| Abstract: | SUMMARIES. Radiographic images of paper are used to study the watermarks and the pattern of chain and laid lines originating from the sieve used in the production of the paper. X-rays with low penetrative power are used to obtain images with sufficient contrast as a result of differences in thickness /density of the paper. This article explains the improvement in contrast which can be obtained by lowering the X-ray tube potential, which reduces the penetrative power of the beam. Lowering the tube potential is limited by the attenuation of the radiation in air, which ultimately becomes so strong that exposure times become unacceptably long. By replacing the air with helium, this attenuation of the X-ray beam can be reduced. A container for helium was developed which had an airtight fit to the exit window of the X-ray tube and has an exit window of a very thin layer of polyethylene. The effect of the technique was investigated by measuring the first and second half value layer (HVL) of the beam expressed in millimetres of high density polyethylene (HDPE).
CONCLUSION. The absorption by air is the main obstacle to the use of radiation generated with X-ray tube potentials below 10 kV for investigation of watermarks in paper. The transmission of 5 keV radiation through 43 cm of air is 10% and through helium, more than 99%. As expected from calculations, the contrast in 10 kV images can be improved by replacing the air between the X-ray tube and the specimen by helium. The contrast obtained with helium at 5 kV is six times higher than at 10 kV without using helium. With use of helium, images can be obtained at 5 kV at a distance of 43 cm from the X-ray tube with a rather short exposure time (Table 1). |
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