Halo nuclei

Another question we might pose to ourselves is whether the neutron and proton distributions in nuclei are the same Modern models for the nuclear potential predict the nuclear skin region to be neutron-rich. The neutron potential is predicted to extend out to larger radii than the proton potential. Extreme examples of this behavior are the halo nuclei. A halo nucleus is a very n-rich (or p-rich) nucleus (generally with low A) where the outermost nucleons are very weakly bound. The density distribution of these weakly bound outermost nucleons extends beyond the radius expected from the R °c A1 /3 rule. Examples of these nuclei are nBe, nLi, and 19C. The most well-studied case of halo nuclei is 1 Li. Here the two outermost nucleons are so weakly bound (a few hundred keV each) as to make the size of 11 Li equal to the size of a 208Pb nucleus (see Fig. 2.12). 

Figure 2.12 Schematic representation of the relative sizes of the halo nucleus nLi and 208Pb. (Figure also appears in color figure section.)...

Figure 2.12 Schematic representation of the relative sizes of the halo nucleus uLiand208Pb.

Another limit of nuclear stability is the extreme of the neutron to proton ratio, N/Z. For certain very neutron-rich nuclei, such as Li, an unusual halo structure has been observed. In halo nuclei, a core of nucleons is surrounded by a misty cloud, a halo of valence nucleons that are weakly bound and extend out to great distances, analogous to electrons surrounding the nucleus in an atom. Halo nuclei are fragile objects, are relatively large, and interact easily with other nuclei (have enhanced reaction cross sections). The halo nucleus Li, which has a Ti core surrounded by a two-neutron halo is shown in Figure 1. Li is as large as ° Pb. Li and other... 

Isotope shifts for the various isotopes of lithium Li to Li have been measured by the group of Kluge et al. at GSI (including a collaboration with TRIUMF for Li) and the results reported in Refs. [66,65]. The result for Li is of special interest because, like He, it is a halo nucleus with a Li core, and so the nuclear radius is very sensitive to the details of nuclear structure. All the experiments involve measurements of the isotope shift for the 2 Si/2-3 Si/2 two-photon transition. As an interesting test of the method. Table 4.9 compares theory and... 

As a consequence of these advances, helium and lithium now join the ranks of hydrogen and other two-body systems as examples of fundamental atomic systems. The high precision theory that is now available creates new opportunities to develop measurement tools that would otherwise not exist. One such example discussed here is the determination of the nuclear charge radius for the halo nuclei He and Li. This opens up a new area of study at the interface between atomic physics and nuclear physics, and it provides important input data for the determination of effective nuclear forces. Other similar experiments have been performed on the lithium isotopes [66], including the halo nucleus Li [65], and further work is in progress on He at Argonne and Be at GSI/TRIUMF. 

Chloromethylation of the aromatic nucleus occurs readily with alkyl and alkoxy substituents accelerating the reaction and halo, chloromethyl, carboxyl, and nitro groups retarding it. 

The imidazole nucleus is often found in biologically active molecules,3 and a large variety of methods have been employed for their synthesis.4 We recently needed to develop a more viable process for the preparation of kilogram quantities of 2,4-disubstituted imidazoles. The condensation of amidines, which are readily accessible from nitriles,5 with a-halo ketones has become a widely used method for the synthesis of 2,4-disubstituted imidazoles. A literature survey indicated that chloroform was the most commonly used solvent for this reaction.6 In addition to the use of a toxic solvent, yields of the reaction varied from poor to moderate, and column chromatography was often required for product isolation. Use of other solvents such as alcohols,7 DMF,8 and acetonitrile9 have also been utilized in this reaction, but yields are also frequently been reported as poor. 

Spectral analysis shows quite clearly that the various types of atoms are exactly the same on Earth as in the sky, in my own hand or in the hand of Orion. Stars are material objects, in the baryonic sense of the term. All astrophysical objects, apart from a noteworthy fraction of the dark-matter haloes, all stars and gaseous clouds are undoubtedly composed of atoms. However, the relative proportions of these atoms vary from one place to another. The term abundance is traditionally used to describe the quantity of a particular element relative to the quantity of hydrogen. Apart from this purely astronomical definition, the global criterion of metallicity has been defined with a view to chemical differentiation of various media. Astronomers abuse the term metaT by applying it to all elements heavier than helium. They reserve the letter Z for the mass fraction of elements above helium in a given sample, i.e. the percentage of metals by mass contained in 1 g of the matter under consideration. (Note that the same symbol is used for the atomic number, i.e. the number of protons in the nucleus. The context should distinguish which is intended.)... 

It is known that the oxygen abundance in the interstellar medium increases all the time this nucleus is produced by type 11 supernovas which, one after the other, also contribute their iron production to the Galaxy (Fig. 8.7). The pO mechanism is thus likely to grow in importance as the Galaxy evolves. In other words, clues to the Op mechanism should be sought in the early phases of galactic evolution, that is, in halo stars. The fact remains that the two mechanisms induce different evolution in beryllium and boron as a function of oxygen. 

There are also other examples where a halogen atom in the 4- or 5-position of the nucleus is involved in a Suzuki reaction <2002SL223, 2003SL1482>. The ort o-brominated pyridazinamines 4-bromo-6-phenylpyridazin-3-amine 177 and A -benzyl-4-bromo-6-phenylpyridazin-3-amine 178 are especially interesting since, as observed for 6-halo-pyridazin-3-amines, no protection of the primary or secondary amino group is required (Equation 33) <2003SL1482>. 

Guanidines with a-halo ketones form 2-aminoimidazoles. a-Hydroxy ketones also react with amidines to form imidazoles, and a variety of substituents can be introduced into the imidazole nucleus (CHEC 4.08). 

The method used depends largely on whether precise localisation is required or whether a certain amount of diffusion of the soluble compounds can be tolerated or is required. In the latter case rapid air drying (at 37°C) of cells labelled with tritiated thymidine allows thymidine phosphates to diffuse a little way to form a halo around the nucleus (Adams, 1969a). For precise localisation it is necessary to freeze the cells in isopentane or freon held at liquid nitrogen temperature and then to subject them to lyophilisation. This drying... 

The size of a cometary nucleus cannot be measured directly, since even in the largest telescopes it remains an unresolved point of light. Photometric brightness measurements of comets still far away from the Sun before a radiating halo has formed, together with a phase law and a plausible value for the albedo, yield diameters of the order of 1-20 km (Roemer ). Periodic comets are, on the average, smaller than new ones, since they lose about 0.1 % of their masses per revolution. 

Plasma cell Dark purple, eccentric nucleus, light to dark blue cytoplasm, distinct perinuclear halo... 

Various groups on the aromatic nucleus including halo, hydroxyl, alkoxyl, and amino groups are stable during reduction of the carbonyl group by one or more of the above procedures. The Clem-mensen reduction of keto acids is treated in method 269. 

Satisfactory procedures are described, however, for the deamination when halo, nitro, or carboxyl groups are on the nucleus. Metals or metallic oxides are sometimes added as catalysts. ... 

Aryl ethers in the presence of a solvent can be preferentially halo-genated in the nucleus. Thus, anisole with phosphorus pentabromide or with iodine monochlotide yields p-bromoanisole (90%) and p-iodo-anisole (46%), respectively. Phosphorus pentachloride has also been used for the halogenation of the nucleus as in the preparation of 4-chloro-biphenyl ether (90%). The action of this reagent with aliphatic and aryl-aliphatic ethers is very complex, giving both cleavage and halogenation products. ... 

The reduction of diazonium salts by sodium sulfite forms monosub-stituted arylhydrazines. An improved procedure for the synthesis of phenylhydrazine in 84% yield is typical. Arylhydrazine salts substituted in the nucleus with halo," ether, carboxyl, or nitro groups have been prepared. The free bases are liberated from the salts by the action of aqueous sodium hydroxide or sodium acetate. 

Several other functional groups may be present on the aromatic nucleus during the sulfonation reaction, including halo, hydroxyl, phen-oxyl, carboxyl, and amino " groups. Sulfonations of aniline and of dimethylaniline take place by different mechanisms. ... 

A carboxyl group is removed from a heterocyclic nucleus in much the same way as from an aromatic nucleus (method 13), i.e., by thermal decomposition. The pyrolysis is catalyzed by copper or copper salts and is frequently carried out in quinoline solution. The reaction is important in the synthesis of various alkyl and halo furans. Furoic acid loses carbon dioxide at its boiling point (205°) to give furan (85%). A series of halo furans have been made in 20-97% yields by pyrolysis of the corresponding halofuroic acids. The 5-iodo acid decarboxylates at a temperature of 140°, whereas the 3- and 5-chloro acids requite copper-bronze catalyst at 250°. ... 

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