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Thickness Gauging of Organic Films on Large Plastic Body Parts With an Xrf Probe Based on a Roomtemperature Mercuric Iodide Detector

  • Pierre F. Buckens (a1), Jan S. Iwanczyk (a1), Yuzhong J. Wang (a1) and Richard S. Marano (a2)

Abstract

The variability of adhesion promoter (primer) film thickness has been identified at Ford Motor Company as an important factor in maintaining the quality of paint on exterior plastic body parts. XRF offers unique advantages for non-destructive film monitoring. Monitoring of large parts for process control requires a non-contact approach using a small mobile probe. Previous attempts to implement a practical XRF gauging probe have been hampered by the choice of available x-ray detection technologies; proportional counters cannot resolve the layer's fluorescent lines properly and suffer from instability, while Si(Li) or Ge detectors need cooling to low temperatures, which makes their usage impractical for mobile probe.

The recent commercial availability of state-of-the-art Mercuric Iodide (HgI2) x-ray detectors led to the development of a lightweight XRF probe that contains an x-ray tube and a HgI2 detector. Thickness measurements of primer films on plastic substrates were characterized, and 15 sec. acquisition yielded precision of 0.225 and 0.525 pm for film thicknesses of 5 and 20 µm respectively. Ti was added to the substrate and Cr to the film as tagging elements. A ratiometric technique, taking into account absorption of the Ti signal in the film, yields excellent linearity and compensates for instrumental errors such as sampleto- probe distance variation and x-ray source fluctuations.

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[1]. Slapa, M., Huth, G., Seibt, W., Schieber, M., and Randtke, P., “Capabilities of Mercuric Iodide as a Room Temperature X-ray Detector,” IEEE Trans, on Nucl. Sci., vol. NS-23, no. 1, pp. 205-211, Feb. 1976.
[2]. Iwanczyk, J.S., Wang, Y.J., Bradley, J.G., Conley, J.M., Albee, A.L., and Economou, T.E., “Performance and Durability of HgI2 X-ray Detectors for Space Missions,” IEEE Trans. NucL Sci, vol. NS-36, no. 1, pp. 841-845, Feb. 1989.
[3]. Iwanczyk, J.S., “Advances in Mercuric Iodide X-ray Detectors and Low Noise Preamplfication Systems,” Nucl. Inst Meth., vol. A283, no. 2, pp. 208214, Nov. 1989.
[4]. Iwanczyk, J.S., Wang, Y.J., Bradley, J.G., Conley, J.M., Albee, A. L. and Schnepple, W.F., “Advances in the Development of Encapsulants for Mercuric Iodide X-ray Detectors,“ IEEE Trans, on Nucl Sci. NS-37, pp. 2214, 1990.

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