Non-destructive, position-selective, and multi-elemental analysis method involving negative muons

Abstract

Many elemental analysis methods that utilize various probes have been developed till date in several research fields. Among them, non-destructive analysis methods are particularly useful, although these have proven to have accuracies inferior to destructive analysis methods in most cases. The usefulness of non-destructive methods stems from the fact that a sample can be utilized for further processes after a round of non-destructive analysis. Therefore, non-destructive methods are advantageous for the analysis of valuable samples. The development of an elemental analysis method that enables the analysis of all elements in a given sample, even bulk samples with high positional sensitivity, is a major goal for analytical scientists at present. Recently, a new elemental analysis method that satisfies these conditions by utilizing a negative muon (MIXE: muon induced x-ray emission) has been developed and applied in several studies. A negative muon is an elementary particle, and intense muon beams generated in an accelerator are used in many studies. With recent advancement in accelerator technology to produce intense muon beams, it has become possible to use muons for practical elemental analyses. A muon’s mass is about 207 times that of an electron (105.658 MeV/c). When a muon is injected into a substance, it gradually loses its kinetic energy due to its interaction with electrons and finally comes to a stop. Because a negative muon has the same charge as an electron, it binds with nuclei. An atom in which a negative muon has replaced an electron is called a muonic atom. The binding energies of a muon in the orbital states of a muonic atom are about 200 times higher than those of an electron with the corresponding principal quantum number. Although much research has been conducted on the muon capture process in an atom, the process is still not completely understood. During the initial stages of muonic atom formation, the captured muons are highly excited. However, then, because there is only one muon in a muonic atom, and all other muon atomic orbits are vacant, the muon immediately de-excites to the muonic 1s state. As a result,, electrons by muon-electron Auger processes and characteristic X-rays (muonic X-rays) are emitted. Since a muon is 200 times heavier than an electron, the muon atomic orbit is much closer to the nucleus than the electron atomic orbit and the binding energy is much larger. Therefore, the energies of muonic X-rays are 200 times higher than those of the characteristic X-rays of electrons. After a muon reaches the 1s muon atomic orbit, it either decays into an electron and two neutrinos within its lifetime, or gets absorbed into the nucleus in the case of heavy nuclei. The muon absorption reaction is very similar to EC (electron capture) decay, leading to the formation of a Z-1 nucleus. Muonic X-rays can be applied to elemental analysis by X-ray spectroscopy as well as X-ray fluorescence. As aforementioned, because muonic X-rays have higher energy than the characteristic X-rays from electrons, muonic X-rays can penetrate samples easily. As a result, the impact of X-ray absorption by the sample on the results can be ignored in a muonic X-ray measurement; this is a significantly large problem in X-ray f luorescence analysis. Further, even light elements such as carbon, that are difficult to analyze using conventional X-ray fluorescence analysis have large muonic X-ray energies. For example, the energy of the Kα X-ray (2p1s transition) of a muonic carbon atom is 75 keV, which is the energy corresponding to the lead KX-ray line in the case of ordinary X-ray fluorescence. The probability of muonic X-ray emission depends on the probability of muonic atom formation. One muonic atom is formed when a muon stops in it, and multiple muonic X-rays are emitted during the muon de-excitation process. Therefore, highly sensitive elemental analysis is possible using muonic X-rays. However, it is difficult to predict muon capture probabilities in each element precisely because the probabilities are slightly altered by the chemical environment of the muon capturing atom. It was revealed in our previous work that the muon capture probability for an element is almost independent of the element concentration (in wt %) in a sample, implying that the analytical sensitivity is hardly be altered. When a muon beam is generated by an accelerator, the incident energy into the sample can be selected by the magnetic system in the beam transportation line. Since the muon stopping depth in a sample depends on the incidence energy of a muon, it is possible to control the position of muonic X-ray emissions, allowing the position of analysis to be selected. For these reasons, we agree that muonic X-ray measurement has superior features for elemental analysis: its Non-destructive, position-selective, and multi-elemental analysis method involving negative muons