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A Gamma-spectrometer for determination of density distribution and
moisture distribution in building materials
-
Ser
I-.
ITHl
N21d
National Research
Conseil national
1.
1302
I*
Council Canada
de recherchg
Canada
c .
2
BLRG
Division of
Division des
-
--Building Research
recherche~
en batiment
A
Gamma-Spectrometer for Determination
of Density Distribution and Moisture
Distribution in Building Materials
by M.K. Kumaran and M. Bomberg
Reprinted from
Proceedings of the International Symposium
on Moisture and Humidity
Washington, D.C., April 15- 18, 1985
p.
485
-
490
DBR Paper No. 1302
Price $1
.OO
Un spectrometre gamma
B
double faisceau a 6te congu et monte,
sa source etant un assemblage de 5 5 ~ s 1 3 7
et de 95~m241. Le
detecteur se compose dlun cristal (thallium)
B
iodure de sodium
et dqun analyseur multivoie, chaque voie balayant une region
precise dtenviron
48
kJ
m
o
l
'
l
du spectre combine.
La
source et
le detecteur sont montes sur des plates-formes distinctes qulil
est possible de deplacer dans une matrice spatiale definie par
des coordonnees verticales et horizontales, avec une precision
meilleure que 0,02 mm.
Le
deplacement des plates-formes est
command6 et synchronise par ordinateur.
Les intensites du
rayonnement gamma dans les regions dtint6r@t (autour des deux
pics caracteristiques de 5,79 et 63,88 GJ m
o
l
'
l
du spectre,
aprss passage dans un milieu homogene c o m e de llair,
de lteau
ou des solutions aqueuses) sont enregistrees, lt6cart type
&ant de 0,2% dans les conditions experimentales optimalis6es.
Des programmes informatiques permettent de definir les
parametres de chacun des essais, dtexecuter ltessai et de
collecter les donnges.
Les coefficients dtattenuation
de masse de lteau pour les deux
regions energetiques ont ete dgtermings et concordent
B
1% pres
avec les valeurs couramment acceptees.
On pr6sente dans cette
etude un profil de la distribution de lthumidit6
dans un bloc
de fibre de verre de masse volumique moyenne en train de
MOISTURE AND H U k
Proceedings of the
International Symposium on Moisture and Humidity
Washington, I
April 15.18,
I
A GAMMA-SPECTROMETER FOR DETERMINATION
OF DENSITY DISTRIBUTION AND MOISTURE
DISTRIBUTION IN BUILDING MATERIALS
M. K. Kumaran
M. Bomber
O ISA, 1985
A GAMMA-SPECTROMETER FOR DETERMINATION OF DENSITY DISTRIBUTION
AND MOISTURE DISTRIBUTION I N BUILDING MATERIALS
M.K. Kumaran and M. Bomberg
Thermal Performance S e c t i o n D i v i s i o n of B u i l d i n g R e s e a r c h N a t i o n a l R e s e a r c h C o u n c i l of Canada
Ottawa KIA OR6
ABSTRACT
A dual-beam gamma-spectrometer h a s been d e s i g n e d and i n s t a l l e d , i t s s o u r c e a n assembly of 5 5 ~ s 1 3 7 and 9 5 ~ m 2 4 1 . The d e t e c t o r c o n s i s t s of a sodium i o d i d e ( t h a l l i u m ) c r y s t a l a n d a m u l t i c h a n n e l a n a l y s e r , e a c h c h a n n e l s c r e e n i n g a s p e c i f i e d r e g i o n a 4 8 kJ mol-l i n t h e combined s p e c t r a . The s o u r c e a n d t h e d e t e c t o r a r e mounted o n s e p a r a t e t o w e r s t h a t can be moved a l o n g a s p a c e m a t r i x d e f i n e d by v e r t i c a l a n d h o r i z o n t a l c o o r d i n a t e s w i t h a p r e c i s i o n b e t t e r t h a n 0.02 mm; movement of t h e t o w e r s i s c o n t r o l l e d a n d s y n c h r o n i z e d by computer. I n t e n s i t i e s of t h e gamma r a y i n t h e r e g i o n s of i n t e r e s t ( r o u n d t h e two c h a r a c t e r i s t i c peaks of 5.79 and 63.88 GJ mol-l i n t h e s p e c t r a , a s p a s s e d t h r o u g h homogeneous media l i k e a i r , w a t e r o r aqueous s o l u t i o n s ) a r e r e c o r d e d , w i t h a t y p i c a l s t a n d a r d d e v i a t i o n o f 0.2% u n d e r t h e o p t i m i z e d e x p e r i m e n t a l c o n d i t i o n s . Computer programs a r e u s e d t o d e f i n e t h e p a r a m e t e r s of e a c h t e s t , t o r u n t h e t e s t , and t o c o l l e c t d a t a . The mass a t t e n u a t i o n c o e f f i c i e n t s of w a t e r f o r b o t h e n e r g y r e g i o n s have b e e n d e t e r m i n e d a n d a g r e e w i t h a c c e p t e d l i t e r a t u r e v a l u e s w i t h i n 1%. A p r o f i l e i s p r e s e n t e d o f m o i s t u r e d i s t r i b u t i o n i n a b l o c k of medium-density g l a s s f i b e r w h i l e d r y i n g . INTRODUCTION I n t e r a c t i o n between e l e c t r o m a g n e t i c r a d i a t i o n and m a t t e r forms t h e b a s i s o f a l l e x p e r i m e n t a l methods of s p e c t r o s c o p y . Gamma r a y s e m i t t e d by r a d i o a c t i v e m a t e r i a l s , b e i n g p a r t of t h e e n t i r e s p e c t r u m of e l e c t r o m a g n e t i c r a d i a t i o n , a r e a l s o known t o i n t e r a c t w i t h m a t t e r t h r o u g h which t h e y p a s s . As a consequence, r a d i a t i o n i s absorbed by m a t t e r , e i t h e r p a r t i a l l y o r c o m p l e t e l y , d e p e n d i n g upon t h e e n e r g y of t h e p h o t o n s , t h e n a t u r e of t h e a b s o r b e r , a n d t h e d i s t a n c e t h e r a d i a t i o n t r a v e l s t h r o u g h t h e a b s o r b e r . Such a b s o r p t i o n may be due t o a number o f d i f f e r e n t p h y s i c a l phenomena, a s summarized by Adams and Dams ( 1 ) . Two of t h e more i m p o r t a n t phenomena a r e p h o t o e l e c t r i c e f f e c t and Compton s c a t t e r i n g ; i f t h e e n e r g y of t h e photon e x c e e d s 98.62 GJ mol-l, t h e phenomenon of p a i r p r o d u c t i o n i s a l s o b e l i e v e d t o p l a y a n i m p o r t a n t r o l e i n t h e i n t e r a c t i o n ( 1 , 2 ) . It i s g e n e r a l l y a c c e p t e d t h a t t h e a b s o r p t i o n of a rnono-energetic gamma r a y p a s s i n g t h r o u g h a homogeneous a b s o r b e r i s e x p o n e n t i a l , e x p r e s s e d by where I. i s t h e i n t e n s i t y of t h e i n c i d e n t gamma r a y , I i s t h a t of t h e t r a n s m i t t e d gamma r a y , d i s t h e t h i c k n e s s of t h e a b s o r b e r , and
u
i s a c o n s t a n t c h a r a c t e r i s t i c o f t h e a b s o r b e r a n d i s c a l l e d a t t e n u a t i o n c o e f f i c i e n t o r a b s o r p t i o n c o e f f i c i e n t . The q u a n t i t y p f o r t h e homogeneous a b s o r b e r i s d i r e c t l y p r o p o r t i o n a l t o i t s d e n s i t y , p , and may be r e p r e s e n t e d as The p r o p o r t i o n a l i t y c o n s t a n t o i n E q u a t i o n ( 2 ) i s known a s t h e mass a t t e n u a t i o n c o e f f i c i e n t o f t h e a b s o r b e r . Its v a l u e depends o n l y on t h e c h e m i c a l c o n s t i t u t i o n o f t h e a b s o r b e r a n d n o t o n t h e p h y s i c a l s t a t e of e x i s t e n c e . E q u a t i o n ( 1 ) may b e r e w r i t t e n a sI f t h e a b s o r b e r i s made of two components, s a y . 1 and 2, t h e n E q u a t i o n ( 3 ) becomes
where s u b s c r i p t s 1 and 2 r e f e r t o t h e two components and pl and p2 a r e t h e a p p a r e n t o r p a r t i a l d e n s i t i e s o f t h e components i n t h e m i x t u r e . E q u a t i o n ( 4 ) i s of g r e a t p r a c t i c a l i m p o r t a n c e ; i f o n e m e a s u r e s , s i m u l t a n e o u s l y , t h e t r a n s m i t t e d i n t e n s i t i e s of two gamma r a d i a t i o n s d i f f e r i n g i n e n e r g y , o n e c a n d e t e r m i n e t h e p a r t i a l d e n s i t i e s pl and p 2 , t h e mass a t t e n u a t i o n c o e f f i c i e n t s ol a n d o2 h a v i n g b e e n d e t e r m i n e d s e p a r a t e l y . T h i s p r i n c i p l e h a s been u s e d e x t e n s i v e l y f o r d e t e r m i n i n g d e n s i t i e s a n d m o i s t u r e c o n t e n t o f s o i l s (3-14). I t s m e r i t s have a l r e a d y been p o i n t e d o u t by F e r g u s o n and G a r d n e r ( 3 ) . The most i m p o r t a n t among them i s t h a t t h e system under i n v e s t i g a t i o n r e m a i n s u n d i s t u r b e d . A number of f a c t o r s ( f o r example, c o l l i m a t i o n , background r a d i a t i o n , r e s o l u t i o n o f t h e a n a l y s e r , "dead
time"* of the measurement, nature of the analyse the transmitted beam. The available 2048
radioactive source, intensity of radiation, channels used in the pulse height analysis mode of
build-up factor) influence the precision of the the equipment are calibrated as follows. A
results, however. The errors introduced by such histogram of the combined spectra is obtained on
factors have been systematically analysed and the equipment screen. The position of the two
appropriate corrections suggested (15-17). Thus, peaks and their separation are adjusted such that
the precision of the measurements depends on how each channel records a successive increment
well one can define and minimize the corrections. -48 kJ mol-l. This is achieved by positioning the
Am peak at the 120th channel and the Cs peak at
THE GAMMA-SPECTROMETER the 1 3 2 4 ~ ~ channel. One of the available
facilities of the equipment allows the user to
The layout of the gamma-spectrometer is shown restrict the measurement to any selected region or
schematically in Figure 1. It was assembled by regions of interest in the histogram.
Foundation Electronic Instruments Inc., Ottawa,
according to the requirements of the Division of The source housing assembly and the detector
Building Research, National Research Council of assembly together with the respective collimators
Canada (NRCC). Construction details of the are mounted on two separate platforms, each of
equipment are available on request from NRCC; only which can be moved vertically and horizontally in
the more important features are summarized below. a 100-cm x 15-cm space matrix for any desired
position within the matrix, defined by a vertical
Source arid a horizontal coordinate with precision better
than 0.02 mm. The resolution of the vertical
The gamma ray source of the equipment consists of coordinate is 0.003 mm and that of the horizontal
1) a composite of americium oxide and aluminium coordinate, 0.015 mm.
oxide, and 2) cesium chloride in a fused silica
matrix. Each is a line source with an active area The gamma-spectrometer is interfaced with a
of 2 mm x 25 mm and is separately housed in PDP11/23 computer system that controls and
double-encapsulated stainless steel tubes. The synchronizes the movements of the two platforms
radioactivity of the
95Ad41
source was carrying the source and detector assemblies.7.4 x lo9 s-I and that of the 5 5 ~ s 1 3 7 source was Programs are used to define parameters for each
3.7 x lo9 s-l, as determined by the manufacturer test, i.e.
,
starting time, vertical and horizontal(Isotope Products Laboratories). Characteristic coordinates, live time*, energy regions of
energies of these isotopes are known to be 5.79 interest in terms of channel numbers, etc.
and 63.88 GJ mol-l
,
respectively. The line Sub-programs are available to modify the testsources are placed in a "source housing assembly." parameters during the test and for data
The gamma rays from the Cs source can be reduced acquisition, display, and printing.
appreciably by use of a manually operated internal
shutter located between the two sources. The EXPERIMENTAL METHODS AND RESULTS
external shutter of the assembly. operated by
electrically activated solenoids, effectively The importance of dead time corrections in
stops all radiation from the two sources. Thus, gamma ray methods is well known (10,lS). The
by an appropriate combination of the two shutters TN-1710 has built-in features to correct for dead
i
the operator can either stop all radiation or have time to some degree. The remaining dead time
a beam from Am or a dual beam from both sources. corrections for both energy regions have been
The beam emerging from the assembly is not determined by various operators of the NRCC
sufficiently collimated. Two successive lead spectrometer, using the method suggested by
block assemblies with 2-mm x 12-mm rectangular Groenevelt (10). Aluminum plates were used as
openings are used to collimate and define the absorbers for these measurements. The dead time
incident gamma ray for any measurement. corrections applied for those reported in this
paper are the most recently determined values,
Detector and Analyser i.e., 3.4 ps count-l for Am and 14.6 us count-l
for Cs. The live time selected for the
The detector and analyser form the most important measurements was 200 s, giving a precision better
parts of a gamma-spectrometer. The detector used than 0.2% with homogeneous absorbers. Shorter
in the NRCC gamma-spectrometer is a 51-m thick live time may be used, but would reduce the
sodium iodide (thallium) crystal with a precision of the counts.
photomultiplier tube (Harshaw model 14SHA8/3.5/x).
The diameter of both the crystal and the Experimental determination of the mass attenuation 1
photomultiplier tube is 89 mm. Before it enters coefficients of water for both energy regions is
the detector the transmitted beam can also be probably the best way to check the spectrometer;
collimated by using lead block assemblies similar theoretical values for these coefficients are
to those used with the source. known (18) to be 0.2049 and 0.0857 cm2mg-l,
respectively, for the Am and Cs characteristic
The multi-channel analyser, TN-1710 from Tracor energies, and several authors have already
Northern, coupled with the detector is used to verified these coefficients experimentally.
*Average time between counts during which analyser "rime during which detector is active and counting
After the NRCC gamma-spectrometer had been constructed and installed, it was found that the manually-operated internal shutter in the source housing assembly could stop only 80% of the Cs radiation. It is to be expected, therefore, that the
Am
readings will be influenced by secondary photons produced as the absorber interacts with Cs radiations. This will naturally depend on the distance through which the radiation travels. To investigate this quantitatively the following measurements were performed. Four rectangular Plexiglas containers were made with internal path lengths 5.05, 7.77, 10.03, and 14.13 cm,respectively. These boxes were filled with distilled water and scanned in the spectrometer at several positions for each path length.
Corresponding reference measurements were also made with empty boxes. Each set of measurements was repeated several times to provide the best average value of the mass attenuation coefficients for each path length. They were carried out in two series; in one, no collimation was used on the detector side, and in the other a rectangular collimation with experimentally optimized dimensions was used. For example, when the source-detector separation was -60 cm, the optimized dimension of the detector collimation was 2.4 x 2 cm. The results of these measurements
are summarized in Table I.
Evgin and Svec (19) have recently used the gamma-spectrometer for a set of measurements carried out on specimens prepared with fine Ottawa sand. For three different specimens with bulk densities of 1550, 1716 and 1802 kg* me3 average values for the logarithm of the ratio (I/Io) for Am were, respectively, -1.704, -1.908 and -1.992 for a constant path length. The general
applicability of Equation (3) may be examined with these values.
The NRCC gamma-spectrometer has been designed with a specific objective: to study moisture transport through building materials. As a preliminary study the following experimental investigations were undertaken. A 9.7 x 9.8 x 16-cm block of medimdensity glass fiber insulation was exposed to laboratory conditions (21°C and 48% relative humidity) until it attained constant mass. The average density of the specimen was determined to be 48.4 k g ~ m - ~ . The block was then scanned with the gamma ray at several positions and an average value for the count of transmitted americium beam was determined to be 2.122 x lo6, with a standard deviation of 6 x lo3 for 200 s live time. After
immersion in distilled water, the block was allowed to drain for 10 min. It was then placed on a flat surface and left at constant temperature (21°C) and relative humidity (48%) for 80 h. At this stage it could be seen from a mass
determination that the sample was holding about 300 cm3 of water, distributed within the available space subject to gradients produced by gravity and evaporation. The system was then scanned through a height of 16 cm on the central area. One typical set of readings is shown in Table 11. No collimation was used on the detector side for these measurements.
DISCUSSION
Mass Attenuation Coefficient of Water
As pointed out earlier, it is anticipated that the magnitude of the mass attenuation coefficient of water for
Am
will be influenced by the presence of Cs. It may be seen in Table I that magnitude depends, as expected, on path length andcollimation on the detector side. Empirically, however, the dependency is linear for both series
(Figure 2). This dependency on path length may be due to the secondary photons produced by water interacting with Cs photons. If this is the case, the value of the mass attenuation coefficient should approach the theoretical value of 0.2049 cm2*g-l when the path length approaches zero. On extrapolation to zero thickness both lines converge to 0.203 cm2wg-l, with a standard error of 0.002 cdog-l that agrees well with the theoretical value (Figure 2).
It may also be seen that the magnitude of the slope of the straight lines (a measure of the intensity of the secondary photons) is smaller when a collimator is used on the detector side. This is not unexpected, since the collimator shields part of the secondary photons from
reaching the detector. The magnitude of the slope may be further reduced by reducing the size of the collimator; beyond the optimum size, however, the number of photons decrease and the uncertainty in the count increases as is natural for radioactive phenomena.
The mass attenuation coefficient of water for Cs from all measurements was found to be
0.085
cm2ag-l ,
with a standard deviation of 0.002 cdmg-l, which agrees very well with the theoretical value of 0.0857 cm2* g-l.
Densities of Ottawa Sand
If p 1 and p2 are densities of two specimens prepared from sand, for a constant path length it follows from Equation (3) that
The ratio p1
lP2
is known from gravimetric measurements and hence can be compared with the three ratios determined from the spectroscopic measurements. Gravimetrically these ratios are 0.860, 0.903 and 0.952. From the gamma ray measurements the corresponding ratios are 0.856, 0.893 and 0.958.Moisture Distribution in Glass Fiber Block The data in Table I1 are used to construct the profile of moisture distribution in Figure 3. A .
noted, the magnitude of the mass attenuation coefficient of water for Am depends on path length. The calculation was therefore carried out in two stages. In the first, the value of the mass attenuation coefficient was assumed to be 0.203 cm2og-l, and from Equation (3) the apparent density of water at different heights was
9.7 cm, was t h e n c o r r e c t e d f o r e a c h h e i g h t by m u l t i p l y i n g by t h e f r a c t i o n = ( a p p a r e n t d e n s i t y l r e a l d e n s i t y o f w a t e r ) . I n t h e second s t a g e t h e c o r r e c t e d p a t h l e n g t h was u s e d t o c a l c u l a t e t h e mass a t t e n u a t i o n c o e f f i c i e n t a t t h e r e s p e c t i v e h e i g h t s and t h e v a l u e r e - s u b s t i t u t e d i n E q u a t i o n ( 3 ) . The whole s e r i e s of c a l c u l a t i o n s was r e p e a t e d t o a c h i e v e c o n v e r g e n c e w i t h r e s p e c t t o d e n s i t i e s . F o r t h e r e s u l t s r e p o r t e d h e r e , one t o t h r e e i t e r a t i o n s were s u f f i c i e n t t o a c h i e v e convergence. A p p l i c a t i o n of t h i s e x p e r i m e n t a l t e c h n i q u e w i l l b e f u r t h e r examined by s t u d y i n g m o i s t u r e d i s t r i b u t i o n i n porous m a t e r i a l s . At t h i s s t a g e of t h e s t u d y , however, i t i s a n t i c i p a t e d t h a t f o r very low m o i s t u r e c o n t e n t s , s a y l e s s t h a n 5 g i n a n a v a i l a b l e volume o f 1 0 0 c m 3 , t h i s method may n o t be s u i t a b l e . ACKNOWLEDGEMENT The a u t h o r s g r a t e f u l l y acknowledge t h e a s s i s t a n c e of M r . R. Marchand i n t h e development of t h e computer s o f t w a r e t o o p e r a t e t h e gamma-spectrometer. T e c h n i c a l h e l p from M r . J.G. T h e r i a u l t and M r s . N. Normandin i s a l s o acknowledged. The c o n t r i b u t i o n s of
M r . E. R o s e n q u i s t and M r . P. Armstrong towards t h e development of t h e computer programs w h i l e t h e y were working a s summer s t u d e n t s d e s e r v e s p e c i a l mention. T h i s p a p e r i s a c o n t r i b u t i o n from t h e D i v i s i o n o f B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of Canada, and i s p u b l i s h e d w i t h t h e a p p r o v a l of t h e D i r e c t o r o f t h e D i v i s i o n . REFERENCES
1. Adams, F. and Dams, R., Revised Second Ed., "Applied Gamma-ray S p e c t r o m e t r y " by
C.E. Cronthamel, I n t . S e r i e s of Monographs i n A n a l y t i c a l C h e m i s t r y , Pergamon P r e s s , 1970, pp. 14-27.
2. D u i v e n s t i j n , A.J. and Venverloo, L.A.J., " P r a c t i c a l Gamma S p e c t r o m e t r y
,"
( I n Dutch) P h i l i p s T e c h n i c a l L i b r a r y S e r i e s , London, C l e a v e r H u m e P r e s s L t d . , 1963, 146 p. 3. Ferguson, H. and Gardner, W.H., "WaterC o n t e n t Measurement i n S o i l Columns by Gamma Ray A b s o r p t i o n , " S o i l S c i e n c e S o c i e t y of America P r o c e e d i n g s , 1962, p. 11-14. 4. Coreg, J . C . , Boulogne, A.R., and
Horton, J . H . , " D e t e r m i n a t i o n of S o i l D e n s i t y and Water Content by F a s t Neutrons and Gamma Rays," Water R e s o u r c e s R e s e a r c h , Vol. 6, No. 1, 1970, p. 223-229.
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Table
I.
Average mass attenuation coefficients a h and a s of water foramericium and cesium radiations as a function o! path length d; the
standard deviation for each oh is 0.001 cm2*g-1 and for each aCs 0.002 cm2-g-1.
-.
Without collimation on detector with collimation on detector
Table 11. The counts IAm for the transmitted americium radiation at various heights, h, from the base of the glass fiber block and the apparent
densities p 1 of water; the live
time is 200 s, the path length is 9.7 cm and the average
reference count is 2.122 x lo6
with a standard deviation of
A AND B - PLATFORMS THAT CARRY DETECTOR AND SOURCE HOUSE ASSEMBLIES: THESE PLATFORMS CAN BE MOVED VERTICALLY AND HORIZONTALLY. S I - AMERICIUM SOURCE; 5 2 - CESIUM SOURCE: C - INTERNAL SHUTTER SEPARATING THE TWO SOURCES; D - EXTERNAL SHUTTER; El. E2. G1 AND G2 - LEAD BLOCK ASSEMBLIES FOR COLLIMATION;
F - SPECIMEN: H - SODIUM IODIDE (THALLIUM) CRYSTAL: I - PHOTOMULTIPLIER TUBE F I G U R E 1 S C H E M A T I C D I A G R A M OF N R C G A M M A - S P E C T R O M E T E R P A T H L E N G T H , c m F I G U R E 2 D E P E N D E N C E O F c r A m O F W A T E R O N P A T H L E N G T H I N T H E P R E S E N C E O F C S R A D I A T I O N . T H E R E L A T I V E I N T E N S I T I E S O F A m A N D C s A R E 4 T O 1. E A C H * O R 0 R E P R E S E N T S T H E A V E R A G E O F 50 TO 1 0 0 M E A S U R E M E N T S . E A C H S T R A I G H T L I N E I S F R O M A L E A S T S Q U A R E S F I T OF E I G H T P A I R S O F D A T A R E P R E S E N T I N G F O U R U P P E R A N D F O U R L O W E R L I M I T S O F E A C H E X P E R I M E N T A L P O I N T H E I G H T . c m F I G U R E 3 P R O F I L E O F M O I S T U R E D I S T R I B U T I O N I N B L O C K O F M E D I U M - D E N S I T Y G L A S S F I B E R I N S U L A T I O N D E T E R M I N E D B Y G A M M A - S P E C T R O M E T R Y
