lecture note radon in geology

Radon in geology
Assistant Professor
Dr. Kamal K. Ali
University of Baghdad-College of
Science – Geology Dept.

The geology of radon
What is Radon?
Radon is a gas produced by the radioactive decay of
the element radium within the decay of U-238 series.
Half life = 3.825 d.
When solid radium decays to form radon gas, it loses
two protons and two neutrons. These two protons
and two neutrons are called an alpha particle, which
is a type of radiation.
Radon itself is radioactive because it also
decays, losing an alpha particle and forming the
element polonium.

Radon
• radon (222Rn): One of the main members of
natural radioactivity of the earth crust.
• It is a noble gas and having a relatively long
life-time (3.8) days.
• It has a great mobility to reach considerable
distances in different geological environments.
• Radon is present everywhere: air, soil, water

Radon
• The concentrations of radon in geological
environments depend mainly on the migration
processes, and the abundance of it parent
nucleus Radium (Ra-226)
• Upward migration of radon gas in soil is
facilitated by the presence of faults.
• It reaches in the atmosphere by diffusion to
the surface, this exhalation forming the radon
flux of the earth crust

Radon
• Radon and radon flux from soil are used as
indicators for some applications such as:
radon risk assessment by the determination of
radon potential of the soil.
identification of the faults.
in applying migration models in soil and
geological environments
and for the transport to the atmosphere and
inside homes

Radon
• There are at least three different issues of great
importance in radon studies:
 The first issue relates to the presence of radon
and radium in ground waters (wells, mineral
springs, geothermal waters, etc)
(In addition to the knowledge of radiation doses
received by population in using these water
sources (by ingestion, inhalation, spa treatment) ,
radon monitoring in groundwaters and
geothermal waters is a great interest in
geophysical studies.)

Radon
• There are at least three different issues of
great importance in radon studies:
The second aspect is related to the radon
potential in soil and the flux from the earth
(soil) surface.
By this, is important that radon anomalies
indicate radioactive accumulation (U, Th) or
the presence of tectonic faults. In such areas,
radon flux from soil is significantly higher

Radon
• There are at least three different issues of great
importance in radon studies:
• The third aspect, is related to radon
concentrations inside homes.
• Outdoor air has an average radon concentration
of 4-8 Bq⋅m-3 that depends on the geological and
meteorological conditions.
• Inside homes, the radon concentration may
produce normal amounts of 20-80 Bq⋅m-3,
through accumulation.

Radon
• In case of high indoor radon levels the main
radon sources are the soil and building
materials, which contain radioactive materials
or uranium waste in uranium areas. These
zones are considered “radon-prone areas”

Radon
• In addition to these important aspects of
radon studies, another research field are the
applications in geophysics
• Radon considered as „trace element” or
„monitoring element”, can give information
about geophysical properties of geological
formations

Radon
• Another important application:
• is the use of radon monitoring techniques in
the studies of volcanic eruptions
• and of seismic activities, where the monitoring
radon concentration variations in bore-holes
and groundwater can be applied to
earthquakes forecast

methods of radon concentration
measurement in soil
There are two basic
method for measuring
radon in soil. Both
methods measure the
alpha particles
produced by the decay
of the radon in the air.
In the first method, a
sample of soil air is
collected from a probe
driven into the ground,
and the radon in the
sample is measured by
using electronic
equipment.

The Rn measuring start
after delay time 5 minutes.
The total time of one
measurement is no more
than 10 minutes.
the detector determines an
average Rn concentration
(corrected from the
background of the of the
cell

Solid state nuclear track detector
• In the other method involves putting an alphatrack detector, in the soil and leaving it open to the
soil air. This method allows long-term
measurements,. It is called Solid state nuclear
track detector
• A solid state nuclear track detector (SSNT) is a sample of a
solid material ( glass or plastic) exposed to nuclear
radiation
( charged particles), etched, and
examined microscopically.

Solid state nuclear track detector
• The basis of solid state nuclear track detection is that
charged particles damage the detector within nanometers
along the track in such a way that the track can be etched
many times faster than the undamaged material. Etching,
typically for several hours, enlarges the damage to conical
pits of micrometer dimensions, that can be observed with a
microscope.
• If the particles enter the surface at normal incidence, the
pits are circular; otherwise the ellipticity and orientation of
the elliptical pit mouth indicate the direction of incidence.

SSNTD
• SSNTDs are commonly used to study radon
concentration in houses, and the age of geological
samples.
• A material commonly used in SSNTDS is CR39. It is
a clear, colorless, rigid plastic with the chemical
formula C12H18O7. Etching is usually performed in
solutions of caustic alkalis such as sodium
hydroxide, often at elevated temperatures for
several hours.

Applications of radon studies in environmental
science, geology and geophysics
Radon studies with applications in risk
assessment of radon from soil: the
assessment of radon risk from soil.
• radon is the main source of natural radiation
for the population, with a contribution of
about 57 % to annual dose.
• In some areas, this can reach towards to the
annual mean exposure of 2.2 mSv. Depending
on geological and meteorological conditions,

• In radon risk areas (”radon-prone areas”) the
concentration of radon gas in atmosphere
and indoor can reach high levels, which is due
to soil and building materials.
• The assessment of the radon risk from soil is
based on the determination of the radon
potential by measuring radon concentration
from soil and the permeability of soil.

• models are used for risk assessment of soil
radon.
The evaluation of radon index (radon risk)

• The soil gas radon concentration is one of the
main parameter in determining the
• radon potential of a building site. Soil
permeability is the second main parameter in
determining the radon potential of a building
site.

• Soil permeability is the second main parameter in
determining the radon potential of a building site.
• High permeability allows an increased transport of radon
from soil and transfer to the building, thus in case of
permeable soils can be estimated an increase radon risk.
• Soil permeability can be determined by in situ
measurements, where the k permeability is given in [m2].
• In situ soil permeability measurements usually is carried
out at a depth of 0.8 m in soil, and the measurement
method consist in measuring the flow rate of the soil gas by
extraction or by pumping in soil at constant pressure.

• According to radon risk assessment, the categories of
soil permeability are the follows:
• k < 4,0⋅10-13 m2 for low permeability;
• 4,0⋅10-13 m2 < k < 4,0⋅10-12 m2 for medium
permeability;
• and k > 4,0⋅10-12 m2 for high permeability.
• The number of in situ permeability measurements are
the same as for soil radon measurements, at least 15
measurements for a building site (≤ 800 m2 area), or
perform measurements in grids of 10x10 m for sites
with area > 800 m2.

The 10 point system to estimate the soil
radon potential

Applications of radon studies geology and
geophysics

Applications of radon studies geology and
geophysics
• radon has a role of trace elements, which can indicate
accumulation of radioactive material in the crust, or
the presence of tectonic faults.
• Faults serving as pathways for the ascendant migration
of these gases towards surface.
• Detection of high thoron activities in soil gas may
indicate a fast migration mechanism from a distant
source, due to the short half-time of thoron (55 sec)
than of radon (3,82 days). This is possible only in
presence of a carrier gas (e.g., CO2) as typically occurs
along faults and fractured rocks

Radon Monitoring for geological
exploration

The mobilization of uranium ore body detected by
radon measurements using SSNTDs

Use of radon monitoring in
hydrocarbon exploration

Sketch of gas seepage from a simplified hydrocarbon
Reservoir. The gas emission at the surface of the earth is
indicated in the graph. “Reservoir” refers solely to
hydrocarbons

Application of radon monitoring in
locating geological faults

Radon Hydrologic Studies
Because of its rapid loss to air and
comparatively rapid decay, radon is used in
hydrologic research that studies the
interaction between ground water, streams
and rivers. Any significant concentration of
radon in a stream or river is a good indicator
that there are local inputs of ground water

Radon emission as precursor of
earthquake
•The strain changes occurring within the
earth's surface during an earthquake is
expected to enhance the radon concentration
in soil gas.
•The principle seems to be simple: Radon gas
which is trapped within the ground, is released
through small fractures resulting from many
changes taking place in the earth's crust in
that region prior to the major physical shock of
an earthquake..

Radon emission as precursor of
earthquake
The stress–strain developed within
earth's crust before an earthquake leads
to changes in gas transportation and rise
of volatiles from the deep earth to the
surface. As a result, remarkable
quantities of radon come out of the pores
and fractures of the rocks on surface

Concentration of radon
• Because the level of radioactivity is directly related
to the number and type of radioactive atoms
present, radon and all other radioactive atoms are
measured in picocuries. For instance, a house
having 4 picocuries of radon per liter of air (4
pCi/L) has about 8 or 9 atoms of radon decaying
every minute in every liter of air inside the house.
A 1,000-square-foot house with 4 pCi/L of radon
has nearly 2 million radon atoms decaying in it
every minute.

Concentration of radon
• Radon levels in outdoor air, indoor air, soil air, and
ground water can be very different. Outdoor air
ranges from less than 0.1 pCi/L to about 30
pCi/L, but it probably averages about 0.2 pCi/L.
Radon in indoor air ranges from less that 1 pCi/l to
about 3,000 pCi/L, but it probably averages
between 1 and 2 pCi/L. Radon in soil air (the air
that occupies the pores in soil) ranges from 20 or
30 pCi/L to more than 100,000 pCi/L
• The amount of radon dissolved in ground water
ranges from about 100 to nearly 3 million pCi/L.

Uranium: The source
To understand the geology of radon where it forms, how it forms, how it
moves - we have to start with its
ultimate source, uranium. All rocks
contain some uranium, although most
contain just a small amount - between 1
and 3 parts per million (ppm) of
uranium. In general, the uranium
content of a soil will be about the same
as the uranium content of the rock from The bright-yellow mineral tyuyamunite is one
which the soil was derived.
of the most common uranium ore minerals,
Pitchbend UO2, Uraninite, Granotite
Some types of rocks have higher than average uranium contents. These
include light-colored volcanic rocks, granites, dark shales, sedimentary
rocks that contain phosphate, and metamorphic rocks derived from these
rocks. These rocks and their soils may contain as much as 100 ppm
uranium.

Radon formation
• Just as uranium is present in all rocks and
soils, so are radon and radium because
they are daughter products formed by
the radioactive decay of uranium.
• Each atom of radium decays by ejecting
from its nucleus an alpha particle
composed of two neutrons and two
protons

Radon formation
• The location of the radium atom in the mineral
grain (how close it is to the surface of the grain)
and the direction of the recoil of the radon atom
(whether it is toward the surface or the interior
of the grain) determine whether or not the newly
formed radon atom enters the pore space
between mineral grains.
• If a radium atom is deep within a big grain, then
regardless of the direction of recoil, it will not
free the radon from the grain, and the radon
atom will remain embedded in the mineral.

Radon formation
• Even when a radium atom is near the surface
of a grain, the recoil will send the radon atom
deeper into the mineral if the direction of
recoil is toward the grain's core. However, the
recoil of some radon atoms near the surface
of a grain is directed toward the grain's
surface. When this happens, the newly
formed radon leaves the mineral and enters
the pore space between the grains or the
fractures in the rocks.

The recoil of the radon atom is quite strong. Often newly formed radon atoms enter
the pore space, cross all the way through the pore space, and become embedded in
nearby mineral grains. If water is present in the pore space, however, the moving
radon atom slows very quickly and is more likely to stay in the pore space. For most
soils, only 10 to 50 percent of the radon produced actually escapes from the mineral
grains and enters the pores

Radon movement
• Because radon is a gas, it has much greater
mobility than uranium and radium, which are fixed
in the solid matter in rocks and soils. Radon can
more easily leave the rocks and soils by escaping
into fractures and openings in rocks and into the
pore spaces between grains of soil.
• The ease and efficiency with which radon moves in
the pore space or fracture effects how much radon
enters a house. If radon is able to move easily in
the pore space, then it can travel a great distance
before it decays, and it is more likely to collect in
high concentrations inside a building.

Radon movement
• The method and speed of radon's
movement through soils is controlled by
the amount of water present in the pore
space (the soil moisture content), the
percentage of pore space in the soil (the
porosity), and the "interconnectedness"
of the pore spaces that determines the
soil's ability to transmit water and air
(called soil permeability).

Radon moves more rapidly through permeable
soils, such as coarse sand and gravel, than
through impermeable soils, such as clays.
Fractures in any soil or rock allow radon to move
more quickly.

• Radon in water moves slower than radon in air. The distance that radon moves
before most of it decays is less than 1 inch in water-saturated rocks or soils,
but it can be more than 6 feet, and sometimes tens of feet, through dry rocks
or soils. Because water also tends to flow much more slowly through soil pores
and rock fractures than does air, radon travels shorter distances in wet soils
than in dry soils before it decays.

lecture note radon in geology

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