Nuclei away from stability

We see that many problems still need to be solved in order to obtain accurate results in Hauser-Feshbach calculations. Some examples are the energy dependence of rotational enhancement of levels in deformed nuclei, the energy and mass dependence of Ml gamma-ray transitions, the importance of E2 transitions, and better estimates of fission barriers. Work in each of these areas will benefit greatly from a better understanding of the discrete levels, particularly in nuclei away from stability. 

Explosive astrophysical environments invariably lead to the production of nuclei away from stability. An understanding of the dynamics and nucleosynthesis in such environments is inextricably coupled to an understanding of the properties of the synthesized nuclei. In this talk a review is presented of the basic explosive nucleosynthesis mechanisms (s-process, r-process, n-process, p-process, and rp-process). Specific stellar model calculations are discussed and a summary of the pertinent nuclear data is presented. Possible experiments and nuclear-model calculations are suggested that could facilitate a better understanding of the astrophysical scenarios. 

Even after several decades of research [BUR57] into the mechanisms by which the elements are synthesized in stars, it is still often true that the degree to which an astrophysical environment can be understood is limited by the degree to which the underlying microscopic input nuclear physics data have been measured and understood. As new and more exotic high-temperature astronomical environments have been discovered and modeled (and as observations and models for more familiar objects have been refined) the needs for more and better data for nuclei away from stability have increased. In this brief overview, we discuss a few of the explosive astrophysical environments which are currently of interest and some of their required input nuclear data. 

Thus, in order to understand such environments it is necessary to calculate complete network of the competitions between neutron capture and beta decay as well as their corrections for the thermal population of excited states. With regard to this latter correction it is particularly important to know the low-energy level structure of nuclei away from stability. This structure will affect the beta decay properties differently from the neutron capture properties. In a separate contribution to this conference, [TAK85] we will discuss the corrections for beta decay. Basically this becomes important if a low-lying excited state can undergo a Gamow-Teller allowed decay. The... 

Two factors affect the stability of this orbital. The first is the stabilizing influence of the positively charged nuclei at the center of the AOs. This factor requires that the center of the AO be as close as possible to the nucleus. The other factor is the stabilizing overlap between the two constituent AOs, which requires that they approach each other as closely as possible. The best compromise is probably to shift the center of each AO slightly away from its own nucleus towards the other atom, as shown in figure 7-23a. However, these slightly shifted positions are only correct for this particular MO. Others may require a slight shift in the opposite direction. 

In this case, the 38i7Cl is an odd-odd nucleus, whereas 3917C1 is an odd-even nucleus. Thus, even though 3917C1 is farther away from the band of stability, it has a slightly longer half-life. Finally, let us consider two cases where both of the nuclei are similar in terms of numbers of nucleons. Such cases are the following ... 

The closer that electrons are to the nucleus, the more stable. An s orbital is closer to a nucleus than a p orbital is, as p orbitals are elongated away from the nucleus. An sp carbanion is more stable than an sp carbanion because the sp carbanion has 33% s character and and the electron pair is closer to the positive nucleus than in an sp carbanion which is only 25% s character. The sp carbanion is easier to form because of its relative stability. 

The major distinction between the model of La Mer and that developed for uniform latex particles lies in the incorporation of colloidal stability of small particles. The La Mer model assumes that each nucleus is colloidally stable and survives at the end of the reaction at the center of a particle. The aggregation models argue that stabilizing primary small particles is difficult, but aggregation does not necessarily result in a broad particle-size distribution. When schemes for control of particle-size distribution are developed, the result of accepting the notion that colloidal stability can play an important role is that attention is focused away from the length of the nucleation period and towards the colloidal properties of the growing particles. 

Plot of the different isotopes of the elements as the NIZ ratio. The solid line is for NIZ= 1 and the dark band of stability rises away from this line as the number of neutrons in the nucleus increases. Elements on either side of the band of stability will undergo spontaneous radioactive decay. [Reproduced from http //en. wikipedia.org/wiki/Table of nuclides (complete) (accessed November 30, 2013).]... 

Just like elements of lower Z, each actinide element also has one or more stable isotopes. Isotopes heavier than the -stable nucleus decay by emission of P particles (electrons) whereas lighter isotopes decay by electron capture (EC). In heavy elements, the /EC ratio is very small and, as a consequence, positron ( ) emission has been observed only in a few nuclei. The p decay energy usually increases as the nucleus gets further away from the line of p stability. A quantity denoted by ft is very useful in classifying p transitions and estimating p decay half-lives. The log ft value, also called the reduced P transition probability, is the energy-independent transition rate. 

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