Neutron halo

A. Baran, K. Pomorski, M. Warda, Neutron halos in heavy nuclei - relativistic mean field approach, Z. Phys. A 357 (1997) 33-38. 

B. Nerlo-Pomorska, K. Pomorski, J. F. Berger, J. Decharge, The neutron halo in heavy nuclei calculated with the Gogny force, Eur. Phys. J. A 8 (2000) 19-29. 

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... 

New forms of radioactivity were reported. Proton emission from ground states, predicted as the simplest decay mode of proton-rich nuclei and long searched for, was observed in 1982 for Lu (81 ms) produced by a heavy-ion reaction (Hofinann et al. 1982). Unusual large nuclear radii were found for some very light nuclei (Tanihata et al. 1985) and later attributed to neutron haloes, e.g., for Li (8.5 ms) to a halo of two neutrons around a i core. Even a new kind of natural radioactivity was discovered in 1984 (Rose and Jones 1984) emission of nuclei fi-om Ra (lid) leading to Discoveries of other rare decay modes involving the emission of a variety of fragments from very heavy nuclei soon followed. 

Note that far from the stability region the separation energy of the outermost nucleons is small. In some cases this may lead to a drastic increase of the size of nucleus the external nucleon may occupy a distant lying orbit. For example, such proton halo exists in B, one-neutron halos in Be and and two-neutron halos in He and Li. The radii of these nuclei are extremely large, comparable with that of Ca. See Suzuki et al. (2003). 

Abundance results for additional clusters are currently underway and include analyses of the neutron-capture elements (in order to trace the onset of contributions from low-mass Type II SNe as well as AGB stars). Combined with their ages, the nucleosynthetic histories of the outer halo clusters will better constrain the timescales of formation and construction of the Galaxy. 

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). 

Neutron scattering experiment on I. a2 Sr CuC>4 has reported that an applied magnetic field enhances the AF correlation in the superconducting state [10]. A checkerboard halo of the local density of states (LDOS) around the core has been reported in Bi2Sr2CaCu208+s. [11] However, the relation between the observed phenomena and antiferromagnetism within vortex cores is not clear, because of lack of a spatial resolution of the magnetism. 

Diffraction patterns from amorphous materials consist of a few vague halos. Accurate measurement of the intensity distributions of these halos in reciprocal space provides information about the distribution of atoms in real space. Reviews of the experimental methods and data handling techniques for X-ray-, electron-, and neutron-scattering is given in the literature The mathematical tool which is used to relate the... 

Ba/Eu] is low in halo stars, indicating that the s-process (which is the main source of Ba) has not had sufficient time to contribute to the abundances in halo stars, while the r-process (the main source of Eu) dominates in metal-poor stars. Indeed, very metal-poor halo giants show evidence for a purely r-process contribution to the abundances of heavy neutron capture elements (Sneden et al. 2000). 

The results of Ref. [67] and Ref. [68] strongly suggest that the HDA-LDA transition is indeed a first-order transition, and that an interface between LDA and HDA is obtained. With time the LDA grows at the expense of the HDA matrix upon decompression. This view is also supported by recent neutron diffraction experiments obtained for the reverse transition from LDA to HDA upon compression. In Ref. [70], it is shown that neutron diffraction patterns of samples obtained upon compression of LDA show a double-peaked halo maximum, one peak being characteristic of LDA and the other peak being characteristic of HDA. Also, Raman spectra obtained under pressure were interpreted in favor of a mixture of HDA and LDA [71],... 

Ratcliffe, C. I., Waddington, T. C., Internal Torsional Modes in Mixed Methyl Halo-genocompounds of Group IV Elements Studied by Inelastic Neutron Scattering, J. Chem. Soc., Faraday Trans. II 72 [1976] 1840/50. 

---