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Technical articles Micro X-ray diffraction of cultural properties Yurika Takumi* and Masataka Maeyama** 1. Introduction Journal No. 11, published in November 1961, entitled There are two basic objectives in scientific analysis of “The Einin Controversy” and giving an overview of this tangible cultural properties: appraisal of authenticity, and incident(1). investigation of materials for purposes of preservation X-ray analysis is a non-destructive method and, or restoration. Targets of these analyses are buildings, since the “Einin Pot Incident,” X-ray techniques such sculptures, paintings, craft objects, old documents and as X-ray diffraction (XRD), X-ray fluorescence (XRF), so on. X-ray transmission (XRT) and X-ray CT (computed Many art and craft items have become the objects tomography) have come to play an extremely important of speculation by collectors throughout the world. As role, particularly in preservation and restoration science. a result, the field has an aspect that drives incidents of These techniques are used for scientific analysis of theft and forgery of and damage to works by improper important cultural properties, including national treasures restoration. In the case of paintings, it is not unusual such as the Shosoin Temple treasures (756 C.E.) and the even with famous works for the artist himself to make wall paintings of the Takamatsuzuka Tomb (694 C.E.). changes in the underlying sketch or picture composition This paper presents examples of measuring prior to completion, or for a third party to work on the cultural properties using the latest Rigaku micro- picture after the artist has died. One such example is XRD instruments, including measurements relating the “Madonna del Granduca”(1505) by Raphael. This to investigation of the Takamatsuzuka Tomb, where image of the Madonna and Child, which was publicly methods of preservation and restoration have been exhibited in Japan for the first time in March 2013, is studied since 1972 and research is still ongoing. known for having a jet-black background—a feature not seen in other works by Raphael. During the exhibition 2. Cultural properties and trace/micro-XRD of this masterpiece of the Madonna and Child painting measurement genre, the results of scientific analysis of the jet-black In order to appraise the authenticity of cultural area were also presented. An X-ray image taken in works and ensure proper preservation and restoration, 1984 showed that Raphael had originally painted a it is crucial to first accurately carry out scientific different scene in the background, but that scene had analysis of the materials of the cultural property, and been covered with jet-black paint by someone in a later then develop a comprehensive understanding of factors generation. such as changes in materials due to the passage of To determine authenticity by analyzing the painting time. On the other hand, a major precondition for the process and the time and place where the materials measurement of cultural properties is ensuring that were produced, it is essential not only to carry out the materials are not destroyed. Measurement can be verification by humanities or social sciences, but also done by setting a large item into an analyzer as is, or to analyze based on the knowledge of natural science by conducting point analysis of a tremendous number using the results available from X-ray equipment. of points using a portable system. In measurements It was in 1960 that Rigaku X-ray instruments were using large instruments, there are strong constraints on used for the first time to investigate a cultural property. the equipment itself, and if a portable device is used, There was a question of forgery regarding a Koseto analysis precision is unlikely to be adequate. However, (old Seto pot from the Kamakura period (1185–1333) the latest XRD equipment enables measurements at an that was designated as an important cultural property in ordinary laboratory even with extremely minute trace 1959. This controversy became known to the public as samples, such as pigments that have peeled from a the “Einin Pot Incident.” When the pot was measured painting due to the passage of time, plaster fallen from with Rigaku’s X-ray fluorescence (XRF) spectrometer, the wall of a ruin due to weathering, or a fragment of it was determined that it was a forgery because some pottery unearthed in an excavation. elements in the glaze and substrate had a composition that did not exist in the Kamakura period. At that time, 3. Requirement of XRD equipment for cultural Yoshimichi Emoto of the National Research Institute properties measurement for Cultural Properties, who entrusted this analysis First, we would like to discuss the requirements for to Rigaku, contributed an article to the Rigaku-Denki carrying out XRD measurements of cultural property samples. Generally speaking, samples of cultural * Global Marketing Headquarter, Rigaku Corporation. properties can only be removed in extremely small ** SBU ROD, X-ray Instrument Division, Rigaku Corporation. amounts, typically a few µg. In addition, grinding Rigaku Journal, 31(2), 2015 1 Micro X-ray diffraction of cultural properties is impossible in some cases, and evaluation must be 3.2. 2-dimensional detector performed on a specific micro region. Therefore, micro- If a point focus X-ray source is used, there is no need XRD is essential because it only requires a sample in to suppress vertical divergence with a slit since there the range of tens of µm for evaluation. To carry out this is no smearing, as there is with a line optic system. kind of measurement, the equipment must satisfy the Therefore, the use of a 2-dimensional detector (2D) with following requirements. a larger detection area has advantages. Examples of 2D detectors include: IP (Imaging Plate), CCD (Charge 3.1. Point focus X-ray source Coupled Device), CMOS (Complementary Metal Oxide To measure small amounts of sample or micro Semiconductor), PSPC (Position Sensitive Proportional regions, it is necessary to collimate the X-ray beam, and Counter), and HPAD (Hybrid Pixel Array Detector). Each thus point focus X-rays are useful. If line focus is used, type of detector has its own features and uses depending the X-ray beam can be formed into a point using a slit, on the application. In trace/micro area measurement, the but in this case only part of the entire X-ray beam is detection area and low noise are crucial elements. Table used as output, and thus the X-ray intensity is only 1/10 1 shows the detection area and noise specifications for or less compared with the point focus. each detector. In trace measurement, an important issue In addition, in recent years, the combination of is how efficiently the weak reflections can be measured. high-brilliance microfocus X-ray tubes and multilayer With a large detection area, it is possible to capture mirrors(2) has become popular with advances in more reflections at once, and measurement can be done multilayer coating techniques that enable high even with weak reflections by integrating. Also, the reflectivity of the X-rays. Microfocus tubes have been detecting area has a relationship to exposure time. A large used for a long time to obtain transmission images, detecting area enables measurement in a shorter time. such as medical X-rays, but the focus size was set to a Since effects of crystal grains size appear with samples few µm to increase spatial resolution. Thus, adequate subject to micro area measurement (ranging from a few X-ray output could not be obtained and the method was µm to a few tens of µm), Debye rings from such a small not sufficient for use in X-ray diffraction. Recently, area are usually recorded like diffraction from a single X-ray tubes with a few tens of µm focus have become crystal and do not provide powder XRD pattern. These commercially available. This allows scientists to obtain diffraction patterns have certain crystal orientations, thus usable diffraction patterns from samples in the range diffraction is not observed on the equatorial line, which 100 µm to a few µm. Figure 1 shows a comparison table means a part of the Debye ring cannot be captured if the of X-ray intensities from a 100 µm sample. In the range detection area is too small. Figure 2 shows an example of 100 µm or less, the microfocus tube exhibits an even of the measurement of a 5 µm iron grain. Measurement greater difference compared to a fine focus tube. was performed with an IP, which has a large detection Fig. 1. X-ray intensity ratio (normalized with fine focus). Fig. 2. Diffraction image of 5 µm iron grain. Table 1. Main specifications of each detector (with camera length of 127 mm). IP CCD CMOS PSPC HPAD Detection area (mm)* 465×256 70×70 100×100 140×140 83×106 Detector shape Curved Flat Flat Flat Flat Measurement range (vertical) －45°＋163° ±15° ±21° ±30° ±22° Measurement range (horizontal) 208° 30° 42° 60° 36° 2θ scan Unnecessary Necessary Necessary Necessary Necessary Dark current No Yes Yes No No * Except for IP and HPAD, average values are given for the detection area. HPAD indicates values for PILATUS3 R 300 K. Rigaku Journal, 31(2), 2015 2 Micro X-ray diffraction of cultural properties area. The white squares in the image show the detection 3.4. Automatic XY stage areas for a CCD and a PSPC, which have relatively small In some cases, especially for minerals, measurement detection areas. As can be seen, since the diffraction must be performed at several positions within the same appears in random positions, these cannot be captured by sample. In trace and micro area measurement, exposure a small area detector, and it is necessary to use a detector times can be very long; therefore, it is best to have an with a large detection area such as an IP.

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