Total Cross Sections Measured With Photo Neutrons

This is an authoritative compilation of information regarding methods and data used in all phases of nuclear engineering. Addressing nuclear engineers and scientists at all levels, this book provides a condensed reference on nuclear engineering since 1958.

Neutron Cross Sections presents the principles of cross-section measurement and use, as well as sufficient theory so that the general behavior of cross sections is made understandable. This compilation is a direct result of experiences connected with the collection and evaluation of cross-section data during the past eight years at ""Sigma Centre"", Brookhaven National Laboratory. Here, experimental results received from laboratories throughout the world are carefully evaluated and compiled in the curves and tables of the large volume Neutron Cross Sections, The most recent version of the compilation, known as BNL 325, appeared 1 July 1955, and Supplement 1 to BNL 325 was published on 1 January 1957. The compilation itself consists almost completely of cross sections at specific energies, shown in the form of curves or tables, with only brief explanatory texts. The text opens with discussions of the general properties of cross sections and principles of nuclear structure that are important to the understanding of cross-section behavior. Separate chapters follow that describe specific techniques for measuring cross sections along with experimental results involving fast neutrons, resonance neutrons, resonances in fissionable materials, and thermal neutrons.

The total cross sections of twelve different elements were measured using the neutron beam from the 184-in. cyclotron, operating with deuterons. Bismuth fission ionization chambers were employed as both monitor and detector in conventional 'good geometry' attenuation measurements in the neutron flux emerging from the 3-in. diameter collimating port in the 10-ft-thick concrete shielding. The mean energy of detection of the neutrons in this experiment is estimated to be 95 Mev. Measurements were also made with a monitor and detector placed inside the concrete shielding where an intense neutron flux over a large area could be obtained. Attenuators of four different elements were placed in front of the detector in a 'poor geometry' arrangement so that attenuation was due essentially to inelastic collisions which degrade the neutron energy below the fission threshold. A second detector was placed outside the concrete shielding In the collimated neutron beam in line with the neutron source, absorber, and first detector. Attenuation in it is caused by both inelastic and elastic scattering. By this arrangement the ratio of inelastic to total cross section can be determined directly in one experiment. The nuclear radii as calculated from the observed cross section, using the theory of the transparent nucleus, vary as 1.38 x 10(exp-13) A(exp(1/3)) cm. In this energy range the ratios of the inelastic to total cross sections are all less than one-half.

The measurement of 28O-Mev neutron cross sections of various nuclei is described. The neutron beam used results from 340-Mev protons in the 184-in. cyclotron striking a 2-in. Be target. The neutron detector consists of a double coincidence anthracene scintillation counter telescope which counts recoil protons scattered at 15 deg from a paraffin cylinder placed in the collimated neutron beam. A 2-in. Cu absorber placed between the counters assures that only protons of energy greater than 250 Mev are counted. The cross sections for all nuclei measured from Li to Pb are smaller than the corresponding cross sections measured at 90 Mev by factors between 0.5 and 0.6.