Recoil technique

Harvey, B. G. Recoil Techniques in Nuclear Reactions and Fission Studies, Ann. Rev. Nucl. Sci. 10, 235 (1960). 

The amount of Es available at this time was very small about N = 10 atoms (a 4- 10 g). At a flux density of a particles = 10 " cm s , a cross section (Ta n = 1 mb and an irradiation time of 10 s a yield N

single atoms, the recoil technique was applied (Fig. 14.6). Es was electrolytically deposited on a thin gold foil. The recoiling atoms of Md were sampled on a catcher foil. After irradiation, the catcher foil was dissolved and Md was separated on a cation-exchange resin. In 8 experiments 17 atoms of Md were detected and identified by their transmutation into the spontaneously fissioning the properties of which were known ... 

In these experiments, the recoil technique was modified into a double recoil technique by application of a moving belt (Fig. 14.7). The recoiling atoms generated by the heavy-ion reaction (first recoil) are deposited on the belt and transported along a catcher foil on which the recoiling atoms from a decay (second recoil) are collected. From the activity recorded as a function of the distance, the half-life can be calculated. 

Figure 14.7. Separation of transuranium elements by means of the double-recoil technique (schematically).

The recoil technique is capable of yielding angular distributions with velocity-selected targets. In addition it is particularly useful in studying the change of spin states in a scattering process, since it is easier to analyse the spin states of scattered atoms than those of electrons. Such measurements are not discussed here but the reader is referred to the articles by Bederson (1968) and Bederson and Kieffer (1971). 

Recoil techniques have been used extensively in the synthesis of higher transuranium elements. As an example, consider the formation of element 103, lawrencium (Fig. 15.3). The nuclides, which were formed in the reaction in the target between Cf and projectiles of recoiled out of the thin target into the helium gas where they were stopped by atomic collisions with the He-atoms. These recoil species became electrically charged cations as they lost electrons in the collision with the gas atoms. They could, therefore, be attracted to a moving, negatively charged, metal coated plastic band, which... 

They assigned an 8.8 0.5 MeV a-decay with a half-life of 2 — 40 s, to element 102. However, their results have been contested by scientists at the Lawrence Berkeley Laboratory (LBL) in California, who produced and identified elem t 102 in 1958 by use of a double-recoil technique The reaction scheme was... 

In all cases the product radioactivity was isolated by recoil techniques. 

The most important mechanistic clue for any chemical reaction is the structure of the reaction products, and a great strength of the recoil technique is that there is no more convenient or effective way to determine what products are formed from a polyvalent atom and a chosen reaction substrate than by examining its recoil chemistry. The harshness of the conditions required to liberate polyvalent atoms chemically severely limits the study of their reactions by other methods (vide infra). 

Atoms. Strictly speaking, all the atoms formed via the nuclear recoil methods are unusual species because they all possess kinetic energies far in excess of those that could be formed by conventional chemical means (5-12). On the qualitative side, such hot atoms will open various reaction channels that are not possible for their lower energy counterparts. On the quantitative side, the cross sections of various reactions for the hot atoms are normally very different from those for the thermal atoms. An additional unique feature of the atoms formed via nuclear recoil techniques is that they invariably include radioactive isotopes and, therefore, they are capable of functioning as tracers for their subsequent interactions. 

Recently Caspar and coworkers pr ared 1,1,1,2-tetramethyldisilane, CH3 SiH2Si(CH8)8, from the reactions of recoil Si atoms with trimethyl-silane 64). This novel compound was first synthesized with such nuclear recoil techniques. Two possible mechanisms could be proposed for its formation. The first is through a CH8 group abstraction by Si ions followed by ion-molecule interactions. Another possibiUty involves the insertion of Si atoms into the Si—bond of trimethylsilane followed by unimolecular decomposition of the intermediate into two silylenes, CH8 SiH and Si(CH8)2- The insertion of CH8 SiH into the Si—bond of another trimethylsilane molecule will result in the formation of this unusual compound ... 

In maintaining the narrow focus of this chapter, several important topics have been omitted. The caged recombination reactions that occur in condensed phase recoil experiments exhibit unusual dynamical characteristics (11,26,28,31,35,96). The energetic F-to-HF process shown in Equation 9 has been particularly important from a theoretical standpoint (22,24,29,30,32,34-37,39,40,43,44,49,52). The moderated nuclear recoil technique has provided accurate kinetic parameters for thermal F reactions (24,42,46,52-65,60) that have facilitated the recent development of a realistic potential extended collision theory for activationless bimolecular reactions (42,103),... 

This chapter summarizes many of the contributions that the recoil technique of generating excited radiotracer atoms in the presence of a thermal environment is making to the field of chemical dynamics. Specific topics discussed critically include characterization of the generation and behavior of excited molecules including fragmentation kinetics and energy transfer, measurement of thermal and hot kinetic parameters, and studies of reaction mechanisms and stereochemistry as a function of reaction energy. Distinctive features that provide unique approaches to dynamical problems are evaluated in detail and the complementarity with more conventional techniques is addressed. Prospects for future applications are also presented. 

A discussion of recent published results on the cyclobutane reaction illustrates the kind of excited molecule dynamical information available from recoil techniques. The primary assumption in the approach reported is that the RRKM (Rice-Ramsperger-Kassel-Marcus) method for describing statistical energy redistribution and calculating decomposition rate constants is valid. TTius, deviations from expected RRKM behavior... 

Important contributions to each of these areas are possible due to specific features of the nuclear recoil technique. Truly isolated molecules are formed and can be studied efficiently because they are labeled with radioactive nuclides. In addition, extra energy available from recoil can be degraded or used effectively for chemical purposes, thus contributing a dynamic species in a stationary relative environment. 

The stereochemistry of certain thermal reactions involving atom addition processes and resultant radical intermediates can also be studied conveniently by recoil techniques. In this type of experiment the recoil atom used to initiate the process must be thermalized before reaction, by diluting the system with a large concentration of nonreactive bath molecules. Two recently reported results on geometric isomerization serve as examples of the type of processes that can be studied (55,56). Both involve recoil chlorine atom reactions with unsaturated substrates. 

Limitations of the Moderated Nuclear Recoil Technique for Investigating Thermal Hydrogen Abstraction Reactions by Atomic Fluorine... 

Wang CS, Bai SJ, Rice BP, Axial compressive strengths of high performance fibres by tensile recoil technique, Polym Mater Sci Eng, 61, 550, 1989. 

The first report of the discovery of element 102 came from the Nobel Institute, Stockholm, Sweden. The research group used the reaction of Cm + in which they found a new a emitter with a half-life of about 10 min. However, experiments neither at Berkeley nor at the Kurchatov Institute in Moscow confirmed the above result. In 1958, Ghiorso et al. (1958) announced the positive identification of No that was supposedly produced in the following reaction by use of the double recoil technique ... 

Fm, the daughter of the new element, was collected using the recoil technique, one atom at a time, and identified as Fm by their position in the cation-exchange elution curve. A half-life of 3 s was assigned to No at that time. However, it is now known that the 3 s radioactivity was No produced in the Cm ( C,4n) reaction the used target contained 20 times more Cm than Cm. No is now known to have 55 s half-life. The errors in this experiment indicate the difficulty associated with one-atom-at-a-time studies. In subsequent chemical experiments, it was found that the most stable oxidation state of element 102 in solution was 2+. The element was named nobelium after Alfred Nobel. 

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