Boudard, A

David, J C

Donadille, L

Leray, S

Volant, C

CERN Document Server

Impact of Various Models on the Prediction of Spallation Residue Yields and TargetActivationL. DONADILLE, A. BOUDARD, J.-C. DAVID, S. LERAY and C. VOLANTDSM/DAPNIA/SPhN, C.E.A. Saclay, F-91191 Gif-sur-Yvette, FranceComparisons between the INCL4 cascade model, recently implemented in LAHET, and Bertini model are made inthin and thick lead targets. The impact of the cascade model used on the estimation of the target activity during itsirradiation by a 1 GeV proton beam and its subsequent decay is discussed.KEYWORDS: cascade model; spallation residue; activityI. IntroductionThe interest in spallation reactions has been renewed by theadvent of the concept of accelerator-driven systems (see1, 2)).Such devices couple an accelerator of particles with a thick tar-get converter to create an intense neutron source, additionallysurrounded by a subcritical assembly. The detailed design ofthese technological applications require powerful and reliablecomputational tools able to properly predict the production ofparticles and nuclides from the spallation target. For example,since most of the residual nuclides produced in the spallationtarget are radioactive, it is necessary to know their productionrate in order to assess potential problems of activity or long-term radiotoxicity.The spallation process is generally described by thecoupling between an intra-nuclear cascade (INC), with anevaporation-fission model. The process ends up with theformation of a spallation residue in its ground state, or twofission products for heavy nuclei.Recently, an improved version of the Lie`ge intra-nuclearcascade (INCL4, see3, 4)) has been developed. The new ingre-dients of this model are: (i) introduction of a smooth nuclearsurface, (ii) a consistent dynamical Pauli blocking treatment,(iii) the division of the nucleons in participants and spectators,(iv) some improvements of the pion dynamics and (v) the ex-tension to incident light clusters. The quality of the predic-tivity of the model, tested over a wide range of experimentaldata, has motivated its implementation in the LAHET CodeSystem5). This offers the opportunity to perform consistentinter-comparisons of the Bertini6) and INCL4 cascade mod-els using exactly the same evaporation code (here Dresner7),coupled with the Atchison fission model8)) and particle trans-port conditions (simulated by LAHET). In all calculations pre-sented hereafter, the Bertini cascade is coupled to the Multi-stage Preequilibrium Model (MPM, see9)) as it gives more ac-curate results in this instance.These comparisons, along with some confrontations to re-cent spallation residue experimental data corresponding to re-actions in a thin Pb target, are made in Section I. In Sec-tion II the two cascades are compared for a thick natPb target.Section III is devoted to the estimation of the induced activityas given by both cascades and finally section IV contains our Corresponding author, Tel. (33-1) 69 08 72 77, Fax. (33-1) 6908 75 84, E-mail: ldonadille@cea.frconclusions and outlooks.II. Residue production in thin targetThe Bertini and INCL4 calculations for p (1 GeV)  Pbare compared in Fig. 1 with the experimental data ofRefs.10, 11), measured in inverse kinematics.As it has been already observed (see, e.g. Ref.10)) theBertini calculation overpredicts the yields of light spallationresidues (around A  140). This effect has been ascribed toa distribution of the excitation energy (E ) too high at the endof the cascade, which results in evaporating more particles andfinally producing lighter nuclides. As a consequence, this ef-fect depletes the region of heavier residues around mass 190which is underestimated. We see that INCL4 gives a muchbetter reproduction of the spallation residue mass spectrum.However, part of the effects that we observe here may be dueto the evaporation model, so conclusions regarding the cas-cade themselves must not be given too quickly since a differ-ent evaporation may lead to different results (see, e.g.4, 10)). Onthe other hand, although fission is beyond the scope of this pa-per, we observe a large disagreement with bothcascade modelsin this part of the spectrum. This is ascribed to the Dresner-Atchison model.Regarding the isotopic distributions, shown here from leaddown to terbium, two observations can be made: both cas-cades give very similar shapes, and the experimental data arepoorly reproduced. The shape of the isotopic distributions ismostly driven by the neutron-proton competition in the evap-oration stage of the reaction. This stage is treated by the samemodel (Dresner) for the two Bertini and INCL4 calculations,hence we have the same isotopic distribution shapes. Further-more, the poor overall reproduction of the data is explained bya wrong descriptionof the neutron-protoncompetition in Dres-ner.III. Residue production in thick targetThe results of calculations of residue production with a1 GeV proton beam bombarding a thick cylindrical natPb tar-get (L   cm, R   cm) are presented in Fig. 2. Inthis calculation the neutrons are tranported with LAHET downto 20 MeV. On the mass distribution, it is clear that we ob-serve much less different results for A   between thetwo calculations than in Fig. 1. On the isotopic distributions,10-111010 250 100 150 200Mass NumberCross section (mb)DataBertini + MPMINCL410-210-111010 210 365Tb 66Dy 67Ho10-210-111010 210 368Er 69Tm 70Yb10-210-111010 210 371Lu 72Hf 73Ta10-210-111010 210 374W 75Re 76Os10-210-111010 210 377Ir 78Pt 79Au10-210-111010 210 3150 175 20080Hg150 175 20081Tl150 175 20082PbMass NumberCross section (mb)DataBert+MPMINCL4Fig. 1 Residue mass (top) and isotopic (bottom) production cross-sections for p (1 GeV) Pb reactions. The predictions ofthe Bertini ( MPM) and INCL4 cascades, coupled with theDresner evaporation code are shown by the dashedand contin-uous lines, respectively. The data (dots), obtained in inversekinematics, are from Refs.10,11).although the integrated production probability of a given el-ement changes in correlation with the mass distribution, wesee that the shapes are not influenced by the change of thetarget thickness. This is due to the fact that in a thick targetmany spaalation reactions occur at lower energies than the ini-tial beam energy. The Fig. 3, which shows the residue masspopulation for different energy bins of the particles inducingthe reactions, illustrates this effect. We can observe that themajority of the residues in the regionA   are produced byreactions in between 20 and 300 MeV, which represent morethan 60% of the total number of reactions, and it appears thatthe Bertini and INCL4 cascade models give similar results atlow energy in this mass region. On the other hand, the nuclides10-410-310-210-150 100 150 200Mass NumberProduction probability per incident protonBertini + MPMINCL410-510-410-310-210-1165Tb 66Dy 67Ho10-510-410-310-210-1168Er 69Tm 70Yb10-510-410-310-210-1171Lu 72Hf 73Ta10-510-410-310-210-1174W 75Re 76Os10-510-410-310-210-1177Ir 78Pt 79Au10-510-410-310-210-11150 175 20080Hg150 175 20081Tl150 175 20082PbMass NumberProduction probability per incident protonBertini+MPMINCL4Fig. 2 Residue mass (top) and isotopic (bottom) production weightsfor p-induced reactions on a thick cylindrical natPb target(L   cm, R   cm) at 1 GeV incident energy. Theweight is the formation probability per incident proton. TheBertini ( MPM) and INCL4 calculations are shown by thedashed and continuous lines, respectively.very far from the target (A  ), can only be producedfrom high energy interactions since high excitation energiesare needed to have important mass loss by particle emission.The size of the target does not affect the isotopic distributionshapes because they are much more correlated to the evapora-tion model used than to the energy of the spallation reaction.IV. Activation and decay estimationsWe have estimated the activation due to the irradiation bythe proton beam and the subsequent decay of the lead target.The ORIGEN2 code12) has been used to calculate the evolu-tion of the nuclides concentrations at a function of time. It usesCountsMass NumberFig. 3 Residue mass spectrum for p-induced reactions in a thickcylindrical natPb target (L   cm, R   cm) at 1 GeVincident energy. The calculation is performed with the Bertinicode. Full line: total spectrum, dashed, dotted and dashed-dotted lines: reactions induced by particles of kinetic energyin the bin 600-1000 MeV, 300-600 MeV and 20-300 MeV, re-spectively.the well known set of coupled differential equations dependingon the individual decay constants, the neutron capture cross-sections, and the neutron flux.A constant feeding rate Fiof a given isotope i can be intro-duced in the calculation. In our case, Fi(g/sec)  C  piI (mA), where C is a normalisation constant, pithe productionprobability per incident proton of producing the i-th isotope,information taken from the result of the LAHET calculation,and I the intensity of the beam.In this work, we did not take into account the neutron fluxterms below 20 MeV, since only the neutrons beyond 20 MeVhave been transported in LAHET, and the fission productshave not been considered. Indeed, Shubin et al.13) have esti-mated that the neutron flux below 20 MeV contributes to lessthan 10% to the total activity for decay times not larger than days and that the fission products contribute to the orderof 10-15%. Finally, we have limited the study to isotopes withproduction probability larger that  to keep the statisticaluncertainty of the calculated results not larger than 14% rela-tive. This means that we take into account about 300 nuclideswith A   and Z  	In Figs. 4 and 5 are presented the activity estimations madefrom the Bertini and the INCL4 calculations, and Table 1 givesthe activity for some selected times. The primary proton beamhas 1 GeV energy, 1 mA intensity and irradiates the cylindrical(R   cm, L   m) lead target. For both results and fora given time the main contributor to the total activity is plot-ted. During the irradiation phase one sees that the total activitysaturates at   Ci after about one month. This value is ingood agreement with the Shubin et al. study13). At this timethe main contributors are Pb and Pb for the Bertini andINCL4 calculations, respectively. However, it can also be seenthat these two nuclides contribute to less than an order of mag-nitude below the total activity. This means that in both casesa large number of different isotopes contribute almost equiva-lently to the total activity. A similar behavior is observed up toalmost a month of decay, meaning that most of these nuclideshave rather short periods (T  d). For longer decaytimes the activity is due to only few long-lived nuclides. Forexample, after 10 years of decay the nuclide Tl represents60% of the activity, and after  years the dominant nuclide,representing 50% of the activity, is Tl, populated by the -decay of the long-lived Pb. Finally one remarks that onlythe heaviest nuclides are the dominant ones.1min 1h 1day 1mth 1y 10y 102y 103y 104y 105y 106yTime10−210−1100101102103104105106107193Pt (50 y)194Au (38 h) + 194Hg (520 y)202Tl (12.2 d) + 202Pb (5.3*104 y)Total1sec 1min 1h 1day 1mth 1y101102103104105106107206Tl (4.2 min)201Pb (9.4 h)203Pb (51.9 h)195Au (186 d)204Tl (3.78 y)Activity [Ci]IrradiationDecayFig. 4 Total and partial activities of a natPb target as a fonction of ir-radiation time (top) and cooling time (bottom) with the Bertinicascade model.Table 1 Total activity (Ci) during the irradiation and decay phasesgiven for some selected times.Irradiation time (sec) 3.2Bertini 7.8 3.4 5.0 5.1INCL4 7.5 3.4 4.8 4.9Decay time (sec)6 6 3.2 3.2Bertini 2.1 3.4 4.7 5.0INCL4 1.9 3.5 5.6 5.4On Fig. 6 is shown the total activity ratio between theBertini and INCL4 calculations. It can be observed that dur-ing the whole irradiation period and up to one year of decaythe two models give activities consistent within 10%. It is inagreement with the previous observation of the fact that the1min 1h 1day 1mth 1y 10y 102y 103y 104y 105y 106yTime10−210−1100101102103104105106107193Pt (50 y)194Au (38 h) + 194Hg (520 y)202Tl (12.2 d) + 202Pb (5.3*104 y)Total1sec 1min 1h 1day 1mth 1y101102103104105106107206Tl (4.2 min)209Pb (3.25 h)203Pb (51.9 h)195Au (186 d)204Tl (3.78 y)Activity [Ci]IrradiationDecayFig. 5 Same as Fig. 4 for a calculation with the INCL4 cascademodel.two cascade models give similar mass distributions in the caseof such a thick target (see Fig. 2). However, some larger devi-ations can be seen between the two cascade models for longerdecay times in Fig. 6. Since only few long-lived isotopes areleft, the activity becomes very sensitive to the detailed of theisotopic distribution shapes and hence to the individual pro-duction probabilities. For example, the ratio of the productionprobabilities between the two cascade models is equal to 0.76for Tl and 1.32 for Pb, very close to the correspondingratio of activity after 10 and  years decay times at whichthis two nuclides are the dominant ones, respectively. The factthat no sensitivity is seen during the irradiation and for decaytimes smaller than one year is due to the coexistence of manyshort-lived nuclides which lead to compensation effects.V. Conclusion and outlooksComparing results of with the Bertini (MPM) and INCL4cascades, coupled to the Drener evaporation model in the LA-HET code has shown that the activity in a thick lead target in-duced by a 1 GeV proton beam during the irradiation phaseand up to 1 year of decay is insensitive (within 10%) to thecascade model. However, the sensitivity increases with decaytime since the differences between the models become veryclose to the differences between the productionprobabilitiesofsome specific long-lived isotopes (larger than  for Pb).This work is still in progress and we intend to investigatethe impact of the evaporation model on the estimation of theactivity. Indeed a different evaporation model will modify theshapes and mean values of the isotopic distributions. So it isexpected to have a stronger impact than the one of the cascademodel. In this aim the new KHS3v code14) is being imple-mented in LAHET. For these studies, the spallation residuestogether with the fission fragments will be taken into account,and since it has a non negligible effect for long decay times1min 1h 1day 1mth 1y 10y 102y 103y 104y 105y 106yTime0.80.91.01.11.21.31sec 1min 1h 1day 1mth 1y0.80.91.01.11.21.31.4Activity Bertini/INCL4IrradiationDecayFig. 6 Ratio of the activities given by the Bertini and INCL4 calcula-tions, during the irradiation (top) and decay (bottom) phases.(t   y, see13)) the activation in the neutron flux with en-ergy below 20 MeV will be considered as well.AcknowledgmentWe are grateful to R.E. Prael of the Los Alamos NationalLaboratory for providing us the LAHET3 source code and forhis help. This work is supported by the HINDAS Collabora-tion, EU Contract N FIKW-CT-2000-00031.References1) A.C. Bowman et al., Nucl. Instr. and Meth., A 320, 336 (1992).2) C. Rubbia et al., CERN/AT/95-44(ET) (1995).3) J. Cugnon, C. Volant and S. Vuillet, Nucl. Phys., A 620, 475(1997).4) A. Boudard, J. Cugnon, S. Leray and C. Volant, contribution tothis conference and submitted to Phys. Rev., C.5) R.E. Prael and H. Lichtenstein, LANL Report LA-UR-89-3014(1989), R.E. Prael, LANL Report LA-UR-01-1655 (2001), andprivate communication.6) H.W. Bertini et al., Phys. Rev., 131, 1801 (1963).7) L. Dresner, ORNL Report ORNL-TM-196 (1962).8) F. Atchison, in Intermediate Energy Nuclear Data: Models andCodes, Proc. of a Specialists’Meeting, OECD/NEA publica-tions, Issy-les-Moulineaux, France, 199 (1994).9) R.E. Prael and M. Bozoian, LANL Report LA-UR-88-3238(1988).10) W. Wlazlo et al., Phys. Rev. Lett., 84 (2000) 5736.11) T. Enqvist et al., contribution to this conference and Nucl. Phys.,A 686, 481 (2001).12) A.G. Croff, Nucl. Technology, 62, 335 (1983).13) Yu.N. Shubin, A.V. Ignatyuk, A.Y. Konobeyev and V.P. Lunev,Proc. Int. Conf. Evaluation of EmergingNuclear Fuel Cycle Sys-tem, Global 1995, Sept. 11-14 (1995).14) A.R. Junghans et al., Nucl. Phys., A 629, 635 (1998).

Impact of various models on the prediction of spallation residue yields and taget activation

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