X-ray spectra of elements

Nor was Mendeleev s revolutionary Periodic Table a help. When he first published his Periodic Table in 1869, he was able to include only lanthanum, cerium, didymium (now known to have been a mixture of Pr and Nd), another mixture in the form of erbia, and yttrium unreliable information about atomic mass made correct positioning of these elements in the table difficult. Some had not yet been isolated as elements. There was no way of predicting how many of these elements there would be until Henry Moseley (1887-1915) analysed the X-ray spectra of elements and gave meaning to the concept of atomic number. He showed that there were 15 elements from lanthanum to lutetium (which had only been identified in 1907). The discovery of radioactive promethium had to wait until after World War 2. 

Henry Moseley s research career lasted only forty months before tragically ending with his death on a Gallipoli battlefield in World War I. But in his classic study of the x-ray spectra of elements, he established the truly scientific basis of the Periodic Table by arranging chemical elements in the order of their atomic numbers. 

Moseley s law Lines in the x-ray spectra of elements have frequencies that depend on the proton number of the element. For a set of elements, a graph of the square root of the frequency of x-ray emission against proton number is a straight line (for spectral lines corresponding to the same transition). The law is named for the British physicist Henry Gwyn Jeffreys Moseley (1887-1915). 

As several of the Gottingen physicists who were exposed to these ideas by Bohr s own lectures later commented, the work rested on a mixture of ad hoc arguments and chemical facts without any strict derivations from the principles of quantum theory to which Bohr frequently alluded. As Kragh writes, it was realized in 1922 that Bohr s theory was not deductive, although admittedly Bohr drew on the observed X-ray spectra of elements which were interpreted with the aid of quantum theory. 

Whereas zirconium was discovered in 1789 and titanium in 1790, it was not until 1923 that hafnium was positively identified. The Bohr atomic theory was the basis for postulating that element 72 should be tetravalent rather than a trivalent member of the rare-earth series. Moseley s technique of identification was used by means of the x-ray spectra of several 2ircon concentrates and lines at the positions and with the relative intensities postulated by Bohr were found (1). Hafnium was named after Hafma, the Latin name for Copenhagen where the discovery was made. 

I9l 3 H. G. J. Moseley observed regularities in the characteristic X ray spectra of the elements he thereby discovered atomic numbers Z and provided justification for the ordina] sequence of the dements. 

The very close similarity between the x-ray spectra of the different elements shows that these radiations originate... 

It has always been difficult to do quantitative work with the characteristic x-ray lines of elements below titanium in atomic number. These spectra are not easy to obtain at high intensity (8.4), and the long wavelength of the lines makes attenuation by absorption a serious problem (Table 2-1). The use of helium in the optical path has been very helpful. The design of special proportional counters, called gas-flow proportional counters,20 has made further progress possible, and it is now possible to use aluminum Ka (wavelength near 8 A) as an analytical line (8.10). 

One problem with Mendeleev s table was that some elements seemed to be out of place. For example, when argon was isolated, it did not seem to have the correct mass for its location. Its relative atomic mass of 40 is the same as that of calcium, but argon is an inert gas and calcium a reactive metal. Such anomalies led scientists to question the use of relative atomic mass as the basis for organizing the elements. When Henry Moseley examined x-ray spectra of the elements in the early twentieth century, he realized that he could infer the atomic number itself. It was soon discovered that elements fall into the uniformly repeating pattern of the periodic table if they are organized according to atomic number, rather than atomic mass. 

The X-ray-spectra of low Z elements clearly show (Fig. 3.25) that detection of XRF-lines down to 1 keV is possible with MIMOS IIA. 

Now if either the elements were not characterized by these integers, or any mistake had been made in the order chosen or in the number of places left for unknown elements, these regularities would at once disappear . We can therefore conclude from the evidence of the X-ray spectra alone, without using any theory of atomic structure, that these integers are really characteristic of the elements. Further, as it is improbable that two different stable elements should have the same integer, three, and only three, more elements are likely to exist between Al and Au. As the X-ray spectra of these elements can be confidently predicted, they should not be difficult to find. The examination of keltium would be of exceptional interest, as no place has been assigned to this element. (For keltium see footnote1)... 

All impactor and filter samples were analyzed for up to 45 elements by instrumental neutron activation analysis (INAA) as described by Heft ( ). Samples were irradiated simultaneously with standard flux monitors in the 3-MW Livermore pool reactor. The x-ray spectra of the radioactive species were taken with large-volume, high-resolution Ge(Li) spectrometer systems. The spectral data were transferred to a GDC 7600 computer and analyzed with the GAMANAL code (1 ), which incorporates a background-smoothing routine and fits the peaks with Gaussian and exponential functions. 

Courtesy of Lyman C. Newell Henry Gwyn Jeffreys Moseley, 1887-1915. English physicist whc studied the X-ray spectra of more than fifty elements and discovered the relation existing between the atomic number of an element and the frequency of the X-rays which it emits when bombarded by cathode rays. At the age of twenty-seven years he was killed while in active service at the Dardanelles. 

Figure 5. High-voltage electron micrographs and x-ray spectra of microparticles collected at Whiteface Mountain in 1983. Scale bars equal 0.5 pm. (a) y-Fe203 sphere collected 28 July II. (b) y-Fe203 sphere cluster collected 28 July IV. (c) Mullite sphere collected 28 July IV. (d). Mixed-element sphere collected 28 July III.

In order to verify the supposed presence of elements around Z=126, monazite inclusions were irradiated with a sharply collimated proton beam and the proton induced x-ray spectra of the elements were recorded. As can be seen in Figure 10, two well separated groups of strong peaks appear [51], the L x-rays of uranium and thorium, and the K x-rays of the lanthanide elements. In between, from about 24 to 29 keV energy, much weaker peaks are identified and assigned to the La[ x-rays of elements 126, 116 and 124. 

Moseley s Table of Atomic Numbers had places for all of these fifteen elements. They fitted beautifully into spaces 57 through 71. His work on the X-ray spectra of the elements had settled once and for all time the position and number of the rare earths. This in itself was a remarkable achievement. 

Main analytical lines in the X-ray spectra of rare earth elements. 

Within the last 25 years of X-ray spectroscopy on fusion devices, the theory of He-like ions has been developed to an impressive precision. The spectra can be modeled with deviations not more than 10% on all lines. For the modeling, only parameters with physical meaning and no additional approximation factors are required. Even the small effects due to recombination of H-like atoms, which contribute only a few percent to the line intensity, can be used to explain consistently the recombination processes and hence the charge state distribution in a hot plasma. The measurements on fusion devices such as tokamaks or stellarators allow the comparison to the standard diagnostics for the same parameters. As these diagnostics are based on different physical processes, they provide sensitive tests for the atomic physics used for the synthetic spectra. They also allow distinguishing between different theoretical approaches to predict the spectra of other elements within the iso-electronic series. The modeling of the X-ray spectra of astronomical objects or solar flares, which are now frequently explored by X-ray satellite missions, is now more reliable. In these experiments, the statistical quality of the spectra is limited due to the finite observation time or the lifetime of... 

Soon afterwards, Moseley wrote a second paper on the X-ray spectra of the elements, in which he stated that every element from aluminium to gold (which marked the boundaries of his studies) was characterized by an integer N which determined its X-ray spectrum. This integer N was the atomic number of the element and it was identified with the number of... 

Recognition of the fact that elements always displayed the same chemical behavior - regardless of their isotopic composition - led to a reformulation of the periodic law. The idea that each element was characterized by a unique number had already been demonstrated experimentally by Hemy Moseley (1887-1915). By studying the X-ray diffraction patterns produced by a variety of elements, he discovered that the frequencies of the K lines differed from element to element in a predictable and consistent fashion. He went on to show that the frequency of any line in the X-ray spectrum is approximately proportional to A(N-b), where A and b are constants and N is an integer that he termed the atomic number of the element. Moseley was able to identify the number N with the number of protons in the atomic nucleus. Plots of the square root of the frequency for the K and L lines in the X-ray spectra of the elements versus their atomic number, reproduced in Figure 5, show almost straight lines. From this work, it became clear why the order in which certain element pairs appeared in the periodic table needed to be reversed. The pairs in question are argon (39.95) and potassium (39.10) cobalt (58.93) and nickel (58.69) and tellurium (127.60) and iodine (126.91), the... 

About 40 years after Mendeleev published his periodic table, an English chemist named Henry Moseley found a different physical basis for the arrangement of elements. When Moseley studied the lines in the X-ray spectra of 38 different elements, he found that the wavelengths of the lines in the spectra decreased in a regular manner as atomic mass increased. With further work, Moseley realized that the spectral lines correlated to atomic number, not to atomic mass. 

Recently, Bergmann et al. (37) measured the A -capture x-ray spectra of Fe metal and FcjO, with high-energy-resolution crystal spectrometer and compared them with the x-ray excited spectra for Mn metal and MnO. Unfortunately they did not evaluate the K0/Ka ratio, but demonstrated the difference in the peak shapes for K0 spectra between two excitation modes. Considering these facts, future experimental studies on the K0 /K a ratio for 3d elements should be performed with high-resolution spectrometers, or at least with careful data analysis of the SSD spectra. 

Moseley s work was based on X-ray diffraction studies that revealed an even more powerful proof of the unity of the elements than Mendeleev s table of chemical characteristics. Moseley had worked with Ernest Rutherford (1871-1937) on radioactivity, but in 1912 he decided to use X-ray diffraction to examine the characteristics of the elements. He received training from Lawrence Bragg (1890-1971), the world s leading X-ray diffraction expert, and in 1913 began charting the X-ray spectra of the metallic elements. 

These elements possess well-developed multiplet structure in the soft X-ray spectra of the condensed phase, which can be compared with the spectrum of the free atom by performing ah initio Dirac-Fock calculations, the actual 4/ occupancy in the solid can be deduced. An example is shown in fig. 11.4 The first point to note is that the multiplet structure of the atom survives in the solid, because of the strong localisation of the 4/ electrons (see section 5.6), so that soft X-ray spectroscopy provides a... 

Bohr has made this conception more precise by assuming that, in the group So to Ni, the series of the 3 - and 32-orbits are completed by 3,-orbits. We shall consider later how such a completion of inner groups can occur for the present it may be mentioned that the interpretation of the complex spectra of Sc to Ni2 fully confirm this assumption. The appearance of the last M-term in the X-ray spectra of Cu (cf. fig. 16, p. 179) shows that 33-orbits are actually present in the interior of the atoms of the following elements. The 33-orbits in the core do not prevent the existence of excited 33-orbits in the exterior, as the table on p. 190 shows for Cu, Zn, Ga, lib. 

Almost precisely this relation was found by Moseley for the X-ray spectra of the elements. All save the lightest emit X-ray spectra on bombardment with cathode rays. The lines of these spectra form various series. The lines of highest frequency for a given element are the lines, and for these Moseley found the relation... 

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