Total neutron cross sections

Recently, very careful measurements have been made from very low energies (50 kev) to 3 Mev for a large number of elements. In these measurements the 

From 3 to 12 Mev, measurements of the total cross sections in a large number of elements have been made by Nereson and Darden using the neutrons emitted by a fast neutron reactor as a source, and a special energy-selective detector. In that instrument, protons projected in the forward direction from a polythene foil were measured by an ionisation chamber connected in coincidence with a proportional counter located between it and the foil, and in anticoincidence with another counter beyond it. The energy of the protons was measured by the size of the pulses they produced in the chamber the energy resolution was about 10%. 

Above 12 Mev, total cross section measurements are less complete. Some measurements have been made by Bonner and his associates in the range 

14 to 18 Mev. The neutrons for this energy range were obtained by the T [d n) reaction. Few accurate measurements have yet been made immediately above 20 Mev though some measurements have been made at 25 Mev by Sherr. There is a gap in the data extending from this energy to the point where measurements have been made with synchrocyclotrons, viz. 60 Mev and upwards. 

Experimental results for neutrons up to 9 Mev are shown in Fig. 40b, p. 99 some higher energy results are shown in Fig. 12, 

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Fig. 2. Total neutron cross sections of silver (—) and cadmium (----) (10). To convert pm to bams, multiply by 10 ...

Assume that the wall and the window of the counter are made of aluminum and are 2 mm thick. Take the total neutron cross section for A1 at 1 eV to be 1.5 b. 

Monoenergetic neutron sources at the energy range provided by crystal spectrometers are necessary for the study of low-energy neutron cross sections with resonances. Consider, as an example, the total neutron cross section of iridium shown in Fig. 14.20. To be able to measure the resonances of this cross section, one needs neutron energy resolution less than 0.1 eV, resolution that can be achieved only with crystal spectrometers or time-of-flight measurements (see Sec. 14.8). 

Figure 14.20 The total neutron cross section of iridium (from Ref. 46).

This is analogous to the relation (177) for total neutron cross sections and enables / to be estimated. This graphical method has been applied to most of the results of the Wisconsin laboratory for the elastic scattering of charged particles in particular to the elastic scattering of protons by 0 and Mg. The absolute cross sections in experiments such as these are... 

Fig. 43. Comparison of total neutron cross section and neutron capture cross section for fluorine (Henkel and Barschall).

The Li levels are obtained from the [dp) reaction, from the excitation function for inelastic scattering of neutrons and from the total neutron cross section. There is evidence for s-wave interaction, leading probably to a 1 level. An odd parity level of this spin also seems to exist in a nearly analogous position in Be according to the results of a study of the BoP[np) reaction (Sect. 41). 

No levels of Na are known, but for F there is unusually extensive information, from the dp) and (rfa) reactions, from the total neutron cross section of fluorine, from the [ny] reaction, observed by means of the F yield, and from the inelastic scattering of neutrons by F , resulting in a yield of 0.1 and 0.2 MeV radiation [32 ], These observations confirm the density of levels already established... 

Bockelman, C. K., D. W. Miller, R. K. Adair and H.H. Barschall Phys. Rev. 84, 69 (1951). — This paper reports the energy variation of total neutron cross section for Li, Be, B , B, C and O which show many sharp resonances. 

Fig. 12. Total neutron cross sections according to Nereson and Darden.

Fig. 13. Theoretical total neutron cross sections according to the strong coupling theory of Feshbach and Weisskopf.

Fig. 14. The square root of the low energy total neutron cross section, divided by the radius, plotted against the radius, according to Adair.

Fig. 27. Nonelastic neutron cross sections plotted against the neutron energy, according to Taylor, Lo.vsjd, and Bonner, full circles crosses, Beyster et al. The dotted line is the elastic cross section obtained by subtracting these results from the total neutron cross section.

Fig. 7. Total neutron cross-sections for C, Al, Cu and Pb as a function of energy. The values were obtained from the references in Table 4.

Table 5. Taylor s values for nuclear radii as determined by a best fit to total neutron cross-sections.

Jastrow points out that the real parts of the forward scattering amphtude are not necessarily the square root of the scattering cross-sections at zero degrees. He tries to obtain information about the nucleon-nucleon potential from the values of n obtained from interpreting total neutron cross-sections. He interprets the value of nr. 0 (i.e. at about 300 Mev as evidence favoring a... 

In general, below about 0.003 eV, the total neutron cross section rises with decreasing energy this rising cross section may be a combination of inelastic lattice vibration scattering and simple neutron capture. Above the neutron diffraction region, 0.03 eV, the cross section... 

Total neutron cross section of (plot of evaluated nuclear data file)... 

Fig. 1. The resonance enhancement of the total cross section difference. er, , and d, , are the total neutron cross section and the difference of the total cross sections for neutrons with opposite helicities P = d/2tr, , E is the neutron energy and v is the weak mixing matrix element, s and p indicate the s- and p-wave compound resonances.

In fundamental reactor science, experimental studies display several essential concepts from neutron and reactor physics. Total neutron cross section is measured by a method involving basic neutron-beam techniques in other exercises, measurements of isotopic neutron cross sections and absolute neutron fluxes are made by use of activation methods in the thermal, resonance, and fast regions of the neutron spectrum. The resonance escape probability and fast fission factor are evaluated in the lattice of the Argonaut reactor. 

The experiment discusses the time-of-flight technique for the selection of neutrons of definite energy ranges from a neutron spectrum. The Fermi slow neutron chopper is described and is used to measure the reactor beam spectrum, as well as the total neutron cross section of several materials as a function of velocity. 

Total Neutron Cross Section and Neutron Temperature Ex-periments of the this manual. 

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