Heavy elements production

To give one a feel for the magnitude of the quantities involved, we outline below a very simple schematic method for estimating heavy-element production cross sections. It is intended to show the relevant factors and should not be taken too seriously, except to indicate the order of magnitude of a particular formation cross section. 

Some astrophysicists believe the r-process mechanism for producing these heavy elements also occurs in supernovae. (R-process reactions are rapid reactions, probably occurring inside supernovae, in which heavy elements are formed as atomic nuclei capture neutrons. This is in contrast to s-process reactions, the slower reactions in giant stars in which heavy elements are created as atomic nuclei capture neutrons.) Supporters of the supernova theory of heavy element production argue that collapses of binary neutron stars happen too infrequently to pro-... 

Actinides served already as targets, when neutron capture and subsequent P decay were used for the first synthesis of transuranium elements. Later, up to the synthesis of seaborgium, actinides were irradiated with light-ion beams from accelerators. At that time it was already known that cold fusion reactions yield higher cross sections for heavy element production. 

Since both the vacuum isolation foil and the target backing foil must hold a pressure difference of greater than 1 bar, relatively thick metal foils, such as 2.5 mg/cm2 Be or 1.8 mg/cm2 HAVAR, have been used. These thick foils are especially attractive when considering the mechanical stability of extremely radioactive actinide targets. Variations on the double-window target system have traditionally been used for heavy element production with actinide target materials. 

Element 113 has been discovered at RIKEN in the reaction B Zn, n) 113 (Morita et al. 2004d). In an irradiation time of 79 days with a projectile dose of 1.7 x 10 the a-decay sequence of one atom had been observed. In a second experiment (Morita et al. 2007b) one more chain was found (O Fig. 19.4). The production cross section is 30 fb, the smallest cross section ever measured for the production of a heavy element. The value is 11.68 MeV and the half-life, 0.24 ms (Morita 2008). The isotope 113 has been produced in the reaction Np( Ca, 3n) (Oganessian et al. 2007), while the isotopes with masses 283 and 284 appear in the decay chains of element 115. The half-lives of the four isotopes range from 0.24 ms to 0.48 s. Element 113 is presently at the experimental limit for heavy-element production by cold fusion, because of the required beam time. A simple extrapolation would require a beam time of the order of 1 year for one atom of element 114. To go beyond, new concepts are needed. 

Negative ion yield is proportional to the electron affinity of the element. Sputter yield depends on the difference between electron affinity of the desired atom and the effective work function. Work function varies upon the environment of the surface of the sample. Physical conditions of the sample affect the properties of atoms on the surface. The probability of negative ion formation is enhanced by the presence of Cs layer at the surface of the sample and electron cloud near the sample surface. Samples are mixed with metallic powder (e.g., Ag or Nb) to improve the thermal and electrical conductivity. Ion-atom collision kinematics reduces the sputter yield for heavy elements. Production of negative ions is at the maximum for normal incidence of the sputtering beam, but the total sputter rate, which means positive, negative, and neutral emission, increases when the angle of incidence is away from the normal. Atomic ion current is very low or zero for some elements. In that case, selection of one molecular ion out of many possible molecular ions (like oxides, hydrides, or carbides) becomes important (Tuniz et al. 1998). 

As described in the previous two sections, the mechanisms of cold and hot fusion of heavy ions are fundamentally different. Of course, there must be some smooth transition in reaction character as the projectile/target asymmetry evolves unit by unit from the irradiation of ° Pb to the irradiation of actinides to make the same heavy-element product. In practice, the reaction mechanisms are distinct because of a lack of suitable target nuclides between ° Bi (Z = 83) and Ra (Z = 88). However, reactions involving the fusion of " Ca ions with actinide target nuclei probe an intermediate reaction mechanism. 

Moller, P., Nix, J.R., Armbruster, P., Hofmann, S., Milnzenberg, G. Single-particle enhancement of heavy-element production. Z. Phys. A359, 251-255 (1997)... 

Gr. technetos, artificial) Element 43 was predicted on the basis of the periodic table, and was erroneously reported as having been discovered in 1925, at which time it was named masurium. The element was actually discovered by Perrier and Segre in Italy in 1937. It was found in a sample of molybdenum, which was bombarded by deuterons in the Berkeley cyclotron, and which E. Eawrence sent to these investigators. Technetium was the first element to be produced artificially. Since its discovery, searches for the element in terrestrial material have been made. Finally in 1962, technetium-99 was isolated and identified in African pitchblende (a uranium rich ore) in extremely minute quantities as a spontaneous fission product of uranium-238 by B.T. Kenna and P.K. Kuroda. If it does exist, the concentration must be very small. Technetium has been found in the spectrum of S-, M-, and N-type stars, and its presence in stellar matter is leading to new theories of the production of heavy elements in the stars. 

Fig. 1. Nuclear reactions for the production of heavy elements by intensive slow neutron irradiation. The main line of buildup is designated by heavy...

It is possible to prepare very heavy elements in thermonuclear explosions, owing to the very intense, although brief (order of a microsecond), neutron flux furnished by the explosion (3,13). Einsteinium and fermium were first produced in this way they were discovered in the fallout materials from the first thermonuclear explosion (the "Mike" shot) staged in the Pacific in November 1952. It is possible that elements having atomic numbers greater than 100 would have been found had the debris been examined very soon after the explosion. The preparative process involved is multiple neutron capture in the uranium in the device, which is followed by a sequence of beta decays. Eor example, the synthesis of EM in the Mike explosion was via the production of from followed by a long chain of short-Hved beta decays,... 

Production in Fission of Heavy Elements. Tritium is produced as a minor product of nuclear fission (47). The yield of tritium is one to two atoms in 10,000 fissions of natural uranium, enriched uranium, or a mixture of transuranium nucHdes (see Actinides and transactinides Uranium). 

Lead (13 ppm) is by far the most abundant of the heavy elements, being approached amongst these only by thallium (8.1 ppm) and uranium (2.3 ppm). This abundance is related to the fact that 3 of the 4 naturally occurring isotopes of lead (206, 207 and 208) arise primarily as the stable end products of the natural radioactive series. Only (1.4%)... 

The H and He produced in the Big Bang served as "feed stock" from which all heavier elements were later created. Less than 1% of the H produced in the Big Bang has been consumed by subsequent element production and thus heavy elements are rare. Essentially all of the heavier elements now in the Earth were produced after the Big Bang inside stars. Following the Big Bang, the universe expanded to the point where instabilities formed galaxies, mass concentrations from which up to stars could develop. 

Figure 1.1 also gives a schematic illustration of the complex interactions between the ISM and stars. Stars inject energy, recycled gas and nuclear reaction products ( ashes of nuclear burning ) enriching the ISM from which other generations of stars form later. This leads to an increase in the heavy-element content of both the ISM and newly formed stars the subject of galactic chemical evolution (GCE) is really all about these processes. On the other hand, nuclear products may... 

Fig. 5.14. Element production in winds and supernova ejecta from stars affected by strong mass loss, as a function of initial mass. Upper panel stars with about 1/20 solar heavy-element abundance. Lower panel stars with approximately solar composition, for which the effects of mass loss are believed to be more drastic. Horizontal shadings indicate outer layers that are expelled in winds prior to SN explosion. After Maeder (1992).

The introduction of EU directives on Waste Electrical and Electronic Equipment and Reduction of Hazardous Substances has highlighted the need for precise and repeatable elemental analysis of heavy metals in the plastics production process. X-ray fluorescence (XRF) spectroscopy has emerged as the most economical and effective analytical tool for achieving this. A set of certified standards, known as TOXEL, is now available to facilitate XRF analyses in PE. Calibration with TOXEL standards is simplified by the fact that XRF is a multi-element technique. Therefore a single set of the new standards can be used to calibrate several heavy elements, covering concentrations from trace level to several hundred ppm. This case study is the analysis of heavy metals in PE using an Epsilon 5 XRF spectrometer. 

Inclusions of the CV3 led to the search for isotopic signatures of individual nucleosynthetic processes, or at least for components closer to the original signature than average solar compositions. They have also begun to demonstrate the isotopic variability of matter emerging from these processes in agreement with astrophysical and astronomical expectations. The principal features of inclusions are an up to 4% 0 enriched reservoir in the early solar system, variations in a component produced in a nuclear neutron-rich statistical equilibrium, and variations in the contribution of p- and r-process products to the heavy elements. 

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. 

Dr. Darleane C. Hoffman of the Nuclear Science Division of the Lawrence Berkeley National Laboratory and Department of Chemistry at the University of California at Berkeley has written and presented several papers documenting her work and that of her team on the laboratory production of transactinide and actinide elements one-atom-at-a-time. She explains the difficulty of determining the chemistry of heavy elements How long does an atom need to exist before it s possible to do any meaningful chemistry on it Is it possible to learn anything at all about the reactions of an element for which no more... 

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