Saturday, August 9, 2008

FTCua Science Lessons 8

Indirect radon measuring method of materials.doc
M. Gawrys, 2008-08-09
1 of 3
Indirect radon measuring method of materials.doc
By Michael Gawrys, M.Sc. in Physics, Gothenburg in Sweden.
(Working part time on Radon Localizer,

I have noticed the ongoing discussion at Radon Professionals and the interest for
measuring radon from materials like granite slabs or ”countertops” and would like to
share with you a method that is recommended by the Swedish Radiation Safety Authority
(SSI) and has been used successfully for a long time, to detect uranium and radium rich
building materials.
To me it seems possible to adapt this current method to the measurements that could be
done by the countertop industry. In this paper, I sketch the method.
Short historical background
In Sweden, we have problems with houses that are built with uranium rich materials. The
main source was blue lightweight concrete called ”blue concrete” that contains shale. It
was produced from the 1920s to the 1970s. In the 1960s a great portion of the so called
”million program”, for building 1 million dwellings, was built with this material. From
the 1970s until now and many years into the future the aftermath of this ”mistake” is
dealt with. The goal is to measure all houses for radon and try to find those that have a
level over 200Bq/m3 and recommend mitigation. For various reasons this has been and
still is a slow process. Presumably, 5% of the risky dwellings have been mitigated,
although the rate of measuring with alpha track film has increased the last decade.
Current method
The method is simple, yet very helpful in making an estimation of how high the levels of
radon contribution might be from the exhalation from building materials. The walls, floor
and ceiling in a room are measured for gammas by holding a gamma meter to the surface.
The gamma rate corresponds to the exhalation rate of the material. The exhalation rate
multiplied with the area for every different material is added together. From the room
inner volume, it is then possible to make an estimation of radon per volume.
Adaptation of the method
1) Detect the amount of gamma radiation γ from the surface centre of the material.
2) Compare the gamma value with a standard table value of the proper materials
corresponding radon exhalation rate E.
3) Measure the surface area A and multiply by two (for upper and bottom side).
4) Measure the volume of the room where the material is present.
5) Make an estimation of the ventilation rate of the room.
6) Use the equation (1) below to estimate the radon concentration level in the room.
Indirect radon measuring method of materials.doc
M. Gawrys, 2008-08-09
2 of 3
Table 1: Exhalation rates compiled data [1], [2] and [3]
Building material Measured
Gamma radiation
Exhalation rate E
of Radon-222
Blue light weight
concrete [2], [3] 0,25-1,20 600-2600 50-200
Blue light weight
concrete [1] 0,8-1,0 2500 160
Blue light weight
concrete [1] 0,6 1500 100
Blue light weight
concrete [1] 0,3 650 40
Concrete [2], [3] - 20-200 2-20
Concrete [1] Background - 10
Bricks [2], [3] - 40-150 1-10
Sand based light
weight concrete [2], [3] - 10-130 1-3
The measured gamma radiation, the materials radium content and the materials exhalation rate
possibly have to be translated from the SI system to the system used locally. The values in the
table might have to be expanded by standardized measurements for the materials radon
exhalation rate, that also is dependent of the surface top finish, but the method is quite straight
forward as you can see in some examples below.
Radon addition equation [2]

( ) ( )

bm A
Cbm is the radon added from the building material in Bq/m3
λ the radon decay constant in h-1 (which can be discarded)
n is the air exchange in the room in h-1
V is the inner volume of the room in m3
Ei is the Exhalation rate of the building part in Bq/(m2h)
Ai is the area of the building part in m2
1) A normal room with good ventilation [3]:
In a normal room with all four walls made of light weight blue concrete with a radium
content of 1460 Bq/kg the radon content will become 120-150 Bq/m3 if the air exchange
rate is 0,5 exch./h and the walls have an ordinary surface finish.

Indirect radon measuring method of materials.doc
M. Gawrys, 2008-08-09
3 of 3
2) A calculated example of a room with poor ventilation and lots of ”blue concrete” [1]:
If all walls, the floor and the ceiling are made of lightweight blue concrete and the floor
area is 3m x 4m, the height of the room is 2,4m, the air exchange is 0,2times/h and the
gamma reading is 0,6µSv/h on all surfaces and with a corresponding exhalation rate of
100 Bq/(m2h). This will result in a radon concentration of 1000Bq/m3.

3) A fictive example of a material with high exhalation rate:
If the granite countertop is 2m long and has 0,6m depth the radon exhalation area will be
2x0,6x2 = 2,4m2, in the same room size as above, the air exchange in the room at
0,2times/h and an radon exhalation rate that is the same as for the highest of light weight
blue concrete, 200Bq/(m2h). This would result in a radon concentration of around
Result and discussion
As you can see, the result of the fictive, but certainly not strange example, shows that the
radon contribution from the material will not be very high. The reason for this is the
small area of radon exhalation in comparison to the volume of the room.
The work still to be done, to make this a good and simple method to use in practice, is to
gather good data and make tables of corresponding values between the gamma
measurements and the materials exhalation rate.
[1] SSI, Radon Mätteknik, course material 7-8 November 2005.
[2] Clavensjö & Åkerblom, Radonboken, Formas: Stockholm 2003.
[3] Statens offentliga utredning, SOU 2001:7
[4], 2008-08-09

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