Neutron-induced reactions

NAA based upon fast neutron-induced reactions has also been intensively utilized. This technique is applied to systems which produce radionuclides by the general reaction A(n, q)B,... 

Tritium is also produced in ternary fission and by neutron-induced reactions with 6Li and 10B. Tritium is a very low energy (3 emitter with a half-life of 12.33 y. The global inventory of naturally produced tritium is 9.6 x 1017 Bq. Tritium is readily incorporated in water and is removed from the atmosphere by rain or snow. Its residence time in the stratosphere is 2-3 y after reaching the troposphere it is removed in 1-2 months. The natural concentration of 3H in streams and freshwater is 10 pCi/L. 

Such behavior of the cross sections for neutron-induced reactions is referred to as 1 /v behavior. 

For slow neutron-induced reactions that do not involve resonances, we know (Chapter 10) that ct ( ) °c 1 /vn so that (ctv) is a constant. For charged particle reactions, one must overcome the repulsive Coulomb force between the positively charged nuclei. For the simplest reaction, p + p, the Coulomb barrier is 550 keV. But, in a typical star such as the sun, kT is 1.3 keV, that is, the nuclear reactions that occur are subbarrier, and the resulting reactions are the result of barrier penetration. (At a proton-proton center-of-mass energy of 1 keV, the barrier penetration probability is 2 x 10-10). At these extreme subbarrier energies, the barrier penetration factor can be approximated as ... 

The fact that neutrons can be absorbed by nuclei without overcoming a threshold (1 = 0 or s-wave reactions) makes neutrons extremely effective nuclear reactants. Neutron-induced reactions are the energy source for present-day commercial nuclear power (fission reactors) while charged-particle-induced reactions remain under study as power sources (fusion reactors). In this chapter we will consider the general features of nuclear fission reactors, following by the general features... 

Srinivasan, B. (1976) Barites Anomalous xenon from spallation and neutron-induced reactions. Earth Planet. Sci. Lett., 31, 129—41. 

A survey of neutron-induced reactions used for the production of radionuclides is given in Table 12.1. (n, 7) reactions are preferably triggered by thermal neutrons giving isotopic products of limited specific activity. 

Table 12.1. Neutron-induced reactions in nuclear reactors (survey).

Stellar nuclear reactions can be either a) charged particle reactions (both target and projectile are nuclei) or b) neutral particle (neutron) induced reactions. Both sets of reactions can go through either a resonant state of an intermediate nucleus or can be a non-resonant reaction. In the former reaction, the intermediate state could be a narrow unstable state, which decays into other particles or nuclei. In general, a given reaction can involve both... 

Pu-Xe dating. Decay of Pu can be used as a chronometer of the first 100 Ma for some specific meteorite parent bodies. Both Pu and the LREE tend to be concentrated in refractory minerals like phosphates. Which LREE is the best proxy for Pu Various authors have suggested Nd (Lugmair and Marti 1977), Sm (Jones and Burnett 1987), or Pr or Ce (Boynton 1978). There are no neutron-induced reactions that produce a rare gas from any of the LREE, but all of these, particularly Nd, do produce the light xenon isotopes like Xe and Xe through cosmic-ray-induced spallation reactions (Wider 2002, this volume). In fact, in many cases, the LREE (and presumably Pu), are probably not fractionated much from each other. Hence, if the cosmic ray dose (i.e., the cosmic ray exposure age) is known, and the production rate of isotopes like " Xe and Xe is also known, then the abundance of the LREE can be calculated. Then the ratio of Xe244 (Pu-... 

Small EE, Anderson RS, Hancock GS (1999) Estimates of the rate of legolith production using " e and A1 from an alpine hillslope. Geomorphology 27 131-150 Small EE, Anderson RS, Repka JL, Finkel R (1997) Erosion rates of alpine bedrock summit surfaces deduced from in situ °Be and A1. Earth Planet Sci Lett 150 413-425 SrinivasanB (1976) Barites anomalous xenon from spallation and neutron-induced reactions. Earth Planet Sci Lett 31 129-141... 

In a point neutron source, surrounded by a moderator, the thermal neutron flux (particles per unit area and unit time) as a function of the distance from the source is found initially to increase and then to decrease see Fig. 12.5. The distance from the source at which the flux is at maximum is given in Table 12.1. This is the optimum position for thermal neutron induced reactions. 

The present section excludes discussion of the two- and three-body problems, and reduces to a minimum the treatment of resonant processes, since these topics are dealt with in other articles in this Encyclopedia (Vol. XXXIX and XL). General information on neutron induced reactions is contained in references [d] and [7] and total cross sections are given by Adair and by Hughes and Harvey [3d]. The reactions observed are listed in Table 7. 

Using the shorthand notation, examples of photon capture, electron knockout and neutron-induced reactions are, respectively ... 

Some of the accessible reactions are directly important in the nucleosynthetic processes while other experiments can only serve as tests of the theoretical approaches (Kiss et al. 2008). The reactions can either be studied in online beam experiments detecting directly emitted y-rays or particles. Another important type of experiment is that of activation (see, e.g., Gyiirky et al. 2006, and references therein). A material sample is activated by neutron, proton, or a beams at the energy of interest and the long-term radiation is counted over an extended period of time. Alternatively, the amount of the nuclei produced by the activation can be measured by the very sensitive accelerator mass spectrometry (AMS), which has become an important tool also for astrophysical measurements. It is especially well suited to study neutron-induced reactions producing different isotopes of the same element. 

While neutrons, charged particles, and gamma photons can be equally used for irradiation, the most frequent choice is the irradiation by neutrons. Since the neutron lacks an electric charge, it is more penetrating and makes the analysis of bulk samples possible. Additionally, the high cross-sections of neutron-induced reactions offer extremely high analytical sensitivities for many elements. 

Reimer P (2002) Fast neutron induced reactions leading to activation products selected cases relevant to development of low activation materials, transmutation and hazard assessment of nuclear wastes. Thesis, D38 Cologne University, Germany, Jul-3980... 

Copper-67 Produced by fast Neutron-Induced Reactions. 1898... 

Lutetium-177 produced by neutron capture reactions Iodine-125 produced by neutron capture followed by Tungsten-188 produced by double neutron capture Tin-117m produced by neutron inelastic scattering Copper-67 produced by fast neutron-induced reactions... 

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