Spectroscopy of Radioactive Elements

The first group comprises high-resolution laser spectroscopy of short-lived radioactive isotopes with lifetimes in the millisecond range. The ions are produced by nuelear reactions induced by bombardment of a thin foil with neutrons, protons, y-quanta, or other particles inside the ion source of a mass spectrometer. They are evaporated and enter after mass selection the interaction zone of the collinear laser [466]. 

Precision measurements of hyperfine structure and isotope shifts yield information on nuclear spins, quadrapole moments, and nuclear deformations. The results of these experiments allow tests of nuclear models of the spatial distribution of protons and neutrons in highly deformed nuclei [467]. In Fig. 4.8 the hyperfine spectra of different Na isotopes are depicted, which had been produced by spallation of aluminum nuclei by proton bombardment according to the reaction Al(/ , 3p,xn) Na [468] and in Fig. 4.29 the hfs of 6 isotopes of the titanium ion H+ illustrates the good signal-to-noise ratio. Such precision measurements have been performed in several laboratories for different families of isotopes [466-469]. 

Besides excitation spectroscopy of bound-bound transitions, photofragmentation spectroscopy has gained increasing interest. Here predissociating upper levels of parent molecular ions M , which decay into neutral and ionized fragments, are 

For illustration, Fig. 4.30 shows the number of ions formed in the photodissociation reaction 

With a properly selected polarization of the laser, the photofragments are ejected into a direction perpendicular to the ion beam direction. Their transverse energy distribution can be measured with a position-sensitive detector, because their impact position X, y at the ion detector centered around the position x = y = 0 of the parent ion beam is given by x = Vx/Vz)z, where z is the distance between the excitation zone and the detector [471, 472]. 

The first group comprises high-resolution laser spectroscopy of short-lived radioactive isotopes with lifetimes in the millisecond range. The ions are 

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The reaction temperatures and some of the activation energies cited above seem to be too low to support a radical-chain reaction mechanism. Guryanova found that exchange of radioactive elemental sulfur with the p sulfur atoms of bis-p-tolyl tetrasulfide proceeds at 80-130 °C with an activation energy of only 50 kJ/mol in the case of the corresponding trisulfide the activation energy was determined as 60 kJ/mol. These data sharply contrast with the observation that liquid sulfur has to be heated to more than 170 °C to detect free radicals by electron spin resonance spectroscopy and the activation energy for homolytic SS bond scission has been determined as 150 kJ/mol (see above). 

For the analysis of the new surface after every removal one may use all the surface techniques already mentioned in Sect. 4.3.1 as long as their information depth does not exceed the thickness of the layer removed Auger and ESCA-spectroscopy, secondary-ion mass spectrometry (SIMS), backscattering, ion-induced X-ray and nuclear reaction analysis. In addition, one may investigate the content of the element of interest in the removed layer. Because of the low absolute concentration of implanted ions most of the standard methods of analysis fail. The best results come from implantations of radioactive elements followed by measuring the radioactivity of the dissolved removed layer. 

The degree of activation of the sample is measured by post-irradiation spectroscopy, usually performed with high-purity semiconductors. The time-resolved intensity measurements of one of the several spectral lines enables to get the half-life of the radioactive element and the total number of nuclear reactions occurred. In fact, the intensity of a given spectral line associated with the decay of the radioactive elements decreases with time as Aft) = Aoexp[—t/r], where Aq indicates the initial number of nuclei (at t = 0) and r is the decay time constant related to the element half-life (r = In2/ /2), which can be measured. Integrating this relation from t = 0 to the total acquisition time, and weighting it with the detector efficiency and natural abundance lines, the total number of reactions N can be derived. Then, if one compares this number with the value obtained from the convolution of... 

Joseph Lockyer (1836-1920) was one of the pioneers of solar spectroscopy. In examining the spectra of solar prominences in 1869, Lockyer noticed an absorption line that he could not identify. Reasoning that it represented an element not present on Earth, he proposed a new element - helium, from the Greek word helios for Sun. This idea failed to achieve acceptance from Lockyer s scientific colleagues until a gas having the same mysterious spectral line was found 25 years later in rocks. The helium in terrestrial uranium ore formed as a decay product of radioactive uranium. Thus, this abundant element was first discovered in the Sun, rather than in the laboratory. Lockyer s cosmochemical discovery was recognized by the British government, which created a solar physics laboratory for him. Lockyer also founded the scientific journal Nature, which he edited for 50 years. 

Several groups at ISOLDE are planning further improvements of their techniques. For each element the most appropriate experimental scheme has to be found. Today, collinear laser spectroscopy is the most general high-resolution and sensitive method for optical spectroscopy on radioactive beams delivered by on-line mass separators. Its sensitivity ranges from 10 - 10 atoms/s depending on the strength and multiplicity of the optical transitions. 

This new kind of tool to find the missing elements was celebrated by Segre at the beginning of one of his papers. [46] He says that every time physics has provided chemistry with a new tool, it allowed the discovery of new elements, and he lists radioactivity, spectroscopy, and now the cyclotron. Nevertheless, the proof of the existence of and the actual detection of the isotopes of element 43 is first of all chemical, very much in the same way as Ida had criticized Fermi s conclusion in 1934. Segre even published the radioactive part in another journal [47]... 

This technique is about 1000 times more sensitive than infrared and magnetic resonance spectroscopy, outlined later. It can also be used to detect different masses of radioactive and nonradioactive isotopes of elements. It can distinguish between the carbon 12 and 13 isotopes and also the oxygen 16 and 18 isotopes. By... 

The methods range from simple, inexpensive absorption spectroscopy to sophisticated tunable-laser-excited fluorescence and ionization spectroscopies. AAS has been used routinely for uranium and thorium determinations (see for example Pollard et al., 1986). The technique is based on the measurement of absorption of light by the sample. The incident light is normally the emission spectrum of the element of interest, generated in a hollow-cathode lamp. For isotopes with a shorter half life than and Th, this requires construction of a hollow-cathode lamp with significant quantities of radioactive material. Measurement of technetium has been demonstrated in this way by Pollard et al. (1986). Lawrenz and Niemax (1989) have demonstrated that tunable lasers can be used to replace hollow-cathode lamps. This avoids the safety problems involved in the construction and use of active hollow-cathode lamps. Tunable semiconductor lasers were used as these are low-cost devices. They do not, however, provide complete coverage of the spectral range useful for AAS and the method has, so far, only been demonstrated for a few elements, none of which were radionuclides. 

Gamma-ray, X-ray, optical and infrared line spectroscopy of SNe and SNRs have been used to observe nucleosynthesis and the abundance distribution of elements freshly synthesized (both radioactive isotopes and stable decay products) and to extract dynamical information about the explosion. In particular the radioactive isotopes provide unique tracers of nucleosynthetic processes (what, where and how much) and its related dynamics. The best examples are the observations of gamma-ray lines in supernovae, but X-ray line spectroscopy of decay products of radioactive nuclei have also been attempted, and specific elements in numerous SNRs and a few SNe identified. 

Basic nuclear science includes the synthesis of radionuclides, production of new elements, generation of radioactive and exotic nuclear beams, determination of nuclear properties, and applications of nuclear spectroscopy. 

The new element was named radium from the Latin radius meaning ray. The birthday of radium was December 26,1898. when the members of the Paris Academy of Sciences heard a report entitled On a new highly radioactive substance contained in pitchblende . The authors reported that they had managed to extract from the uranium ore tailings a substance containing a new element whose properties are very similar to those of barium. The amount of radium contained in barium chloride proved to be sufficient for recording its spectrum. This was done by the well-known French spectral analyst E. Demarcay who found a new line in the spectrum of the extracted substance. Thus, two methods—radiometry and spectroscopy—almost simultaneously substantiated the existence of a new radioactive element. 

One of the new fields of actinide chemistry since the 1990s is an application of X-ray absorption spectroscopy (Teo 1986) especially by use of the synchrotron radiation. XAFS (X-ray absorption fine structure) is a powerful technique for characterization of the local structure of specific elements, even radioactive elements such as actinides, and their electronic states. XAFS contains two fundamental information, EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near-edge structure). O Figure 18.27 shows a fundamental pictorial view of the XAFS process (Koningsberger and Prins 1987). 

Step 2. After irradiation and appropriate radioactive decay of the sample and standards, these are measured on either a well-t3q)e or a coaxial large volume Ge(Li) y-ray detector system coupled to a PC-based y-spectroscopy system (explained in Sect. 6.3.4) to find out the energies of the y-rays in the observed spectrum (and hence the corresponding isotopes of the elements present in the sample). The distance of the sample fi om the detector (and hence the sohd angle) is adjusted depending on the coimt rate which should be about 100-500 counts per second. 

In his career, before the discovery of europium in 1901, he had developed spectroscopy in order to make it more effective for the identification of RE metals. He used a special induction coU, which generated a very high spark temperature. The material of the electrodes was especially pure platinum. Thus all foreign spectral lines except those of platinum were eliminated. With this spectroscope Demarfay made a special contribution to science. Pierre Curie had brought to him a sample of barium, in which lines of radium could be observed for the first time (see Chapter 52 The Radioactive Elements). After the discovery of europium Demarfay had only a few years to live. He died in 1903. In ref. [17.19] James and Virginia Marshall have described the life of Demarfay and his discovery of europium. 

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