Evaluations of Photonuclear Cross Sections for Actinides M.-L. Giacri-Mauborgne¤, M. B. Chadwick†, J.-C. David¤, D. Doré¤, D. Ridikas¤, A. Van Lauwe¤ and W. B. Wilson† ¤

†

CEA Saclay, DSM/DAPNIA/SPhN, 91191 Gif/Yvette, France Los Alamos National Laboratory (T-16), Los Alamos, NM 87545, USA

Abstract. This article presents calculations of photonuclear reaction cross sections for actinides by using different reaction codes, namely an improved version of HMS-ALICE and GNASH. The GDR parameters from the RIPL2 data library are used to describe the total photo-absorption cross section. In general, our results show satisfactory agreement with experimental data and IAEA evaluations. We plan to include our GNASH evaluations for 235U, 238U, 237Np, 239Pu, 240Pu and 241Am into a new release of the ENDF-B/VII data files. Keywords: photonuclear activation file, photofission. PACS: 85.Jg, 25.20-x

INTRODUCTION A renewed interest in photonuclear process is motivated by a number of different applications where progress in high-intensity electron accelerators was awaited [1]. Major problems in modeling photonuclear reactions are the lack of photonuclear data on corresponding cross sections despite the huge effort of the IAEA [2], where data are available for 164 isotopes only. Up to now no material evolution code including photonuclear reactions is available. Therefore, in close collaboration with LANL, we have been working on the development of a photonuclear activation file (PAF) to be included into the material evolution code CINDER’90 [3]. The reaction codes HMSALICE [4] and GNASH [5] were used to calculate photonuclear reactions for more than 600 isotopes. More information on this effort can be found in [6]. In the case of actinides, the PAF includes nine actinides evaluated by IAEA [2]. We complete the library by calculating the remaining cross sections with the HMS-ALICE reaction code. In this work we discuss in detail our results of HMS-ALICE calculations for 235U and 238U, where separate reaction channels as (γ,n), (γ,2n), and (γ,fiss) are compared with IAEA evaluations. For some high priority actinides we do more accurate evaluations using the GNASH reaction code, which was routinely employed to calculate neutron induced reactions on actinides. The GNASH evaluations for 237Np, 240Pu, 241Am are performed for the first time, while the re-evaluations for 235U, 238U and 239Pu are also made. All these cross sections agree well with experimental data. These results will also be included in ENDF-B/VII.

RESULTS OF HMS-ALICE Fig. 1 presents an improvement we made in modeling the total photo-absorption cross section with HMS-ALICE. In brief, the sum of two Lorentzians is indispensable to reproduce correctly experimental data for deformed nuclei [7, 8]. The GDR parameters from the RIPL data library are used to describe the total photoabsorption cross section in the above parameterization. Equally, the improvement in predicting the FIGURE 1. Comparison of different partial photonuclear reaction cross sections is parameterizations of the total photoabsorption cross sections in HMS-ALICE evident as presented in Fig. 2 (upper part – for 238 for 235U with corresponding data sets. U and lower part – for 235U). 800

HMS-ALICE (old) HMS-ALICE (new) Caldwell data (1980) Gurevich data (1976)

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FIGURE 2. Comparison of HMS-ALICE predictions (dash line: old version, solid line: new version) and IAEA evaluations (bold line) for partial photonuclear cross sections in the case of 238U and 235U.

Note that the IAEA evaluations are based on experimental data and serve as a reference in this case. Similar quality of the predictive power of the improved HMSALICE was observed for other nine actinides provided by the IAEA evaluated data library. Therefore, we used HMS-ALICE to predict photonuclear cross sections of actinides for which no measurements-evaluations existed at all. For other nuclei such as 235U, 238U, 239Pu, 237Np, 240Pu and 241Am, we performed new evaluations with GNASH.

GNASH CALCULATIONS The major advantage of making the evaluations with GNASH is that the cross sections become available in the ENDF format and can be used in transport codes. For

more details on the photo-nuclear cross section evaluations with GNASH we refer the reader to Ref. [9]. γ+

235

Results for 235U, 238U and 239Pu

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GNASH evaluations for partial cross sections on 235U are given Fig. 3. Comparable results were also obtained for 238U and 239Pu. We conclude that GNASH can be successfully used to evaluate photonuclear cross sections both for actinides [9] and non-actinides [10]. We also noticed that the GNASH results were quite sensitive to the fission barrier. Experimental data can be not well reproduced by using the same parameters as for FIGURE 3. GNASH evaluations for 235U. neutron induced reactions. 400

1n, GNASH calc. Caldwell (1980) 2n, GNASH calc. Caldwell (1980) F, GNASH calc. Caldwell (1980) Varlamov (1987)

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Predictions for 240Pu, 237Np and 241Am 400 GNASH (γ,1n) GNASH (γ,2n) GNASH (γ,fiss) Soldatov (97)

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FIGURE 4. GNASH evaluations for 240 Pu.

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Photonuclear cross section data for 240Pu are very scarce. We found the only experiment on photo-fission up to 11 MeV [11]. In addition, no integral measurements were done for this nucleus. Again as for other nuclei, we used input parameters from neutron induced reactions on 239 Pu adjusting the fission barrier to reproduce the available data as shown in Fig. 4. For the second chance fission we employed the same input values as for the first chance fission of 239Pu.

Therefore, even with the lack of data we think that GNASH should provide reasonable predictions for the neutron production channels as ( ,n) and ( ,2n) as we experienced in the case of 239Pu. TABLE 1. Comparison of relative photo-fission rates: 237Np versus 238U. Energy 11.5 MeV 17 MeV 20 MeV Data 2.71±0.08 2.39±0.1 2.40±0.11 GNASH 2.55 2.24 2.11

As it is presented in Fig. 5 for photo-fission of 237Np, the two sets of experimental data disagree. The GNASH evaluation reproduces perfectly low energy region and for TABLE 2. Comparison of relative photo-fission higher energies stands between two rates: 241Am versus 238U. sets of data. Note that for the Energy 11.5 MeV 14.5 MeV 20 MeV remaining channels as ( ,n) and ( ,2n) Data 2.45±0.07 2.42±0.06 2.18±0.12 the evaluation reproduces well the data GNASH 2.54 2.51 2.34 (not shown here). Another validation (Table 1) of the GNASH predictions for the photo-fission cross section is to compare them with integral fission rates of 237Np measured in [12, 13] with different end point Bremsstrahlung energies. Again a reasonable agreement is obtained for all energies considered.

Similarly as for 237Np, in Table 2 and Fig. 6 we give the GNASH results for 241Am. Unfortunately, there is no measurement for photo-neutron production in this case. The photo-fission cross section was measured by [14, 15]. The integral fission rates [12, 16] presented in Table 2 are rather well reproduced for all energies, i.e. the photo-fission cross section presented in Fig. 6 should be rather accurate. γ+ Np reactions 237

400 F, GNASH calc. Bermann, Caldwell (1986) Veyssiere (1973) Soldatov (1992) Zhuchko (1978)

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FIGURE 5. GNASH evaluation for photo-fission of 237Np.

FIGURE 6. GNASH evaluation for photo-fission of Am.

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CONCLUSIONS The photonuclear cross section evaluations for 237Np, 240Pu and 241Am were performed for the first time. The GNASH reaction code was used for this purpose. These new results in addition to the re-evaluations for 235U, 238U and 239Pu will be included in the new release of ENDF-B/VII. The photonuclear activation file (PAF), based on compilation of the IAEA evaluations, HMS-ALICE predictions, and GNASH evaluations, by now includes the cross section for more than 600 nuclei. The release of the first version of PAF is planned by the end of 2005. We also add that new measurements of photonuclear cross sections for 240Pu and 241Am are urgently needed to confirm our evaluations.

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