Fast neutron reactions

The nonelastic neutron cross sections. Until recently the production and detection of monoenergetic beams of neutrons was a matter of great difficulty. The d, n) reaction with deuterium and tritium, and the (/ , n) reactions with tritium and lithium, now provide suitable sources of neutrons with energies ranging from a few kev to 20 Mev or more. The technical details of these sources and their use has been discussed by Hanson, Taschek and Williams [11]] methods of flux measurement have been discussed by Barschall, Rosen, Taschek and Williams [12], and a very complete account of the techniques involved in the measurement of nonelastic and elastic cross sections has been written very recently by Cranberg et al. [13], 

We have seen that direct measurements of the transmission of a neutron beam through different materials give, in general, the total cross section, cr. The nonelastic cross section, defined in Sect. 5, is equal to the difference between the total cross section and the elastic scattering cross section. It can be obtained by a measurement of this difference, or by modifications of the transmission method. 

In the first method, the total cross section is first measured then the differential elastic scattering cross section is measured, integrated, and subtracted from the total cross section  

The most recent measurements of this kind are those of Walt and Beyster. These authors used a collimated beam of 4.1 Mev neutrons produced in the H (i, n) He reaction and measured the elastic scattering of the neutrons produced from targets placed in the neutron beam. Their detector consisted of a number of spherical plastic scintillators separated from each other by quartz plates and placed at the end of a light guide. This method of detection, first described by McCrary, Taylor, and Bonner, by limiting the size of each detecting unit, 

At low energies the nonelastic cross section cannot be much greater than the cross section for inelastic scattering, and it will differ from the reaction cross section only by the compound-elastic scattering cross section. At these low energies the number of states which can be excited by 

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This NAA technique is based on the nuclear reactions 23Na(n,7)24Na and 41K(n/y)42K. Half-lives of the activated products are 15.0 hrs and 12.4 hrs, respectively. For Na analysis, the samples were irradiated in a specially designed thermal column to suppress the fast neutron reaction of 27Al(n,a)24Na which interferes with the reaction for Na. For K analysis, the proplnt samples were irradiated at a standard irradiation position of the reactor. For the Na irradiations, the neutron flux in the thermal column was in the order of 1010, whereas for the K assays it was approx 1012 neutrons/cm2-sec... 

The tritium from fast-neutron reactions with B is estimated to be about 0.6 Ci/Mg of graphite, and tritium from Li and other sources is even less. 

Figure 4. Yatsevich and Honda (1997) calcnlate the energy spectrum of neutrons produced from U and Th (a,n) reactions in an homogenous mantle over 4.5 Ga. They calculate a lower energy spread than the simplified calculation of Rison (1980) because of the inclusion of the effect of elastic and, to a lesser extent, inelastic scattering of the neutrons. Calculated yields for fast neutron reactions, such as with that have a 2-4...

Fast neutron reactions are a possible source of error in the results. For example, °Co, used to determine Co, may be produced by the fast neutron reactions... 

E10.05 E0266-92 Standard Test Method for Measuring Fast-Neutron Reaction Rates by Radioactivation of Aluminum... 

Berkeley. Studying fast-neutron reactions, Seaborg notes, was a prerequisite to the design of an atomic bomb. Oppenheimer had found a place for himself on the ground floor. 

Lines.The activity of the water leaving the reactor is due mainly to the N activity formed from the reaction 0 (n, p )N. This is a fast-neutron reaction having a threshold at about 9.5 Mev and a cross-section- of about 1 barn. Since a. small fraction (about 1/50,000) of the fission- neutrons have energies above 9.5 Mev, the effective cross - section per fission neutron is about 0.014 mb or 1.4 lO" cm . The activity of- the water leaving the reactor is given by... 

Not much has been reported in the literature on the P-sensitivity of CaS04 Dy phosphors. Pradhan et al. (1978) also used the fast neutron reaction S(n,p) P. To increase the efficiency, the phosphor was mixed with sulfiir powder and pressed into pellets. After the neutron irradiation the pellets were heated on an Al planchette and the accumulated TL due to the P activity was used for the P dosimetry. The sensitivity is reported to be 100 times that of the conventional CaS04 Dy phosphors. 

For the BeO systems, since the Be(n, 2n), Be(n, a), and 0(h, a) are all competing fast neutron reactions, the e indicates the net fast multiplication effect of these three reactions therefore, the indicates just the thermal... 

A number of small peaks appear in long background spectra because of activation or excitation of the germanium of the detector. Table 13.8 shows the isotopic composition of germanium, together with some of the nuclear reactions that could be stimulated. There may also be evidence in the spectrum of fast neutron reactions with components of the shielding, lead, copper, cadmium, iron, for example. Table 13.9 lists some of the more prominent peaks that may be detected with an assignment to particular nuclear reactions. A number of these peaks are of particular interest ... 

Perturbation theory can be extended to a two-group or multigroup model. This is necessary if the perturbation involves change in epithermal or fast neutron reaction rates (fuel element variation, for example). These problems are difficult and solutions are not described here. Some information is presented below on the effect of natural uranium columns, air columns and water columns. 

This is a fast neutrons reaction, with threshold energy of 10 5 Mev, and is applicable only if sufficient high-energy neutrons are available. The other two neutron reactions which could be possibly used. 

Second, although only a small amount of tritium is produced from the pure water, there are two other sources of tritium that will produce substantial amounts in a PWR. When boron is used as a chemical shim, fast neutron reactions with boron will produce tritium. Also, tritium is produced as a fission product, and it can escape through clad leaks into the water. Finally, use of Li OH for pH control can contribute tritium. The boron reactions are responsible for most of the tritium, however. 

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