OSTI. Los Alamos, National Laboratory New Mexico NOV DISCLAIMER. G. Young, T-2 LA-U R-

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1 LA-U R Los Alamos National Laboratory is operated by the University of California for the United States Department of Energy under contract W-745-ENG-36 NOV OST TTLE: STATUS OF OPTCAL MODEL ACTlVTES AT LOS ALAMOS NATONAL LABORATORY DSCLAMER AUTHOR(S): G. Young, T-2 This report was prepared as an amunt of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thcreof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name. trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. SwmED To: To be presented at the Second Research Coordination Meeting on "Development of Reference nput Parameter Library for Nuclear Model Calculations of Nuclear Data" October 3-November 3, 1995, Vienna AEA Headquarters By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive. royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for US. Government purposes. The Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. FORM NO. 836 R4 ST. NO /81 Los Alamos, National Laboratory New Mexico 87545

2 DSCLAMER Portions of this document may be illegible in electronic image products. mages are produced from the best available original document.

3 STATUS OF OPTCAL MODEL AcTvTlES AT LOS ALAMOS NATONAL, LABORATORY P. G. Young Theoretical Division Los Alamos National Laboratory ABSTRACT An update will be given of activities at Los Alamos National Laboratory aimed at developing optical model potentials for applied calculations. Recent work on a coupled-channels potential for neutron reactions on 241s243Am and spherical neutron potential updates for 56Fe and 59Co will be presented, together with examples of their application in nuclear reaction calculations with the GNASH code system. New potentials utilized in evaluations at Livermore for 12C, 14N, and l6 are described and additional potentials from earlier analyses at Los Alamos of Ti V, and Ni data are made available for possible inclusion in the Reference nput Parameter Library ( UPL) for nuclear model calculations of nuclear data. Specific activities directed at development of the optical potential segment of the RPL will be summarized.. NTRODUCTON At the First Research Coordination Meeting on Development of Reference nput Parameter Library for Nuclear Model Calculations of Nuclear Data in 1994, a number of spherical and coupled-channels optical model potentials developed at Los Alamos National Laboratory for applied calculations were presented.1 n this paper we update the parameterizations for two materials that were included in the previous paper and provide additional ones that have been employed both at Los Alamos and Livermore in nuclear data calculations. We present selected comparisons with experimental data to demonstrate the validity of the potentials. 11. SPHERCAL OPTCAL MODEL POTENTALS A. Neutron and Proton Potentials for?, 14N, and l6 Target Nuclei Recently new evaluations have been completed at Lawrence Livermore National Library for neutron- and proton-induced reactions 14N, and 16 for neutron and proton radiotherapy applications. The neutron evaluations2s3 cover the energy range up to 1 MeV, and the proton evaluations4 cover the range to 25 MeV. We include here the neutron and proton potentials used for major parts of those evaluations. The neutron potentials used for 12C are taken from the analysis of Arthur5 for neutron energies below 1 MeV and from Dimbylow6 for neutrons in the 1-65 MeV range. Above 65 MeV the global potential of Madland7 is used. The proton potentials were obtained by applying the Lane isospin model to the neutron potentials. The parameters for these potentials are included in Table 1. 1

4 Table 1. Spherical optical model potentials for neutron and proton reactions with over the incident energy range 1 kev 5 Efl,p 1 MeV. Above an energy of 65 MeV, the Madland global potential7 is used. Note that the symbol WD indicates a gaussian form factor for the surface derivative potential. NEUTRONS BELOW 1 MeV Well DeDth &leq VR = En + O.ooOo8 EJ WD = = (En - 6) wv =. Vso = 7. Range &lev) Geometrv (fm) O<E,<lO rr = 1.35 ar =.6 O<E,<6 rd = 1.26 ad =.45 65E,<1 O<En< 1 O<En< 1 rso = 1.3 as =.66 NEUTRO NS FROM 1 TO 65 MeV Well Deuth MeV) VR = & WD = En wv =om Vso = 7. Range (MeV) 1 & < 65 15E,<65 15E,<65 15E,<65 Geometry (fm) rr = 1.23 rd = 1.23 ar =.6 ad = 1.2 rso = 1.23 as =.6 PROTONS BELOW 1 MeV Well Depth (MeQ VR = Ep + O.ooOo8 Ep2 WD = Ep = (Ep - 6) wv =. Vso = 7. -e (M ev) o<ep<lo O<Ep<6 61Ep<1 O<Ep<lO o<ep<lo rso = 1.3 as =.66 PROTONS FROM 1 TO 65 MeV Well Denth Mew VR = Ep WD = E$ wv =. Vso = 7. Range MeV) 1 Ep < 65 1 Ep < 65 1 Ep < Ep < 65 Geometry (fin) rr = 1.23 rd = 1.23 rso = 1.23 ar =.6 ad = 1.2 as =.6 2

5 For 14N and 16, the neutron potentials are taken from Arthur5 below 2 MeV for 14N and below 1 MeV for 16, and from slam et al.8 in the energy range 2-6 MeV for 14N and 1-5 MeV for 16. At higher energies, the Madland Semmering potential7 is again employed. The Lane isospin model is again used to determine the proton potentials. The parameterizations of these potentials are included in Table 2 for 14N and Table 3 for 16. B. Neutron Potential for Analysis of n Fe Reactions The neutron potential included in our previous CRP paperl for 54956Fe targets was based on an analysis by Arthur and Young9 but including modifications that were suggested by the ENDF/B-V evaluation.1 Over the past year we have discovered that our modifications lead to poorer agreement with elastic scattering angular distributions at energies above -8 MeV than was previously obtained with the Arthur and Young potential. Consequently, we have revised the 54~56Fe potential to rely on the original Arthur and Young potential to 26 MeV but have introduced new modifications in the volume imaginary potential that improve the reaction cross section at higher energies. We now utilize this revised potential to an incident neutron energy of 52 MeV and switch to the Madland Semmering potential7 at higher energies. The problem in the n + Fe elastic scattering angular distributions with our previous CRP potential is shown in Fig. 1, where calculated angular distributions using our previous and present potentials are compared with experimental data at 14.1 and 24.8 MeV. The revised potential clearly leads to improved agreement with the measurements. The calculated total and reaction cross sections for n + 56Fe are compared to experimental data for natural Fe in Fig. 2. The revised neutron optical model parameters are included in Table 4. C. Neutron Potential for Analysis of n + 59Co Reactions Similar to the above calculations on Fe, optical model parameters that were given in our earlier paperl for n + 59Co reactions have been modified slightly on the basis of an analysis of recent measurements from Weapons Neutron Research facility at Los Alamos. n particular, it is not possible to achieve suitable agreement with new measurements of neutron-induced alpha particle emission spectra by Grimes et a1.11 at energies above 3 MeV with the previous set of parameters, as is shown in Fig. 3. t was found that, although the Madland Semmering potential7 was derived for energies above 5 MeV, it results in much better agreement with the alpha emission measurements even at lower energies. Because the Madland potential produces elastic and reaction cross sections that are consistent with the lower energy 59Co potential12 near 26 MeV, the transition to the Madland Semmering potential is now made at that energy. Calculations of angle-integrated alpha emission cross sections with the revised potential are included in Fig. 3 The revised n + 59Co parameters are listed in Table 5. Calculated neutron total and nonelastic cross sections are compared to experimental data in Fig. 4. t should be noted that although the reaction theory and optical model calculations using these parameters are only shown here to - 5 MeV, reasonable total and reaction cross sections result from the combination of this potential with the Madland Semmering potential for neutron energies to 1 MeV or higher. 3

6 Table 2. Spherical optical model potentials for neutron and proton reactions with 14N over the incident energy range 1 kev MeV. Above an energy of 6 MeV, the Madland global potential7 is used. NEUTRONS BELOW 2 MeV Well Denth (MeV) Range (MeV) Geometrv (fml VR = En -.8 F$ O<E,<2 rr = 1.35 ar =.7 WD = < & e 4.8 rd = 1.26 ad =.51 = En < 2 wv =om < En < 2 Vso = 7. o<e,<2 rso = 1.31 as =.66 NEUTRO NS FROM 1 TO 6 MeV Well Depth (MeV) Range (MeV) Geometrv (fm) VR = E, 2 5 E,5 6 rr = ar =.593 WD = & rd = ad =.449 wv =. 2 5 En < 22 rv = av =.449 = En 22b6 Vso = rso = 1.1 as =.5 PROTONS BELOW 2 MeV Well Depth (MeV) Range (Mem Geometry (fm) VR = Ep O<Ep<2 rr = 1.25 ar =.65 WD = 13.5 O<Ep<2 rd = 1.25 ad =.47 wv =om o<ep<2 vso = 7.5 < Ep < 2 rso = 1.25 as =.65 PROTONS FROM 2 TO 7 MeV Well Denth MeV) VR = Ep WD = Ep wv =. = Ep Vso = 6. Range MeV) 2 5 Ep Ep Ep < E$ Ep 5 7 Geometry (fm) rr = rd = ar =.593 ad =.49 r~ = av =.449 rso = 1.1 as =.5 4

7 Table3. Spherical optical model potentials for neutron and proton reactions with 16 over the incident energy range 1 kev 5 1 MeV. Above an energy of 5 MeV, the Madland global potential7 is used. NEUTRONS BELOW 2 MeV well DeDth MeV) VR = En + O.ooOo8 En2 WD = & = 6. - O.O2(En - 14) wv =om Vm = 7. Range (M em Geometry (fml <&<2 rr = 1.35 ar =.6 O<E,<14 rd = 1.26 ad = < 2 < En < 2 < En < 2 rso = 1.3 as =.66 UTRONS FROM 2 TO 5 MeV Well Depth MeV VR = En WD = En = wv =. = En Vso = 4.31 RanEe MeV) 25&55 2 < En55 2 E, < <En En 5 Geometrv (fm) rr = rd = rv = rso = ar =.646 ad =.473 av =.473 as =.45 PROTONS BELOW 1 MeV Well Depth MeV) VR = Ep - O.oooO8 Ep WD = Ep = 4. - O.O2(Ep - 6) wv =om Vso = 7. Range (Me V O<Ep51 O<Ep6 6<Ep1 o<ep5 1 <Ep1O rso = 1.3 as =.66 PROTONS FROM 2 TO 7 MeV Well Denth (MeV) VR = Ep WD = Ep = Ep wv =. = J3p Vso = 4.31 Range MeV) 1 < Ep 5 1 < Ep < Ep55 1 e Ep e <%5 1 < Ep 5 5 Geometry (fm) rr = ar =.646 rd = ad =.473 rv = av =.473 rso = 1.11 as =.45 5

8 Table 4. Spherical optical model potentials for 54956Fe + n calculations over the incident neutron energy range 1keV E, 1 MeV. Above a neutron energy of 52 MeV, the Madland global potential7 is used. NEUTRONS TO 52 MeV Well DeDth (MeV) VR = En -.3 WD = & = (En - 6) =o wv = o m = -.27 = Vso = & & Range (MeV) < En 5 2 O<E,<6 65&<26 26 En 5 52 < < & < &552 < En 5 52 Geometry ( in1 rr = ar =.56 rd = ad =.47 r~ = rso = 1.12 av =.47 as =.47 Table 5. Spherical optical model potentials for 59Co + n calculations over the incident neutron energy range 1 kev 5 & 1 MeV. Above a neutron energy of 27.6 MeV, the Madland global potential7 is used. NEUTRONS Well Depth (MeV) VR = En -.3 En2 WD = En = (En - 6) wv =o.oo = Vso = En Range (MeV) < 27.6 O<En<6 6 En 27.6 < E, <.5.5 & < E, Geometry (fm) rr = a R =.561 rd = a D =.473 rv = rso = 1.12 av =.473 as =.47 D. Neutron Potentials for Analysis of n + Ti and n + V Reactions A study of neutron-induced activation cross sections of 45746Ti and 5751V was reported in 1984 by Muir and Arthur.l3 n order to carry out the calculations, optical model parameters were obtained for both elements by fitting resonance data (s- and p-wave strengths and scattering radii) and neutron total and scattering cross sections. Good agreement was obtained between measurements of various reaction cross sections and calculations with the GNASH code.14 Muir and Arthur's analysis only extended to 2 MeV. Presumably it would not be difficult to extend the results to a slightly higher energy where a match with the Madland Semmering potential should be possible. Muir and Arthur's neutron potential for Ti isotopes is given in Table 6 and the potential for V isotopes is given in Table 7. 6

9 Table6. Spherical optical model potentials for neutron reactions on Ti isotopes over the incident neutron energy range 1 kev 2 MeV. UTRONS TO 2 MeV Well Depth MeV) Range MeV) Geometry (fm) VR = En O<E,2 rr = ar =.6 WD = E, OeE,e6 rd = ad =.42 61&52 = (En - 6) wv =o.oo e En e 1.4 -V = av =.6 = En 1.4 E, 5 2 Vm = 6.2 E$, 5 2 rso = 1.12 as =.47 Table 7. Spherical optical model potentials for neutron reactions on V isotopes over the incident neutron energy range 1 kev 5 E, 2 MeV. NEUTRO NS TO 2 MeV Well Depth (MeV) Range (MeV) Geometrv (fml VR = En +.3 En2 < En 2 rr = ar =.676 WD = O<E,<6 rd = ad =.429 = (E,- 6) 65E,52 wv =. = En e e En 5 2 r~ = av =.676 Vso = 6.2 O<E,2 rso = 1.12 as =.47 E. Neutron Potentials for Analysis of n + 587mNi Reactions Measurements of activation cross sections for 14.2-MeV neutrons on targets of 27Al, 5*Ni, 93Nb, and 197Au are compared to calculations with the GNASH code in a 1982 publication by Harper and Alford.15 The analysis that led to the optical model parameterizations for Ni isotopes was performed at Los Alamos under the tutelage of E. D. Arthur. The optical model analysis was carried out in the same manner as described in Section D for Ti and V isotopes. The potential was validated by calculating a number of reaction cross sections for which experimental data exist. Good agreement with experimental was obtained in most cases, for example, the 58Ni(n,2n), 58~6Ni(n,p), 58Ni(p,pn), 54Fe(a,n), and 5gCo(p,xn) cross sections. The optical model parameterizations that resulted for neutron, proton, and alpha reactions to 2 MeV are given in Table 8. Again, these parameterizations could probably be extended to higher energies by making use of the Madland Semmering potential? 7

10 Table 8 Spherical optical model potentials for neutron reactions on Ni isotopes over the incident neutron energy range 1 kev MeV. UTRONS TO 2 MeV Well DeDth (MeV) Range (Mew Geometrv (fm) VR = En < En 52 rr = ar =.56 WD = En O<En<6 3 = ad =.47 = (En - 6) 6E,2 wv =o.oo & e.5 rv = av =.56 = E$,.5 5 E$, 5 2 Vso = 6.2 < S 2 rso = 1.12 as =.47 PROTONS TO 2 MeV (rc = 1.25 fm) Well Depth VR = Ep WD = Ep wv= Vm = 7.5,ALPHA PARTCLES (rc = 1.4 fm) Well Depth (MeV) VR = Ea WD Z. WV = Ea Range MeV) Geometry (fm) O<Ep<2 rr = 1.25 ar =.65 o<ep52 rd = 1.25 ad =.47 O<Ep52 o<ep52 rso = 1.25 as =.47 Rance (MeV)_ Geometry (fm) O<Ea52 rr = 1.37 ar =.56 O<Ea52 < Ea 2 rv = 1.37 av = COUPLED-CHANNELS OPTCAL MODEL POTENTAL FOR Am SOTOPES n our previous paper' we presented a coupled-channels potential for neutron reactions on w1am. Since that time we have performed an analysis of neutron cross sections on 243Am. For the new work, we formulated the 241Am potential in an isospin-dependent form and used it for 243Am. The generalized Am coupled-channels neutron potential is given in Table 9. The results of the fission cross section analysis utilizing transmission coefficients from this potential are compared in Fig. 5 with experimental data and the ENDFB-V evaluation. Similarly, the calculated 243Am(n,y) cross section is compared to measurements and the ENDFB-V evaluation in Fig. 6. Finally, the calculated a3am + n total and (n,2n) cross sections are compared to the ENDFB-V evaluation in Fig. 7.

11 Table 9. Coupled-channelsoptical model and deformation parameters for neutron reactions with Am isotopes to 3 MeV. The lowest five members of the ground-state rotational band are coupled in the calculation for each isotope. The quantity q is given for each isotope by q = ( N - Z ) A. - n Arn Parameters (E, = 3 MeV) Well Depth (MeV) WgeWeV) E S 3 <E< 8 8 S E 3 <E< 8 8 S E 3 E S 3 VR = q -.3E WD = E = q -.46(E - 8) + wv= = E Vso = 6.2 Geometry (fin) rr = 1.25 ar =.6 rd = 1.24 ad =.55 = 1.24 av =.55 rso= 1.1 aso=.75 Deformation Parameters sotope p2 p4 a am a3am V. STATUS OF THE REFERENCE NPUT PARAME'ER LBRARY n the time period since the first research coordination meeting organized y t h e L EA for development of the Reference nput Parameter Library (RD?L),1 a preliminary format for the RlpL has been developed and a number of spherical and coupled-channels optical model parameterizations have been cast in the format and provided to the Nuclear Data Section. More details on this activity are given in the paper by S. B. Garg and A. Kumarl6 in this meeting. V. CONCLUSONS n our previous paper,l we presented a variety of optical model potentials used in reaction theory analyses at Los Alamos National Laboratory. n this paper we have refined the Fe and Co potentials and included additional potentials for Ti, V, Ni, and Am isotopes. Additionally, we have included potentials from Lawrence Livermore National Laboratory that are highly successful in reproducing measured data for neutron and proton reactions on %, 14N, and 16. As was pointed out in our previous paper, we expect that refinements and improvements can be made to all these potentials. Our hope again is that the present parat-neterizations will be adequate with minimal revision for some applications and will provide a starting point for future detailed analyses. 9

12 As was also mentioned in our previous paper, it is our view that substantial additional work is needed at higher energies in order to put optical model characterizations on a sound basis. We feel that a systematic study utilizing both a Schrodinger and Dirac approach is needed to develop a global nucleon-nucleus optical model potential that is reliable into the medium energy region. Significant progress has been made over the past year in specifying and initiating a library of reference input optical model parameters for nuclear model calculations. After approval of a format, we will complete a base library and begin considering default or recommended initial parameters for a variety of applied problems. REFERENCES P. G. Young, "Experience at Los Alamos With Use of the Optical Model for Applied Nuclear Data Calculations," nternational Atomic Energy Agency report NDC(NDS)-335 (1994) p. 19. M. B. Chadwick, L. J. Cox, P. G. Young, and A. S. Meigooni, "Calculation and Evaluation of Cross Sections and Kerma Factors for Neutrons Up to 1 MeV on Carbon," to be published in Nucl. Sci. Eng. (1995). M. B. Chadwick and P. G. Young, "Calculation and Evaluation of Cross Sections and Kerma Factors for Neutrons Up to 1 MeV on 16 and 14N," submitted to Nucl. Sci. Eng. (1995). M. B. Chadwick, Lawrence Livermore National Laboratory, personal communication (1995). E. D. Arthur, Los Alamos National Laboratory progress report LA-9841-PR (1983). P. J. Dymbylow, Phys. Med. Biol. 25, 637 (198). D. G. Madland, "Recent Results in the Development of a Global Medium-Energy Nucleon- Nucleus Optical-Model Potential," Proc. Specialists' Mtg. Preequilibrium Nuclear Reactions, Semmering, Austria, 1-12 February 1988 [Ed: B. Strohmaier, NEANDC-245 'U'(1988)l p. 13. M. S. slam, R. W. Finlay, J. S. Petler, J. Rapaport, R. Alarcon, and J. Wierzbicki, Phys. Med. Biol. 33, 315 (1988). 9. E. D. Arthur and P. G. Young, "Evaluated Neutron-nduced Cross Sections for 54756Fe to 4 MeV," Los Alamos National Laboratory report LA-8636-MS (ENDF-34) (198). 1. C. Y. Fu and D. M. Hetrick, "Update of ENDFB-V Mod-3 ron: Neutron-Producing Reaction Cross Sections and Energy-Angle Correlations," Oak Ridge National Laboratory report OR.NL,/TM-9964 (1986). 11. S. M. Grimes, C. E. Brient, F. C. Goeckner, F. B. Bateman, Mc B. Chadwick, R. C. Haight, T. M. Lee, S. M. Sterbenz, P. G. Young,. A. Wasson, and H. Vonach, "The 59Co(n,a) Reaction from 5 to 5 MeV," Nucl. Sci. Engr., to be published (1995). 1

13 12. E. D. Arthur, P. G. Young, and W. K. Matthes, "Calculation of 59Co Neutron Cross Sections Between 3 and 5 MeV, "Proc. Symp. on Neutron Cross Sectionsfrom 1 to 5 MeV, BNL (May 198), p D. W. Muir and E. D. Arthur, J. of Nucl. Materials 122 & 123, 158 (1984). 14. P. G. Young, E. D. Arthur, and M. B. Chadwick, "Comprehensive Nuclear Model Calculations: ntroduction to the Theory and Use of the GNASH Code," Los Alamos National Laboratory report LA MS (1992). 15. R C. Harper and W. L. Alford, J. Phys. G: Nucl. Phys. 8, 153 (1982). 16. S. B. Garg and A. Kumar: "Starter File of Optical Model Potential Parameters and Nuclear Data Computations of Nd-sotopes," Second Research Co-ordination Meeting on Development of Reference nput Parameter Library for Nuclear Model Calculations of Nuclear Data, September 1994, Vienna, Austria (present meeting). 11

14 CROSS SECTON (b/sr) lo- loo rn 3 --L P A 3 2 CROSS SECTON (b/sr) 3*1-1-* lo- loo s 4 U bl P N U 6 h3 ul b u1 b

15 Fe+n Total Cross Section OptMod OptMod. x Peterson Cierjacks, 1969 A Larson i a -: - a * Taylor, Brady, 1979 Z Q.t. --- W v) v,2 v, E up X Lebedev, Pasechnik,1955 Mac gregor, Degtyarev, pt.mod pt.mod. Fe+n Nonelastic Cross Section a NEUTRON ENERGY (MeV) Fig. 2. Calculated and measured neutron total and reaction cross sections from n + Fe reactions. 13

16 - - 7 i NEUTRON ENERGY (MeV) 5. Fig. 3. Comparison of measured and calculated alpha-production cross sections from the Wo(n,xa ) reaction. 14

17 1 ~ Co+n Total Cross Section pt.mod. -_ pt.mod. x Peterson, Cierjacks, 1969 A Foster, Co+n Nonelastic Cross Section pt.mod pt.mod. Mac gregor, NEUTRON ENERGY (MeV) Fig. 4. Calculated and measured neutron total and reaction cross sections from n + 59Co reactions. 15

18 E Am(n,f) Cross Section 7 FOMUSHKN,1984 FOMUSHKN 1967 FOMUSHKN A FURSOV, ZO a- i= W cn cn v) 9- CT KNTTER, 1988 FOMUSHKN # 1984 X BUTLER,1961 V KANDA # 1987 A FURSOV, GNASH, SEEGER, 197 ENDF/B-V 243 Am(n,f) Cross Section NEUTRON ENERGY (MeV) Fig. 5. Comparison of experimental 243Am(n,f) cross section data with the ENDFB-V evaluation (dashed curve) and the results of the GNASH calculations (solid curve). 16

19 T i t e. 1 l l l l ( ll v1~-3 lo-* lo loo lo NEUTRON ENERGY (MeV) Fig. 6. Comparison of the GNASH calculation of the 243Am(n,y) cross section with the experimental data base and with the ENDF/B-V evaluation (dotted curve). 17

20 243 Am + n Total Cross Section * oq cn v, 8" l l l l 1 llll 1-3 o-? " 1' Am(n,Zn) & (n,3n) Cross Sections f\ \A(n'2n) GNASH u/.,2n) END F/B-V NEUTRON ENERGY (MeV) Fig. 7. Comparison of the optical model calculation of the n + 243Am total cross section (upper half) and the GNASH calculations of the (n,xn) and (n,3n) cross sections (lower half) with the existing ENDFB-V data evaluation of these quantities. 18