Accuracy in Randon Work

The technique of randon is an elaborate one and demands a high degree of skill from those engaged in it. The volume of radon in equilibrium with one gram of radium element (1 curie) is almost exactly 0·6 c.mm. and as many of the needs of the radiotherapist are in terms of millicuries it is obvious that we are dealing with much smaller volumes than are met with in most gaseous processes. Under ordinary working conditions if a particular requirement of X millicuries is in question, then it can be said that X millicuries is in question, then it can be said that X millicuries within a very few per cent. will be supplied. If, however, this quantity X has to be divided among N items, the question arises as to how the accuracy suffers in the subdivision, as it undoubtedly does. The small quantities mostly involved are called “seeds” their dimensions vary in length from 5–17 mm., the average bore of the glass capillaries is 0·45 mm. diameter external and 0·2 mm. internal. The method of making the seeds is first to draw out a length of capillary tubing from 20–40 cm. long; purified radon under less than atmospheric pressure is then passed into the capillary tube which has previously been exhausted to a pressure of about ·001mm. Hg as measured on a Pirani gauge.