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Fast Neutron Cross Sections of 2L*°Pu; Measured Results and a Comparison

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Fast Neutron Cross Sections of 2L*°Pu; Measured Results and a Comparison Fast Neutron Cross Sections of 2l*°Pu; Measured Results and a Comparison with an Evaluated File À. B. Smith, P. P. Lambropoulos and J. F. Whalen Argonne National Laboratory Argonne, Illinois Abstract Fast neutron total cross sections and elastic- and inelaetic-scatter- ing cross sections of 2**°Pu are reported. The total cross sections are measured from neutron energies of 0.1 to 1.5 MeV in increments of *»* 25 keV. The elastic-scattering cross sections are measured at ^ 50 keV intervals from incident neutron energies of 0.3 to 1.5 MeV. The inelastic neutron excitation cross sections of states at 4 2 + 5 , 140 + 10, 300 + 2 0 , 600 + 20 and 900 + 50 keV are measured. The experimental results are discussed in the context of the optical, compound-nucleus and direct-reaction nuclear models including the effects of resonance width fluctuations and the fission process. The measured results are critically compared with the corresponding quantities in the evaluated nuclear data file ENDF/2. I. INTRODUCTORY REMARK The isotope 2t*°Pu is a major constituent of many fast-breeding reactors wherein the plutonium fuel may consist of £ 20 percent 2lf0Pu.^ Therefore fast neutron interactions with this isotope are a considera­ tion in the neutronic design of these systems. Despite this applied importance the experimental microscopic fast neutron cross sections of 2*°Pu are relatively unknown and a number of requests for measured in- 2 ' formation are outstanding. Major reliance continues to be placed upon evaluated data sets based largely on nuclear-model estimates. The fission neutron cross section of 2tfl*Pu has been reasonably well 3 4 5 measured. * * It is large with a relatively low energy threshold. At low energies the fission cross section is of interest in the context of g fission theory due to its sub-threshold characteristics. Experimental total cross sections and elastic- and inelastic-scattering cross sections of 21*°Pu above * 100 keV are experimentally essentially unknown. This ignorance is, in part, due to the limited availability of suitable samples and to experimental problems associated with the high spontaneous fission rate of the material.^ The present work was undertaken in an effort to generally improve experimental understanding of the fast neutron cross sec­ tions of 2**°Pu by direct measurement of total and of scattering cross sec­ tions to 1.5 MeV and to specifically satisfy requests for fast neutron 2<*°Pu data. It was also the objective to*, provide a reasonable experimen­ tal foundation for the nuclear models employed in the interpolation and ex­ trapolation of measured quantities, and to obtain additional insight into nuclear structure and fission properties in the trans-uranlum region* II. EXPERIMENTAL METHODS The experimental measurements were made possible through the avail­ ability of a 54 gm sample of plutonium ^ 100% enriched in the isotope 2**°Pu. The material was a metal foil 98.7 weight-percent plutonium and 1.3 weight-percent aluminum. It was formed into a cylindrical sample 2.0 cm in diameter by pressing the foil into a 0.013 cm thick stainless steel can. The uniformity of the sample density was governed by the pressing procedure* The sample was, in itself, an appreciable fast neutron source due to spontaneous fission. Total neutron cross sections were deduced from the observed trans­ mission of essentially mono-energetic neutrons through the 2**°Pu sample assuming a uniform sample density« The measurements were made at ^ 5 keV intervale with 'v 5 keV resolution from incident neutron energies of 0.1 to 1.5 MeV. Sample transmissions were ^ 75%. The statistical precision of the measured cross sections was 1%. Small corrections were made for "in-scattering" and background contributions and the fidelity of the apparatus verified by determination of the well known total neutron cross 3 8 sections of carbon. * The neutron scattering cross sections were measured using the pulsed- beam fast time-of-flight technique. The apparatus consisted of collimated detectors which concurrently measured neutrons scattered at eight labora­ tory angles. The 7Li(p,n)7Be neutron source was so arranged as to provide an incident neutron resolution at the scattering sample of ^ 20 keV. The scattered neutron velocity resolution was generally ^ 1.5 nsec/M. All scattering cross sections were determined relative to the known differ- 9 ential elastic scattering cross sections of carbon and corrected for multiple-scattering and other experimental perturbations. The specific details of the total and scattering apparatuses and methods are described elsewhere. III. EXPERIMENTAL RESULTS® A. Total-Neutron Cross Sections The measured total cross sections displayed considerable energy- dependent structure which was well correlated with known prominent reso­ nances in the total cross section of aluminum. The primary results were corrected for the 1.3 weight-percent aluminum content of the sample using £ A numerical tabulation of all measured values is given in the Appendix and all experimental results have been transmitted to the National Neutron Cross Section Center, Brookhaven National Laboratory. aluminum total cross sections measured at this Laboratory and smoothed by averaging the corrected values over 25 keV intervals. The final cor­ rected and averaged results are shown in Fig. 1. The remaining structure near 530 keV is believed a residual artifact due to uncertain corrections for the effects of the large aluminum resonance in this region and to have no physical significance. The errors associated with the results were largely of a systematic nature and, particularly, are due to the uncertain transmission-deneity of the sample. The combined total-cross-section error was conservatively estimated at five percent and the mean deviation of the measured values for a smooth curve was generally appreciably less. No previously reported total cross sections of 2**°Pu in the energy range of the present experiments were found in the literature. The meas­ ured partial elastic- and inelastic-scattering cross sections (see below) 3 were confined with the reported fission cross section for comparison with the directly measured total cross sections. As shown in Fig. 1, the agreement was good with no discrepancy greater than ъ 300 mb. This con­ sistency indicates that both total and partial cross sections have been reasonably determined. B. Biastic-Neutron-Scattering Cross Sections The differential elastic-scattering cross sections were deduced from the measured time-of-flight (TOF) spectra with careful attention to back­ ground effects. Each TOF distribution was corrected for non-saraple associ­ ated backgrounds inclusive of contributions due to the sample container using direct experimental measurements. After this correction there re­ mained an appreciable fission-neutron background, time-uncorrelated from 2-°Pu spontaneous fission and time-correlated from neutron-induced fission. Both fission-neutron backgrounds were estimated using a least-square fitting procedure. Time intervale were selected from each TOF distribution in such a manner that elastic- and/or inelastic-scattered neutrons were either physically inadmiesable (for example» at times before detection of the elastic event) or were judged to make no significant contribution to the selected Interval. The measured data in these selected time intervals was fitted with a low-order power series in time (usually quartic) Which rea­ sonably Interpolated the fission-neutron background over the entire TOF distribution. The fission-neutron contribution determined from the fitting procedure was subtracted from the measured TOF spectra. The 2<f0Pu elastic-scattering component was obtained in such a manner as to include neutrons elastically-scattered from the aluminum contaminant of the sample. At forward scattering angles (ь 30 deg.) the aluminum contribution was indistinguishable from the primary 2l*°Pu elastic events. At backward scattering angles (y 155 deg.) and, particularly, at higher energies the two elastic components were well separated due to the differ­ ent energy transfer to the recoiling nucleus. In these instances consid­ erable attention was given to the correct evaluation of the aluminum elas­ tic contribution in the presence of inelastic neutrons resulting from the excitation of the 42 and 140 keV states in 2<*°Pu. The combined 2tf0Pu and aluminum differential elastic scattering cross sections were determined from incident energies of 0.3 to 1.5 MeV in incre­ ments of 50 keV and at eight scattering angles between 25 and *v* 155 degrees. The measurement angles varied slightly from distribution to dis­ tribution but were typically 28, 38, 53, 68, 84, 114, 128 and 154 degrees. The resulting differential cross sections were least-square fitted with the expression dû “ 4тГ (1 + wipi> where a (angle-integrated cross section) and coefficients were deter­ mined from the fitting procedure and P^ are Legendre polynomials expressed 451 in the laboratory system. The differential elastic-scattering croes eec- tione» expressed in the form of Eq. (1)» were corrected for the aluminum contamination using the aluminum elastic-scattering cross sections of Ref. 13. The scattered neutron resolution was not generally sufficient at incident energies of £ 1.0 MeV to differentiate the elastic neutron group from inelastic neutrons resulting from the excitation of the 42 keV state in 2<f0Pu. Careful measurements at selected angles with an improved resolu­ tion of 0.75 nsec/M qualitatively established the
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