Neutrons slow, thermal

For the neutron porosity measurement, fast neutrons are emitted from a 7.5-curie (Ci) americium-beryllium (Am-Be) source. The quantities of hydrogen in the formation, in the form of water or oil-filled porosity as well as crystallization water in the rock if any, primarily control the rate at which the neutrons slow down to epithermal and thermal energies. Neutrons are detected in near- and far-spacing detectors, located laterally above the source. Ratio processing is used for borehole compensation. 

Naturally occurring boron consists of approximately 20% of 10B and 80% of UB, leading to an average atomic mass of 10.8 amu. Because 10B has a relatively large cross-section for absorption of slow (thermal) neutrons, it is used in control rods in nuclear reactors and in protective shields. In order to obtain a material that can be fabricated into appropriate shapes, boron carbide is combined with aluminum. 

As a result of slow (thermal) neutron irradiation, a sample composed of stable atoms of a variety of elements will produce several radioactive isotopes of these activated elements. For a nuclear reaction to be useful analytically in the delayed NAA mode the element of interest must be capable of undergoing a nuclear reaction of some sort, the product of which must be radioactively unstable. The daughter nucleus must have a half-life of the order of days or months (so that it can be conveniently measured), and it should emit a particle which has a characteristic energy and is free from interference from other particles which may be produced by other elements within the sample. The induced radioactivity is complex as it comprises a summation of all the active species present. Individual species are identified by computer-aided de-convolution of the data. Parry (1991 42-9) and Glascock (1998) summarize the relevant decay schemes, and Alfassi (1990 3) and Glascock (1991 Table 3) list y ray energy spectra and percentage abundances for a number of isotopes useful in NAA. 

Spallation occurs when a high-energy cosmic ray breaks a target nucleus into two or more pieces. These interactions commonly eject neutrons. The secondary neutrons slow down to thermal energies and eventually react with other nuclei in the target material to generate heavier species. Production of cosmogenic nuclides by secondary neutrons increases with depth to a peak at between 0.5 and 1 m below the surface. Therefore, in order to get an... 

Although Muraour and Ertand stated that they had substantiated the results of Bowden and Singh (Refs 28 35), a different environment was used. It should be noted that the former utilized reactor irradiation at higher dose rates and doses as compared to the slow thermal neutron irradiation for the latter. For example the thermal neutron dose rate for Pb azide was 4.2 x IQ9 compared to 2 x 107n/cm2/sec and the total dose was 3 x 1014 compared to 7.2 x 10lon/cm2 ... 

NAA is gamma ray spectroscopy that uses the slow thermal neutrons from a nuclear reactor to excite the nucleus of an atom. When an atom absorbs a thermal neutron, its atomic mass increases by one and the nucleus becomes unstable. One or more nuclear reactions then take place that release gamma-rays with energies characteristic of the particular nuclear decay reactions, along with other radiation (Fig. 4.14). While... 

Fast moving neutrons emitted from a radioactive source (usually Radium-Berrylium or Americium-Beryllium) upon collision with a particle having mass nearly equal to its own, like hydrogen atom in the soil, release their energy and gets thermalized or slowed down. The thermalized neutrons are detected by a detector and recorded on a scalar. Usually BFg gas is used as detector of slowed down neutrons. Increased thermalization indicates higher water content of the soil. The zone of influence is normally about 15-20 cm arormd the detector. 

It is obvious that the neutron energy spectrum of a reactor plays an essential role. Figure 19.4 shows the prompt (unmoderated) fission neutron spectrum with 2 MeV. In a nuclear explosive device almost all fission is caused by fast neutrons. Nuclear reactors can be designed so that fission mainly occurs with fast neutrons or with slow neutrons (by moderating the neutrons to thermal energies before they encounter fuel). This leads to two different reactor concepts - the fast reactor and the thermal reactor. The approximate neutron spectra for both reactor types are shown in Figure 19.4. Because thermal reactors are more important at present, we discuss this type of reactors first. 

Bowden and Singh [14] used the electron beam of an electron microscope (75 kV, 200 nA) and found that lead and silver azide could be made to explode in the beam, but this was partly due to heating of the crystal. Fluxes of slow thermal neutrons up to 10 m and of fast (2 MeV) neutrons up to 10 m do... 

Fission reactors consume the only naturally occurring fissile isotope, namely, U-235. Fission occurs as the result of absorption of slow (thermal) neutrons. Uranium ores contain the following isotope distribution 6 x 10 of U-234, 7.11 X 10- of U-235, and 0.99283 of U-238. Chattanooga shale is a typical deposit it was formed 33-29 million years ago and is widely distributed in Illinois, Indiana, Kentucky, Ohio, and Tennessee (Swanson, 1960). The Gassaway member of this shale deposit is about 16 feet thick. 

Moderator. A material used in an atomic reactor to reduce the speed of fast neutrons to slow (thermal) velocities. 

Thermal reactor. Atomic reactor which slows emitted neutrons to thermal velocities by using a moderator (q.v.). 

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