THE METHOD FOR RECAL I BRA TION OF THORON ... - doiSerbia

Y. Tan, et al.: The Method for Recalibration of Thoron Con cen tra tion Read ing ...92 Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2013, Vol. 28, No. 1, pp. 92-96THE METHOD FOR RECALIBRATION OF THORONCON CEN TRA TION READ ING OF RAD7 AND OB TAIN INGTHE THORON EX HA LA TION RATE FROM SOIL SUR FACEbyYanliang TAN 1,2* and Detao XIAO 21 De part ment of Phys ics and Elec tron ics, Hengyang Nor mal Uni ver sity, Hengyang, China2 School of Nu clear Sci ence & Tech nol ogy, Uni ver sity of South China, Hengyang, ChinaTech ni cal pa perDOI: 10.2298/NTRP1301092TThoron ex ha la tion rate can be ob tained through the com bi na tion of the “ac cu mu la tion cham -ber” tech nique and RAD7. Thoron’s rapid de cay causes the in take path and the air flow rateto be come im por tant fac tors in cal i bra tion. In field con di tions, since the flow rate of the in ter -nal pump in RAD7 will change as the volt age of the bat tery de creases, the big dry ing tube ismore suit able for a long mea sure ment than the small dry ing tube. We de vel oped the methodfor recalibration of the thoron con cen tra tion read ing of RAD7 based on the cal i bra tion fac torfor 222 Rn, and ob tained the thoron ex ha la tion rate from soil sur face near by the Ra don Lab o -ra tory of the Uni ver sity of South China. This method can be ap plied to de velop and im proveinstruments for measuring the radon exhalation rate.Key words: thoron ex ha la tion rate, soil, RAD7INTRODUCTION220 Rn, also known as “thoron” is a nat u ral de cayprod uct of tho rium. It has a half-life of 55.6 sec ondsand emits al pha ra di a tion. In door sur veys in Eu ropeand Asia re vealed that the dose con tri bu tion due to thein ha la tion of 220 Rn and its prog eny can equal or evenex ceed that of 222 Rn and its prog eny [1]. Now a days,the significance of 222 Rn has been rec og nized and thein ter est in the mea sure ment of the in door 220 Rn con -cen tra tion has been in creased [2-4]. Ma jor part of220 Rn co mes from the top layer of the earth. Mea sur ingthe exhalation rates for thoron indicates significantpresence of thoron in in door en vi ron ment which isalso sup ported by in door mea sure ments of thoron andits prog eny [5]. Thoron exhalation rate measurementsmay be per formed by the “ac cu mu la tion cham ber”tech nique [6]. The ex per i men tal set-up for thoron ex -ha la tion rate mea sure ment is con ducted by a solid statenu clear track de tec tor. How ever, this method needs along time to ob tain the re sults.RAD7 is much less sus cep ti ble to radon-thoronin ter fer ence due to its abil ity to dis tin guish the iso topesby their unique al pha par ti cle en er gies. It sep a rates ra -don and thoron sig nals and counts the two iso topes atthe same time with a lit tle in ter fer ence from each other.Some re search ers had used RAD7 in a lab o ra tory to de -terminate the 222 Rn and 220 Rn exhalation rates from* Corresponding author; e-mail: hytyl@163.combuilding materials, simultaneously [7]. As the manualof RAD7 in di cates, the RAD7 cal cu lates thoron con -cen tra tion on the ba sis of the count rate of the 6.78 MeVal pha line of 216 Po. Thoron’s rapid de cay causes the in -take path and the air flow rate to be come im por tant fac -tors in calibration. The RAD7 factory calibration forthoron is based on a stan dard RAD7 in let fil ter, a stan -dard 3-foot long, 3/16 inch in ner di am e ter vi nyl hose,and a stan dard small (6 inch) dry ing tube. The thoroncon cen tra tion in the in ter nal cell of RAD7 is about 50%of the orig i nal sam ple; the thoron read ing is in tended tocom pen sate, al ready, for the loss of the sam ple by de -cay. It means that the read ing value of thoron concentrationfrom RAD7 is the dou ble of the real thoron con cen -tra tion in the in ter nal cell. De vi a tion from thisar range ment can change the thoron re sults.Gen er ally, a small dry ing tube is suit able formea sur ing thoron con cen tra tion. In this case, theinterspace vol ume of the small dry ing tube is so smallthat can be ig nored. The man ual of RAD7 also in di -cates that the big dry ing tube will last for days un dercon tin u ous op er a tion at high hu mid ity in the NOR -MAL mode, be fore it needs re gen er a tion. But thesmall dry ing tube con tain ing 30 g of desiccant willlast for only a few hours. The big dry ing tube is moresuit able for long mea sure ments than the small dry ingtube.The man ual of RAD7 in di cates that RAD7 hasenough bat tery ca pac ity to go for two to three dayswith out any ex ter nal power source. So, RAD7 can be

Y. Tan, et al.: The Method for Recalibration of Thoron Con cen tra tion Read ing ...Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2013, Vol. 28, No. 1, pp. 92-96 93used to per form the thoron exhalation rate measure -ments from soil sur face in field con di tions. How ever,the flow rate of the pump will change as the volt age ofthe bat tery de creases, and the big dry ing tube is moresuit able for long time mea sure ments than the smalldry ing tube. In this pa per, we will pres ent the methodfor recalibration of thoron con cen tra tion read ing in or -der to ob tain the ac cu rate thoron exhalation rate. Allthe us ers of RAD7 can use this method and the data torecalibrate the thoron con cen tra tion read ing of RAD7with out a stan dard thoron source.METHOD FOR MEASURINGTHORON EXHALATION RATEFROM SOIL SURFACEThe ex per i men tal set-up for thoron ex ha la tionrate mea sure ment con ducted by RAD7 is pre sented infig. 1. Be cause the linked pipe line is short, the thoroncon cen tra tion in the ac cu mu la tion cham ber is de ter -mined by the fol low ing dif fer en tial equa tiondCVdtJS ( l lleak lb ) CVVLV CVLV CReVLl 0 (1)where C V is the thoron con cen tra tion in the ac cu mu la -tion cham ber, C R – the thoron con cen tra tion in the in -ter nal cell of RAD7, J – the thoron ex ha la tion rate, S –the area of the cham ber bot tom, V – the vol ume of theaccumulation chamber, V 0 – the in ter nal hose vol umefrom the in ter nal cell to the ac cu mu la tion cham ber, L –the flow rate of the pump, l – the de cay con stant ofthoron, l leak – the cham ber leak age co ef fi cient, and l b– the back-dif fu sion co ef fi cient. JS/V pres ents theyield of the thoron ex ha la tion, (ll leak l b C V pres -ents the yield of the thoron de cay, leak age andFig ure 1. Thoron ex ha la tion rate mea sure ment scheme.(the vol ume of the ac cu mu la tion cham ber is 4 L, theinterspace vol ume of the big dry ing tube and the pipe vol -ume from the ac cu mu la tion cham ber to the in let orRAD7 is 450 ml, the in ter nal hose and dust fil ter vol umesin the RAD7 is 29 ml, the vol ume of the in ter nal cell ofRAD7 is 0.723 L)back-dif fu sion, (L/V)C V pres ents the yield of thorondrawn to the in ter nal cell of RAD7 by the in ter nal( V / L)pump, and ( L/ V ) CRe l 0pres ents the yield ofthoron sent back from the in ter nal cell by the in ter nalpump.As far as thoron ex ha la tion rates are re garded,the ef fects of leak age and back dif fu sion were not ob -served and eval u ated be cause of the short half-life ofthoron. The half-life is only 55.6 s and we con firm thatl .l leak + l b (2)Since V 0 is small, eq. (1) can be pre di gested asdCV JS L l CV ( V RdtV V C C ) (3)Ac cord ing to the smooth flow equa tion [8], thethoron con cen tra tion at the in let of the RAD7, C 1 , canbe ex pressed asV1C CL1 V e l (4)where V 1 is the interspace vol ume of the big dry ingtube and the pipe vol ume.The sam ple de cays slightly as it passes from theRAD7 in let to the in ter nal cell, and the air flow is also asmooth flow and the in ter nal cell in let con cen tra tion,C 2 , can be cal cu lated asC2V2 CL1 e l (5)where V 2 is the in ter nal hose and dust fil ter vol ume,which is 29 ml.Within the RAD7 in ter nal cell, the air flow is tur -bu lent. Ac cord ing to the tur bu lent flow equa tion [8],the equi lib rium thoron con cen tra tion, C R , will be de -ter mined by the for mulaV1 V2 lCL2 eCR CV(6)lV3 lV311LLwhere V 3 is the vol ume of the in ter nal cell of RAD7.Equa tion (6) is just the same as the for mula given in theman ual of the RAD7.When the thoron con cen tra tion in the ac cu mu la -tion cham ber reaches the steady-state,dC V 0 (7)d tThus, eq. (3) can be re writ ten asVJ LCS V C Cl V ( V R )(8)Equa tion (6) can be re writ ten asCVCRlV1Le3V1 V2 lL(9)By in sert ing eq. (9) into eq. (8) the fol low ingequa tion is ob tained

Y. Tan, et al.: The Method for Recalibration of Thoron Con cen tra tion Read ing ...94 Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2013, Vol. 28, No. 1, pp. 92-96VCJ SRlV1Le3V1 V2 lLV V lL LLel V V lV 1L1 23 (10)The equa tion can be used for es ti ma tion of thethoron ex ha la tion rate from soil sur face in the ac cu mu -la tion cham ber.THE METHOD FOR RECALIBRATIONOF THE READING OF RAD7 ANDESTIMATION OF V 1The read ing of thoron con cen tra tion from RAD7is dou ble of the real thoron con cen tra tion in the in ter -nal cell. Some re search ers pro posed that the mea sure -ment sen si tiv ity of the RAD7 for 220 Rn is about 30%less than its sen si tiv ity to 222 Rn. The re ports of Jap a -nese Institute of Radiological Sciences (NIRS) indicatethat the cal i bra tion fac tor of the RAD7 for 220 Rn isroughly 1.25-1.3 [9, 10]. The re ports of the Uni ver sityof South China in di cate that the cal i bra tion fac tor is1.40 [11]. In this pa per, we de ter mined that the cal i bra -tion fac tor of RAD7 for 220 Rn is 1.35 and it is the meanvalue of the re ports of NIRS and Uni ver sity of SouthChina.Thus, eq. (10) can be writ ten as135 . VCJ 2SRlV1Le3V1 V2 lLV V lL LLelV V lV 1L1 23 (11)A kind of self-made thoron flow-through solidsource was used as thoron stan dard, which is sta bleenough with a rel a tive de vi a tion of 2.5% over 5year, with rel a tive hu mid ity range from 50% to 90%.The em a na tion per cent age of this thoron source is96.5 + 3.0%.For the in clud ing the interspace vol ume of thebig dry ing tube filled with desiccant, the ac cu rateinterspace vol ume of the desiccant is not known. Fig -ure 2 shows the solid flow-through thoron source [8]was used to ac cu rately mea sure the interspace vol umeof the big dry ing tube and the pipe vol ume. V 1 can bede ter mined by eq. (12)CRALV1 V 2eLlV1L3(12)where is A is the thoron ac tiv ity em a nated from thesolid flow-through thoron source.Fig ure 2. V 1 mea sure ment schemeEXPERIMENT AND RESULTSRAD7 with calibration factor for 222 Rn of 1.12 isused to per form the thoron ex ha la tion rate from soilsurface measurement near by the Radon Laboratory ofthe Uni ver sity of South China. The area of the cham -ber’s bot tom is 430 cm 2 , whereas the vol ume is 4 L; thevol ume of the in ter nal cell of RAD7 is 0.723 li ters(Pro vided by Dr. Derek Lane-Smith, the pres i dent ofDURRIDGE Com pany, Inc.); the de cay con stant ofthoron is 0.756 min –1 .The set tings of RAD7 are as fol lows: cy cle, 30min; mode, sniff; pro to col, thoron. Be cause there is noex ter nal power source in field con di tion, this mea sure -ment was per formed only by the bat tery in the RAD7.Since the flow rate of the pump changes with de creas -ing of the bat tery volt age, we re corded the volt age ofthe bat tery in each cy cle, and ob tained the flow rates ofthe in ter nal pump for var i ous bat tery volt ages, in thelab o ra tory.Be cause suf fi cient time is needed for the thoroncon cen tra tion in the cham ber and the in ter nal cell ofthe RAD7 to achieve equi lib rium, the first da tum isdis carded. Ta ble 1 lists the ex per i ment data and thethoron ex ha la tion rates ob tained by eq. (11).From tab. 1, we can find that the thoron ex ha la tionrates from soil sur face are rel a tively sta ble when theweather conditions are relatively stable. The average thoronex ha la tion rate in two hours is 2.85 0.23 Bqs/m 2 .Some re search ers in di cated that thoron dif fu sionlength in the air is very short (2.9 cm); there fore, inusual cham bers with di men sions of 10 cm mag ni tudeor der, uni for mity of thoron con cen tra tion is not fea si -ble [6]. How ever, in tab. 1, a cor re la tion be tween theflow rate and thoron ex ha la tion rate do not ex ist.Maybe the in ter nal pump of RAD7 forces the air mix -ing in the ac cu mu la tion cham ber.V 1 450mL (13)

Y. Tan, et al.: The Method for Recalibration of Thoron Con cen tra tion Read ing ...Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2013, Vol. 28, No. 1, pp. 92-96 95Table 1. Experiment data and the thoron exhalation ratesCycleMeasured value ´1.12qm 3 ]Air pressure[hPa]Temperature[°C]Relation humidity[%]Battery voltage[V]Flow rate Thoron exhalation rate[mlmin –1 ] [Bqs –1 m –2 ]2 580 186 1017 8.4 73.2 6.18 430 2.70 0.873 542 180 1017 6.7 78.7 6.12 415 2.65 0.884 556 182 1017 6.5 82.6 6.09 395 2.91 0.955 582 186 1017 5.9 83.1 6.06 385 3.15 1.01CONCLUSIONSThe al pha de tec tor, which is much less sus cep ti -ble to ra don-thoron in ter fer ence due to its abil ity to dis -tin guish the iso topes by their unique al pha par ti cle en -er gies, can be used to mea sure the thoron con cen tra tionin the “accumulation chamber” and the thoron exhala -tion rate can be ob tained. In field con di tions, the flowrate of the in ter nal pump in RAD7 will change whilethe volt age of the bat tery de creases, the big dry ing tubeis more suit able for long mea sure ments than the smalldry ing tube. These pa ram e ters are very im por tant formea sur ing thoron con cen tra tion. Thus, the cal i bra tionfac tor for thoron is not suit able to cal i brate the thoroncon cen tra tion read ing of RAD7. We de vel oped themethod for recalibrating the thoron concentration read -ing of RAD7 based on the cal i bra tion fac tor for 222 Rn.ACKNOWLEDGEMENTPro ject is sup ported by the Na tional Nat u ral Sci -ence Foun da tion of China (Grant No. 11075049), theEx cel lent Tal ents Pro gram of Hengyang Nor mal Uni -ver sity of China, the con struct pro gram of the key dis -cipline in Hunan prov ince and the Sci ence Foun da tionof Hengyang Nor mal Uni ver sity of China (Grant No.11B41). The au thors would also like to thank Dr. DerekLane-Smith for pro vid ing all the di men sions in theRAD7.[3] Calamosca, M., Penzo, S., A New CR-39 Nu clearTrack Pas sive Thoron Mea sur ing De vice, Ra di a tionMea sure ments, 44 (2009), 9-10, pp. 1013-1018[4] Yonehara, H., et al., Thoron in the Liv ing En vi ron -ments of Ja pan, In ter na tional Con gress Se ries, Vol -ume 1276, Feb ru ary 2005, pp. 58-61[5] Sharma, N., Virk, H. S., Ex ha la tion Rate Study of Ra -don/Thoron in Some Build ing Ma te ri als, Ra di a tionMea sure ments, 34 (2001), 1-6, pp. 467-469[6] Uji}, P., et al., Stan dard iza tion and Dif fi cul ties of theThoron Ex ha la tion Rate Mea sure ments Us ing an Ac -cu mu la tion Cham ber, Ra di a tion Mea sure ments, 43(2008), 8, pp. 1396-1401[7] Tuccimei, P., Moroni, M., Norcia, D., Si mul ta neousDe ter mi na tion of222 Rn and 220 Rn Ex ha la tion Ratesfrom Build ing Ma te ri als Used in Cen tral It aly withAc cu mu la tion Cham bers and a Con tin u ous SolidState Al pha De tec tor: In flu ence of Par ti cle Size, Hu -mid ity and Pre cur sors Con cen tra tion, Ap plied Ra di -a tion and Iso topes, 64 (2006), 2, pp. 254-263[8] Qiu, S., Cal i bra tion of a 220 Rn Flow-through Source,Ra di a tion and En vi ron men tal Bio phys ics, 45 (2006),3, pp. 215-220[9] Kobayashi, Y., et al., Prac ti cal ity of the Thoron Cal i -bra tion Cham ber Sys tem at NIRS, Ja pan, Pro ceed -ings, 6 th In ter na tional Con fer ence on High Lev els ofNat u ral Ra di a tion and Ra don Area, Osaka, Ja pan,Sep tem ber 6-10, 2004, Int. Cong. Ser., 1276, 2005,pp. 281-282[10] Sorimachi, A., et al., Per for mance of NIRS ThoronCham ber Sys tem, AIP Conf. Proc., Au gust 7, 2008,Vol. 1034, pp. 206-209[11] Qiu, S., Xiao, D., Liu, L., First Intercomparison ofTn–Rn Discriminative Mea sure ments in China, Ra -di a tion Pro tec tion Do sim e try, 28 (2008), 1, pp.124-127AUTHOR CONTRIBUTIONSThe o ret i cal anal y sis was car ried out by Y. Tanand experiments were car ried out by D. Xiao. Themanu script was writ ten by Y. Tan and the fig ures werepre pared by D. Xiao and Y. Tan.Re ceived on Sep tem ber 17, 2012Ac cepted on No vem ber 20, 2012REFERENCES[1] Steinhäusler, F., En vi ron men tal220 Rn: A Re view, En -vi ron ment In ter na tional, 22 (1996), 1, pp. 1111-1123[2] Yamada, Y., et al., Ra don-Thoron DiscriminativeMea sure ment in Gansu Prov ince, China, and TheirIm pli ca tion for Dose Es ti mates, Jour nal of Tox i col -ogy and En vi ron men tal Health, 69 (2006), 7, pp.723-734

Y. Tan, et al.: The Method for Recalibration of Thoron Con cen tra tion Read ing ...96 Nu clear Tech nol ogy & Ra di a tion Pro tec tion: Year 2013, Vol. 28, No. 1, pp. 92-96Janliang TAN, Detao KSIAOMETODA REKALIBRACIJE RAD7 ZA O^ITAVAWE KONCENTRACIJETORONA I ODRE\IVAWE BRZINE EKSHALACIJE TORONASA POVR[INE ZEMQI[TABrzina ekshalacije torona mo`e se odrediti kombinacijom “akumulacione komore” iRAD7. Toronov brz raspad razlog je za{to su putawa unosa i brzina protoka vazduha bitni faktoriza kalibraciju. U terenskim uslovima, brzina protoka unutra{we pumpe u RAD7 mewa sesmawewem napona baterije, te je velika cev za su{ewe pogodnija za dugotrajna merewa u odnosu nakratku cev. Razvili smo metodu za rekalibraciju o~itavawa koncentracije torona zasnovanu nakalibracionom faktoru za 222 Rn i odredili brzinu ekshalacije sa povr{ine zemqi{ta u bliziniRadonske laboratorije Univerziteta Ju`ne Kine. Ova metoda mo`e se primeniti za razvijawe iunapre|ewe instrumenata za odre|ivawe brzine ekshalacije radona.Kqu~ne re~i: brzina ekshalacije torona, zemqi{te, RAD7