natural radioactivity of building materials

J.RADIOANAL.NUCL.CHEM.,LETTERS 103 /4/ 231-240 /1986/

NATURAL RADIOACTIVITY OF BUILDING MATERIALS

E. Tamez, M.T Olgu[n, N. Segovia, S. Bulbulian, F. Abascal

Instituto Nacional de Investigaciones Nucleares, Agricultura No. 21, Col. Escandon, C.P. 11800,

Mexico D.F., Mexico

Received 6 November 1985 Accepted 12 November 1985

Experiments were designed to measure trace uranium concentration and the rate of radon exhalation from masonry structural materials, both bare and surface finished and coated. LRII5 cellulose nitrate track detectors were used to record the alpha emission from structural material surface. Fission track, neutron activation and fluorometric analysis methods were used to determine the uranium content. Most types of paints studied will reduce alpha contribution and radon emanation from building materials.

INTRODUCTION

It has been known since the beginning of the centu-

ry that uranium is found in trace amounts in most of

the earth substances, including rocks and soils. The

element is relatively scarce in distribution, approximate-

ly 2 ppm /Ref. i/. 222Rn is a noble and alpha radio-

active gas, a member of the natural radioactive ura-

nium series. Building materials are commonly made

from the natural materials /sand, rocks, cement/ which

231 E~ev~r Sequom ~ A., Lausanne Ak~dmmi K ~ 6 , B ~ e s t

TAMEZ et al. : RADIOACTIVITY OF BUILDING MATERIALS

will contain traces of natural radioactive isotopes.

The half life of radon, 3.8 days, is long enough for

part of it to diffuse from the building material to

the inside of the room. The radon gas naturally

arising from the earth below, is added to those ex-

haled from the dwelling materials. The internal wall

decoration used to cover the building materials will

act in such a way as to decrease or increase this

emanation; this depends on whether the material acts as

a sealer against radon emanation or as a source of

radon.

A first mapping of radon in household dwellings in

Mexico City and neighbouring cities showed maximum 2

quantities in houses constructed within volcanic zones

In order to measure the radon contribution arising

from the building materials, an alpha emission survey

was made of the most commonly used masonry structural

materials in Mexico and the surface finishes and coat-

ings.

We report the effects of using different types of

paints, wall paper and plasters on the amount of alpha

contribution and radon emanation from building

materials.

EXPERIMENTAL

Masonry structural materials

Structural materials selected for study were: ordi-

nary kiln-fired bricks, cinder block, hollow compressed

kiln-fired bricks and quarried volcanic rock, both

as purchased from distributors and covered with dif-

ferent kinds of paints, plaster and plastic wall plaper

coating.

232


The structural materials were studied both coated

and bare except for the volcanic rock samples, which

were not coated since they are commonly used without

such finishes. 66 sample types resulted from varying

the combinations of masonry with surface covering

/5 types of masonry and 12 surface finishings plus

1 volcanic rocks/ as shown in Table i.

Analytical techniques

Track detection, activation analysis and fluoro-

metric techniques were used to determine radon and

uranium in samples.

A Triga Mark III nuclear reactor facility with -2 -i a thermal flux of approximately 1013 n cm .s was

used for uranium determination in masonry samples by

fission track technique and activation analysis.

Radon exhalation sampling by track detection. Measuring

arrangement

Disc-shaped film track detectors /LR!I5 from Kodak-

Pathe/ were utilized for measuring the alpha emission

from the masonry samples. The track detectors, red

dyed cellulose nitrate foils /adhered to a polyester

support i00 ~m thick/ were 12 ~m thick. Two sizes of

can 3 radon capture containers were used:

- The first one was a 32.7 cm 3 can where a detector

was placed upon a 3 mm high PVC supporting ring

which was set upon the surface of each masonry

sample. The internal diameter of the ring was 1.7

cm and the outer 2.7 cm. The sensitive side of the

detector faced downward to record alpha particles

emitted from the sample surface.

- The second container was a 103.7 cm 3 can. Detectors

were placed at the top of the can facing the mason-

233

TAMEZ et al.: RADIOACTIVITY OF BUILDING MATERIALS

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Dimens ions in cm

�9 containers: A/ 32.7 cm can. B/ Fig. 1 Radon capture 3

103.7 cm ~ can. a - radon capture containers, b - PVC rings, c - track detectors, d - sensitive side, e - masonry structural material

ry. 222Rn and daughters alpha particles were re-

corded in this assemblage. Both sizes of can were

sealed to the surface of each masonry sample /Fig. i./.

The detectors were left for exposure for 55 days.

After this period all cans were removed at the

same time.

Track etching and track c0untin q

After exposure, to enlarge the alpha tracks the

detectors were immersed in an etching solution of

2.5N NaOH at 56~2 ~ for 3 h. The newly-etched

detectors were then stripped from the support layer

and the tracks counted with a spark counter 4. A

uniform residual thickness of the detectors had to

be achieved in all etched detectors for consistency

of results.

235


The results of track counting of the detectors are 2

reported in number of tracks per cm per week.

Uranium determination

Uranium content in the samples was determined by

fission track, neutron activation and fluorometric tech-

niques.

Fission track technique

Powdered brick samples of 250 mg were~pressed into

pellets and placed in an irradiation container in contact

with policarbonate detector for fission track recording.

The samples and detectors were irradiated in the nuclear

reactor for 15 min. After irradiation the detectors were

etched for 60 min in a 2.5N NaOH solution at 56+2 ~

and the etched tracks microscope counted. Calibration

of the method was performed with NBS glass standards

with uranium contents of 0.02 and 1 ppm.

Neutron activation and fluorometric techniques

Uranium was chemically separated from the masonry

samples. They were crushed and powdered and uranium

extracted by a leaching out method previously reported 5.

After separation the uranium fraction was used for

analysis by neutron activation and fluorometry techniques.

RESULTS AND DISCUSSION

The results of the alpha track survey experiments

from bare masonry samples and from the 12 combinations

of surface finishings and coatings are summarized in Figs

2. and 3. Figure 2. shows results from track detectors

placed at 3 mm upon the surface of each masonry sample.

236


1 2 3 4 5 6 7 8 9 10 11 12 13 Codes

Fig. 2. Alpha record /tracks cm-2.week/ from masonry

structural materials and coatings in bottom track detectors. Masonry and coatings are codified as sDecified in Table i. a - TRN; b - TRH; c - TRC; d - TOC; e - T; f - L

-~ 250

200

~ 150

lie

I 2 3 4 5 6 7 8 9 10 11 12 13

Codes

Fig. 3. Alpha record /tracks cm-2.week/ from masonry structural materials and coatings in top track detectors. Masonry and coatings are codified as specified in Table i. a - TRN; b - TRH; c - TRC; d- TOC; e - T

3 237


In this figure the ordinate is the number of tracks -2

cm .week while the abscissa corresponds to the codes

defined in Table 1 for the superposition of plasters,

paints, varnish and plastic wall paper coatings on

the ordinary kiln-fired brick, cinder block and hol-

low compressed kiln-fired brick structural materials

/except volcanic rock that appears uncoated/. The high-

est alpha contribution values are observed for uncoat-

ed hollow compressed kiln-fired bricks /code 1/I \

Comparing the effects of different types of paints

with that of alternative covers on the alpha measure-

ment it appears that yellow, green and blue enamel

paints are most efficient in reducing the alpha emission.

Generally paints /codes 4-9/ reduced the alpha emission

to at least one fourth of the bare brick values, while

the white vinyl base paint reduced it only to one half

in the hollow compressed kiln-fired bricks /TRC and

TOC! and did not reduce alpha emission for the other

structural materials. The likely speculation is that white

paint contains enough natural alpha active materials

which compensate for the alpha absorption from structural

materials. Using varnish and plastic wall paper cuts

the alpha contribution in a similar way as the coloured

paints showing only a slight alpha contribution.

Figure 3. shows the results in the detectors exposed

at 5 cm upon the surface of each masonry sample. This

analysis indicates a reduction in radon emanation when

masonly samples are covered.

Theoretical estimation of the LRII5 des

response using the relations reported by Somogyi et al 6 .

indicates that for the 103.7 cm 3 can, a record of 29.82 -3

tracks cm .30 days, corresponds to a radon concentra-

tion of 1 pCi i-i; therefore the maximum radon concentra-

tion /for the TRC structural material/ is approximately

2 3 8


-i 32 pCi 1 . The study of the uranium content in the

masonry materials showed that uranium concentration for

TRC and TOC samples were found to be at least two times

higher /in average 1.6 and 1.2 ppm, respectively/ than

the other structural materials /which had in average

0.6 ppm/. From the previous observations, the more like-

ly explanation for the effect of paint on reducing ra-

don emanation from the bricks is that it acts as a

sealer to the pores between the grains, thus reducing

the gas emanation.

Figure 3. shows that TRC, the material with higher

uranium content, produces higher radon emission; those

structural materials which have lower uranium content

favour the radon emission, particularly TRN ordinary

kiln-fired bricks, probably as an effect of the high

porosity of the material Comparison of Figs 2. and 3.

indicates that the number of tracks registered in the

bottom detectors is a measure of 238U, 234U and other

alpha emitting elements of the natural radioactive

chains, including 222Rn, present in the upper layer of

the masonry materials while the top detectors show the

measurements of the 222Rn gas liberated from the mason-

ry materials and the coatings.

ONCLUSIONS

Building materials studied showed an alpha contri-

bution that depends on the uranium content of the

samples. Coatings reduced the alpha emission except

in finishes that contribute by themselves to the alpha

emission. In general radon emission depends on the

uranium content of masonry but porosity of the ordi-

nary kiln-fired bricks favours the radon emission

~* 239


compensating in this way the effect of low uranium

content. Coatings reduce the radon emission.

Results obtained in the present work show that

contribution of radon in household dwellings as re-

ported in Ref. 2 can be partially explained by the

emission from building materials.

x

The authors wish to thank the Triga Reactor staff,

from the Nuclear Center of M~xico for their colla-

boration in irradiation and activation analysis sup-

port; Dr. C. Collins for her relevant suggestions

and C. Martlnez for technical assistance.

REFERENCES

i. D.K. Evans, Health Physics, 17 /1969/ 229.

2. N. Segovia, J. Cejudo, Nucl. Tracks Rad. Meas., 8 /1984/ 407.

3. F. Abu Jarad, H. Fremlin, Health Physics, 44 /1983/ 243.

4. L. Maldonado, R. Linares, T. Morales, N. Segovia, Technical Report, ININ AII-82-19, 1982.

5. J. Korkisch, F. Abascal, C. Aguilar, Rev. Lat. Am. Quim., 4 /1973/ 165.

6. G. Somogyi, B. Parip~s, Z. Varga, Nucl. Tracks Rad. Meas., 8 /1984/ 423.

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natural radioactivity of building materials

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