Krivosheev, O E

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DISTRIBUTION OF ABSORBED DOSES IN THE MATERIALSIRRADIATED BY “RHODOTRON” ELECTRON ACCELERATOR:EXPERIMENT AND MONTE CARLO SIMULATIONS∗A. Kolchuzhkin‡, S. Korenev, O. Krivosheev† , I. Tropin‡FNAL, Batavia, IL 60510, USA STERIS Corporation, USA ‡ Tomsk Polytechnic University, RussiaAbstractThis paper describes the experimental setup and presentsstudies of absorbed doses in different metals and dielectricsalong with corresponding Monte Carlo energy depositionsimulations. Experiments were conducted using a 5 MeVelectron accelerator. We used several Monte Carlo codesystems, namely MARS, MCNP, and GEANT to simu-late the absorbed doses under the same conditions as in ex-periment. We compare calculated and measured high andlow absorbed doses (from few kGy to hundreds kGy) anddiscuss the applicability of these computer codes for ap-plied accelerator dosimetry.1 INTRODUCTIONThe prediction of correct distribution of absorbed dosesin the irradiated product is very important for applied ra-diation technologies. Application of Monte Carlo energydeposition computer simulation allows us to increase theefficiency of the product irradiation. Different materials(from metals to dielectrics), different geometries with vari-ation of density of irradiated product lead to complex prob-lems of measurements and calculations of absorbed doses[1]. Present status of computer simulation for radiationtechnologies is not quite simple. Two main calculationmethods were developed - analytical solutions and statisti-cal simulation also known as Monte Carlo method. MonteCarlo method has been developed quite extensively latelyand several large code systems are available for compari-son and evaluation. We present an attempt to make com-parison of three main computer codes (MCNP, GEANT3and MARS14) with experimental data taken from indus-trial electron accelerator ”Rhodotron”.2 EXPERIMENTAL SETUPThe experiments were conducted using CW”Rhodotron” Electron Accelerator [3] manufacturedby IBA(Belgium) [2] with the main parameters listed inTable 1.Accelerator is able to operate in either static or scanningmode. The static mode is used only for sample irradiation.The scanning mode of electron beam in the area of irradia-∗Work supported by Universities Research Association, under contractDE-AC02-76CH03000 with the U. S. Department of Energy.† kriol@fnal.govTable 1: Main Parameters in the experimentsElectron kinetic energy 5 MeV;Beam current 2-16 mA;RF operating frequency 107.5 MHz;Beam repetition rate 100 Hz.tion leads to pulsed regime of accumulation of the absorbeddoses under the adiabatic conditions [4].Electron beam propagates in air over the 70 cm distanceon its way from accelerator foil window to the sample po-sition. Time of irradiation determined the different levelof absorbed doses, and by increasing irradiation time from5 sec to 1 min or more we covered all interesting dosesfrom several kGy to hundreds kGy.Samples were made from plates of particular materialwith film dosimeters placed in between. The plates hadvarying thickness for different materials but with constantdimensions 22 cm2 in the direction transversal to thebeam propagation.Dielectrics such as Teflon and glass, and metal (Alu-minum) were used as materials for samples. The stan-dard Cellulose TriAcetate (CTA) film dosimeters (FTR-125) were used for the dose measurements. We em-ployed ”Beckman DU640” spectrophotometer with thewavelength of 280 nm in order to get dose readings fromCTA films.3 COMPUTER CODES ANDNORMALIZATION PROCEDUREThree popular computer codes: MCNP v.4C [5],GEANT3 v3.21 [6] and MARS v.14 [7] were used in thisstudy. These codes have long history and are applicable indifferent research areas. MCNP was started as pure neu-tron transport code and only lately the electron, positronand gamma transport were added from Integrated TigerSeries developed by M. Berger and S. Seltzer. MCNPteam concentrated on low energy transport above 1 keVbut below 20 MeV. MCNP has highly advanced biasingtechnique which allows for optimization of data scoringand efficiency. Another MCNP advantage is the abilityto use PVM with multiple processors or network of work-stations to speed-up the calculations. GEANT was devel-oped as Monte Carlo tool for high energy physics detectors.Main interest area is high-energy processes, so GEANTcan track electrons, positrons, and photons only down to0-7803-7191-7/01/$10.00 ©2001 IEEE. 2500Proceedings of the 2001 Particle Accelerator Conference, Chicago10keV. 3D geometry block is included but GEANT re-quires a lot of programming skills in order to set up the cal-culations. MARS was started as tools for accelerator andshielding studies and from middle 70s has been developedinto full scale Monte Carlo code. It got advanced electro-magnetic module last year [8]. As in GEANT, program-ming skills are required to set up the calculations. Particlescan be tracked down to 1keV for photons and 10keV forelectrons. Electromagnetic module can be used either sepa-rately or embedded into software controlling technologicalprocess. MARS has biasing technique implemented to in-crease calculation efficiency.Monte Carlo accuracy, in general, depends on the num-ber of particles tracked through the setup since the statis-tical error goes down as inverse square root of number oftracked particles. All codes have systematic errors due tocross-section data uncertainty around few percent. We con-tinued tracking until statistical error was below 1% in allbins. This study considers the electron source with real-istic parameters, angular and spatial distribution of beam,sample geometry, and normalization condition for scanningwork mode of the accelerator.Normalization procedure is quite complicated and takesinto account beam size, sample size, current, scanner po-sition, speed, and frequency. We also take into accountelectron struggling through the air and angular and spatialspread due to multiple scattering. For normalization pur-poses we derived complicated expression which accountsfor all above mentioned factors and calculates the numberof electrons which actually hit the target. It is available onrequest and will be published in extended version of thispaper.4 RESULTS0 1 2 3 4 5 6Distance (mm)0102030Dose (kGy)Aluminum5mA, 10 secondsExperimentMCNP 4CGEANT 3.21MARS 14Figure 1: Aluminum sample, 5 ma current, 10 sec irradia-tionTypical distributions of measured and calculated ab-sorbed doses for different materials and irradiation timesfor aluminum, Teflon and glass are presented on Figs. 1-6Variation of irradiation time allows us to generate differentlevels of accumulated absorbed doses without overheatingthe samples under our adiabatic conditions. The results ofcomputer simulation and experiments show that for con-ducting materials such as aluminum (Figs. 1-3) we havealmost perfect linear dependence between irradiation timeand absorbed dose for all interesting levels starting with0 1 2 3 4 5 6Distance (mm)01020304050Dose (kGy)Aluminum5mA, 20 secondsExperimentMCNP 4CGEANT 3.21MARS 14Figure 2: Aluminum sample, 5 ma current, 20 sec irradia-tion0 1 2 3 4 5 6Distance (mm)0102030405060708090100110120130140150Dose (kGy)Aluminum5mA, 60 secondsExperimentMCNP 4CGEANT 3.21MARS 14Figure 3: Aluminum sample, 5 ma current, 60 sec irradia-tion0 1 2 3 4 5 6 7 8 9 10Distance (mm)0102030405060708090Dose (kGy)Glass5mA, 20 secondsExperimentMCNP 4CGEANT 3.21MARS 14Figure 4: Glass sample, 5 ma current, 20 sec irradiation0 1 2 3 4 5 6 7 8 9 10Distance (mm)010203040506070Dose (kGy)Teflon5mA, 15 secondsExperimentMCNP 4CGEANT 3.21MARS 14Figure 5: Teflon sample, 5 ma current, 15 sec irradiation0 1 2 3 4 5 6 7 8 9 10Distance (mm)04080120160200240280320Dose (kGy)Teflon5mA, 60 secondsExperimentMCNP 4CGEANT 3.21MARS 14Figure 6: Teflon sample, 5 ma current, 60 sec irradiation2501Proceedings of the 2001 Particle Accelerator Conference, Chicagolow dose (up to 20 kGy) up to high doses (around 150-200kGy). The results for dielectric materials for these levelsof absorbed doses have strong nonlinear behavior. Also wecan clearly see the shift of the position of the dose maxi-mum. For the high level of doses in Teflon 6 one can seeclose to 30-40% difference between experimental data andsimulation results. The average error of simulation is be-low 5% if we take into account statistical and systematicAl errors combined. The total average experimental er-ror for absorbed doses on the level of 15% for both min-imum and maximum doses. Monte Carlo calculations forall three codes were done using the same source and ge-ometry terms and were traditionally done per one incidentparticle. Given the irradiation time and using normaliza-tion procedure mentioned above we were able to compareand plot together calculated and experimental data.5 DISCUSSIONIn course of our research we found good correlation forconducting materials such as aluminum for all levels ofdoses. The results for measured and simulated absorbeddoses in Al confirm it. The dose growth has linear de-pendence versus irradiation time or beam current. Wecould also see that MCNP and MARS reproduce the doseshape reasonably well with about 20% difference, whichwe could attribute to possible normalization uncertaintiesand systematic errors. In the case of dielectric materials(glass and Teflon) we have good correlation with the dataup to medium doses. But in the case of high doses whichare quite interesting range of doses for radiation technolo-gies, the results of simulation show the difference on thelevel of 30-40% after the dose maximum. We believe itcan be explained by the properties of dielectrics. Effectof dielectric charging by electrons could be responsible formuch of the difference, as was clearly demonstrated andsummarized earlier in the monograph [9]. The position ofdose maximum in dielectric materials relative to the met-als can be explained by effect of electrical field inside ofdielectric produced by stopping electrons during the irra-diation. Analysis of experimental data and computer sim-ulations shows that for low and middle level of doses forconducting and dielectric materials we can use these threecomputer codes. For conducting materials we also can usethese codes for all level of absorbed doses. But for di-electrics we need to understand and introduce practical cor-rections in the algorithm used to propagate electron throughmedia and dissipate its energy.6 CONCLUSIONSOur future plans include an investigation of the influenceof dielectric constant on the value of absorbed doses andan investigation of effects of electrical discharge inside ofthe dielectric materials. The effect of internal dischargewas observed for high current electron beam hitting the di-electrics with high dielectric constant [10]. The other ef-fect of anomaly conductivity for dielectrics also could haveplace [11]. Other interesting experiments which we areplanning to conduct are using DC and RF Linacs (differenttime scale of beam parameters) for comparison with resultsof computer simulation by those 3 codes.As a result of this study we can make following mainconclusions. The computer codes could give the user av-erage error on the level 12-15% for conducting materialsfor all levels of absorbed doses. The computer codes couldproduce results with average error on the level 15-20% fordielectric materials for absorbed doses up to 70 kGy. Thedistribution of absorbed doses in dielectrics has differentposition of the dose maximum in comparison with con-ducting materials. The MCNP, GEANT3 and MARS canbe used for simulation of absorbed doses for conductingmaterials. The MCNP code is quite accurate for practi-cal applications and is probably more useful for radiationindustry. Correct normalization procedures for industrialscanning electron source are very important for compari-son with those computer codes.7 REFERENCES[1] S.A. Korenev, A.E. Ligachev, I.N. Meskov, V.I. Perevod-chikov ”The status of beam technologies”, Proc. of 1stInternational Symposium on Beam Technologies (BT’95),Dubna, Russia (1995).[2] J. Pottier “A new type of RF electron accelerator: TheRhodotron”, NIM, B40/41, p. 943-945 (1989).[3] Y. Jongen, M. Abs, T. Delvigne, A. Herer, J.M. Capdev-ila, F. Genin, A. Nguyen “Rhodotron accelerators for indus-trial electron-beam processing”, Proc. of International Con-ference on the Application of Accelerators in Research andIndustry, Denton, USA (1992).[4] S.A. Korenev, ”Electron beam curing of composites”. Proc.of VIth International Conference on Electron Beam Tech-nologies (EBT’2000), Varna, Bulgaria (2000).[5] J. F. Briesmeister. Ed., ”MCNP - A General Purpose MonteCarlo N-Particle Transport Code, Version 4C”. 2000. Man-ual LA-13709-M, April 2000, LANL.[6] ”GEANT, Detector Description and Simulation Tool”,CERN Program library Long Writeup W5013, (1994).[7] N.V. Mokhov, O.E. Krivosheev, “MARS Code Status”,Fermilab-Conf-00/181 (2000).[8] O.E. Krivosheev, N.V. Mokhov, “MARS ElectromagneticPhysics Status”, Proc. of International MC2000 Conference,Lisbon, Portugal (2000).[9] V.V. Gromov, “Electrical charge in the irradiated materials”,Energoizdat, Moscow (1982).[10] S.A. Korenev “Determination of admittance irradiationloads by electron beam on the BaTiO3”, Proc. of All-Union Conference on Physics of Dielectrics, Tomsk, Russia(1988).[11] D. Vaisburd, B. Semin, S. Kharitonova, “Intrinsic radiolu-minescence of non-ionized electrons and auger-free holes indielectrics excited by high-current-density electron beams”,Proc. of 11th International Pulsed Power Conference, Balti-more, USA (1997).2502Proceedings of the 2001 Particle Accelerator Conference, Chicago

Distribution of Absorbed Doses in the Materials Irradiated by "RHODOTRON" Electron Accelerator: Experiment and Monte Carlo Simulations

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