Real-Time Microradiography of Pearls: A Comparison Between Detectors

Abstract

ries have been using film-based microradiography to reveal minute structures that separate natural from cultured pearls, sometimes alongside other methods such as Laue diffraction and endoscopy (e.g., Galibourg and Ryziger, 1927; Anderson, 1932; Alexander, 1941; Webster, 1954; Farn, 1980; Hänni, 1983; Poirot and Gonthier, 1998; Scarratt et al., 2000; Sturman, 2009). Today, real-time X-ray microradiography (RTX) is the foremost testing method for carrying out this vital work. It can also be paired with the more timeconsuming X-ray computed microtomography for challenging identifications and research purposes (Karampelas et al., 2010; Krzemnicki et al., 2010). With film-based microradiography, pearls are sometimes placed in direct contact with a high-resolution film cassette or immersed in a scatter-reducing liquid (e.g., lead nitrate solution or carbon tetrachloride, both of which are hazardous). X-ray scattering may also be reduced by simply surrounding individual pearls with a thin lead sheet or film. Regardless of the technique, a darkroom and a series of chemicals are necessary for film processing. It takes approximately 20 minutes to select the film and secure the pearls to the film cassette. At the same time, the exposure must be determined assuming an average-sized sample and keeping in mind that pearls in a strand are often graduated in size and that spheres require longer exposure times through the centers than the edges. Next, the film must be developed, fixed, and dried. All told, it is a time-consuming process. The last 20 years have seen RTX steadily replace film-based radiography in the medical sector, and over the last decade most gemological laboratories have followed this trend by partially or fully adopting RTX. This method has several important advantages. No hazardous liquids are needed, and it gives immediate or nearly immediate results, which are easier to store and share among a team of technicians or in publications and presentations. RTX microradiography also requires a lower total amount of radiation than filmbased microradiography, though similar X-ray generating tubes may be used without a need to reduce X-ray scattering. Some gemological laboratories initially employed RTX units with image intensifier (II) technology generally associated with digital cameras to acquire RTX microradiographs. More recent units have employed flat panel detectors (FPD), which can be of larger dimension with high resolution. However, FPD costs approximately US$25,000, about 40% more than II units and cameras. An II is a vacuum tube device that converts invisible X-rays transmitted through the sample into visible light by a cesium iodide (CsI) scintillator. The visible light is then converted into electrons at the photoelectric surface and emitted inside the vacuum tube. The emitted electrons are accelerated and focused by electrodes, which act as an electron lens, onto the output screen and converted into bright visible light that is captured by a digital camera (figure 1, left; Ide et al., 2015). On the other hand, an FPD is a thin, flat solid sensor where invisible X-rays transmitted through the sample are converted into visible light, also by a CsI scintillator. The visible light is REAL-TIME MICRORADIOGRAPHY OF PEARLS: A COMPARISON BETWEEN DETECTORS