Monte Carlo simulations and measurements

Dose rate determinations in the Portuguese GammaIrradiation Facility:Monte Carlo simulations andmeasurementsC.Oliveira*,J.Salgado,A.Ferro de CarvalhoNuclear and Technological Institute,Est.Nac.10,2685±953,SacaveÂm,PortugalReceived22October1998;accepted30September1999AbstractA simulation study of the Portuguese Gamma Irradiation Facility,UTR,has been carried out using the MCNP code.The work focused on the optimisation of the dose distribution inside the irradiation cell,dose calculations inside irradiated samples and dose calculations in critical points for protection purposes.Calculations were carried out at points inside and outside the irradiation cell,where di erent behaviour was expected(distance from the source,radiation absorption and scattering in irradiator structure and walls).The contributions from source,irradiator structure,sample material,carriers,walls,ceiling and¯oor to the photon spectra and air kerma at those points are reported and discussed.Air kerma measurements were also carried out using an ionisation chamber.Good agreement was found between experimental and calculated air kermas.#2000Elsevier Science Ltd.All rights reserved.Keywords:Dosimetry;Air kerma;Monte Carlo;Irradiation facility;Co-60(cobalt-60)1.IntroductionIndustrial60Co irradiation facilities are widely used for irradiation purposes.Suitable dosimetry procedures must be carried out in order to establish plant oper-ational parameters,such as dose uniformity,source utilisation e ciency and maximum and minimum dose positions.A detailed dose mapping of the irradiation cell can be performed using calculations,which reduces signi®cantly the monitoring with dosimeters.Di erent mathematical methods for dose predictions have been developed(Herring and Saylor,1993).One of them is based on the point kernel approach(Mosse et al.,1988;Shultis and Faw,1996;Pina-Villalpando and Sloan,1998);another uses the Monte Carlo method.The most used Monte Carlo codes are the EGS4(Nelson and Rogers,1988;McIntyre et al., 1990;Raisali and Sohrabpour,1993),the ITS (Halbleib,1988;Weiss et al.,1997)and the MCNP (Briesmeister,1997;Oliveira et al.,1999).Saylor and Jordan(1999)are developing a software package, which provides the end-user with a robust geometry input and output platform from which various forms of point kernel and Monte Carlo algorithms can be launched.The accuracy of the Monte Carlo method depends on the physical interactions considered,the cross sec-tion library and the correctness of the incorporation of the source,geometry,composition,dimensions andRadiation Physics and Chemistry58(2000)279±2850969-806X/00/$-see front matter#2000Elsevier Science Ltd.All rights reserved.PII:S0969-806X(99)00462-/locate/radphyschem*Corresponding author.Tel:+351-1-9550021;fax:+351-1-9941525.E-mail address:coli@itn1.itn.pt(C.Oliveira).density of the irradiated material.However,themethod is computer time intensive.The use of some variance reduction techniques can enhance the e -ciency of the calculations,although its use can intro-duce some errors.Validation measurements must be performed to check the suitability of the model for the speci®c application.Several points were chosen inside and outside the ir-radiation cell,where di erent behaviour in ¯ux and dose distributions was expected (distance from the source,radiation absorption and scattering in irradia-tor structure and walls).The calculations using the MCNP-4B2code (Briesmeister,1997)provide the con-tributions from the source,irradiator structure,sample material,carriers,walls,ceiling,¯oor and air to the photon spectra and air kerma 1at those points.Air kerma measurements were also carried out using an ionisation parison between exper-imental and calculated air kerma revealed good agree-ment.2.The irradiation plantThe Portuguese g -irradiation facility is designed mainly for sterilisation of medical devices and other products (Cavaco et al.,1991;Botelho et al.,1993).The irradiator is a rectangular chamber (8Â3.9m)with high-density concrete walls (1.8±2m thick).Thesource rack has an area of 0.9Â0.9m,and contains thirty stainless steel tubes.One hundred and ®fty six 60Co sources,doubly encapsulated in welded stainless steel (81mm long Â11mm diameter)were loaded into the source rack so that two tubes in the middle of the rack contain eight 60Co sources each while seven tubes to the left and right of the middle ones contain ten cobalt sources each.The source area is 0.421m 2(height=0.81m and width=0.52m).When not in use the sources are stored in dry storage.The actual ac-tivity (Feb/98)is (3.4210%)Â1015Bq.The product to be irradiated is transported by forty carriers sus-pended from a monorail conveyor.Each carrier can be loaded with four product boxes,each of size 0.4Â0.4Â0.4m and a maximum weight of 120kg.The ir-radiation of the boxes is carried out in two rows on each side of the source plaque.There are seven ir-radiation positions (i.e.7carriers)in each row.An ir-radiation cycle corresponds to twenty-eight irradiation positions.Fig.1presents a schematic view of the ir-radiation cell,showing the source rack and the associ-ated iron structure (12mm thick),which supports the mechanisms for moving the rack,as well as the pos-ition of several points where simulations and exper-imental measurements were carried out.The input of the code includes a complete description of the source rack (each Co source is individually modelled),the iron structure,the carriers loaded with boxes and the concrete walls.The measurements were carried out using ``dummy''boxes loaded with layers of folded and crumpled newspapers,with a density of 0.11215%g cm À3.This relatively large error is due to the bad reproducibility of the box ®lling process.The points are located on a plane 100cm above the ¯oor.3.Simulation resultsThe platform used for the MCNP code was an Alpha Personal Workstation running Digital Unix with 64Mbytes RAM and a 433MHz CPU.The code simulated the transport of photons and electrons.The detailed physics treatment for photon interactions was used (photoelectric e ect,Compton e ect,pair pro-duction).It accounts also for ¯uorescence photon emission after photoelectric absorption.The generation of electrons from photons is considered as well.The model generates electrons that are banked for later transport.The electron interactions include the pro-duction of bremsstrahlung photons and electron-induced ¯uorescent X-rays.Once the cut-o energy is reached (50keV),the electrons deposit their energy locally.Some variance-reducing techniques,such as geome-try splitting with Russian roulette and the pointdetec-Fig.1.Schematic representation of the irradiationcell.ÓÉ Foxit Readei ±à¼-°æȨËùÓÐ(C) Foxit Èí¼þ¹«Ë¾,2005-2006½öÊʺÏÆÀ¹ÀʹÓá£tor estimator,have been used to reduce the computertime required to obtain results with statistical standard deviation less than 5%,and the variance of variance less than 10%.The other statistical checks for the esti-mated results included in the code were also satis®ed.The point detector is a deterministic estimate;contri-butions to the point detector are made at source and collision events throughout the random walk.As is suggested in the MCNP manual,the radius of the sphere of exclusion in air (region with very few col-lisions)was set to zero.The number of histories per minute was dependent on the number and position of the detectors used in each run,and on the cell import-ance;it ranged between 500and 2500histories/min.The total number of histories per run necessary to satisfy the statistical checks lay between 15000and 50000.Fig.2shows gamma spectra for points P2,P3,P4and P8.These points,due to their localisation relative to the source,are extreme cases.Spectrum P4clearly reveals the presence of 60Co peaks;33%of the pho-tons have arrived directly from the source.In spectrum P2,the 60Co peaks are strongly reduced.The photons emitted by the source in the direction of this point are mostly absorbed or scattered by the iron structure.Photons that reach point P8,in the corridor,su ered multiple scattering in the walls.The low-energy com-ponents in the 60Co spectra may have important e ects in the absorbed dose enhancement in a material (ASTM,E 1249-93,1998;ASTM,E 1250-88,1998).The material where the photons had the last inter-action before reaching a given detector is depicted in Table 1.Points P4and P6are symmetrical relative to the source plane.The contributions from di erentori-Fig.2.Photon spectra in air at P2,P3,P4and P8.Table 1Percentage of photons having last interactions in materials shown before reaching points P1to P8P1P2P3P4P5P6P7P8Co sources2531933233226±Irradiator structure 6336472±Samples 14151522172217±Carriers 691011111215±Walls 2645341730202276Ceiling 453±2138Floor 10121158596Air 23312±29Others75253141C.Oliveira et al./Radiation Physics and Chemistry 58(2000)279±285281ÓÉ Foxit Readei ±à¼-°æȨËùÓÐ(C) Foxit Èí¼þ¹«Ë¾,2005-2006½öÊʺÏÆÀ¹ÀʹÓá£gins are similar.A more detailed analysis shows the contributions for point P8of the scattered photons in walls:39%su er the last scattering in wall D,32%in wall E,3%in wall F and 2%in wall A.In order to compare the ionisation chamber measurements with the simulated results,the MCNP code directly converts the ¯ux estimates to air kerma rates by using the photon energy ¯ux and the mass energy absorption coe cients (Whalen et al.,1991;Higgins et al.,1992).Fig.3shows the air kerma rate per energy interval in the same points as a function of the photon energy.Fifty eight percent of the air kerma rate of P4originates from photons coming directly from the source,while only 14%of these photons con-tribute to the air kerma at P2.The air kerma rate distribution inside the irradiation cell is not homogeneous;the air kerma rate at P2is only about 20%of that at P4.Air kerma rate calcu-lations were also performed in a square grid of nine points,5cm apart,around each simulated point.Fig.4shows the air kerma rates around point P6.The error bars are the absolute statistical errors.In each line parallel to the source plane,the air kerma rateisFig.3.Air kerma rate per interval of energy at P2,P3,P4andP8.Fig.4.Air kerma rate distribution around point P6as a func-tion of horizontal distance at a height of 100cm above ¯oor level.C.Oliveira et al./Radiation Physics and Chemistry 58(2000)279±285282practically constant.However the air kerma rate is a function of the distance from the source,showing a gradient of the order of 1mGy s À1cm À1.Similar results were obtained for other points.Pro®les of the air kerma rate in the vertical line pas-sing through the simulated points were also calculated.Fig.5shows,as an example,the pro®le around point P6.The error bars are the absolute statistical errors.The range of the corresponding relative errors lies between 1and 4%.The curve has a maximum at height=100cm,which corresponds to the centre plane of the source.The curve is symmetrical relative to the middle point.The air kerma rates corresponding to heights of 40and 160cm are equal.A detailed analysis of the results shows that the contribution to the pho-ton spectra of the ¯oor is 14%and 3%for heights of 40and 160cm,respectively;however,as this di erenceexists in the lower energy region of the spectra it has little e ect in air kerma calculations.A linear ®t Ðair kerma rate as a function of the distance to the source Ðwas adjusted to the simulated values in the grid around each height.The gradient of this curve,associated with the respective error,is rep-resented as a function of the height in Fig.6,for point P6.The maximum of the curve is also obtained at the middle point of the source.4.Experimental resultsExperimental measurements were performed using a 2571,0.6cm 3Farmer ionisation chamber.This chamber consists of a thin walled high purity graphite thimble and a pure aluminium electrode.Previous to the measurements,the response of the ionisation chamber with build-up cap on and o was investi-gated.The results showed that the cap is indispensable for air kerma measurements in high-energy photons from 60Co,while the presence of the cap did not dis-turb measurements in 100keV photons.The air kerma calibration of the ionisation chamber was then carried out in a Standards Laboratory in photon beams of 100keV (X-ray),662keV (137Cs)and 1250keV (60Co).Calibration factors were calculated with an overall uncertainty of 2%.Fig.7shows the chamber response relative to energy.It can be seen that the response of the ionis-ation chamber with build-up cap on is practically inde-pendent of photon energy in the range from 100to 1250keV.parison of simulated and experimental resultsIn order to validate the applied model,some measurements have been carried out at points with di erent photon spectra and air kermarates.Fig.6.Variation of the gradient of the air kerma rate around point P6as a function of height above ¯oorlevel.Fig.5.Air kerma rate pro®le around point P6as a function of height above ¯oorlevel.Fig.7.Relative response of the NE 2571ionisation chamber at three photon energies.C.Oliveira et al./Radiation Physics and Chemistry 58(2000)279±285283Fig.8shows a comparison between the calculated and the experimental results.The straight-line y=x is also shown.The di erences are always smaller than 12%.Taking into account that the source intensity and the density of the dummy boxes have estimated accuracies of 10%and 15%,respectively,these results show good agreement.The mean value of the ratio ``Calculated values/Experimental values ''is 0.968,and its correlation coe cient is r=0.998.Such satisfactory agreement con®rms that the simu-lation can be used to predict the dose distribution in the irradiation cell.6.ConclusionsThe work focused on air kerma calculations and measurements at several points in the Portuguese Gamma Irradiation Facility.The relative contributions to the photon spectra and air kermas from the source,irradiator structure,sample material,carriers,walls,ceiling,¯oor and air depend on the point considered.Most photons have one or more scattering events before reaching a given detector.The maximum air kerma rate is obtained at the centre height of the source.The satisfactory agreement between calculations and measurements con®rms that the simulation model can contribute to the dose mapping data inside the ir-radiation cell,and thus to optimising the irradiation process.The simulation can also be used to optimise the source reloading process,in order to attain a better dose distribution.ReferencesASTM E 1249-93,1998.Standard practice for minimising do-simetry errors in radiation hardness testing of silicon elec-tronic devices using Co-60sources.Annual Book ofStandards 12.02,658±672.ASTM E 1250-88,1998.Standard test method for application of ionising chambers to assess the low energy gamma com-ponent of cobalt-60irradiators used in radiation-hardness testing of silicon electronic devices.Annual Book of Standards 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