SAASTE Science/Radon Measurement

SAASTE Science

Scientific Method - Guidelines for Scientific Investigations - Ideas for Class Projects - Radon Measurement - Road Markings

MEASUREMENT OF INDOOR RADON IN PAARL

SCIENCE PROCESS SKILLS PROJECT'

REASON FOR STUDY

Paarl is situated about 60 km from Cape Town and is built on the slopes of a hill consisting granite rock. Granite rock typically has a higher than average uranium content. Radon is a colourless, tasteless and odourless gas that is a decay product in the uranium decay chain in soil and rock . Radon diffuses easily through cracks in walls and floors and may therefore accumulate in homes with poor ventilation. Radon gas has been found to be a major cause of lung cancer and contribute to at least 15 000 deaths in the United States annually. Radon measurements have been performed previously in Paarl and elevated levels were found in some older homes 1. It was therefore proposed that further measurements are needed for a broader study of radon gas concentrations in soil and inside homes in Paarl.

HYPOTHESIS

The Radon concentrations in houses and soil in areas with high granite deposits are higher than in houses and soil in areas with low granite deposits.

RADON

What is Radon?

Radon is a naturally occurring radioactive gas that is produced in the uranium decay chain. Radium is a product of the radioactive decay of uranium. Radon is then formed during the decay of radium. Radioactive decay is a natural, spontaneous process in which an atom of one element breaks down to form another element by losing atomic particles. When solid radium decays to form radon gas it loses two protons and two neutrons. The two protons and two neutrons are called an alpha particle. An alpha particle is identical to the nuclei of helium-4 atoms. Radon itself eventually loses an alpha particle to form the element polonium.

Radon is considered a noble gas and is thus very reluctant to react with other elements. Radon is not detectable through human senses; the only way to determine radon occurrence is by measurement.

The rate of radioactive decay is measured in terms of half-life, the time it takes for half of a sample of an element to decay. While uranium has a half-life of 4.4 billion years, the half-life of the most common isotope of radon is only 3.8 days. This means that 50 % of a given volume of radon will decay in 3.8 days to form decay products like polonium, bismuth and lead. When we inhale high concentrations of radon, the gas molecules may decay to the above mentioned elements, which may stick to the epithelial cells of the lung. The presence of these elements may eventually cause lung cancer.

Radon formation

All rocks contain some uranium-most of them in the region of 1 to 3 parts per million (ppm) of uranium. Granite rocks, as volcanic rocks, dark shale and certain sedimentary rocks, have higher than average uranium content, as much as 100 ppm. The higher the uranium content in rocks and soil in an area, the greater the chances of higher radon levels indoors.

Radon movement and diffusion

Radon is released from mineral grain via alpha recoil. As each radium atom decays by ejecting an alpha particle, the newly formed radon atom recoils in the opposite direction. By recoiling the radon atom may then enter the pore space between grains in a fractured rock. The radon atom could also recoil towards the interior of the grain and not enter the pore space.

Because radon is a gas it moves more easily than radium that is fixed in rocks and soil. Radon easily escapes into fractures in rocks and pore spaces between grains of soil. The ease of movement of radon affects how much radon may enter a home. Radons movement is dependent on porosity of soil, soil permeability and soil moisture content. Radon moves rapidly through permeable soils, such as sand and gravel, than through impermeable soils like clay. Fractures and pores allow radon to diffuse more quickly. In water, however, radon moves slower than in air.

For these reasons, homes in areas with drier, highly permeable soils and fractured bedrocks may have higher levels of indoor radon. The air pressure in the ground around most houses is said to be higher than the air pressure inside the house. Air tends to move through cracks in the foundation and walls of houses. Soil air may thus diffuse easily into houses with low indoor pressures, poorly sealed foundations and several entry points for soil air, leading to higher than normal radon levels.

LOOKING AT THE ACTUAL INVESTIGATION

Equipment used

There are a few methods to measure radon indoors, but this procedure explains the handling and usage of a standard (S) chamber E-PERM monitoring device. This device takes the measurement over a few days. To measure radon with this method, the equipment used were:

A SChamber E-PERM
An electret to fit the S Chamber
A special reader to read voltage on electret
A Mylar bag (A type of plastic bag)
Data Logging Sheet

Method of data collection

When an electret (ST) is fitted to a S-chamber, the E-PERM configuration is called a SSTE-PERM.

1. All electrets are covered by a protective cap when in storage. Unscrew the cap and place the electret face down in the slot provided on the electret reader to measure its Initial Voltage (I). Record the value on the data logging sheet provided.

2. Screw the electret into the bottom of the S-Chamber. Make sure not to touch the white surface of the electret.

3. When you are ready to start measuring, turn the E-PERM to the on position, by unscrewing the top screw cap.

4. Place the E-PERM at the location to be monitored. Make sure to read the instructions on where to place the detector or where not to in a later section in the supplement. Record the date and time of the placement.

5. After the exposure period (about seven days), close the cap of the E-PERM which returns it to its off position. Record the date and time on the logging sheet when the measurement was stopped.

6. Read the Final voltage (F) on the electret thereafter with the reader and record it.

7. The radon concentration is then calculated with a few equations.

footnote

[1] Lecture, Professor Robbie Lindsay, Department of Physics, University of the Western Cape

The uranium decay chain can thus be summed up as follows:

Data from the project

In xls format from SPSP-DATA & GRAPHS-Radon-WW.xls Note: We need to load this file up to a server, so people can download it --Riaan 07:14, 26 Jan 2005 (UTC)