Light element production

The light elements present in cosmic rays are partly thermalized, i.e. brought down to low velocities by ionization losses, and thus make a minor contribution to their abundance in the ISM (perhaps about 20 per cent). The main source is usually thought to come from reactions of cosmic-ray protons and a-particles with stationary nuclei of He, C, N and O in the ISM. 

About 80 per cent of the cosmic-ray flux is at energies above 150 MeV per nucleon where the cross-sections are more or less constant (Fig. 9.4). 

A simple calculation neglecting GCE refinements is based on the assumption of a constant production rate  

3 Curiously, there are some nearby interstellar diffuse clouds displaying anomalously low isotope ratios for 7Li/6Li, with a ratio apparently as small as 2 in one case (Lemoine et at. 1994 Knauth, Federman Lambert 2003), compared to the Solar-System (and more usual interstellar) ratio of 12 the anomaly here is that the low ratio in such clouds is consistent with cosmic-ray spallation whereas that in the Solar-System is not. 

the ratio nB/10B is underpredicted by a smaller factor 1.5 which could also come from stellar production, notably from the neutrino process in corecollapse supernovae (Woosley Weaver 1995 Timmes, Woosley Weaver 1995), the efficiency of which is quite uncertain.4 

---

In this textbook many exciting topics in astrophysics and cosmology are covered, from abundance measurements in astronomical sources, to light element production by cosmic rays and the effects of galactic processes on the evolution of the elements. Simple derivations for key results are provided, together with problems and helpful solution hints, enabling the student to develop an understanding of results from numerical models and real observations. 

Fields, B.D., Oliva, K.A., Casse, M. Vangioni-Flam, E. 2001 Standard cosmic ray energetics and light element production. A A 370, 623. 

Sepa.ra.tion of Plutonium. The principal problem in the purification of metallic plutonium is the separation of a small amount of plutonium (ca 200—900 ppm) from large amounts of uranium, which contain intensely radioactive fission products. The plutonium yield or recovery must be high and the plutonium relatively pure with respect to fission products and light elements, such as lithium, beryUium, or boron. The purity required depends on the intended use for the plutonium. The high yield requirement is imposed by the price or value of the metal and by industrial health considerations, which require extremely low effluent concentrations. 

The light elements (D to B, apart from 4He) have such fragile nuclei (see Table 9.1) that they tend to be destroyed, rather than created, in thermonuclear burning, although certain special processes can lead to stellar production of 3 He, 7Li and nB. 

Fig. 9.4. Reaction cross-sections, as a function of energy per nucleon, for the production of light elements for some typical cases. Adapted from Read and Viola (1984). Courtesy Vic Viola.

Neutron depth profiling technique (NDP) [13]. NDP is a speeial method for depth profiling of few light elements, namely He, Li, B and N in any solid material. The method makes use of speeifie nuelear reaetions of these elements with thermal neutrons. The samples are plaeed in the neutron beam from nuclear reactor and the charged products of the neutron indueed reactions (protons or alpha particles) are registered using a standard multiehannel spectrometer. From the measured energy spectra the depth profiles of above mentioned elements can be deduced by a simple computational procedure. 

The leach liquor is first treated with a DEHPA solution to extract the heavy lanthanides, leaving the light elements in the raffinate. The loaded reagent is then stripped first with l.Smoldm nitric acid to remove the elements from neodymium to terbium, followed by 6moldm acid to separate yttrium and remaining heavy elements. Ytterbium and lutetium are only partially removed hence, a final strip with stronger acid, as mentioned earlier, or with 10% alkali is required before organic phase recycle. The main product from this flow sheet was yttrium, and the yttrium nitrate product was further extracted with a quaternary amine to produce a 99.999% product. 

Np through Lr are all prepared artificially by bombardment with neutrons and/or light element ions (He-4, B-10, B-11, C-12,0-16,0-18, Ca-48, Fe-56). Some routes are presented in Table 18.1. The elements have been separated from the targets and other product species by redox reactions, ion exchange, and solvent extraction. In a typical separation, a sulfonic acid ion exchange resin is placed in a column, the tripositive ions of Am through Lr are poured into the column where they are taken up, then the column is eluted with a solution of ammonium a-hydroxybutyrate. As elution proceeds, the An+ ions come off in this order Lr-Md-Fm-Es-Cf-Bk-Cm-Am. They are detected by the distinctive energies of their radioactive emissions. 

---