Precursor nuclide

In the last column of Table 7.1, the most popular radioactive precursor nuclide is given together with the nuclear decay process (EC = electron capture, = beta decay) feeding the Mossbauer excited nuclear level. 

Fig. 1. Ratio of experimental delayed neutron emission probability to calculated emission probability plotted versus neutron number of the precursor nuclide. Calculated values are based on Liran and Zeldes mass formula [LIR76],...

The latter course was followed in this work since the Pb activity deep in the core reaches a level that is higher than Ra—its long-lived precursor nuclide. The empirical, or self-defined, supported Pb activity of... 

These early spectrograms shown in Figs. 4 and 5 were obtained with a 180° gas-type mass spectrometer, using fission gas samples of 0.1 to 1 mm3 in volume at N.T.P. (105). The importance of this method of investigating fission products was immediately recognized. Stable and long-lived isotopes of xenon and krypton which are end products of fission chains, were identified for the first time. Since these products were extracted from the uranium several years after the neutron irradiation and since the precursor nuclides were known to he all relatively short-lived, it could be assumed that the... 

A Mossbauer spectrum arises from the recoil-free emission and resonant absorption of a 7-ray by a nuclide. The intensity of the radiation emitted by a source containing the radioactive Mossbauer precursor nuclide, and transmitted through a solid absorber containing the Mossbauer nuclides in... 

The Ra isotopes in the other decay series can be evaluated similarly. Ra in the series (Table 1) is the product of the third a decay, and so the effects of near-surface deletion or decay of recoiled precursors must be calculated accordingly. Ra in the series is also the product of the third a decay. Further processes that may be considered where circumstances warrant include nonsteady state conditions or removal by precipitation at rates that are fast compared to the decay rate of the Ra nuclides. 

Less-refractory fission products condense later onto the surface of the particles. Those with gaseous precursors, for example Sr and 137Cs, condense as they are formed by decay of their parent nuclides. The... 

The mass distribution curves in Figs. 8.13 to 8.15 give the total yields of the decay chains of mass numbers A. The independent yields of members of the decay chains, i.e. the yields due to direct formation by the fission process, are more diflicult to determine, because the nuclides must be rapidly separated from their precursors. Only a few so-called shielded nuclides (shielded from production via decay by a stable isobar one unit lower in Z) are unambiguously formed directly as primary... 

The classic idea of a cosmic-ray exposure (CRE) age for a meteorite is based on a simple but useful picture of meteorite evolution, the one-stage irradiation model. The precursor rock starts out on a parent body, buried under a mantle of material many meters thick that screens out cosmic rays. At a time fj, a collision excavates a precursor rock—a meteoroid. The newly liberated meteoroid, now fully exposed to cosmic rays, orbits the Sun until a time ff, when it strikes the Earth, where the overlying blanket of air (and possibly of water or ice) again shuts out almost all cosmic rays (cf. Masarik and Reedy, 1995). The quantity ff — h is called the CRE age, f. To obtain the CRE age of a meteorite, we measure the concentrations in it of one or more cosmogenic nuclides (Table 1), which are nuclides that cosmic rays produce by inducing nuclear reactions. Many shorter-lived radionuclides excluded from Table 1 such as Na (ff/2 = 2.6 yr) and °Co ty = 5.27 yr) can also furnish valuable information, but can be measured only in meteorites that feu within the last few half-Uves of those nucUdes (see, e.g., Leya et al. (2001) and references therein). 

Natural radioactive nuclides in the atmosphere have two principal sources— radon and its progeny derived from Earth s surface and cosmic-ray-produced nuclides. Dust from the elevation of soils can also provide secondary sources of these nuclides. Suitable accommodation for these sources must be made if only those species having gaseous precursors are to be considered. 

However, we may wish to calculate nuclide amounts in a chain wherein some members may be formed by neutron reactions with their individual precursors. We define here a linear chain as one in which each nuclide other than the first is formed directly only from a single precursor, illustrated as follows ... 

The equations of Sec. 6.2 give the number of atoms of each fission product after a reactor has been run at stated conditions for a specified time. If the reactor is then shut down, the fission products build up and decay in accordance with the laws of simple radioactive decay, which were outlined in Sec. 3. If the nuclides in the decay chain are removed orJy by radioactive decay during reactor operations, the equations of Sec. 3 describe the changes with time of the number of atoms of any nuclide in the decay chain. If a member of a fission-product decay chain or its precursors in the decay chain are removed by neutron absorption, equations for the amount of each nuclide present at time t after shutdown may be obtained by applying the equations of radioactive decay to the amount present at shutdown. 

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