Self-absorption on radioactive air sample filters should be evaluated on acase-by-case basis.

Results of a Self-Absorption Studyon the Versapor 3000 47-mmFilters for Radioactive ParticulateAir Stack SamplingJ.M. Barnett,* V.I. Cullinan,† D.S. Barnett,* T.L.T. Trang-Le,* M. Bliss,*L.R. Greenwood,* and M.Y. Ballinger‡Abstract: Since the mid-1980’s the PacificNorthwest National Laboratory (PNNL) hasused a value of 0.85 as a correction factor forthe self absorption of activity for particulateradioactive air samples collected from buildingexhaust for environmental monitoring. Morerecently, an effort was made to evaluate thecurrent particulate radioactive air sample filters(Versapor® 3000, 47-mm diameter) used atPNNL for self absorption effects. There weretwo methods used to characterize the samples.Sixty samples were selected from the archive foracid digestion to compare the radioactivitymeasured by direct gas-flow proportional count-ing of filters to the results obtained after aciddigestion of the filter and counting again bygas-flow proportional detection. Thirty differentsample filters were selected for visible lightmicroscopy to evaluate filter loading and par-ticulate characteristics. Mass-loading effectswere also considered. Large error is associatedwith the sample filter analysis comparison andsubsequently with the estimation of the absorp-tion factor resulting in an inadequate method toestimate losses from self-absorption in the sam-ple filter. The mass loading on the sample filteras determined after digestion and drying was�0.08 mg cm�2; however, this value may not

represent the total filter mass loading given thatthere may be undetermined losses associatedwith the digestion process. While it is difficultto determine how much material is imbedded inthe filter, observations from the microscopyanalysis indicate that the vast majority of theparticles remain on the top of the filter. Incomparing the results obtained, the continueduse of 0.85 as a conservative correction factor isrecommended. Health Phys. 97(Supplement 3):S161–S168; 2009

Key words: operational topics; air sampling;correction factors; radioactivity, environmental

INTRODUCTIONSince the mid-1980’s, the Pa-

cific Northwest National Labora-tory (PNNL) has used a value of0.85 as the correction factor forself absorption of alpha particlesemitted from the particulate ma-terial collected on building ex-haust air sample filters (Higby1984). This correction factor hasalso been equally applied atPNNL to samples analyzed forbeta particles. This value ac-counts for activity that cannot bedetected by direct counting of

alpha and beta particles. Emis-sions can be degraded or blockedby filter fibers for particles buriedin the filter material or by inac-tive dust particles collected withthe radioactive particles. ANSI/HPS N13.1-1999, Sampling andMonitoring Releases of Airborne Ra-dioactive Substances From theStacks and Ducts of Nuclear Facili-ties, recommends that for filtermedia, if the penetration of ra-dioactive material into the collec-tion media or self-absorption ofradiation by the material col-lected would reduce the countrate by more than 5%, a correc-tion factor should be used (ANSI1999).

Over the past 6 years, the Efflu-ent Management group coordi-nated an effort to evaluate thecurrent particulate radioactive airsample filters (the Versapor3000,§ 47-mm diameter) used atPNNL for self-absorption effects.The two methods employed were(1) a comparison between themeasured gross alpha and grossbeta activity deposited on samplefilters as measured with a gas flowproportional detector against themeasured activity after the sam-ple filters were acid digested and

* Pacific Northwest National Laboratory, PO Box999, Richland, WA 99352; † Battelle Marine Sci-ences Laboratory, 1529 W Sequim Bay Rd, Sequim,WA 98382; ‡ Battelle Seattle Research Center,1110 Dexter Ave N, Suite 400, Seattle, WA 98109.

§ Pall Gelman Versapor Membranes, Krackeler Sci-entific, Inc., 57 Broadway, Albany, NY 12202.

Matthew Barnett is the Radioactive Air Task Lead at the Pacific Northwest NationalLaboratory in Richland, Washington; he is also an adjunct faculty member at theWashington State University Tri-Cities. He is also active in supporting the U.S. Departmentof Energy activities associated with 40 CFR 61, Subpart H and ANSI N13.1-1999. Hisprofessional interests include low-level radiation detection, liquid scintillation spectrome-try, and environmental sampling. He received his MSE and BSE degrees in engineering fromArizona State University. His personal interests include snowboarding and following thestock market. His email address is matthew.barnett@pnl.gov.

Operational Topic

Operational Radiation Safety S161

the sample material dried onto aplanchet and analyzed againwith a similar counting geometryby a gas flow proportional detec-tor and (2) reflected light micros-copy. Mass loading was also con-sidered as a part of the filteranalyses conducted.

Versapor® membrane filters arecomposed of an acrylic copoly-mer on a nylon substrate. Theyare manufactured in pore sizesranging from 0.2 to 3 �m, andthe filter diameters range from 25to 293 mm. For uses in air filtra-tion, the typical flow rates rangefrom 1.8 to 52 L min�1 cm�2. Thetypical filter thickness is 190 �m(Krackeler Scientific, Inc. 2008).

The performance characteris-tics of Versapor 3000 filters (3 �mpore size) with a 47-mm diameterwere investigated. When in-stalled in the filter holder (Fig. 1),they have a �41-mm active di-ameter (13.2 cm2 active samplingarea). Samples are collected over a2-wk period. Prior to samplingradioactive air emissions, theexhaust air passes through aminimum of one high-efficiencyparticulate air (HEPA) filter. How-ever, in some cases unfilteredbuilding air and the filtered ex-haust may be combined creatinga mechanism for additional unfil-tered non-radioactive particles toalso be sampled. Sample flowrates generally range from 28 to85 L min�1, which is on the lowend of the maximum rated flowrate of 900 L min�1 (52 L min�1

cm�2). Face velocities range from0.35 to 1.1 m s�1. Barnett and

Kane (1993) have previouslyshown that the Versapor 3000 fil-ters may be operated in this rangeof unfiltered systems withoutsampling volume degradationdue to particulate loading. At theend of the sampling period, thesample filter is removed and sub-mitted for gross alpha and grossbeta analysis.

The self-absorption correctionfactor of 0.85 was based on dataobtained from the direct alphacounting and photon spectrome-try of glass-fiber filter samples(Higby 1984). The filters were ex-posed to 0.66 �m, 1.72 �m, and3.07 �m monodisperse aerody-namic diameter particles overface velocities between 0.5 and2.0 m s�1 incremented in 0.5 ms�1 steps. Three tests were con-ducted at each of the four facevelocities used for each particlesize available resulting in a totalof 36 tests conducted. Monodis-perse particles of 239PuO2 weresuspended in a solution of am-monium hydroxide with a pH of10.0. Test aerosols were generatedfrom these solutions using a com-pressed air aerosol nebulizer. Thestudy involved conditions whichwere designed to maximize theeffects of particle size and veloc-ity on burial depth in glass-fiberfilters. The smallest particle size(0.66 �m) was chosen for use atPNNL because it was closest tothe theoretical typical penetrat-ing size for fiber filters (0.1 to 0.3�m) and could potentially ac-count for the burial losses at thevelocities tested. There were foursets of data at the smallest parti-cle size yielding an overall self-absorption factor of 85% � 9%(1�) as determined by the re-ported mean fraction detected re-sults. Higby concluded that “acorrection which assumes 10 to15% losses would ensure thatconcentrations of airborne alphaemitting radionuclides would notbe underestimated by collectionand analysis on glass-fiber fil-ters.” Considering all of the re-

ported mean fraction detected re-sults for the 0.66 to 3.07 �mrange, a self-absorption factor of88% � 13% (1�) was obtained.

In addition to Higby (1984),other studies by Haung et al.(2002), Luetzelschwab et al.(2000), and Stevens and Toureau(1963) have discussed major fac-tors affecting measurements ofradioactivity on air samples col-lected on filters. These factors in-clude collection face velocity,particle size, type of filter, andfilter loading. Higby showed thatsmaller particles have higherlosses due to burial in the filtermatrix and that losses are less asvelocity and particle size increasedue to inertial impaction on thetop layer of the filter; he reportedthe minimum burial depth for analpha particle to be lost due toabsorption is �3.7 mg cm�2. Thisis the calculated range for 239Puparticles in glass so alpha parti-cles would be difficult to count atthe bottom of loadings greaterthan this value. Stevens and Tou-reau (1963) investigated filtertypes, dust loading, and particlesize on the energy spectrum andnoted that glass fiber filters havea near surface collection with de-tection efficiencies of 85 to 100%of 0.2 to 1.0 �m radioactive par-ticles under low face velocities of0.35 m s�1. The tendency is forhigher losses in alpha radiationas dust loading on the filterincreases. Luetzelschwab et al.(2000) recommended assuming28% losses for a loading of 2.3 mgcm�2 and 40% when the loadingis increased to 3.3 mg cm�2.Haung et al. (2002) showed thatloading is not important if theradioactive layer is a thin layer ontop of the nonradioactive dust(e.g., detecting a sudden pulserelease of radioactivity) ratherthan distributed throughout. Thesefactors are considered in thisstudy for radioactive air samplingfrom PNNL building stacks, andresults from actual stack samplesare presented.

Figure 1. Sample filter on fixed head ra-dioactive air stack sampler.

J.M. Barnett et al. Results of a self-absorption study

S162 November 2009

METHODSSample filter count and acid digestcount comparison

Sixty samples were selectedfrom the archive of sample filtersthat showed both a sample par-ticulate loading and had also re-turned a positive result for radio-active material. These sampleswere sent back to the laboratoryfor further analysis; first bycounting the filters again andthen dissolving the particulate(radioactive and non-radioactive)material using acid digestion, dry-ing the solution onto a planchet,and conducting a second count.The assumption was that digest-ing the sample filter would re-cover all radioactive material,even that embedded in the filter,and the self-absorption factorcould be determined by takingthe result of the first filter countand dividing by the result of thedigested count. An average self-absorption factor could be deter-mined from the results.

The selected sample filters werecounted 500 min each in a 2�geometry by gas-flow proportionalcounting using a LB4100/W LowBackground Alpha/Beta Count-ing System** as if they had justbeen collected. The 47 mm filteris mounted directly on a planchet(no chemical processing) withthe loaded face of the filter ex-posed. Two-sided tape is placedon a 50.8-mm-diameter by 3.2-mm-deep stainless steel dish andthe filter is lightly tapped downusing tweezers or small glass rod.The filter may not extend beyondthe lip of the planchet dish andadditional tape may be used tohelp hold down a curled filter.Commercial grade P-10 gas (90%argon and 10% methane) is usedwith the detector system duringcounting. Alpha and beta instru-ment crosstalk is controlled byadjusting the instrument discrim-

inator settings. The results werelogged, and then the individualsamples were run through anacid-digest process to separatethe radioactive material from thefilter and other non-radioactiveparticulate matter.

The individual digested sam-ples were again counted 500 mineach by gas-flow proportionalcounting. The sample material isfirst dissolved in a small amountof 16 M HNO3 and then trans-ferred to a tared sample planchet.The transfer vessel is rinsed atleast three more times using 2- to8-M HNO3 and each time addingthe rinsate to the planchet to as-sure quantitative transfer of thesample material. The solution isevaporated under a heat lamp todryness and heated until thereare no visible fumes. The taredsample planchets are then weighedagain to determine the mass of thesample material deposited. Driedsamples are then counted usingthe same LB4100/W Low Back-ground Alpha/Beta Counting Sys-tem and the results were logged.Sample process controls for theacid digestion included preparingsample blanks and sample spikesfor analysis with the regularsamples; these results are usedin determining the digestionprocess efficiency. Results fromboth counting processes werecompared.

The LB4100/W Low Back-ground Alpha/Beta Counting Sys-tem has the capability to count16 samples at one time. The aver-age alpha detector efficiency is35 � 3% (1�), and the averagebeta detector efficiency is 55 �3% (1�).

There are several sources of er-ror associated with the sampleresults and are compensated forin determining the overall sam-ple activity uncertainty. For thestandard counting of a filter sam-ple, these include net uncertaintyin counts, decay factor uncer-tainty, detector efficiency uncer-tainty, and particulate abundance

uncertainty. When counting di-gested samples that are evaporatedonto a sample planchet, additionalerrors may be introduced includ-ing those associated with the diges-tion process, sample size, massloading, moisture absorbed by thesample residue after preparationand prior to counting, and non-uniformity of the sample residueon the sample planchet. Other ef-fects affecting the sample result in-clude the count time, background,and counting efficiency.

Mass loading evaluation

An effort to weigh the samplefilters in an attempt to measurethe weight of the sample materialanalyzed was evaluated. Becausethe archived filters had not beenweighed before use in the field,50 new filters were weighed in aneffort to determine an averagefilter tare weight (Mettler-ToledoAT400†† analytical balance) thatcould be used as a basis to esta-blish a probable sample masson each filter. In addition toevaluating the 50 new filters,the sample planchets were alsoweighed before and after the so-lution was dried down to esti-mate an average mass loading for20 samples.

Reflected light microscopy

A set of 30 random sample fil-ters were selected from the ar-chive for evaluation. The samplefilters had a varying degree ofparticulate loading where the se-lection of light, medium, andheavy particulate loading was de-termined by visual appearance.These 30 sample filters were com-pared to a set of unused new filters.A reference image was taken ofthe entire area of each samplefilter using a digital camera. Mag-nified images of the surface werecollected using an Olympus BXSeries‡‡ microscope.

** Canberra Industries, Inc., (formerly Oxford Nu-clear Measurements Group, Oxford InstrumentsLtd.), 800 Research Parkway, Meriden, CT 06450.

†† Mettler-Toledo, Inc., 1900 Polaris Parkway, Co-lumbus, OH 43240.‡‡ Olympus America, Inc., 3500 Corporate Park-way, Center Valley, PA 18034.

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Table 1. Stack air sample filter analysis results showing gross alpha and gross beta filter results from direct gas flowproportional counting and again after acid digestion.

Sample id.

Alpha Beta

Initial filter results(mBq sample�1)

Digested results(mBq sample�1) Ratio

Initial filter results(mBq sample�1)

Digested results(mBq sample�1) Ratio

3047 54 47 1.15 75 213 0.353048 25 19 1.32 40 142 0.283049 3 2 1.43 3 64 0.043050 73 42 1.72 113 58 1.973051 36 25 1.42 59 147 0.403052 1 1 1.06 4 30 0.153053 3 4 0.83 21 12 1.773054 11 11 0.99 17 13 1.283055 50 38 1.32 68 88 0.783056 10 5 1.90 16 21 0.773090 10 5 1.97 15 5 2.813091 4 4 1.24 9 79 0.113092 5 8 0.70 9 8 1.093093 3 4 0.90 4 6 0.673094 57 49 1.16 105 99 1.063095 33 26 1.25 56 45 1.243096 34 33 1.02 51 49 1.043097 79 62 1.27 124 107 1.163098 6 3 1.77 11 4 2.773099 283 172 1.65 407 283 1.443180 11 4 3.01 16 13 1.223181 462 115 4.04 610 339 1.803182 2 2 1.50 75 4 16.613183 264 30 8.73 380 245 1.553184 312 38 8.13 466 143 3.263185 123 71 1.73 175 79 2.223186 137 39 3.54 189 55 3.473187 31 13 2.30 46 42 1.093188 76 25 3.04 107 72 1.493189 44 15 2.86 68 46 1.483190 24 10 2.48 33 27 1.203191 14 8 1.85 39 21 1.853192 6 4 1.60 31 10 3.273193 249 51 4.91 383 187 2.043194 754 212 3.56 1,031 363 2.843195 289 193 1.49 394 195 2.013196 32 10 3.21 49 15 3.263197 40 12 3.31 47 24 1.993198 22 11 1.97 34 34 1.003199 22 7 3.22 33 22 1.463348 242 205 1.85 358 295 1.653349 426 282 1.65 577 425 2.683350 133 65 0.97 186 95 1.053351 15 10 1.47 24 9 1.203352 51 23 1.10 78 37 1.373353 116 74 1.23 173 102 1.633354 52 24 1.43 69 28 1.323355 35 22 1.46 49 38 1.313356 64 36 3.04 91 72 6.753357 263 156 1.29 364 271 1.323358 312 169 1.18 448 271 1.223359 88 53 1.51 109 41 1.363360 171 177 2.05 262 250 1.953361 107 73 1.46 139 116 2.653362 6 5 2.19 13 10 2.113363 84 69 1.57 128 79 1.693364 218 153 2.13 297 225 2.503365 228 157 1.60 318 242 1.283366 55 18 1.81 66 10 1.263367 17 13 1.69 24 18 1.34

Average 2.09 1.90Standard deviation 1.46 2.182� 2.86 4.27Max 8.73 16.61Min 0.70 0.04

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S164 November 2009

The Olympus BX Series micro-scope system consists of OlympusMicroSuite software, a digital SoftImaging System ColorView II™digital visible light camera, a 20Xobjective lens calibrated with astage micrometer, and a Prior Sci-entific ProScan II™ motorizedmicroscope stage. The OlympusMicroSuite software calculates andinserts the scale bar based upon areference measurement. The Mi-croSuite software controls theProScan™ stage which allows thesoftware to create a surface recon-struction, called the Extended Fo-cal Image, from multiple imagescollected in the z-dimension. Thesoftware also uses the images col-lected in the z-dimension to createa topographic 3D rendering of thesurface. A series of images werecaptured for each sample over adistance of approximately 0.03mm in the z-dimension. Both theExtended Focal Image and 3D ren-derings were necessary for evalua-tion. The 3D rendering can beused to exaggerate the z-dimensiontopography.

The upper illuminator of theBX series microscope did not pro-vide adequate image contrast, sotwo auxiliary lights were placedabove either side of the micro-scope stage. Typical particle sizeswere measured on select samplefilters. Although a range of parti-cle sizes were measured, distribu-tion statistics were not acquired.Particle shapes were too irregularto assume equivalent sphericaldiameter. Dimensional measure-ments of representative capturedparticles were in the micrometerrange for these samples. The goalwas to obtain a range of the typ-ical particle sizes of sample mate-rial and to estimate the extent ofmaterial burial within the filter.

RESULTSSample filter count and acid digestcount comparison

Because of the low emissionrate, the radioactivity on stack

samples from PNNL facilities areoften below detection limits. Forthis study, archive samples wereselected so that results were likelyto be above the critical level§§based on previous readings andthus would have detectable ra-dioactivity. The measurements ofradioactivity on the filter beforeand after digestion are shown inTable 1 for the 60 samples ana-lyzed. Results indicate that the al-pha results yielded a before/afterdigestion ratio (self-absorption fac-tor) of 2.1 � 2.9 (2�) and a median

value of 1.7. Thus, on average,more activity was measured by di-rectly counting the filters beforedigestion than by counting afteracid digestion. Only 5 samples hadgreater activity after acid digestionthan before. The minimum ratiowas 0.7, and the maximum ratiowas 8.7 with a wide variation en-compassing the value of 1 whereno difference can be detected be-tween the two separate counts(no self-absorption), 0.85 (currentself-absorption correction factor),and many other values.

Similar results were seen forbeta analysis and these are alsopresented in Table 1. The betaresults yielded a before/after di-

§§ The critical level is the net sample count (grosscounts minus background counts) which must beexceeded before the sample is said to contain anymeasureable radioactive material above background(Currie 1968).

0

50

100

150

200

250

300

350

0 100 200 300 400 500 600 700 800 900

Initial Alpha (mBq sample-1)

Fina

l Alp

ha (m

Bq

sam

ple-1

) R2 = 0.6636

Figure 2. Plot of the filter data counted before and after digestion—alpha activity.

0

100

200

300

400

500

600

0 200 400 600 800 1000 1200

Initial Beta (mBq sample-1)

Fina

l Bet

a (m

Bq

sam

ple-1

)

R2 = 0.7583

Figure 3. Plot of the filter data counted before and after digestion—beta activity.

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gestion ratio of 1.9 � 4.3 (2�) anda median value of 1.4. It is againshown that more activity wasmeasured by directly countingthe filters before digestion thanby counting after acid digestion.There were 8 samples where theactivity after digestion wasgreater than the initial activitymeasure in the sample. The min-imum ratio was 0.1, and the max-imum ratio was 16.6.

The before and after digestionresults were plotted against eachother to further explore the data.Figs. 2 and 3 show the plots andthe coefficients of determination(R2 values) for alpha results andbeta results, respectively. A rela-tionship can be seen in both fig-ures, but the R2 values (the pro-portion of variance explained bythe model) are weak at 0.66 foralpha results and 0.76 for betaresults.

The method used did not pro-vide results suitable to quantifyself-absorption in actual stacksamples. As mentioned earlier, er-rors associated with the initialsample filter counts and the addi-tional errors from the acid diges-

tion process may, in part, ac-count for the low correlation inthe data.

Mass loading evaluation

Fifty new glass fiber filters wereweighed to establish a standardtare weight. The average mass ofthe 50 new filters was 0.102 �0.008 (1�) g and the range was0.09 to 0.12 g. The ratio of themaximum to minimum weighedfilter was 1.39. These effectsraised concern as to whether thetare weight of the archive samplematerial could be preciselyenough established without hav-ing pre-weighed them individu-ally before use. Filter weight gainhas been reported as 5 to 30 mg(Luetzelschwab 2000) for envi-ronmental air samples with aone-week collection period; thisgain is in the noise of samplefilter tare weight uncertainty.

Twenty of the archive sampleswere weighted prior to the aciddigestion process (Table 2). Forthese 20 samples, the averageloaded filter weight was 0.104 �0.006 (1�) g and the range was0.09 to 0.12 g, which is very sim-ilar to the unused new filter data

above. The ratio of the maximumto minimum archive sample filterweight was 1.25. For comparison,the results of six filter blanks thatwere also analyzed in this processresult in an average filter weightof 0.109 � 0.003 (1�) g.

The weight of the 20 samplesremaining after digesting and dry-ing onto the planchets averaged1.1 � 0.6 (1�) mg per sample (Ta-ble 2). Assuming that this also rep-resents the sample catch beforedigestion, the mass loading wouldhave averaged 0.08 � 0.05 (1�) mgcm�2 on the active area of the sam-ple filters. For the digested filterblanks, the average net filter blank

Figure 4. Reflected light microscopy ex-tended focal image showing particle sizeson a used sample filter with light particu-late loading.

Table 2. Mass results of sample filters and net weight results of digested samples.

Sample id. Filter weight (g) Planchet gross weight (g) Planchet tare weight (g) Net sample weight (g)

3047 0.1112 7.7116 7.7102 0.00143048 0.1092 7.6891 7.6884 0.00073049 0.1016 7.7208 7.7197 0.00113050 0.1046 7.6825 7.6821 0.00043051 0.1066 7.6917 7.6911 0.00063052 0.1005 7.6641 7.6631 0.00103053 0.1003 7.7008 7.6989 0.00193054 0.1067 7.6450 7.6442 0.00083055 0.1024 7.7026 7.7006 0.00203056 0.0997 7.7184 7.7178 0.00063090 0.1064 7.6615 7.6607 0.00083091 0.0945 7.6835 7.6826 0.00093092 0.1032 7.7379 7.7349 0.00303093 0.1084 7.6925 7.6912 0.00133094 0.1166 7.6561 7.6553 0.00083095 0.0979 7.7040 7.7033 0.00073096 0.1000 7.7050 7.7040 0.00103097 0.0935 7.6708 7.6700 0.00083098 0.1047 7.7167 7.7158 0.00093099 0.1117 7.6676 7.6672 0.0004

Average 0.1040 7.6911 7.6901 0.0011Standard deviation 0.0058 0.0246 0.0243 0.00062� 0.1166 7.7379 7.7349 0.0030Max 0.0935 7.6450 7.6442 0.0004Min 0.1112 7.7116 7.7102 0.0014

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weight after digestion was 0.8 �1.0 (1�) mg; two of the filter blankshad a negative net sample weight.Given that on average about halfthe radioactivity was measured af-ter digestion vs. before digestion, itis possible that some losses oc-curred in the digestion and dryingprocess which would affect thepre-digestion mass estimates.

Based on these results, it wouldbe difficult to ascertain filterweight gain in the archived sam-ples with such a wide range oftare weight possibilities as evi-denced in the contrast between1) the new and archive samplefilter results, and 2) the netplanchet weights of the archivesamples and the filter blanks.While definitive sample massloading cannot be determined,for the 20 samples evaluated, amass loading of �0.08 mg cm�2

per sample was obtained.

Reflected light microscopy

It was observed from the visi-ble light microscopy that thevast majority of the particulateloading rests on top of the filterfiber media and it is difficult todetermine how much material,if any, is imbedded in the filter.The structural fibers within thefilter appear to be �30 �m indiameter. Particle sizes on thesurface were generally from 1 to10 �m with the typical size be-ing from 2 to 6 �m as shown inFig. 4. There is no indication ofparticulate overloading or parti-cles obscuring other particles.The 3D rendering (Fig. 5) allowsthe viewer to look topographi-cally at the fibers on the sampleface. The dark color on the edgeof the 3D renderings representsthe color of the sample surfaceat the edge of the image.

It is likely that radioactive ma-terial is well distributed through-out the particle loading as op-posed to a thin layer on top of aparticle layer as evaluated byHuang et al. (2002). However, thesize of radioactive particles com-pared to the size of nonradioac-tive particles is unknown. Theradioactive particles may be inthe smallest size range becausethey are emitted from HEPA-filtered locations before reach-ing the stack sampling systemwhereas other nonradioactivecontributions to the stack sam-ple may not be HEPA-filteredbecause they originate down-stream of filtration.

The downstream side of a sam-ple filter with heavy particulateloading was viewed under the mi-croscope in an effort to determineif any of the particulate penetratedthrough the sample filter. This

Figure 5. Composite view of a used sample filter with medium particulate loading showing the extended focal image, photo of actualsample filter, and 3D rendering.

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Operational Radiation Safety S167

evaluation found the downstreamside of the sample filter to be freefrom particulate matter. This is incontrast to the particles that wereroutinely found on the face of thesample filters.

CONCLUSIONLarge error is associated with

the sample filter analysis compar-ison and subsequently with theestimation of the absorption fac-tor resulting in an inadequatemethod to estimate losses fromself-absorption in the sample fil-ter. Issues that influence thedata results and the ability tomake a determination of the self-absorption factor include the sen-sitivity of the instruments used,the filter paper variations, massloading, and detector efficiencies.

Mass-loading of filters couldnot be determined because thetare weights of the archive filtershad not been previously mea-sured and a generic tare weighthad too much variability. Themass loading on the sample filteras determined after digestion anddrying was �0.08 mg cm�2. Thevalue may not be indicative ofthe total filter mass loading giventhat there may be undeterminedlosses associated with the diges-tion process.

While it is difficult to deter-mine how much material is im-bedded in the filter, observationsfrom the microscopy analysis in-

dicate that the vast majority ofthe particles remain on the top ofthe filter. Particle sizes collectedat face velocities of 0.35 to 1.0 ms�1 were typically in the 2- to6-�m range on sample filtersevaluated. There is no indicationof particulate overloading or par-ticles obscuring other particlesthat would create significant self-shielding. No particles were seenon the downstream side of thefilter as would be expected ifthere were significant particulatepenetration through the filter.

Sample analysis, mass load-ing, and reflected light micros-copy were used to obtain agreater understanding of theself-absorption of activity on Ver-sapor 3000, 47-mm-diameter fil-ters, used at PNNL for particulateradioactive air sampling collectedfrom building exhaust for envi-ronmental monitoring. The sam-ple analysis comparison indicatesmore activity is measured by di-rectly counting the sample filterrather than counting after theacid digestion process, however,the results were insufficient toquantitatively verify the currentself-absorption factor or to rec-ommend a different value. Com-paring analytical results with theresults from the mass loading andreflected light microscopy stud-ies, and recognizing the compet-ing factors between the massloading on the filter and the par-ticle penetration in the filter, the

continued use of a 0.85 self-absorption factor is a conserva-tive correction factor.

Acknowledgment: This work was conducted at the Pacific

Northwest National Laboratory which is operated for

the U.S. Department of Energy by Battelle under Con-

tract DE-AC05-76RL01830.

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