Number
Short-Term Detectors (within %48 of actual levels)	$	(days)	(days)
Air Chek	10	4-7	9-13	800 247-2435
First Alert	18	2-3	8-11	800 323-9005
RTCA	25	2-5	4-13	800 457-2366
Home Diagnostics	28	7	9-14	212 308-1222
Teledyne	30	4	5-10	800 666-0222
Long-Term Detectors (within %166 of actual levels)
Key-Trac-Kit	19	90	22-29	800 523-4964
RadTrak	25	90	16-23	708 755-7911

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