Neutron interactions

Greater detail in the treatment of neutron interaction with matter is required in modem reactor design. The neutron energy distribution is divided into groups governed by coupled space-dependent differential equations. 

The real component of the neutron refractive index 8 is related to the wavelength X of the incident neutrons, the neutron scatterir length (a measme of the extent to which neutrons interact with different nuclei), the mass density and the atomic... 

Of these neutron interactions, those that produce prompt-7 rays were evaluated as the most feasible for mine detection. As discussed in the Introduction, we define a prompt 7-ray as one which is produced as a direct result of the primary neutron interaction, usually within nanoseconds of that initial event. Such reactions are obviously attractive because they can best satisfy the desired rapid sweep rate over a minefield. The three specific neutron-prompt gamma reactions that were intensively investigated by the US Army are listed below ... 

SANS is a valuable research technique, since it provides information about the size and shape of the molecular scale structures in the size range 1-10 nm found in polymer or dendrimer systems. Neutrons interact with atomic nuclei, hence are sensitive not only to the details of the way such nuclei are arranged but also to the presence of different isotopes. 

Proportional counters are extremely sensitive, and the voltages are large enough so that all of the electrons are collected within a few tenths of a microsecond. Each pulse corresponds to one gamma ray or neutron interaction. The amount of charge in each pulse is proportional to the number of original electrons produced. The proportionality factor in this case is the gas amplification factor. The number of electrons produced is proportional to the energy of the incident particle. 

Proportional counters can also count neutrons by introducing boron into the chamber. The most common means of introducing boron is by combining it with tri-fluoride gas to form Boron Tri-Fluoride (BF3). When a neutron interacts with a boron atom, an alpha particle is emitted. The BF3 counter can be made sensitive to neutrons and not to gamma rays. 

The BF3 proportional counter is used to monitor low power levels in a nuclear reactor. It is used in the "startup" or "source range" channels. Proportional counters cannot be used at high power levels because they are pulse-type detectors. Typically, it takes 10 to 20 microseconds for each pulse to go from 10% of its peak, to its peak, and back to 10%. If another neutron interacts in the chamber during this time, the two pulses are superimposed. The voltage output would never drop to zero between the two pulses, and the chamber would draw a steady current as electrons are being produced. 

Both gammas and neutrons interact in the boron-coated chamber. 

Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched U235. The neutrons interact with the U235, causing fission. One of the two fission fragments enters the chamber, while the other fission fragment embeds itself in the chamber wall. 

Neutrons interact with the U235 coated chamber causing fission of the U235. 

As discussed in Chapter 10, there are other reaction mechanisms besides fission when neutrons interact with heavy nuclei. They include (a) elastic scattering where <2 = 0 and kinetic energy is conserved. However, the target nucleus recoils in each... 

Impurities in the water and water activation products also contribute to the radioactivity of the coolant water. Tritium is produced as a low yield ( 0.01%) fission product that can diffuse out of the fuel, by activation of boron or fiLi impurities in PWRs. 24Na and 38C1 are produced by neutron activation of water impurities. In BWRs, the primary source of radiation fields in the coolant and steam systems during normal operations is 7.1s 16N. This nuclide is produced by 160(n, p)16N reactions from fast neutrons interacting with the coolant water. This 16N activity can exist as N07, NO in the coolant and NHj in the steam. 

The total neutron interaction cross section, atotal( ), is the sum of the various reaction cross sections ... 

As we have assumed that the region over which neutron interaction with the nucleus is appreciable is much smaller than the neutron wavelength, we are dealing with a true quantum phenomenon and semiclassical methods are not valid. It is interesting that a calculation which assumes that the nuclear potentials are averaged over the bulk and in which semiclassical methods are adopted leads to a result which has a superficial resemblance to (2.51) but with a plus sign before the second term on the right hand side. 

In this report we discuss the extension of this concept to odd-odd nuclei. Odd-odd nuclei provide richer and more complex structure, and the residual proton-neutron interaction appears explicitly in the boson-fermion interaction. 

The proton single-particle energies become modified due to the proton-neutron interaction in the following way... 

Cosmic ray neutrons interact in the upper atmosphere with nitrogen, producing 15N, which is radioactive and disintegrates into common carbon (12C) and tritium ... 

Carbon-14 is formed in the upper parts of the atmosphere from secondary neutrons formed by cosmic ray interactions with the atmosphere. The neutrons interact with common nitrogen ... 

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