AN EVALUATION OF NEUTRON AND GAMMA-RAY-PRODUCTION CROSS SECTIONS FOR LEAD [report]

C Fu, F Perey
1972 unpublished
This addendum describes improvements made to the evaluat ion uocuntsnt ed in the main body of this report. One of the known deficiencies of the evaluation was the capture crosssection file above thermal. At the time the evaluation was done, we knew that some high-resolution measurements of the capture cross sections for the lead isotopes were in progress at ORELA by Allen et^ ai. Now that the data have become available, we have gone back and performed a reevaluation of the capture cross-section
more » ... file. With the new data we have been able to improve also the representation of the resonances in the total and elastic-scattering cross-section files. It did not appear necessary to us to represent explicitly in the file all of the capture resonances reported by Allen et_ al. (numbering more than 500). Table 1 lists only the capture resonances which could be corre lated with observed resonances in the total cross sections and the other capture resonances having areas greater than 5 barn-eV in natural lead. All areas in Table 1 are given in terms of natural lead using isotopic abundances of 0.0124, 0.236, 0.226 and 0.523 for the isotopes 204, 206, 207, and 208 respectively. Our aim was to represent these resonances in both the total and capture cross-section files in such a manner that upon subtraction of these files tuc resonances would have a reasonable representation in the elar.tic cross-section file. The remainder of the small capture resonances, together with the 1/v component, were averaged to form a smooth cross section to which the large capture resonances could be added. Total widths and res onance areas in the total cross sections were calculated from resonance param eters compiled in BNL-325 (Ref. 2) and are compared in Table 1 with those mmmmu & THIS mm.m i' mmm h A4 obtained from Good's data. The adopted resonance energy, E, total width, T, and resonance areas, A~t are averages of the two sets with some exceptions. Below about 50 keV, the resonance areas in Good's data are generally 501 smaller than those reported In BHL-325. Because Good's measurements have not yet been documented, wa aaopred the BKL-325 values whenever availab?.e and in creased by SOX the areas of the other resonances in Good's data. Most of the total widths in Allen et al.'s data are given as upper limits. Because the capture measurements had the better energy resolution, we felt that these upper limits should not be exceeded. Thus when the parameters from BNL-325 and the widths from Good's data gave values of the resonance widths larger than the upper estimates obtained in the capture cross-section measurement, we adopted the later values as the resonance widths. For the capture res onances which had areas greater than 5 barn-eV but could not be correlated with total cross section resonances, the total resonance area A_ was set equal to the capture resonance area A . For some of the resonances for which A^ and F appeared reasonably certain, we could ex'.ract V and these are given in the last column of Table 1. The resonance at 1.685 keV, vhlch is below the lover limit of the capture measurement, is the lowest energy resonance observed so far in the lead isotopes. Since it has the same J and approximately the same V and A_ as the resonance at 2.485 keV, we acopted the same A for the file. The average capture cross sections for the lead isotopes, natural lead and the smooth capture cross sections upon which the resonances were built are 204 listed in Table 2. Except for Pb above 50 keV, the average capture cross sections for the isotopes are simply a sum of the resonance cross sections in ABSTRACT A survey was made of the available information on neutron and gamma-r ay-product ion cross section measurements of lead. Evaluated nuclear data se^s in the ENDF/B format were prepared for lead covering the energy range from 0.00001 eV to 20.0 MeV. The cross section sets were based on experimental results avail able to June 1971 and on nuclear model calculations. This evaluation received MAT. No. 4136 in the DNA Library and 1136 for CSEWG. Recent measurements with much improved techniques have shown a great many new resonances in the total cross sections. The gross structure of 3 the total cross sections agrees quite well with the previous evaluations. However, many of the newly observed resonances have peaks and valleys differing from the gross structure by as much as one barn. This may bear some significance in neutron transport calculations. Changes in (n,Y), (n,n % ) and (n,2n) cross sections were small; except the level excitation cross sections in (n,n*) reaction were given in much more detail. (n,3n) cross sections have been added. Elastic-scattering cross sections, as the total cross sections, contain many more resonance structures than those previously reported. Angular Distribution of Secondary Neutrons The angular distributions of elastically scattered neutrons are given for five neutron energies in the AWRE evaluation and for 378 energies in the UNC report. The present evaluation, in terms of the first four Legendre expansion coefficients, agrees with the latter to within 20% above 1 NeV and 30Z below 1 MeV. The angular distributions of secondary neutrons from inelastic excita tion of 35 levels up to 4.4 MeV have been added. Energy Distribution of Secondary Neutrons The energy distributions of secondary neutrons from elastic scattering, and inelastic scattering exciting the discrete levels, may be obtained analyti cally from the angular distributions. Thus the situation here is the same as in the above section. The energy distribtuions of secondary neutrons from (n,n* continuum) and from (n,2n) reactions are believed to be much improved. Those from (n,3n) reactions are also included.
doi:10.2172/4668599 fatcat:j6xkoabqqzdsrjjfpiejqj7qpe