Indirect radon measuring method of materials.doc

M. Gawrys, 2008-08-09

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Indirect radon measuring method of materials.doc

By Michael Gawrys, M.Sc. in Physics, Gothenburg in Sweden.

(Working part time on Radon Localizer, www.radonlocalizer.com)

Introduction

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.

Idea

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

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Table 1: Exhalation rates compiled data [1], [2] and [3]

Building material Measured

Gamma radiation

[µSv/h]

Corresponding

Radium-226

concentration

[Bq/kg]

Corresponding

Exhalation rate E

of Radon-222

[Bq/(m2h)]

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

Note:

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]

( ) ( )

∑

+

=

i

i

i

V

n

bm A

E

C

λ

1

(1)

where

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

Examples

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

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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

83Bq/m3.

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.

References

[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] www.stralsakerhetsmyndigheten.se, 2008-08-09

## Saturday, August 9, 2008

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