Buttar, C M

Dawson, I

Moraes, A

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Estimating induced-activation of SCT barrel-modulesC. Buttar, I. Dawson, A. MoraesDepartment of Physics and Astronomy, University of Sheffield,Hicks Building, Hounsfield Road, Sheffield, S3 7RH, UKc.m.buttar@sheffield.ac.uk, Ian.Dawson@cern.ch, a.m.moraes@sheffield.ac.ukAbstractOperating detector systems in the harsh radiation envi-ronment of the ATLAS inner-detector will result in the pro-duction of radionuclides. This paper presents the findingsof a study in which the radioactivation of SCT barrel mod-ules has been investigated.I. INTRODUCTIONOne of the consequences of operating detector systemsin the harsh radiation environments of the ATLAS inner-detector [1] will be the radioactivation of the components.If the levels of radioactivity and corresponding dose ratesare significant, then there will be implications for any ac-cess or maintenance operations. In addition, maintenanceoperations may be required on nearby detector or machineelements, for example beam-line equipment, so the im-pact of the SCT has to be considered. A further motivationfor understanding SCT activation concerns the eventualstorage or disposal of the SCT-modules at the end of thedetector lifetime, in which any radioactive material willhave to be classified.Radionuclides are produced in the SCT modules viathe inelastic interactions of hadrons with the various tar-get nuclei comprising the modules. The hadrons originatefrom: 1) secondaries from the p-p collisions, dominatedby pions, and 2) backsplash from hadron cascades in thecalorimeters, mainly neutrons. Therefore, the calculationof radionuclide production requires a detailed inventory ofthe target-nuclei comprising the SCT modules and a goodknowledge of the corresponding radiation backgrounds.II. MODULE DESCRIPTION ANDMATERIAL INVENTORYIn order to make activation estimates, it is necessaryto obtain a detailed inventory of the materials involved inthe construction of a module. The relevant information isnow becoming available as SCT modules go into the pro-duction phase [2]. Of particular importance is knowledgeof elements containing isotopes with high neutron capturecross sections. For example, 197Au has a thermal neutroncapture cross section  100 b for the (n,) process and evensmall quantities of such an isotope can contribute consid-erably, or even dominate the activated environment.In the SCT barrel system there are 4 layers of cylinders,comprising 32, 40, 48 and 56 rows of 12 modules respec-tively. Each SCT barrel module comprises silicon sensors,baseboard with BeO facings, ASICs and a Cu/polymidehybrid [3], connected to opto-packages and dog-legs [4].Each row of modules has associated with it a coolingpipe [5] and power tape. In total, the module mass is cal-culated to weigh 34.6 g and contain 15 different elements.Details of the elemental breakdown are given in Table 1.Unfortunately, material details are not always givenin their elemental form and sometimes have to be de-rived. Assumptions are therefore unavoidable and include:1) Thermal adhesives assumed to be 70% polyimide and30% BN; 2) Epoxy films, polyimide layers with adhesivesand PEEK are chemically described as C22H10N2O5; 3) Thesolder composition is assumed to be 59% Pb, 40% Sn and1%Ag; 4) Capacitors made of AlO and resistors 20% C and80% AlO. Perhaps the most important assumption con-cerns the possible use of silver loaded conductive glues.Silver is important due to the high thermal neutron capturecross section and long half-life of 110mAg. In the currentstudy no silver has been assumed in the glues.III. RADIONUCLIDE PRODUCTIONRadionuclides are produced in the SCT modules via theinelastic interactions of hadrons with the various target nu-clei comprising the modules. However, radionuclides withshort half-lifes can be neglected as access to the irradiatedSCT material is unlikely for at least several days. In thecurrent study, radionuclides are only considered of radi-ological interest if they have half-lifes greater than 1 hourand less than 30 years.Radionuclide production can be divided into two cate-gories:Table 1: Table of barrel module element masses.Elements SiliconsensorsBaseboardwith BeOfacingsASIC’s Hybrid Coolingpipe withcoolantPowertapeOpto Pack-age / DogLegTotal (g)H 0.001 0.008 0.004 0.034 - 0.012 0.035 0.094Be - 0.597 - - - - - 0.597B 0.010 0.054 0.027 0.185 - - - 0.276C 0.037 4.672 0.099 3.024 0.821 0.306 0.926 9.885N 0.017 0.092 0.045 0.311 - 0.032 0.324 0.821O 0.011 1.119 0.037 0.442 - 0.093 0.304 2.006F - - - - 3.462 - - 3.462Al - 0.118 0.028 0.393 1.166 0.78 0.434 2.919Si 10.812 - 0.742 0.187 - - 0.029 11.770Ni - - - 0.025 - - 0.015 0.040Cu - - - 1.501 - - 1.071 2.572Ag - - - 0.001 - - 0.0004 0.0014Sn - - - 0.049 - - 0.012 0.061Au - - - 0.011 - - - 0.011Pb - - - 0.073 - - 0.024 0.097Total (g) 10.888 6.660 0.982 6.236 5.449 1.223 3.174 34.6121. Production via low energy (< 20MeV) neutron inter-actions, eg (n,), (n,p), (n,), (n,np) etc.. The cross sec-tions for these neutron interactions are well knownfor the target nuclei being considered [6]. The pro-duction probability for each radionuclide of interest,per target nuclei, can be obtained by convolving theenergy dependent cross sections with neutron energyspectra. The radiation environment of the SCT envi-ronment has previously been studied [1] and neutronenergy spectra obtained. According to these studies,thermal neutrons account for more than a half of thetotal neutron fluences in and around the SCT barrelsystem. In the current study, the radionuclide pro-duction probabilities were evaluated for all the low-energy neutron interactions and, as expected, thedominant process was found to be thermal-neutroncapture (n,).Values of radionuclide production per p-p event permodule from (n,) interactions are obtained simplyfrom n where n is the number of atoms of a givenisotope in the module and  is the correspondingthermal neutron cross section, obtained from [7];  isthe thermal neutron fluence which, according to [1],has the value 2 106 n cm 2 s 1 over the whole SCTbarrel system. It should be noted, however, that theinner detector thermal neutron rates are likely to besmaller in reality than those predicted in the originalfluence simulations. This is because elements such asboron, xenon, silver, gold etc., which have very highthermal neutron capture cross sections, had not beenincluded.2. High energy inelastic interactions, or spallation. Un-like neutron interactions, radionuclide productioncross sections from spallation are not available forall the target nuclei, and are often scarce for parti-cles such as pions. In this study, hadron interactionmodels in the Monte Carlo particle transport codeFLUKA is used [8]. The results then depend on thequality and coverage of the physics models, in partic-ular: nuclear evaporation, the intranuclear cascade,nuclear fission and nuclear fragmentation. Of thesethe first three are considered to be well modelledbut fragmentation, which is important for the heav-ier target nuclei, is not included in the code. Thegeneral features of residual-nuclei production pre-dicted by FLUKA are in reasonable agreement withexperimental data [9], exept for light-nuclei produc-tion from heavy targets where fragmentation effectsstart showing up. Fortunately, the bulk of the tar-get nuclei in SCT modules are in the light to mediummass range.IV. EVALUATING ACTIVITYKnowledge of the various radionuclide productionrates along with their half-lifes allows the calculation of ra-dioactivities; defined as the number of decays per second.The build-up and decay of activity, for each radionuclide,is given by:A = N  (1  e ti) e tc (1)where tiand tcare irradiation and cooling times respec-tively. N is the number of produced radionuclides per p-pevent per module,  is the average number of p-p eventsper second and  is the decay constant. In going fromradionuclide production per p-p event to activity, it is nec-essary to make certain assumptions about the p-p eventrates. The design luminosity of the LHC is 1034 cm 2 s 1,resulting in a p-p interaction rate of 8 108 s 1 as pre-dicted by PHOJET [10]. However, the average luminosityover longer timescales will be less due to beam-lifetimesetc. and an averaged luminosity value of 5 1033 cm 2 s 1is assumed [11].(a) (b)Figure 1: Dominant activities produced by (a) (n,) interactions and (b) spallation.Shown in Figure 1 are the dominant contributions to ac-tivation from (n,) and spallation reactions. Presented inTable 2 are activities obtained 1 day, 1 week and 1 monthafter shutdown, for the two cases of one-year of high-luminosity running and ten-years of high luminosity run-ning. A high-luminosity year is defined as 180 daysof running (assuming the average beam-luminosity of5 1033 cm 2 s 1) followed by 185 days of shutdown.V. CALCULATING DOSE RATESDose rates are obtained at distances of 10 cm, 30 cm and100 cm from the centre of a barrel-module. The calculationsare facilitated by assuming each module is a point sourceof radioactivity. This assumption is good for the distanceslarger than the dimensions of the module (ie 30 cm and100 cm) and will be conservative by some  30 % for thevalue obtained at 10 cm.A. Dose rates from -emittersThe dose rate Dfrom a -emitting nuclide can be ob-tained [12], for a point source, from:D=A:E7d2Sv=h (2)where A is the activity in MBq, E is the sum of  energiesin MeV weighted by their emission probabilities and d isthe distance from the source in metres. The above formulais valid for photons in the range 0.05 to 2 MeV, which is therange covering most of the emitted s.Using the activity values given in Table 2 and the rel-evant gamma decay and energy information, the totalgamma dose is obtained by summing the contributionsfrom each radionuclide calculated using equation 2 above.The results are given in Table 3.B. Dose rates from -emittersThe dose rate Dfrom a -emitting nuclide can be ap-proximated [12] by:D=10 Ad2Sv=h (3)This expression assumes no absorption of the s, whichcan be appreciable depending on the  energy. For ex-ample, the average  energy in 3H decay is 5.68 keV. Therange in air of such s is a few mm and will not contributeto external -doses. However, most of the emitted s havemuch higher energies, with ranges in air going up to sev-eral metres. In order to avoid overly overestimating the-doses, the maximum ranges in air of all -emitters havebeen obtained and, if the range is shorter than the distanceat which the dose is calculated, then it is not included inthe total -dose estimate. The results are given in Table 3.The above strategy will still overestimate -doses for tworeasons; 1) the ranges are obtained assuming the maximum-energy and 2) self-shielding of the module material hasbeen neglected. However, the emphasis of the -dose cal-culations is to provide reliable upper-limits.Table 2: Dominant radionuclides contributing to module activity.180 days irradiation 10 years irradiationcooling times cooling timesRadionuclide 1day 1week 1month 1day 1week 1month3H 477.87 477.43 475.74 3759.17 3755.69 3742.427B 4139.23 3828.54 2838.78 4175.48 3862.06 2863.6422Na 476.32 474.59 466.35 1894.87 1886.59 1855.1924Na 505.16 0.64 - 505.16 0.64 -31Si 3.78 - - 3.78 - -32P 13.31 9.95 3.26 13.31 9.95 3.2648V 45.20 34.97 13.07 45.35 35.01 13.1154Mn 114.69 113.18 107.55 206.61 203.88 193.7456Co 106.19 100.73 82.28 110.65 104.96 85.7457Co 153.14 150.81 142.19 252.27 248.43 234.2358Co 541.38 510.49 407.57 557.01 525.23 419.3359Fe 51.79 47.17 32.97 51.97 47.33 33.0860Co 14.24 14.21 14.09 84.72 84.54 83.8564Cu 22114.09 8.54 - 22114.09 8.54 -110mAg 253.97 249.78 234.35 398.90 392.32 368.07113Sn 9.81 9.64 8.24 11.04 10.65 9.27123Sn 3.10 3.01 2.66 3.61 3.50 3.09125Sn 65.14 42.32 8.09 65.14 42.32 8.09198Au 5260.37 1127.36 3.07 5260.37 1127.36 3.07C. Dose rates from BremsstrahlungWhile most of the energy of electrons or positronsis lost through ionisation, there will also be somebremsstrahlung, depending on the energy of the particleand the atomic number Z of the absorbing medium. Ac-cording to [13], the total bremsstrahlung dose rate from apoint source is given by the equation:6 10 4AE2md2(Z + I) Sv=h (4)where A and d are as before, E2mis the maximum  en-ergy in MeV, I takes into account internal bremsstrahlungand  is the mass energy absorption coefficient of x-rays incm2 g 1. Assuming values of 7, 5 and 0.03 for Z, I and respectively [13] for bremsstrahlung in air, it can be seenthat equation 4 remains several orders of magnitude lowerthan equation 4 for all distances and energies. Dose ratesfrom bremsstrahlung can therefore be neglected.D. Total dose ratesfor whole barrel systemIt is also of interest to estimate dose rates for the en-tire SCT barrel system. This has been done assuming ev-ery module in the barrel system is a point source of identi-cal activation. While this assumption is reasonable for the(n,) activation, it will overestimate spallation related doserates as the relevant particle rates are higher in the first SCTbarrel than in the other three barrels. Two ‘access scenar-ios’ are considered, shown in Figure 2; the correspondingresults are given in Tables 4 and 5.Table 3: Total  () dose rates (Sv/h) resulting from the activation of a single barrel module.180 days irradiating 10 years irradiatingDistance cooling times cooling timesfrom the source 1day 1week 1month 1day 1week 1month10 cm 0.161 (28.1) 0.049 (1.3) 0.037 (0.13) 0.215 (28.4) 0.102 (1.4) 0.089 (0.24)30 cm 0.018 (3.1) 0.005 (0.15) 0.004 (0.015) 0.024 (3.3) 0.011 (0.16) 0.010 (0.027)1 m 1.6110 3 (0.28) 0.4910 3 (0.013) 0.3710 3 (0.001) 2.1510 3 (0.28) 1.0210 3 (0.013) 0.8910 3 (0.001)VI. CONCLUSIONSConcerning (n,) activation the dominant radionu-clides are 64Cu, 198Au and 110mAg (see Figure 1). However,64Cu and 198Au have half-lifes of 12.7 hours and 2.7 daysrespectively. Therefore, assuming access cannot be madeto the SCT within 1 week of shutdown, module activitywill be dominated by the 110mAg which has a half-life of249.9 days. Considering the small amount of silver as-(a)rr01.50m(b)r0yxzFigure 2: Considered access scenarios (a) Scenario 1 and (b) Scenario 2.sumed in the module construction ( 1 mg), it is clear thata design using a higher silver content will result in a pro-portional increase in activities.Concerning spallation induced activation, the domi-nant radionuclides are 3H, 7Be and 22Na. However, whenconsidering external dose rates from activation, 3H can beneglected as discussed in Section V.B. Also, 7Be only de-cays  10% of the time into gammas, leaving 22Na as themost important radionuclide resulting from spallation. In-terestingly, the target nuclei mainly responsible for 22Naproduction is silicon and is therefore unavoidable.Inspection of Table 3 shows that even after 10 yearsof LHC running and at a distance of 10 cm, the mod-ule -doses will be about  0.1Sv/h. The correspond-ing -doses (Table 3) are much higher but after 1 weekdrop to about 1Sv/h. According to the CERN radiationsafety manual [14], dose rates less than 0.1Sv/h at 10 cmare considered non-radioactive, while dose rates above0.1Sv/h but less than 10Sv/h at 10 cm are consideredslightly-radioactive. These low dose rates mean that ifextraction were necessary then modules could be storedsimply in supervised areas [14]. After a cool down period ofone month, -dose and -dose values of  0.1Sv/h and0.24Sv/h at 10 cm are given respectively. These valuesare similar to those of natural background activity.Inspection of Tables 4 and 5 show that dose rates fromthe barrel ensemble will approach 10Sv/h at 10 cm after a1 month cool-down time. As the predicted values are con-sidered upper-limits, if barrel extraction were necessarythen it would probably only need be kept in a supervisedarea. If the dose rates were higher than 10Sv/h but lessthan 100Sv/h then the barrel ensemble would have to bestored in a ‘controlled’ area [14].Finally, it should be stressed that the current study hasassumed a low-silver module design. If the silver contentis much higher, as would be the case if silver-loaded con-ductive glues are used, then the results concerning 110mAgshould be scaled accordingly. However, the radiologicalimplications would have to be reevaluated.Table 4: Maximum  () doses (Sv/h) for Scenario 1.180 days irradiating 10 years irradiatingApproaching cooling times cooling timespoint 1day 1week 1month 1day 1week 1month10 cm 6.2 (1084) 1.9 (50) 1.4 (4.1) 8.3 (1088) 3.9 (53) 3.4 (7.4)30 cm 4.0 (695) 1.2 (32) 0.92 (2.4) 5.3 (697) 2.5 (34) 2.2 (4.1)100 cm 1.3 (184) 0.38 (9.5) 0.29 (0.35) 1.7 (184) 0.79 (9.6) 0.69 (0.47)Table 5: Maximum  () doses (Sv/h) for Scenario 2.180 days irradiating 10 years irradiatingApproaching cooling times cooling timespoint 1day 1week 1month 1day 1week 1month10 cm 9.2 (1598) 2.8 (74) 2.1 (6.4) 12 (1603) 5.8 (79) 5.1 (12)30 cm 5.1 (888) 1.6 (41) 1.2 (3.4) 6.8 (891) 3.2 (43) 2.8 (5.7)100 cm 1.5 (254) 0.45 (11) 0.34 (0.48) 1.99 (254) 0.95 (12) 0.83 (0.66)VII. REFERENCES[1] I.Dawson, Review of the Radiation Environment inthe Inner Detector, ATL-INDET-2000-006[2] SCT Barrel Module, Final Design Reviews: SCT-BM-FDR-1, SCT-BM-FDR-2, SCT-BM-FDR-3 and SCT-BM-FDR-4. Available from:http://atlasinfo.cern.ch/Atlas/GROUPS/INNERDETECTOR/SCT/module/SCTbarrelmod.html[3] Table of material for the Cu/Polyimide hy-brid 12 m Copper, version 4. Available from:http://jsdhp1.kek.jp/unno/si hybrid/k4/KhybridXo01feb26.pdf[4] Private communication, T.Weidberg[5] T.Niinikoski, Evaporative Cooling - Conceptual De-sign for ATLAS SCT, ATL-INDET-98-214[6] Evaluated Nuclear Data Files (ENDF), obtainedfrom: http://www-nds.iaea.org/ndsstart.html[7] Chart of the Nuclides, produced by Knowles AtomicPower Laboratory; 14th edition - revised to April1988. (Using National Nuclear Data Center (NNDC)files.)[8] A.Fasso, A.Ferrari, J.Ranft and P.Sala, Full de-tails can be found at FLUKA official website;http://fluka.web.cern.ch/fluka/[9] A.Ferrari and P.Sala, The Physics of High Energy Re-actions, ATLAS Internal note, PHYS-NO-113, 1997[10] R.Engel and J.Ranft, Hadronic photon-photon interac-tions at high-energies, Physics. Rev. D54:4244-4262,1996[11] K.Potter and G.Stevenson, Average Interaction Ratesfor Shielding Specification in High Luminosity LHC Ex-periments, CERN AC/95-01, CERN/TIS-RP/IR/95-05.[12] K.J.Connor and I.S.McLintock, Radiation Protection,HHSC Handbook No. 14, 1997, ISBN 0-94237-21-X[13] I.S.McLintock, Bremsstrahlung from Radionuclides,HHSC Handbook No. 15, 1994, ISBN 0-948237-23-6[14] Radiation Safety Manual, CERN – TIS/RP, 1996

Estimating induced-activation of SCT barrel-modules

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