Neutron Physics (eBook)

M Yeater (Herausgeber)

eBook Download: PDF | EPUB

2012 | 1. Auflage
318 Seiten
Elsevier Science (Verlag)
978-0-323-15624-0 (ISBN)

Infos

Nuclear Science and Technology, Volume 2: Neutron Physics provides information pertinent to neutron and reactor physics. This book presents a discussion of the general area of energy sources, surveying the fusion problem. Organized into 16 chapters, this volume starts with an overview of the broad range of other research related to nuclear technology, radiation effects, solid state work, chemistry, and materials research. This book then examines the experimental data for the cross sections and fission parameters of the fissile nuclides. Other chapters outline the role of fast choppers in time-of-flight spectrometers and consider the total cross section measurements. This book discusses as well the various experiments performed to test the operation of the system. The final chapter deals with the long-range prospects of fusion power. This book is a valuable resource for graduate students, physicists, nuclear engineers, researchers, scientists involved in fusion research will find this book extremely useful.

Textauszug

The Low Energy Cross Sections of Fissile Nuclides

B.R. LEONARD, Jr.,     Hanford Laboratories, General Electric Company, Richland, Washington

ABSTRACT

A survey has been made of the experimental data for the cross sections and fission parameters of the fissile nuclides: U233, U235, and Pu239 for the Neutron Physics Symposium at Rensselaer Polytechnic Institute, May 5-6, 1961. The best available data have been used to construct the most probable behavior of the low energy cross sections which are presented graphically. It is shown that the most probable 2200-meter/sec parameters of U233 and U235 form a set of values which are consistent with the results of ratio measurements. The values which are provisionally derived are for U233: σnT = 587 ± 3, σnn = 12 ± 3, σnx = 575 ± 4, σnγ = 49 ± 2, σnf = 526 ± 4, α = 0.0935 ± 0.0038, ν = 2.502 ± 0.014, and η = 2.288 ± 0.010, and for U235: σnT = 694 ± 4, σnn = 16 ± 3, σnx = 678 ± 5, σnγ = 99 ± 4, σnf = 579 ± 6, α = 0.172 ± 0.007, ν = 2.434 ± 0.019, and η = 2.077 ± 0.010. It is recommended that a least squares adjustment similar to that made for U233 by Evans and Fluharty should be made for the U233 and U235 parameters as a single set. For Pu239 the lack of precise determinations of 1 + α preclude the development of a set of parameters in the same manner as done for U233 and U235. The Pu239 parameters of σnf, v, and η which can be derived from ratio measurements with U235 are shown to have an internal inconsistency of 4.3% when combined with the most probable value of σnx. On the assumption that the measurements of η are most likely discrepant, the most probable values of the 2200-meter/sec parameters for Pu239 are taken to be: σnT = 1018 ± 5, σnn = 10 ± 3, σnx = 1008 ± 6, σnγ = 254 ± 11, σnf = 754 ± 9, α = 0.337 ± 0.017, ν = 2.89 ± 0.05, and η = 2.16 ± 0.05.

Introduction

During the past 2 years a number of experiments have been completed which were designed to determine low energy neutron cross sections and fission parameters of the important fissile nuclides with high precision. I refer especially to the efforts at obtaining precise total cross sections at Columbia University and BNL by Safford and Havens; at ORNL by Block, Slaughter, and Harvey; and at the MTR by Simpson, Moore, and Simpson. Also, important experiments on precise determinations of η and ν have been made at ORNL by the Saussure, Macklin, Gwin, Magnuson, and others. In addition, a considerable effort has been expended in critical review of the status of knowledge of these parameters. In this connection I would particularly mention the review of U233 cross sections by Evans and Fluharty1 and of the U235 cross sections by Safford and Havens.2 It is appropriate at this time to combine the results of recently completed experiments with these reviews and survey the status of our present knowledge of these neutron cross sections. The results of this survey will show that the low energy neutron parameters of U233 and U235 form a very consistent set of numbers, at least at the level of about 1% precision. For the case of Pu239 the situation is not nearly so cheerful as the best neutron parameters which have been obtained are of the order of 4% discrepant. However, the outlook for the near future is bright as several definitive experiments on Pu239 parameters are now in progress.

In the present survey the best available information on the low energy behavior of the total and fission cross sections has been compiled and presented in graphical form. It is intended that this presentation will demonstrate the extent with which the results of separate measurements of the same cross section are consistent with each other. These comparisons show that small systematic effects are frequently present in any single measurement such that it is necessary to consider the resulting dispersions of data to assign best values of cross section behavior. In this way also reliable estimates can be made of the uncertainties which are involved in the use of these cross section curves in practical applications.

The survey also studies the question of the best values of the 2200-meter/sec neutron parameters of these nuclides. In this study the logical approach which is attempted leans heavily on the outstanding review article on the evaluation of the U233 neutron parameters by Evans and Fluharty. One aspect of this approach is the solution of the problem which arises from the overdetermination of the neutron parameters because of the experiments which involve the determination of combinations of parameters. Closely related to this aspect is the suggestion of Evans and Fluharty that U233 is a promising standard for comparison measurements because of the near 1/ν behavior of its thermal cross sections. Actually, the present survey will show that the U235 cross section behavior is sufficiently close to 1/ν that its neutron parameters are determined to the same order of precision as has been possible for U233.

The present study determines the separate parameters for U233 and U235 from the following procedure: The best value of σnx is determined from the total cross section and the estimated scattering cross section. The best value of α is obtained from experiment and a best value of η is also obtained from experiment. These three parameters: absorption, α, and η then determine the remaining parameters of fission cross section, capture, and ν The resulting derived parameters are then compared with the values obtained in direct measurement and with the values of the U235: U233 ratios of parameters obtained in direct measurements. The comparison will be seen to be an excellent one except for the case of the absolute values of the fission cross sections. For the study of the parameters of Pu239 this procedure cannot be used because of the lack of precision determinations of α. The Pu239 results must be obtained primarily from ratio measurements with U235. As will be seen this procedure yields Pu239 parameters which are badly discrepant.

There have been three recent measurements of the total cross section of U233 for low neutron energies. The results of these measurements are shown in Fig. 1 in the form where a value of 12 barns has been used for σnn. The scale is such that the small deviation from 1/ν of the absorption cross section can be clearly seen. The solid points represent the data of Simpson et al.3 obtained with a fast chopper using a metallic sample. Each point represents an average of several experimental values in a very small energy interval. The open circles represent the data of Block et al.4 also obtained with a fast chopper and a metallic sample. The points below 0.035 ev represent the average of several experimental values in a very small energy interval. The other data are those of Safford et al.5 obtained with a crystal spectrometer. The triangular points were obtained using a metal foil. The three lowest energy values are at wavelengths beyond the Bragg cutoff for uranium so that no scattering cross section has been subtracted from these values. The other data of Safford et al. were obtained with a liquid sample so that the 12-barn scattering cross section was subtracted from all of the data. The liquid sample data were taken so that the irregular effects of Bragg scattering would not be present. However, the data obtained show a much larger dispersion from a smooth curve than the data of Block et al. and Simpson et al. and consequently the data of Safford et al. have not been included on the graph for energies above 0.035 ev. In addition, a systematic inconsistency is noted between the metal and liquid sample measurements at the lowest energies with the liquid sample results being much higher. This indicates the possibility of coherent liquid scattering effects at low energies so that the primary emphasis is given to the results obtained with metallic samples. A similar systematic effect is noted in the identical experiments performed on U235.

Fig. 1 Absorption cross section data for U233 from 0 to 0.1 ev. A value of 12 barns for σnn has been subtracted except for the metal samples beyond Bragg cutoff. The data of Block et al. and Simpson et al. represent averages of data over small energy intervals. The solid curve estimates the most probable smooth behavior of the cross section. The dashed lines represent 0.5% deviations from the smooth curve.

The solid line drawn through the data is a smooth curve which has only been fitted by eye. It follows most closely the MTR data which have by far the smallest dispersion from a smooth curve. The dashed lines represent deviations of ±0.5% and demonstrate that the shape of the absorption cross section is probably well determined within these limits.

The three experimental groups have each...

Produkt-Details

• Erscheint lt. Verlag: 2.12.2012
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Atom- / Kern- / Molekularphysik
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Hochenergiephysik / Teilchenphysik
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Quantenphysik
• Themenwelt: Technik ► Bauwesen
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-323-15624-X / 032315624X
• ISBN-13: 978-0-323-15624-0 / 9780323156240