P-nuclides

Fig. 12. Decomposition of the solar abundances of heavy nuclides into s-process (solid line), r-process (black dots) and p-process (open squares) contributions. The uncertainties on the abundances of some p-nuclides due to a possible s-process contamination are represented by vertical bars (from [32]). See Figs. 13 - 15 for the uncertainties in the s- and r-nuclide data...

From the above short description of the splitting strategy between s-, r-and p-nuclides, it is easily understood that uncertainties affect the relative s-and r-(p-)process contributions to the SoS abundances of the sr(p)-nuclides. Even so, they are quite systematically put under the rug. This question clearly deserves a careful study, especially in view of the sometimes very detailed and far-reaching considerations that have the s-r SoS splitting as an essential starting point. 

The splitting of the SoS s-, r- and p-nuclide abundances has been reviewed in some detail by [32,24]. In view of its importance, we repeat here most aspects of the procedure. 

In practice, the question of the evolution of the galactic content of the nuclides heavier than iron concerns the s- and r-nuclides only. It is traditionally assumed indeed that the p-nuclides are just as rare in all galactic locations as in the SoS. In such conditions, the p-nuclide abundances outside the SoS are out of reach of spectroscopic studies. On the other hand, the s-process and its contribution to the galactic abundances at different epochs are discussed elsewhere in this volume, so that we focus here only on the evolution of the r-nuclide galactic content. 

Quite clearly, the neutron-deficiency of the p-nuclides forbids their production in neutron capture chains of the s- or r-types. In contrast, and as already proposed very early in the development of the theory of nucleosynthesis [29], they could well be synthesized from the destruction of pre-existing s- or r-nuclides by different combinations of (p,y) captures, (7,n), (y,p) or (7, a) reactions. Some /J-decays, electron captures or (11,7) reactions can possibly complete the nuclear flow. These reactions may lead directly to the production of a p-nuclide. In most cases, however, they are synthesized through an unstable progenitor that transforms into the stable p-isobar through... 

Fig. 32. Schematic representation of some simple possible nuclear routes through which seed s- or r-nuclides (black dots) can be transformed into a p-nuclide (black square). Unstable nuclei are represented by open dots. Routes (1) and (2) are made of a succession of (p,y) and (y,n) reactions leading directly to the p-nuclide. Slightly more complicated chains involve (p,y) reactions followed by (3-decays (route (3)), or a combination of (y,n) and (y,p) or (7,a) and (3-decays (route (4)). More complicated flow patterns involving combinations of the represented ones can also be envisioned. The p-nuclide destruction channels are not represented...

Note that the above considerations concern only thermonuclear modes of p-nuclide synthesis. Some non-thermonuclear scenarios have in fact been proposed, like production by spallation reactions in the interstellar medium [71,72], or photonuclear reactions triggered by non-thermalized photons [73]. These models suffer from either too low efficiencies, or from constraints that limit their astro-physical plausibility. They are not discussed further here. 

In fact, what is basically required is just a successful explosion with a typical SN kinetic energy [32]. Another essential requirement is to have enough s- (or possibly r-) process seeds for the production of the p-nuclides. The s-process accompanying core He burning in massive stars can do the job. It strongly enhances the 60 < A < 90 s-nuclei, while the abundances of the heavier nuclei are only very weakly increased. 

Fig. 33. Location in the (N, Z)-plane of the stable isotopes of the elements between Fe and Bi. The p-isotopes are represented by black squares, while both the s-, r-, sr- or sp-isotopes are identified with open squares (see Figs. 13 - 15 for details). The p-nuclides are the progeny of unstable neutron-deficient isobars located on the down-streaming p-process flow (thick black line for more details on the p-process flow, see [32]). Some possible r-process flows derived from a high-temperature parametric model (Sect. 7) are also shown, as well as the up-streaming s-process flow (thin black line) confined at the bottom of the valley of nuclear stability. The proton and neutron drip lines correspond to the locations of zero proton and neutron separation energies...

Figure 33 provides a schematic view of a typical SNII p-process flow. Its distinguishing feature is that it evolves from heavy to light nuclides as a result of the dominant action of photodisintegrations. Figure 34 displays p-nuclide yields in the form of normalized overproduction factors computed for a variety of SNII explosions of solar-metallicity stars (results for each star... 

The impact on the p-nuclide yields displayed in Fig. 34 of a change of the SNII explosion energy, of metallicity, and of reaction rate uncertainties is also discussed by [32],... 

Fig. 35. Abundances of the p-nuclides in a SNIa deflagration model. They are calculated with solar s-seeds (black squares), or with a seed distribution which is representative of the s-process in solar-metalhcity AGB stars (open squares) (see [32] for details)...

Fig. 37. Overproduction of the p-nuclides resulting from the He-detonation considered in Fig. 36. The full and open symbols correspond to SoS s-seed abundances, and to SoS seeds increased by a factor 100, respectively (from [32])...

Figure 37 shows that almost all the p-nuclei are overproduced in solar proportions within a factor of 3 as a combined result of the p- and pn-processes. This includes the puzzling Mo and Ru p-isotopes, as well as 138La, which appears to be overproduced at the same level as the neighbouring p-nuclides. This contrasts with the situation encountered in SNII (Sect. 11.1) or SNIa (Sect.11.2) explosions. 

An embarrassment with the SCWD scenario may come from the fact that the Ca-to-Fe nuclei are predicted to be overabundant with respect to the p-nuclei, but 78Kr, by a factor of about 100. Taken at face value, this situation implies that the considered He detonation cannot be an efficient source of the bulk SoS p-nuclides (see [32] for details). In order to cure this problem, one may envision enhancing the initial abundance of the s-seeds, which has already been seen in Sect. 11.1 to be an essential factor determining the level of production of the p-nuclides. Figure 37 shows that an increase by a factor 100 of the s-nuclide abundances over their SoS values makes the overproduction of a substantial variety of p-nuclides comparable to the one of 78Kr and of the Ca-to-Fe nuclei. The factor 100 enhancement would have to be increased somewhat if the material processed in the core of the CO-SCWD by a C-detonation were ejected along with the envelope. At this point, one essential question concerns the plausibility of the required s-nuclide enhancement. 

There is now observational evidence for the existence of isotopic anomalies involving the p-isotopes of Kr, Sr, Mo, Xe, Ba and Sm in various meteoritic materials [32]. These anomalies manifest themselves as excesses or deficits of the abundances of the p-nuclides with respect to the more neutron-rich isotopes, when comparison is made with the bulk SoS mix. In addition, two isotopic anomalies are attributable to the now extinct neutron-deficient radionuclides 92Nbg and 146Sm which have decayed in the meteoritic material where excesses of 92Zr and 142Nd are observed. 

As far as the p-process is concerned, the calculations conducted in realistic models for the various proposed sites, and in particular for the O-Ne-rich layers of SNell, indicate, roughly speaking, that they may all provide yields enriched with a suite of p-nuclides at a level that is compatible (within a factor of 3 or so) with the solar values. 

A detailed interpretation of the meteoritic isotopic anomalies involving p-nuclides still eludes us to a large extent. This concerns in particular the Xe-HL case. It is also concluded that no reliable p-process chronometry can be built yet on the p-radionuclides 92Nbg or 146Sm in view of the uncertainties still affecting their production in each of the considered p-process sites and of the possible variations of their yields from one site to the other. 

Some progress in the description of convection in advanced stages of the evolution of massive stars, and in particular in their O-Ne layers. The possibility of survival at the explosion stage of at least a fraction of pre-explosively produced p-nuclides remains an uncertain issue. 

The modeling of the p-process in massive stars with a quite broad range of metallicities. Yields from such stars are needed to predict the evolution of the p-nuclide content of galaxies. 

Other useful classes of basic probe molecules used to examine silica, alumina, and silica-alumina surfaces (as well as zeolite systems) include small organic phosphines and phosphine oxides, which rely on the highly convenient P nuclide (/ = 1/2, 100% natural abundance). As Lunsford and coworkers demonstrated for zeolites [89], the P NMR signal of trimethylphosphine is a useful probe for Bronsted acid sites on surfaces. The basis for this approach is the formation of R3P -H B( ) sites at surface Bronsted acid sites, H-B(. ... 

Within the separated uranium fraction, the Th and 234(m)p nuclides will re-grow to their secular equilibrium values relatively quickly but, once separated, Th, the long lived daughter of is, to all intents and purposes, lost forever. That means that chemically separated samples of uranium will only contain nuclides within the first four steps of the chain. Nuclides such as Ra, and Pb, which are often measured to estimate activities, will be completely absent. For this reason, one cannot expect the gamma-ray spectrum of laboratory reagent uranium salts to be the same as natural uranium. 

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