R ad io a ct iz, ity & R adio cb emistry

Vol. 8, No. 3, 1997

FROM THE COUNTINGROOM

Determination ol Beta-Particle Counting Elficiency for Wipe-Test Samples

Robert C. McFarlandAnalytics, Inc.

he purpose of this experiment was to establish a realistic counting efficiency using a gas-flowproportional counter for counting low-energy (approximately 300-keV maximum) beta-particle-

emitting radionuclides when performing removable contamination "wipe" tests. The sample countingefficienry was compared with the counting efficienry obtained from mylar-covered standard sources. Severalfactors were investigated to determine their effect on beta-particle counting efficienry such as: material of thewipe either paper or heavy cloth; force applied in wiping; the presence of various amounts of dirt on thesurface; and geometrical effects in counting.

Introduction\(hen performing removable contamination tests, whether assource leak tests, laboratory work-surface contaminationdeterminations, or to satisfy shipping regulations, it isimportant for the analysis to provide a realistic value. If thevalue determined from the analysis grossly overestimates theactual value, much time may be wasted decontaminatingobjects that really do not require decontamination. If theanalysis grossly underestimates the real value,,contaminationmay be allowed to spread throughout the work area orregulatory action may result if a contaminated shipment isreleased.

This study concentrated on determining a realisticcounting efficienry, recognizing that many other factors notconsidered can have large effects on the accvr^6/ of these typeof tests such as the technique of wiping and the choice of the"representative" place to wipe. A low-energy beta-particle-emitting radionuclide was chosen for study since it wasexpected to show the most variation in counting efficienrywith attenuation in the wipe sample and standards of low-enerry beta-particle-emitting radionuclides were expected toshow the most decrease in counting efficienry with increasingself-absorption in any covering or dead layer. The radionuclidechosen was eTc which has a maximum beta-particle energy of292keY and an average beta-particle energy of 101 keV.

Wipe-sample preparationIn order to prepare realistic wipe samples it was decided toevaporate gravimetrically dispensed aliquots of a solution ofDTc on plastic plates and wipe the dry surfaces as one wouldin actual practice. A plastic surface was chosen because itsimulates laboratory bench-top material. The effea of surfaceroughness was not investigated.

To determine the fraction of the activity of.nTc transferredto the wipe sample, a short-lived (6.007 h) gamma-ray-emittingtracer fhTc) was mixed with the beta-particleemitting eTc.

The mixture of hTc and eTc was prepared gravimetricallyfrom calibrated solutions. The use of the shortlived, gamma-ray-emitting tracer allowed the use of higher activity, thusshortening the counting times and improving the statistics forthe wipe-transfer fraction determination.

The first step in the preparation of the wipe samples wasthe gravimetric deposition of approximately 0.1 mL of thecalibrated solution of ehTc and eTc in a 25-mm-squarepattern of 5 drops in the center of a 10 x 10 cm plastic plate. Aweighed amount of pulverized soil was added to some of thesamples to simulate a dLrry surface. The plate was then &iedunder a heat lamp. After drying, the plate was centered on thecap of a FIPGe garnm -ray spectrometer and the 140.5-keVgamma ray f.rom e-Tc was measured. The plate was returnedto the laboratory and wiped for removable contamination

using either a 47-mmdiameter filter paper or cloth wipe.\7ipes were manually performed by making a series of fivesmall circles through the deposited activiry using the specifiedamount of force on the wipe. One series of wipes wasperformed using a moderate force equivalent to a 40Gg mass,estimated by pressing on a top-loading balance. Another seriesof wipes was performed using a havy force equivdent to a700-9 mass. The force was varied to determine if pressing theactive material into the wipe material would affect thecounting efficienry through self-absorption.

After the wiping, the plate was recounted for hTc usingthe same position on the FIPGe detector. The wipes werecounted 2 cm above the cap of the detector. The fraction ofthe activity left on the plate after wiping was determined bycomparing the counts of the plate before and after wiping.lfiping changed the distribution of the activiry on the platefor the after wiping count, possibly introducing a 1.0olouncertainty in the determination of the aaivity remaining onthe plate. This uncertainty would contribute less than 3olo tothe final uncertainty in the transfer fraction since the majorttyof the activity was transferred to the wipe. The wipingremoved between 75 and96o/o of. the aaivity. The agreementbetween determination of the transfer fraction from platecounting and from wipe counting was generally better than5oh. The uncertainty due to the unknown distribution ofac:ivity on the wipe makes that count less accurate fordetermining the transfer fraction. Once the transfer fractionwas determined, the activity of eeTc on each wipe wascalculated from the gravimetric data.

Beta-particle counting of wipe samplesAfter the gafiLma-ray determination of the wipetransferfraction, the wipes were allowed to decay for approximately72 h to remove any interference from the %Tc tracer. Eachwipe was placed in a stainless steel planchette. The wipes wereheld flat by a small amount of spray adhesive which had beenpreviously applied to the planchette. Each wipe was countedin a gas-flow beta-particle counter.

The beta-particle counting system used was a CanberraModel 2401 gas-flow proportional counter with a 57-mmdiameter, 16A ng/cr* gold-mylar window. The model 2401isa manual qfstem with a slide which holds one 51-mm (2 i".h)diameter planchette. The samples were counted in flat-bottomstainless steel planchettes having a 7-mm-high rim. The model2401 has some capabilrry for alpha-lbeta-particlediscrimination by pulseheight analysis. Alpha-/beta-particlediscrimination was not used in this experiment to avoid thecomplication of spill-over of beta-particle counts into thealpha channel which occurs with low-energy beta particles.lBeta-particle counting of the samples was performed with1650 V applied to the detector and the discriminator windowfully open.

R adio act io ity & Radio ch emistry

Vol. 8, No. 3, 1997

ResullsCounting efficiencies were determined for filter-paper andcloth wipes and taken from clean and dirry surfaces. The tworypes of wipes were used to determine if the radioactivematerial could work down into the more open cloth surfaceand thus give a lower counting efficiency. Duplicatedeterminations were performed on the clean surface and theaverages are reported. The effect of force on the wipe was alsostudied since it could cause the radioactivity to penetratedeeper into the wipe material. Between 6 mg and 21 mg ofpulverized soil was added to several of the samples todetermine the attenuation of dirt on the wipe. Table 1 showsthe beta-particlrcounting efficiencies determined for the filterpaper and cloth wipes under different conditions of force anddirt loading. Table 2 shows the counting efficiencies obtainedfor mylar covered simulated wipe standards. Standards wereprepared by gravimetric deposition of a large number ofmicroliter sized drops of a cdibrated solution on mylar.2 Themylar was placed with the activity down on a filter and fixedin a flat-bottom stainless steel planchette. Results are given forseveral different diameter sources and two thicknesses of mylarcovering.

GEIGER.TIUEllERRADIATIOI{ DEIECIORS

L N D m a n u f a c t u r e s a w i d e r a n g eo f G e i g e r - M u e l l e r ( G M ) a n d E n e r g yC o m p e n s a t e d G M D e t e c t o r s f o r y o u rs c i e n t i f i c , h e a l t h p h y s i c s , m e d i c a l , a n dinstrumentation aoolications.

LND's dependab le and opera t iona l l yp r o v e n d e t e c t o r s a r e d e s i g n e d a n dengineered for a long and stable countingl i fe. Our exacting manufacturing procedures and str ict,aud i ted Qua l i t y Assurance po l i c ies meet DCAS MIL-Q9 8 5 A , M I L - E - 1 , a n d A p p e n d i x B o f 1 0 C F R 5 0 Q u a l i t yC o n t r o l S t a n d a r d s A l l L N D d e t e c t o r s p a s s a r i g i d ,

T h i s i q \ / n r r r 2 q q r , r 2 n a c O funequalled periormance. Callor write for information.

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Reader Service No. 14

Radioactioity & Radio cbemistry

Vol . 8, No. 3, 1997

Filter paperFilter paperFilter paperFilter paper

ClothClothClothCloth

Table 1 Counting efficiencies for wipe samples.

0.5u.)0.850.850.85

40 mm on a side. The data shows that all of the mylar-coveredstandards give counting efficiencies in the range of theefficiencies determined from actual wipes counted on this gas-flow counter. The standards covered with 0.5 mg/cm2 mylarapproximate a clean-surface wipe and the standards coveredwith 0.85 mg/cr* mylar approximate the dirty surface wipes.In order to be conservative for health-physics purposes andeconomical at the same time, one 47-mm standard coveredwith 0.85 mg/cm2 could be used to determine the countingefficienry for wipes and air-filter samples counted on a gas-flow proportional counter. Calibrating with this one standardwould give accurate results when counring samples withmoderate to heavy dirt loadinp and give probably no morethan a l7o/o overestimation of the activitv in the case ofcounting a clean wipe.

References1. Alpha,/Beta Systems Model 2400 Operator's Manual,

Canberra Industries, Inc. 1984.

2. R.C. McFarland, "Geometric Considerations in theCalibration of Germanium Detector for Filter-PaoerCounting," Rad.ioact Radiochem., 2 (I), 4, tg9l.

BiographyRobert C. McFarlandis founder and President of Analytics, Inc. He holds six

patents in the area of radionuclide calibrationstandards. Prior to founding Analytics, he was amember of the faculty of the Georgia Institute ofTechnology Nuclear Research Center. He has a B.S.in Chemistry, and a M.S. in Physics from the GeorgiaInstitute of Technology.

Analytics, Inc.1380 Seiboard Industrial Blvd.Atlant4 GA 30318Phone: (404) 352-86nFax: (404) 452-2837

R&R

ModerateHeavyHeavyHeavy

Moderate

CleanClean

0.2330.2280.2t60.2050.2270.2330.20t0.r92

618

CleanClean

102t

HeavyHeavyHeavy

454L47J )

)4

0.2tt0.2300.193A 1a''

0.229

Table2 Counting efficiencies for standards,

GonclusionsFrom the results in Table 1 it can be seen rhar the countingefficienry for beta-particles of approximately 300-keV marci-mum energy does not vary significantly with the type of wipe(filter paper or cloth) nor with the force applied to rhe wipewhen wiping a smooth surface. The average countingefficiency for the four clean-surface wipes is 0.230 with astandard deviation in the group of only t.2Vo. The counringefficienry does depend on the dirt loading. Only 6 mg of dinon the surface lowers the counting efficiency on the wipe byabour 60/o and2l mg lowers the efficienry by I7o/o.If a typicalwipe is taken from 100 cm'� of surface, these dirt loadingswould correspond to 0.06 mg/cnf up to 0.21 mg/cm2 of.surface dirt loading. A wipe of a typical "clean" floor generallypiclrs up 1-3 mg of dirt. Dirt loadings of LS-ZO mg should notbe encountered in routine counting as wiping dirq' surfaces isnot recommended due in part to the adverse effects onremoval efficienc'J. Heavy dirt loadings were used here todemonstrate the effect on counting efficienry.

The results in Table 2 from counring mylar-coveredstandards show that the counting efficienry varies with thediameter of the standard and with the thickness of the mvlarcovering. The geometry of the active material on the actualwipe samples is approximately a square of between 35 and

a