An Improvement in Grenz Radiography of Paper to Record Watermarks, Chain and Laid Lines.

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 he 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) the beam expressed in millimetres of high density polyethylene (HDPE). [ABSTRACT FROM AUTHOR]