Stability, valley

Unstable nuclei decay to an isobar in the fi stability valley by or emission or electron capture ... 

The VB simplified model of ground-state potential energy surface H3 system considered as transition state and stabilization valleys of the H + H2 reaction is also an early problem, belonging to the history of physical chemistry under the name London-Eyring-Polanyi-Sato (LEPS) model that continues to serve as basis of further related developments [17,18], The actual analysis is a new a focus on the JT point of this potential energy surface able to absorb results of further renewed CASCCF type calculations on this important system. 

Positron emitters cannot be produced by n-irradiation from Figure 4.8 it is seen that only charged particle irradiation (using H, He, etc) can result in product nuclei on the proton rich side of the stability valley, for which positron emission is the main decay mode... 

We have already shown how one model for the nuclear structure, the liquid drop model, has helped us to explain a number of nuclear properties, the most important being the shape of the stability valley. But the liquid drop model fails to explain other important properties. In this chapter we shall try to arrive at a nuclear model which takes into account the quantum mechanical properties of the nucleus. 

Mean binding energy per nucleon Eb/A for nuclei lying in the stability valley as a function of the mass number A. The approximate contribution of different terms in Eq. (2.3) is also shown (Based on Kravtsov 1965)... 

The root-mean-square (charge) radius J r.m.s. depends both on Z and N. At the bottom of the stability valley (see Sect. 2.2.4), the atomic number Zstab and mass number A tab are related by... 

The stability valley of nudei. The vertical projection of the bottom of the valley on the Z x A plane follows the line of stable nuclei (cf. Fig. 2.7 in Chap. 2) at an "altitude" expressed by the ratio of the nudidic mass (M) and the mass number (A). The horizontal projection of the bottom of the valley onto the (M/A) x A plane is essentially the same as the graph labeled "experimental" in Fig. 2.3 in Chap. 2, except that it is upside down. The isobaric cross sections of the valley are parabolic as a consequence of the Weizsacker formula (2.3) in Chap. 2. (See also the legend of Fig. 7.2 in this chapter, as well as Fig. 14.5 in Chap. 14, Vol. 2.) The local dips marked are related to magic numbers (Ca N=20 Fe-Ni A/=28, Z= 28 Y A(=50 Ba Af=82). Beta decay occurs all along the valley. On the near side of the valley decay is predominant, on the far side decay and electron capture (EC) compete with each other as indicated on the parabola. Alpha decay occurs for A > 60 mainly on the far side of the valley... 

Beta emitters are found all along the stability valley (see Pig. 7.13) starting from jH up to... 

There are two basic timescales in this scenario of heavy-element nucleosynthesis by neutron captures (1) the P-decay lifetimes, and (2) the time intervals between successive captures that are inversely proportional to the neutron capture reaction rates and the neutron flux. If the rate of neutron capture is slow compared to the relevant P decays, the synthesis path will follow the bottom of the stability valley very closely. On the other hand, if the rate of neutron capture is faster than the relevant P decays, highly neutron-rich nuclei will be formed. After the neutron flux has ceased, those nuclei will be transformed to stable nuclei by a series of P decays. The above two processes are called s- and r-process, respectively, according to their slow or rapid rate of neutron capture. The observed abundances of nuclei in the solar system, especially in the regions of closed-shell nuclei, suggest that the s- and r-processes contributed more or less equally to the formation of the elements above the iron region (see Fig. 12.12). 

Figure 1.2 The beta stability valley at low Z. Adapted from a figure published by New Scientist, and reproduced with permission...

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