Iron as an Impurity in Colloidal Graphite: Its Effect on Thimble Ionization Chamber Performance

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Abstract
Small thimble chambers of the “compensated” type, consisting of a graphite-coated plastic cap with an aluminium collecting electrode of appropriate size, have been employed at the London Hospital for almost all routine measurements such as those of percentage depth dose. The plastic material used for the caps has been Perspex and the graphite coating has been obtained by painting the inside of the cap with a colloidal graphite preparation to a thickness of several thousandths of an inch. The size of the aluminium peg has then been adjusted empirically to provide the desired quality dependence characteristics. Until recently, Aquadag had been used for coating the cap, this being one of the preparations supplied by Messrs. Acheson Graphite Ltd. Graphite coatings formed from this preparation showed some tendency to flake off,* and a change was made to Dag in resin-alcohol in order to minimise this tendency. Recently a chamber cap was broken during the course of some experimental work, and was replaced by a new one, coated with Dag in resin-alcohol. It may or may not be relevant to state that this preparation was in fact supplied in an iron can, and had been in stock in our laboratory for several months. During subsequent experimental work with this chamber some suspicion was aroused that its response showed untoward quality dependence.

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