The x-process

8-10 mass units below the nuclides associated with the s-process maxima (Fig. 1.5). It has also been suggested that neutron-rich isotopes of several of the lighter elements might also be the products of an r-process, e.g. Ca, Ca and perhaps Ti, Ti and Ti. These isotopes, though not as abundant as others of these elements, nevertheless do exist as stable species and cannot be so readily synthesized by other potential routes. 

The problem of the existence of the heavy elements must also be considered. The short half-lives of all isotopes of technetium and promethium adequately accounts for their absence on earth. However, no element with atomic number greater than ssBi has any stable isotope. Many of these (notably 84P0, 85 At, sfiRn, 8 Fr, 89Ac and 91 Pa) can be 

Proton capture processes by heavy nuclei have already been briefly mentioned in several of the preceding sections. The (p,y) reaction can also be invoked to explain the presence of a number of proton-rich isotopes of lower abundance than those of nearby normal and neutron-rich isotopes (Fig. 1.5). Such isotopes would also result from expulsion of a neutron by a y-ray, i.e. ()/,n). Such processes may again be associated with supernovae activity on a very short time scale. With the exceptions of In and Sn, all of the 36 isotopes thought to be produced in this way have even atomic mass numbers the lightest is I Se 

One of the most obvious features of Figs. 1.1 and 1.5 is the very low cosmic abundance of the stable isotopes of lithium, beryllium and 

6-12 suggest the need for a low-temperature low-density extra-stellar process. In addition to spallation, interstellar (p,Q ) reactions in the wake of supernova shock waves may contribute to the synthesis of boron isotopes  

before it decays, Li is struck by a prevalent He nucleus then B can be formed ( Li 4- He - B 4- n) and this will survive longer than in a proton-rich environment ( B 4 p - 3 He). Other neutron-rich species could also be synthesized and survive in greater numbers than would 

ReevlvS, Origin of the light eiement.s, A. Rev. Asiron. Astrophys. 12, 4,37-69 (1974). 

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The x-process. This is defined as the process of transferring an s molecule from an ideal gas at 1 atm pressure to a hypothetical dilute-ideal solution in which the mole fraction of s is unity. (The temperature T and pressure of 1 atm are the same in the two phases). ... 

The relation between the standard Gibbs energy of the x-process and the solvation Gibbs energy is obtained from the general expression (7.16) for the chemical potential. Thus,... 

Equation (6.13.27) is the required connection between the Gibbs energy of the x-process and the solvation Gibbs energy. Again, we note that this equality holds only for the ideal-gas and ideal solutions. To obtain the corresponding relations for the other thermodynamic quantities associated with the processes defined above, we start from the general equation, Eq. (6.13.1), for the chemical potential. The partial molar (or molecular) entropy of s is obtained from... 

Thus, information on Kh is essentially equivalent to information on the Gibbs energy change for the x-process. 

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