Heavy-Element Collisions

Figure 4.19. The LEIS spectrum of a CU/AI2O3 catalyst illustrates that Ions lose more energy in collisions with light elements than with heavy elements. Note the step In the background at the low kinetic energy side of...

Although much of the preceding discussion involved the synthesis of new molecules by organic and inorganic chemists, there is another area of chemistry in which such creation is important—the synthesis of new atoms. The periodic table lists elements that have been discovered and isolated from nature, but a few have been created by human activity. Collision of atomic particles with the nuclei of existing atoms is the normal source of radioactive isotopes and of some of the very heavy elements at the bottom of the periodic table. Indeed nuclear chemists and physicists have created some of the most important elements that are used for nuclear energy and nuclear weapons, plutonium in particular. 

Figure 4.19 The LEIS spectrum of a Cu/Al203 catalyst illustrates that ions lose more energy in collisions with light elements than with heavy elements. Note the step in the background at the low kinetic energy side of the peaks. The high peak at low energy is due to sputtered ions. The low energy cut-off of about 40 eV is indicative of a positively charged sample (courtesy of J.P. Jacobs and H.H. Bron-gersma, Eindhoven).

As indicated above, a combination of reactor and cyclotron irradiations is used to prepare most radionuclides. While many of these radionuclides are available commercially, some are not. In addition, nuclear structure, nuclear reactions, and heavy-element research require accelerator or reactor irradiations to produce short-lived nuclei or to study the dynamics of nuclear collisions, and so on. One of the frequent chores of radiochemists is the preparation of accelerator targets and samples for reactor irradiation. It is this chore that we address in this section. 

Heavy elements can also be produced in particle accelerators, which accelerate ions to high speeds, causing collisions that generate the new elements. Technetium, for example, is not found in nature but was first produced in 1937 when high-energy deuterons were directed at a molybdenum source ... 

Theoretical and experimental investigations of relativistic and QED effects in atomic physics and chemistry have increased continuously during the last decade. As a consequence of this interest in various relativistic phenomena and in their empirical manifestations a diverse field of research has developed linking together widespread activities ranging from high-energy heavy-ion collision physics, atomic or molecular physics and chemistry of heavy elements to solid-state physics. 

Some elements do not occur naturally, but can be synthesized. They can be produced in nuclear reactors, from collisions in particle accelerators, or can be part of the fallout from nuclear explosions. One of the elements most commonly made in nuclear reactors is technetium. Relatively large quantities are made every day for applications in nuclear medicine. Sometimes, the initial product made in an accelerator is a heavy element whose atoms have very short half-lives and undergo radioactive decay. When the atoms decay, atoms of elements lighter than the parent atoms are produced. By identifying the daughter atoms, scientists can work backward and correctly identify the parent atoms from which they came. 

The cycle of birth and death of stars that is initiated by population III stars constantly increases the abundance of heavy elements in the interstellar medium, a crucial prerequisite for terrestrial (rocky) planet formation and subsequently for the origin of life (75). Metals dispersed in the interstellar gas or incorporated into micron-sized dust particles and molecules like CO and water, have the ability to cool the interstellar gas much more efficiently than molecular hydrogen does for population III stars. These elements and molecules are also excited through atomic and molecular collisions and their return to lower lying energy levels releases energy via far-infrared and sub-millimeter radiation below... 

Research Chemist Some nuclear chemists specialize in studying the newest and heaviest elements. To produce heavy elements, a nuclear chemist works with a large team, including physicists, engineers, and technicians. Heavy elements are produced by collisions in a particle accelerator. The nuclear chemist analyzes the data from these collisions to identify the elements and understand their properties. For more information on chemistry careers, visit glencoe.com. 

Values for AXj) are low for heavy elements (e.g. Au(II) at 200.08 nm has a value of 0.8 pm, whereas AXj) values are high for light elements such as Be(II) at 313.11 nm that has a value of 5.9 pm. Collisional broadening results due to collisions among analyte ions, atoms, and neutral Ar atoms and has also been called pressure broadening. Doppler broadening is dominant near the center of the band, whereas collision broadening dominates near the tails. AE linewidths dictate what resolution is needed to resolve one AE emission line from another. For each transition metal, there is a plethora of lines to consider. 

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 in ISS, elemental compositional analysis is achieved by measuring the energy losses suffered by light primary ions in single binary collisions. Although in ISS the primary species is usually He, in RBS either H or He is used exclusively, and at very much higher energies, typically 1 -2 MeV. Consequently, due to the penetration of the primary ion, both outer atomic layers and subsurface layers of a material are accessible the surface sensitivity comes when the material in the outer few monolayers consists of heavy elements. 

Riedel, C., Norenberg, W. Theoretical estimates for the production of transuranium elements in heavy-ion collisions. Z. Phys. A290, 385-391 (1979)... 

Photoionization detection in a buffer gas has also been used to study the properties of superheavy (transuranium) elements with charge numbers Z > 92. Isotopes of such elements can only be produced by fission reactions in heavy-ion collisions or by transfer reactions using radioactive targets. The elements produced can be placed in an optical buffer-gas cell for the purpose of laser resonance photoionization spectroscopy. This was successfully demonstrated with atoms of such radioactive elements as americium (Z = 95) (Backe et al. 2000), einsteinium (Z = 99) (Kohler et al. 1997), and fermium (Z = 100) (Sewtz et al. 2003). 

A related technique is Auger emission spectroscopy (AES). Here, the surface is bombarded by a beam of electrons of known energy (20 eV-2 keV). The bombardment ejects an electron, whose collision with neighbors emits a photon, which finally causes ejection of yet another electron, referred to as an Auger electron. Usually K- and L-shell electrons are involved. The technique is very useful with heavy elements, and could be used to identify Pb, As, Ba, among others, on composite surfaces (as it is now used with semiconductor surfaces). AES can penetrate to no more than 50 A (5 nm). Suitable equipment is supplied by, among others, JEOL. 

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