Pertanika 9(3), 369 - 373 (1986)

Gamma Ray Attenuation in Hevea Latex and itsApplication to DRC Measurement

ELIAS SAlON, DEOU SEANG SIN, ZAINAL A. SULAIMAN,SABIRIN H. IKHSAN* and HUSIN WAGIRAN*

Department of Physics,Faculty of Science and Environmental Studies,

Universiti Pertanian Malaysia,43400 Serdang, Selangor, Malaysia.

Key words: Gamma ray attenuation; coefficient; Dry Rubber Content (DRC); Total Solid Content

(TSC); Hevea latex.

ABSTRAK

Kertas ini melaporkan suatu pengukuran kandungan pepejal getah Hevea dengan mengguna­kan teknik pengecilan sinar gama. Nisbah komponen bagi jumlah iszpadu air dan bahan pepejalgetah dalam lateks ditentukan dengan menggunakan pengesan Si(Lz) yang mempunyai peleraiantinggi bagi sinar gama bertenaga rendah. Keputusan ini dibandingkan dengan Kaedah Piawai dan di­dapati pekali korelasi ialah 0.998. Berdasarkan kepada tindak balas sinar gama dengan jirim, bersan­darkan kepada nombor atom bahan pada suatu tenaga tertentu, teknik ini berkeupayaan mengesanbahan asing dalam lateks tetapi penyelidikan yang mendalam masih dzperlukan untuk menentusah­kan keadaan ini.

ABSTRACT

This paper reports the measurement of the total solid content (TSC) offresh Hevea latex by thegamma ray attenuation technique. The component ratios of the total volume of water and solidmaterial in multzphase system of latex were determined by the measurement ofattenuation coefficientof latex with high energy resolution Si(Lz) detector at low gamma ray energies. The result was com­pared against the Standard Laboratory Method and the correlation coefficient was found to be 0.998.Based on the behaviour that the interaction ofgamma ray with matter at a given energy depends uponthe atomic number of the material, the present technique may be able .to detect adulterants in thelatex, but investigations are needed to verify this.

INTRODUCTION

Accurate measurement of dry rubbercontent (DRC) of field latex is essential to latexprocessing factories for economic reasons. As theDRC of latex varies widely according to clonetypes, soil conditions, seasonal variations,weather, tapping systems and yield stimulationuse (De Jonge, 1965; Abraham, 1970 and Ng,Ping and Lee, 1970) the actual rubber content

must be determined. Some of the methods suchas Rapid Method, Chee Method and Hydro­metric Method are commonly used in determin­ing DRC but their accuracies however vary from1% to about 12% deviation from a Standard­Laboratory Method (Chin, 1979). Recently ourcolleagues have successfully introduced a newmeasuring instrument to determine DRC basedon the microwave attenuation technique (Kaida,1982 and Kaida and Mahdi, 1983). The instru-

*jabata~ Fizik, Fakulti Sains, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia.

ELIAS SAlON, ~:>EOU SEANG SIN, ZAINAL A. SULAIMA~, SABIRIN H. IKHSAN AND HUSIN WAGIRAN

where a i is the fraction of the total volumeoccupied by the ith component of the mixture,equation (3) becomes

nIlmix = . L at' Ill' (5 )

1 = 1

where No is Avogandro's number, A is theatomic mass number, Pmix is the density oflatex and a is the atomic cross-section for theremoval of gamma ray photons. Since thedensity of the mixture can be written in the form

P = nmix i ~1 <; ,Ii i. (4 )

where a, is the atomic cross-section for removalofgamm~ ray photons from the beam by atom oftype i, N i is the number of atoms of type i perunit volume, Z is the atomic number of thematerial and E is the energy of the incidentgamma ray photon. Consider Hevea latex as amixture consisting many components which dis­tribute uniformly throughout the irradiatedvolume. The attenuation coefficient of themixture can be employed as

ment is inexpensive, simple, portable, rapid andaccurate to less than .I % deviation to the Stan­dard Laboratory Method. The only disadvan­tage of this instrument 'is its inability to detectforeign materials present in the latex.

Gamma ray attenuation technique has beenused in multiphase system of oil-water-air todetermine the component ratios of the pipelineflow system (Abouelwafa and K~ndall, 1980).We have employed a similar principle of physicsto determine the attenuation of gamma ray inHevea latex. Low energy gamma rays have beenselected to give the attenuation which is mainlydue to photoelectric absorption and Comptonscattering. The DRC has been derived from themeasurement of the component ratios of latex atdifferent concentrations. The measurement withStandard Laboratory Method is used for compa­rison. This present technique is able to produceresults of a level of 2% unit of DRC. Based onthe behaviour that gamma ray attenuation co­efficient of rubber materials varies with theiratomic compositions (Elias et at., 19~3), it isbeiieved that this technique may be able todetect adulterants added to the latex providedthat their effective atomic numbers are differentfrom the effective atomic number of rubber.

Ilmix =(No fA) a Pmix (3)

Gamma Ray A ttenuation in Latex

The attenuation of monoenergetic gammaray photons in a good geometry experiment isdescribed by the following relationship

(1)

where I and I are the transmitted and incidentintensities of the gamma ray respectively, d is theabsorber thickness and/i is the linear attenua­tion coefficient of the absorbing materials. Thetotal attenuation coefficient depends upon theenergy of the incident gamma rays and atomicnumber of the material. For a compoundmaterial, the attenuation coefficient is related toits atomic composition and is given by

(2)

where J..Li is the linear attenuation coefficient ofelement i. Equation (5) describes the gamma rayattenuation coefficient of a multicomponentsystem of a mixture at a given energy. For sim­plicity, we consider Hevea latex as consisting of atwo-component system of water suspension, thatis water and solid material (rubber and non­rubber solid). Equation (5) simply becomes atwo-component system of a mixture and can bereduced to

(6)

where a'l and a 2 are the fractions of the totalvolume occupied by water and solid material res­pectively and J..L

1and J.l

2are the attenuation co­

efficients of water and solid material respec­tively. The above model assumes that the latexsuspension layer of thickness d may be consider­ed as consisting of two separate layers of thick­ness, d I and d 2' of water and solid material res-

370 PERTANIKA VOL. 9 NO.3, 1986

GAMMA RAY ATTENUATiON IN HEVEA LATEX

pectively, i.e. d = d1

+ d 2. Equation (1) cannow be written for latex as

The volume fractions of water and solid materialcan easily be shown as

~In(I/IZ)

at .-'in(i~/I;) (8)d

az = ~z = ~ili.Il~ (9)d In(Iz/I t )

Where 11 is the transmitted gamma ray intensitywhen the sample thickness d is full of water (i.e.d = d l' 12is transmitted gamma ray intensitywhen the sample thickness d is full of solidmaterial (i.e. d = d 2) and I is the transmittedgamma ray intensity when the sample thicknessis full of latex.

In practice a\ and a2

may not bedetermined from equation (8) and (9) simplybecause the intensities of gamma rays I \ and 1 2could not be measured directly from the use oflatex as sample. By measuring at two differentenergies for various concentrations of latex, thevalues of J.Lj and IJ. 2can be evaluated. The atte­nuation coefficients of latex for two gamma rayenergies E \and E 2can now be written as

immediately diluted with distilled water in thelaboratory. This was to prepare the samples withlow DRC. The samples with DRC ranging-from5·% to 35 % were put into s~all plastic bottlesand sealed to minimise the loss -of water due toevaporation. For each concentration, at leastthree samples were prepared from different bulksamples. The height of each sample in the bottlewas kept to 2 cm. They were then ready for theattenuation measurement onthe following day.DRC measurements by the Standard LaboratoryMethod were carried out based on MalaysianStandard MS 3:35:1975 (SIRIM, 1975).

The linear attenuation coefficients of thesamples of different DRC were determinedseparately by transmission measurements of thecollimated gamma ray beam. The experimentalarrangement employed a lithium drifted siliconSi(Li) solid state detector, a preamplifier, anamplifier and a multichannel analyser (MCA) asin the earlier experiment elsewhere (Elias et ai.,1983'). The radioactive source of Am was avail­able at low intensity of about 1.0 X 10 6 Bqwhich emits low energy photons at 17.7 keY and26.4 keV. The counting time of about 20minutes has been employed although this couldeasily be reduced by the use or a radioactivesource of higher activity. The sample was placedhalf-way along the beam which has a fixedgeometry corresponding to 5 mm in collimatordiameter and 8 cm source-detector distance.

(10RESULTS AND DISCUSSION

The component ratios of a\ and a:2

by latexcould be obtained by solving equations (10) and(11).

MATERIALS AND METHODS

Field Hevea Brasiliensis latex samples werecollected from the Universiti Pertanian Malaysiafarm during nannal tapping., These sampleswere taken iIi five working days at differentlocations. On each day at least 10 rubber treeswere randomly selected to obtain the requiredamount of bulk sample. The samples were

A total of 27 samples of various DRC levelswas used in the experiment. These samples wereprepared from five bulk samples of latex. Theirlinear attenuation coefficients of gamma ray areshown in Figure 1. Each point on the graphrepresents an average reading from three dif­ferent samples of the same DRC. Using a leastsquare curve fitting method, two straight lineswere drawn to represent the attenuations of latexat gamma ray energies 17.7 keY and 26.4 keYrespectively. These lines were used to calcul~te

the linear attenuation coefficients of water andsolid material of latex. Their values are shown inTable 1. Solid natural rubber samples suppliedby the Ru:bber Research Institute of Malaysia

PERTANIKA VOL. 9 NO.3, 1986 ·371

ELIAS SAlON. DEOU SEANG SIN. ZAINAL A. SULAIMAN. SABIRIN H. IKHSAN AND HUSIN WAGIRAN

plex compositions are known, we simply cannotignore these two competing values as they arefound in similar manner for water of latex anddistilled water. We were unable to determine thelinear attenuation coefficients at higher DRCconcentrations due to the fact that the basiccOInponents of freshly tapped natural rubberlatex constitute about 20% to 48% solidmaterials other than water (Chin, 1979). Laticeswith higher DRC values may be obtained bycentrifuge but could not be employ.ed herebecause water is not easily separated from latex.The measurement would give false attenuationcoefficients at higher concentrations if serumwere to be separated from the bulk samples.

o~,t·, '-_IfI_____.

...c.~. 1°"

i~ 0"

uco .,';:;co::::JC I"

~~ o··r--'-T-r-''-r-"''--..-."''--.....,~~ ~ ••....y-:.J

Dry Rubber Content by Weight

Fig. 1: Gamma ray attenuation coefficients ofHevea Latex versus DRC by weight (%)

TABLE 1Calculated values of linear attenuation coefficients

of water and solid materials

Linear attenuation coefficient11 (cm -I) + 0.005 I

Component materials E = 17.7 keV E = 26.4 keV

The attenuation coefficients of water andsolid material were then used to determine thevolume fractions of water and solid materialusing equations (10) and (11). Here, 32 moreDRC and attenuation measurements were per­formed using field latices taken from other bulksamples. Figure 2 shows the plot of percentage ofthe actual DRC against the DRC measured bythe attenuation technique. The correlation co­efficient is 0,998 and the standard deviation isabout 2% unit of DRC. All calculations wereperformed on the UNIVAC computer system ofthe Computer Centre, Universiti PertanianMalaysia.

Water (in latex) 0.998 0.418

Solid material 0.464 0.163

(in latex)

Distilled water 0.996 0.442

Natural rubber 0.479 0.216

(RRIM samples)

(RRI) and distilled water were used for compa­rison. We have a good agreement at the lowerenergy but a deviation at higher gamma rayenergy. The reason for this could be due to un­certainty in the counting statistics at the higherenergy with the use of a weak radioactive source.The attenuation coefficients of solid material oflatex was slightly lower than th.at of RRIMrubber sample. This difference may be cOI)Sider­ed insignificant in view of their differences inatomic compositions. Nevertheless, if their com-

g a

';:;CO::::JC

~CO

>~CO .0

EECO .1OJ

>.0 •

Ucca •

Fig. 2:

o 0

DRC by Standard Laborator.y Method

The correlation of DRC measured bygamma ray attenuation method versus DRCby Standard Laboratory Method.

372 PERTANIKA VOL. 9 NO.3. 1986

GAMMA RAY ATTENUATION IN HEVEA LATEX

CONCLUSION

We have demonstrated the measurement ofthe component ratios of the total volume ofwater and solid material in multiphase system oflatex by gamma ray attenuation technique. Themethod proposed in this preliminary work gives an

indication that the possibility of non-destructiveand direct determination of TSC of the Hevealatex which could be improved to determineDRC in the presence of adultrants in latex if themultiphase system is applied. However, moreinvestigation is required to establish the capa­bility of this technique in cooperation with asimple detection system which could replace thepresent sophisticated one.

ACKNOWLEDGEMENTS

We would like to thank Dr. Kasiran Buangof the Faculty of Science, Universiti TeknologiMalaysia for the use of gamma ray detectionsystem and Dr. Mohd Yusof Sulairrian and Dr.

.Abdul Halim Shaari of the Faculty of Science andEnvironment Studies, Universiti PertanianMalaysia for their valuable support in thisproject.

We are most indepted to Mr. Lee Siak Kuanof the Nuclear Physics Laboratory I UniversitiTeknologi Malaysia for his technical assistancethroughout the project. The cooperation of theRubber Research Institute of Malaysia in supply­ing the natural'rubber is appreciated.

REFERENCES

ABOUELWAFA. M.S.A. and KENDALL. E.J.M. (1980):The measurement of component ratios in multi­phase systems using gamma ray attenuation. j.Phys. E: Sci. Instrum. 13: 341 - 345.

ABRAHAM. P.D. (1970): Field trail with ethrel. Plr'sBull. Rub. Res. lnst. Malaya. Ill: 365 - 369.

CHIN. H.C. (1979): Analysis of latex concentration.RRIM Training Manual of Analytical ChemistryLatex and Rubber Analysis, RRIM. KualaLumpur. 77.

DE JONGE, P. (1965): Report on Chemaro tappingexperiments conducted in cooperation withRRIM. Plr's Bull. Rub. Res. lnst. Malaya. 80:195-198.

ELIAS. S.. ZAINAL ABIDIN. S.. ANUAR. A. and HUSIN.W. (1983): Determination of effective atomicnumber of rubber. Pertanika 6(3): 95 - 98.

KAIDA. K. (1982): Determination of dry rubbercontent of Hevea latex by microwave technique.Pertanika 5(2): 192 -195.

KAIDA. K. and MAHDI. A.W. (1983): Microwaveattenuation of fresh Hevea latex and measure­ment of ORC. J. Rubb. Res. lnst. Malaysia,31(3): 145 - 150.

Nc. E.K., P'NG. T.C. and LEE. C.K. (1970): Tappingsystem for young trees. Plr's Bull. Rub. Res. lnst.Malaya, Ill: 285 - 289.

SIRIM (1975): Methods of sampling and testing con­centrated natural rubber latices MS: 3 : 35 :1975.

(Received 16 Februar)1, 1985)

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