Thermoluminescence dosimetry is the principal means of measuring ambient $gamma$ radiation at the Lawrence Livermore Laboratory. These dosimeters are used at 12 perimeter locations and 41 locations in the off-site vicinity of the Laboratory, and are exchanged quarterly. Control dosimeters are stored in a 75- mm-thick lead shield located out-of-doors to duplicate temperature cycling of field dosimeters. Effect of dosimeter response to radiation in the shield is determined each quarter. Calibration irradiations are made midway through the exposure cycle to compensate for signal fading. Terrestrial exposure rates calculated from the activities of naturally occurring uranium, thorium, and potassium in Livermore Valley soils vary from 3 to 7 $mu$R/hr. Local inferred exposure rates from cosmic radiation are approximately 4 $mu$R/hr. TLD measurements are in good agreement with these data. Off-site and site perimeter data are compared, and differences related to Laboratory operations are discussed. (auth

Lindeken, C.L.

Sundbeck, C.W.

Toy, A.J.

White, J.H.

UNT (University of North Texas) Digital Library

^ PREPRINT UCRL- 77106 Lawrence Livermore Laboratory AMBIENT ENVIRONMENTAL RADIATION MONITORING AT THE LAWRENCE LIVERMORE LABORATORY C. L. Lindeken, J. H. White, A. J. Toy and C. W. Sundbeck November 21, 197 5 This paper was prepared for presentation at the Health Physics Society Ninth Midyear Topical Symposium on Operational Health Physics, Denver, Colorado, February 9-12, 1976. This is a preprint of a paper intended for publication in a journal or proceediTffe. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author. DISTRIBUTION OF. THIS DOCUI'.ir,'.; lo UNLIMITED rmmr EWIFONMEOTAL RADIATION KWITORMG AT THE LAWREUCE LTVEFWJEE LABORATORY* C. L. Llndeken, J . H. White, A. J . Toy, and C. W. Sundteck Lawrence Livermore Laboratory, UrJ.versi.ty of California P.O. Box 308, Livermore, California, 9*550 Abstract Thermoluminescence dosimetry Is the principal means of measuring ambient ganira radiation at the Lawrence Livermare laboratory. These dosimeters are used at 12 perimeter locations and 41 locations in t;ie off-site vicinity of the Laboratory, and are exchanged quarterly. Cc.trol doslTeters are stored i/i a 75-mm-thick lead shield located out-of-doors to d e l i c a t e tenperature cycling of field dostiteters. Effect of dosimeter response to radiation in the shield is determined each quarter. Calibration lrradiatior.s are made midway through the exposure cycle to compensate for signal fading. TerrestrL-.l exposure rates calcu­lated from the ac t iv i t ies of naturally occurring uranium, thorium, and potassium in Uvernore , J alley so i l s vary from 3 to 7 pR/hr. Local Inferred exposure rates from cosMc radiation are approximately 4 uR/hr. TLD measurements are in good agreement with these data. Off-site and s i t e perimeter data are conparad, and differences related to Laboratory operations are discussed. Introduction Today's eiphasis m environmental monitoring i s primarily due to public concern for environmental quality, which gained national attention during the la te 1960s. For nuclear ins ta l la t ions , ambient gsr*sa radiation measurements are normally an integral part of environmental monitoring. Typically, therraluTi-nescence dosimetry is the principal means of making these measurements. The Lawrence Livermare Laboratory (LLL) locally operates a number of nuclear fac i l i t i es having a potential for radiological impact. Our ambient monitoring pr gram assesses any such Impact. At LLL, thermoluminesQence dosiiaaters are used at 12 perimeter locations and m legations in the off-s i t e vicinity of the laboratory. This paper describes the TLD procedures which have been developed over the last six years. In addition to local ambient ganra monitoring, the same basic technique has beer, applied in a survey of geographical variations in environmental radiation background in '-'is United S t a t e s / 1 ) variations in environmental radiation between res idences ,^) and radiation sLJveys both a~. EnewetakO) and Bikini a to l l s . W Dosimetry Procedures Selection of Phosphors The environmental TLD package contains two typej of phosphors: CaFjiDy (TLD-2QQ} and 'LA? (TUWDG). The CaFjsDy phosphor was chosen because of i t s greater sensiti\»ity con&ared with LiF. The Li? Is included in the package to provide a more refined analysis of the field exposure (see Discussion) and '<LiF CTLD-fOft} was use-i because of I t s low response to neutrons compared with r&turally abundant lithlu.7.. The OaFj an^ ^ P phosphor1? In the fom of 3-mni square chips were purchased ir. le ts of 1CCC. When received, they were annealed at #25°C for 1-1/2 h r s cooled at ambient tenpersture for ^0 TTJ.I, and then post annealed at 80DC for a 2H hr period. After cooling and irradiat ing the phosphors with D 3 C O , the annealing steps are repeated, followed by re- i rradiat ion. After a *(6 hr delay to allow low temperature traps to decay, the phosphors are then read using the LLL developed hot gas reader.(5) Tne phosphors selected for- environmental monitoring have light output signals within +_ 5% of the respective se t s . Dosimeter Package To take advantage of automated processing capabillttes developed for the.Laboratory's personnel dosimetry program,^; ^ 33.30 y i S & 3tandard LLL doataeter holders for our envi~ ronmental package. This Injection maided plast ic holder shown In F i ^ a e 1 I s 350 mn in diameter and 3 m thick. The two se ts of holes around the circumference are a binary encoding system to identify the phosphor and monitoring location. Fleshly annealed chips are placed in the three central recesses ai>i are teld in place by plast ic coverB. Although the recesses have a alight negative draft and the covers are stressed to make a tight f i t , weatliering LLL environmental TLD holder. •Work performed under the auspices of the U.S. ERDA under contract No. W-7*(05-Eng-iJ8. in the field frequently caused the covers to pop out resulting in loss of the phosphor. To eliminate such losses, xe rrw* use two facing holders held together with plast ic t i es as shown in Figure 1. This double badge containing one holder with three CaFo:£y phosphors and one holder with three LIP phosphors war i'.'rst used In an Er.iMetak radiation survey.ul Siting, of TU3s locations of perimeter and off-site TW monitoring stations are shown in Figures 2 and 3, respec­t ively. Dosimeters are exposed for a period of three months. Perimeter dosimeters are attached to the woven wire Site boundary fence at a neigit of approximately 1 metre above the ground. As the vicinity or the laboratory i s essentially ru ra l , the off~site dosimeters are usually secured to roadside wire fencing. Pig. 2. Lawrence Livertrore Laboratory on-site environmental 3anp We locations. Storage of Control i&sireters Control dosimeters are stored In a 75-mm-thiek lead shield equipped with a Cd-Cu "graded" liner. A lead shield of this trucfcrass constitutes an acceptable storage area since the radiation Inside is j:rima~ r i ly due to the *\ani exponent of cosmic radiation ; lus the contribution of any contanlnants in the shiek iraterlal. Ihe internal exposure rate Is therefore relatively low arid constant. The shlold is sh^wn in Figure ft. Calibration A portion of the control dosimeters i s removed midway th.*oi&h the field exposure- cycle. Usini; a Co source, the individual dosi.<nesers are irradiated at doses frets ID rsrad to 1Q0 mrad. Directly following tills irradiation procedure the dosimeters are returned to the shield and are read along with the field dosimeters at the end or the exposure cycle. Fig. 3. Ijocation of thermoluminescent doslneters in the vicinity of the Lawrer.o-Livermore Laboratory. Dosljneter Readout After collection from the field, the exposed dosimeters are placed in the lead shield for 48 hrs to permit decay of the low temperature traps. Ihe environmental package is then taken apart and the Ca?2 and LiF holders are loaded into the automatic hot gas reader in separate runs. Calibration phosphors arc interspei,3ed within each group. The operating voltage on the photomultiplier tube is increased over that used in personnel dosimetry to provide the increased sensitivity required in meas­uring environmental background radiation. Ihe reader shown in Figure 5 Is then started, and in sequential steps the dosimeter numbers are decoded, the plastic caps are removed, the chips are trans­ferred to the counting chamber and heated, and the light output is recorded. An IBM ca^d is simulta­neously prepared identifying each phosphor, its field location, and its light output. These cards are then used in a computer program, which computes mrem denes and confidence limits for these doses. Fig. U. Lead shield for eonti*ol dosimeters. Results and Discussion Figure 6 represents distribution plots of annual LLL per ',-ieter and off-site dose rates oti alned fivr. 1U) data during 197^.(7) Ihe plots show median doses of 69 and 66 rm-esn for the perimeter ;J.V. oiT-sltt locations, respectively. These a m comparable to the doses observed during 1973.") An elevated rerin-eter dose rate (137 mrem) wa3 due to operation of a lh MeV neutron generator. A single dosimeter i:iat Fig. 5 . feitorrated 11D reader. recorded the highest dose (23.0-215 mrsm) i s located near an off-s i te industrial ; larr where radiography with a 19?Ir scarce is frequently performed. Ihersoltniiinss;er.ce doslswzry measurements do not distlr.£uisn betweon co'^-re.: >;' r e l a t i on - As Wc are primarily c-->rcerr.ea with the iicpact of Tan-isade radla t i* . , it is oft-T: '.,•- ••'zr.-n-*; : t.f able to sub­tract the contrLi-i-i?r. due to the natural radiation back^rou-i 1. Torres tr i ' i i :•*! I! ;* \ :. "r-jrr. naturally occurring uranixi, tnorium, and potassium in the soil plus cosrxic radiation :ire +.:«•• ; riricipai sources of natural radiation. Ihe t e r r e s t r i a l exposure ra te can be approximated fvT, **>« sp*-,.*!-'!.' ac t iv i t ies of naturally occurring sama emitters in the s c d l / ^ ) Q^ -j-^, Cosi?i.j radiat! a. -r-C. 'v-. -ip; j-ofLjaated froM thv elevation and geor.arr.et.ic lati tude -jf the area using the techniques of I/"*.'d-r a:.; N-.v..*- '• 'lamp, spec-tr^I y=a3urenier.ti; vfer̂ r rr&de at 37 jf the off~site locations <A3l:u; * ho UI* i'.-v-.-!-1 •-J fl'-.d leCM j;.iec ."•^r-e'.er. i 3-~ J ?err~s\rial exposure ra'er, frori uraniiT., thorium, and po* aiisium calculated from these spectra shower a ranee jf 3-0 to 6.9 uR/hr, v/ith a median of i .v uR/nr- -^ J 7;^ inferred Iceal exposure rate freer, cosrdc ray:; Lz l.z u.Vhr, cssed on »n averse s i t« elevation tf ~_z.-. .t. These emirates then Indi­cate that the :;^\ a'-.', vii-Ier.* fjjirrA radiation ba:v-<r,~ to ID.t jp/rj- wi-.r. a rr-'Jiar; of ':.? uR/hr. This nedian corresponds to an anr..ml oose af 6<i mrerr: in good acre«vf£t with the TLD data. High pressure ion chaster nfc-asurto'.'iits nit- periodically nade at Labora­tory perimeter monitoring locations, arid the agr-ee-siwrtt between the 3*? n-.-asurements and those uairig ?̂ D i s generally good. The ad*-antage of the ion ebas&er is that it provides real tine- data, which Is useful in investiRatL'ie possible transitory fluctuations in ba-:kijrour<d radia* i^n, However, our reporting require ss&otij in^ly dat-- refleating annual average.-.. Conse­quently, i-ote^i-atlrit: devices, sacn as TLD, are per-na; s better suited for rout Lie envl'^mental irnMtori/SjS. 14)31 users of CaF? Incorporate energy f i l t e r s In thei r dosimeter- package to flatten the phosphor's en­hanced response be2o>, 203 keV. At LLL, we nave elect ed to -utilize this characteristic of Ca?2 t o at. ect the presence of low energy photons. To do this our doslmeisrs also contain LIP v&ose response Is corppar-atlvely energy independent. An increase in the CaF^/LlF response is then an Indication of low energy a 65 10 30 50 70 90 Cumulative frequency less than — % Fig. 6. Laboratory perimeter anci Livermore Valley environment'i 1 radiation dose rates - 197*1. radiation. In addition, since U F is approximately tissue equivalent, dividing the CaFp mrad values by the CaFi/LLF ratio converts those doses to mrem. This ratio, which averages 1.3 is obtained by averaging the da' ̂  from the off-site dosL*neters. Because this ratio is based on a large number of measurements each exposure period, the dose values so derived are usually more precise than individual LiF values. Similarly this manipulation permits us to obtain mrem data with better statistics due to the greater sensitivity of CaF?. Appreciable signal fading of CaFjiDy - approximately 30% over a period of three months — is a problem that is not encountered with LIF. Rather than apply fading corrections, we have compensated for fading by irradiating the calibration dosimeters midway through the field exposure cycle. This method assumes a linear farting and a constant radiation field during the exposure period. When the monitoring program began, the 75-nm-thick lead storage shield was located within a labora­tory maintained at approximately constant temperature (23°C). During the fourth quarter of 1972 an anom­alously high respond- was observed in all field CaF2:Dy dosimeters. This increased response was attri­buted to reduced signal fading of the field dosimeters at the unseasonably low temperatures during that quarter. In order that the control dosimeters might more closely duplicate the temperature cycling of the field dosimeters, the shield was moved out-of-doors. The shield in its present location is shown in Figure 4. i"b jeteniLine the effect of seasonal variations on dosimeter response in the storage shield, a freshly annealed dosimeter is placed in the shield each week of the field exposure cycle. These dosimeters are read out along with the others at the end of the exposure cycle. An Internal dose rate is then calculated fmn the calibration and the data from these weekly dosimeters. Li the siting of our thermoluminescent dosimeters, the height above ground is maintained at approx­imately 1 metre at the perimeter locations. As the vicinity of the laboratory is essentially rural, dosimeters are secured to rcadside fencing, variations in which make it difficult to maintain the same height as at the perimeter locations. Concern over these differences in height, which varied about ± 0-30 metres, prompted a test exposure consisting of dosimeters suspended at heights from 25 to 305 centi­metres. Results of this three month test, which are shown in Table 1, confirm that dosimeter height is not a critical parameter. Table 1. Effect of Dosimeter Height In addition to detecting non-environmental doses at a particular dosimeter site, groups of Ca?2 dosimeter sites may be compared to see if the means Height, cm rrrad/90 days mrem/90, days of those groups are statistically different. Con­sidering the dose to TLBs in a group to be log-13.5 normally distributed and applying a priori tests as 14.3 described by Toy and Li^deken^J and Sokal and 14.8 Rohlf.t11*) we have determined that our program is 13.5 more than adequate to detect 3 difference of 5 mrem 14.2 per year between the nean at our perimeter and the 14.4 mean In the Llvermore Valley. 14.1 13-7 14.7 14.1 15-3 14.3 15.1 15-9 14.5 Summary Although supplemented by other technique's, tnermoluminescence is the principal means of measuring ambient f,amma radiation at the Lawrence Liveitnore Laboratory. The procedures employed for environmental monitoring with TLD are designed to take advantage of the automated processing capabilities developed for the Laboratory's personnel dosimetry program. The environmental dosimeter contains both CaFpiDy (TLD 200) and 'LiF (TLD 700). The CaFj/LiF mrad response ratio is used to detect low energy photons in the radia­tion field, as well as convert the more sensitive CaF? data tc equivalent mrems. Using doslne'ers at 12 locations on the site perimeter and 41 locations in the off-site vicinity, provides adequate sampling to detect a difference of 5 mrem between these two populations. References 1, Lindeken, C. L., Petersen, K. R., Jones, D. E. and McMillen, R. E., "Geographical Variation? in Environmental Radiation Background in the United States," in Proceedings of the Second Lnternational Symposium on the Natural Radiation Environment, August 7-11, 1972", Houston, Texas, CGtfF 720805-P1, p. 319-:5 17.9 ^v 19.0 (.-. 19.7 •-•. 17-9 10'. l i . 9 Wj 19-2 Vr, ]>.8 IV; I P . 1 ?y. 19.5 z?*- 1«.7 ?"b *Q.k ;-'.>5 19.7 ?55 20.1 305 21.2 19.2 2. Lindeken, C. L., Jones, D. E. and McHlllen, R. E., "Environmental Radiation Vai'iv kins !*" we:. Residences," Health Physics, 2k: 8 l , 1973. 3- Beck, H. L., Mclaughlin, J . E., Gudiksen, P. K., Jones, D. E., Lynch, o. D. T. ana Sf.«x±r-, 7. : . , Enewetak Radiological Survey-External Dose Estlnate, NVO-l̂ O, vol. 1, October 1973-4. Crites, T. R. and Gudiksen, P. H., "A Oorparison of Ganma-Ray Exposure RaU- .'-leas-a.*?::*-::':'• v 3ikini Atoll," In Proceeding; Health Physics Society Ninth Midyear Tbplcal Synposlur., Denver, ;:..:.>;-.; , February 9-12, 197oI ~ ~ 5- Petrock, K. F. and Jones, D. E-, "Hot Nitrogen Gas Tor Heating Thentiolumines^r.T. Vus'.::, •••::•," '.:-. Proceedines 2nd International Conference Luminescence Dosimetry, p . 652, Gatllnburg, Tennessee, .'— .'•' September 1968. 6. Jones, D. E., Petrock, K. P . , Samardzich, B. G. and Shapiro, E. G., Itennolumlnes^-rc^ I'•>.••'.::•.•:•.• r System for Personnel Monitoring, Lawrence Livennore Laboratory, Kept. WRL-TiOBU (1971). 7. Silver, W. J . , Llndeken, C. L., Meadows, J . W., W'illes, E. H. and Mclntyre, D. R., Envlr-tJ'r-.-iV-.'. i-fanltfiring at the Lawrence Liverrore Laboratory-ig?*! Annual Report, Lawrence Livemnne I/ib-u-sTor-v, £•-;>:.. X.TL-50027-71 (1975). 8. Silver, W. J . , Undeken, C L., Meadows, J . W., Kutchin, W. H. and Mclntyre, D. R., Envlpjrmi'n'-hl. Levels of Radioactivity in the Vicinity of the Lawrence Llvermore Laboratory-1973 Annual Repoi--, I.'iWivri?* liverscre Laboratory, Rent. UCHL-51547 (197 4). 9- Beck, H. L., DeCampo, J and Gogolak, C , In Situ Ge(Ll) and Mal(Tl) Garma-Ray Sjiectmnf ry. United States Atomic Energy Health and Safety Laboratory, HASL-256, September (1975). 10. Lowder, W. M. and Beck, H. L., "Cosmic-Ray Ionization in the Lower Atmosphere," J . Geophy.;. Res., 71: 4661 (1966). l i . Anspaugh, L. R., Phelphs, P. L., Gudiksen, P. H., Lindeken, C. L. and Huckabay, G. W,, T:.-- I:. Situ Measurement of Radionuclides in the Bwlronment With A Ge(Li) Spectrometer, Lawrence Livf-mm1'-laboratory, Sept. UCRL-73904, January 2t, 1973-12. Lindeken, C. L., Gudiksen, P. H., Meadows, J- W., Harney, K. 0. and Anspaugh, L. R., 'rjivi:-.:-rfrv •»: Levels ?f Fiadloactlvlty lr. Liveinore Valley Soi ls , Lawrence Llvermorv- Laboratory, Rep'. VCRL-v - •."••• '<'•'•.'. 13- Iby, A. J . and Lindeken, C. L., Implications of SanplLig From a lor-'.lcnr&'l P-.j*--uiat I-r., r.'iwr--:r-Llvermore laboratory, jtept. UCRL-76936 (1975/. I'l. Sokal, R. R. and Rohlf, F. J . , Biometry (W. H. Freeman and Company, 1969', p- ?W. 

Ambient environmental radiation monitoring at the Lawrence Livermore Laboratory

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Ambient environmental radiation monitoring at the Lawrence Livermore Laboratory

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