3D confocal microbeam X-ray fluorescence; element distribution; in situ nondestructive analysis; rock and mineral analysis

A three-dimensional confocal microbeam X-ray fluorescence spectrometer was built by using a capillary X-ray lens. The polycapillary X-ray converging lens in the excitation channel and the polycapillary X-ray parallel beam lens in the detection channel were in a confocal state. The confocal structure The background of the X-ray fluorescence spectrum is reduced, which is conducive to reducing the detection limit of X-ray fluorescence analysis technology. In the above confocal structure, the focal spots of the polycapillary X-ray converging lens and the polycapillary X-ray parallel beam lens overlap to form a confocal micro-element, and the detector can only detect the X-ray fluorescence signal from the confocal micro-element. When the confocal micro-element moves in the sample, the three-dimensional X-ray fluorescence information inside the sample can be obtained in situ without damage by using the confocal technology. The polycapillary X-ray converging lens used in the confocal technology has a power magnification of the order of 10³, which reduces the dependence of the confocal technology on high-power X-ray sources, that is, the capillary tube can be designed by using a low-power common X-ray source. X-ray lens confocal X-ray fluorescence technology. The above confocal microbeam X-ray fluorescence spectrometer was used to conduct three-dimensional nondestructive analysis of two rock and mineral samples. In one of the rocks, it was found that the concentration of Cu in the area with high Fe concentration was also high, which to a certain extent reflected the natural characteristics of rock and mineral. mechanism of growth. The experimental results show that the confocal X-ray fluorescence technology can analyze the composition of elements and the three-dimensional distribution of elements in rock and mineral samples without destroying the samples. The confocal three-dimensional microbeam X-ray fluorescence technology has potential applications in the fields of ore exploration, jade material selection and identification, stone edible utensils, “stone gambling” and household stone floor detection.

X-ray fluorescence (XRF) spectroscopic analysis technology has the advantages of rapidity, simplicity, good precision, high accuracy and non-destructive measurement. It has been widely used in routine measurement, online analysis, process control, archaeological research, environmental monitoring and physics. , chemical prospecting and other fields. Among them, mineral analysis is an important branch of mineralogy, which can provide quantitative or qualitative methods to predict and monitor mineral structure, formation and composition. There are many mature analytical methods for mineral analysis, such as atomic fluorescence spectroscopy (AFS), flame atomic absorption (FAAS), proton excited X-ray fluorescence (PIXE), inductively coupled plasma (ICP) and synchrotron radiation XRF. For the analysis of mineral samples by atomic fluorescence spectrometry, flame atomic absorption, and inductively coupled plasma, it is necessary to pre-process the mineral samples before analysis. Analytical method.

Proton-excited XRF and synchrotron radiation XRF can perform non-destructive analysis of mineral samples, but the related equipment is huge, the operation cost is high, and it is not convenient to do on-site in-situ analysis. XRF technology based on common X-ray tube has a wide range of applications in mineral analysis. In recent years, confocal microbeam XRF technology based on capillary X-ray lens has attracted much attention.

The characteristics of this technology are: a capillary X-ray converging lens is placed in the excitation channel, and a capillary X-ray parallel beam lens is placed in the detection channel. The lens is in a confocal state to form a confocal micro-element, and the detector can only detect the XRF signal from the confocal micro-element. By moving the confocal micro-element inside the sample, the three-dimensional XRF information of the sample can be obtained non-destructively. The confocal XRF technology has been widely used in the fields of electrochemistry, biology, atmospheric science and so on.

In this paper, in situ nondestructive analysis of elemental composition and three-dimensional distribution of two rock and mineral samples was carried out by three-dimensional confocal microbeam XRF technology.

Figure 1 is a schematic diagram of a three-dimensional confocal microbeam XRF spectrometer, which consists of a polycapillary X-ray converging lens in the excitation channel.

Fig. 1 Scheme of confocal micro XRF spectrometer

(PFXRL) and a polycapillary parallel beam lens (PPXRL) in the detection channel. The X-rays from the X-ray source are focused on the sample by PFXRL. PPXRL and PFXRL were brought into confocal state using a liquid secondary target adjustment method. In this confocal setup, only the X-ray fluorescence signal in the confocal cell formed by the overlapping of the exit focal spot of PFXRL and the entrance focal spot of PPXRL is detected by the detector. This capillary X-ray lens confocal micro-beam XRF technology is different from the micro-beam X-ray fluorescence technology formed only based on PFXRL, it can move the sample by adjusting the frame to make the confocal micro-element move in the sample, so as to realize the point-to-point three-dimensional image of the sample. In situ nondestructive analysis.

In this XRF spectrometer, the light source used is a common laboratory X-ray molybdenum target light source, and the detector is a silicon drift detector with a resolution of 150eV at 5.9 keV. At 17.4 keV, the confocal cell sizes along the X, Y and Z directions (Fig. 1) are 39.5, 56.3, 56.3 μm, respectively, and the magnifications (power density gains) of PFXRL and PPXRL are 3 100 and 9. When analyzing rock and mineral samples with the three-dimensional confocal microbeam XRF spectrometer constructed in this way, the voltage and current used are 20kV and 1mA, respectively.

The XRF photons in the confocal micro-element diverge in all directions, and within a certain test time t, the number Ni of fluorescent photons of element i received by the detector can be written as

Among them, j₀ is the incident primary X-ray photon density, ΔΩ is the receiving solid angle of the opposite PPXRL and the detector, V is the volume of the confocal microelement, and μ_0i is the primary X-ray mass of the element i at the confocal microelement Absorption coefficient, ρ is the density of the sample, χ_i is the concentration weight percentage of element i, ε_i is the detection efficiency of the detector, η_i is the transmission efficiency of PPXRL, and E_i is the excitation factor that emits X-ray fluorescence radiation of element i. For the K_α line, E_i can be written as

where
ω_k is the fluorescence yield of the K-lineage line, and
(r_k-1)/r_k is the absorption limit transition factor of the K-lineage transition.
(K_α/K_β) and the divergence ratio of K_α.
The exponential part represents the absorption attenuation on the path of the primary incident X-ray reaching the confocal element and the absorption attenuation on the exit path of the radiated X-ray fluorescence, ignoring the absorption of air, Λ₀ and Λ_i can be expressed as

where
μ_0p is the mass absorption coefficient of the primary X-ray at point p on the incident path,
d is the distance from the incident X-ray to the confocal micro-element, and
μ_ip is the mass absorption of the radiated X-ray fluorescence at point p on the exit path. coefficient,
d’ is the distance between the X-ray fluorescence from the microelement to the PPXRL.
Therefore, the photon intensity I_detector detected by the detector can be written as

The minimum detection limit (mMDL) of the confocal microbeam XRF spectrometer was measured using a thin film sample (Micro-matter). As shown in Table 1, the minimum detection limit is

Table 1 Minimum detection limit (MDL) of confocal micro XRF spectrometer(20 kV, 1 mA, 400 s)

In this paper, two rock and ore samples were tested, marked as A and B, and the relatively flat surface areas of A and B were selected for 3D scanning analysis of the two rock and ore samples according to the directions shown in Figure 1. The acquisition time of each scan point was 60 s. The scanning area of the plane element distribution on the XY surface of the rock sample A is 400 μm × 400 μm, and the scanning step is 20 μm; a rock and ore samples were also scanned by XY depth plane element distribution, the scanning area was 1000 μm × 150 μm, and the scanning steps were 100 and 15 μm, respectively. For the XY surface element distribution of the B rock and ore samples, the scanning area is 400 μm × 400 μm, and the step distance is 15 μm;

Figure 2 is the corresponding XRF spectrum of rock A.

Fig 2 The XRF spectrum of mineral A

The characteristic peaks of elements Fe, Ti and K in the fluorescence spectrum are very obvious, and the corresponding fluorescence intensity is relatively large, indicating that rock A contains more Fe, Ti and K elements. Elements Cu, Ni, Mn, Gr and V etc. can also be detected, but their content is less. Fig. 3 is the XRF intensity distribution curves of Fe, Ti and K elements in the depth direction along the Z axis (Fig. 1) inside the sample. It can be seen from Figure 3 that, considering the self-absorption of the sample, the confocal technology can only nondestructively detect Fe, Ti and K elements about 150 μm below the surface of sample A.

Fig 3 Elemental depth profiles of Fe,Ti and K

Figure 4 shows the XRF intensity distribution of Fe, Ti and K elements for rock A in selected areas of the surface. It can be seen from Figure 4 that the distribution of Fe and Ti elements is relatively uneven, and some regions are rich in Fe and Ti elements, and the regional distribution of K element concentration is relatively uniform. In order to analyze the concentration distribution of elements Fe, Ti and K in the rock A in the depth plane perpendicular to the sample surface, the confocal microelement scans the depth plane of the sample in the XZ plane.

Fig 4 Two-dimensional XRF intensity maps for the elements Fe,K and Tifor mineral A surface along XY plane

Figure 5 shows the regional distribution of Fe, Ti and K in the depth direction. It can be seen from Figure 5 and Figure 3 that the concentration distribution of Fe, Ti and K in the depth plane perpendicular to the surface of rock A sample is also different. average.

Fig 5 Two-dimensional XRF intensity maps for the elements Fe, K and Ti for mineral A along XZ plane

A similar three-dimensional elemental analysis of rock and mineral sample B was carried out using a confocal microbeam XRF spectrometer. Figure 6 is the XRF spectrum of rock mine B, it can be seen from the fluorescence spectrum that rock mine B contains more Fe and Cu and less K.

Fig 6 XRF spectrum of mineral B

Fig. 7 shows the one-dimensional depth scanning results of rock and ore B along the Z axis. It can be seen that Fe and Cu have similar distribution trends in the depth direction, and the depth Z positions corresponding to their respective peaks are basically the same, indicating that rock and ore B The places with more iron in the middle have more copper content, which reflects the natural formation characteristics of rock mine B. In order to further understand the concentration distribution characteristics of elements Fe, Cu and K in rock B, we carried out a two-dimensional scan on the surface of rock B.

Fig 7 Elemental depth profiles of Fe and Cu

Figure 8 shows the regional distribution of Fe, Cu and K element concentrations on the surface of rock B. It can be seen that the concentration of Cu in the area with high Fe concentration is also high, which is basically consistent with the law shown in Figure 7; Inhomogeneity in the distribution of elements Fe, Cu and K in the rock B sample. The above-mentioned confocal microbeam XRF spectrometer can not only non-destructively analyze the distribution characteristics of elements inside rocks and mines, but also has the characteristics of being portable, and can perform in-situ non-destructive analysis of minerals and rocks.

Fig 8 Two-dimensional XRF intensity maps for the elements Fe, K and Cufor mineral B surface along XY plane

Using confocal three-dimensional microbeam XRF technology, the elemental composition characteristics of these two rock and mineral samples were identified and analyzed without destroying the samples: sample A contains more elements Fe and Ti, sample B contains more Fe and Ti. Cu, the spatial distribution of these elements in the sample is relatively non-uniform. For sample B, the concentration of Cu in the area with high Fe concentration is also high, which reflects the natural growth mechanism of rock and ore to a certain extent. The confocal XRF technology has potential applications in the field of in-situ non-destructive testing such as ore exploration, jade material selection and identification, stone edible utensils, “gambling stones” and household stone floors.