Report on a radium safe and lead testing system.

Abstract

A radium safe has been constructed, using 46 standard 2 × 4 × 8-inch lead bricks stacked together. Troughs were dug in four of the bricks and fitted with plastic trays. These constitute the drawers which are surrounded with aluminum shells to prevent marring and are fitted with stops so that they cannot come completely out. The drawers are separated by 1/8-inch steel plate, and the entire safe is surrounded by the same material. The outside surface is an aluminum shell, and the door is 1/8-inch steel fitted with a rubber gasket so that the safe is almost air-proof. The base is 1/2-inch steel plate, and the safe sits on a dolly with four wheels mounted in high-strength bearings so that the 1,200 pound unit can be moved. A small hole was drilled in the top of the safe (through the aluminum and steel sections) for connection to a vacuum pump. Short lengths of plastic tubing are connected to both the safe and the pump. The two pieces of plastic tubing are in turn connected with aluminum tubing which is fitted into holes drilled through two rubber stoppers. These stoppers will fit into either end of a 3/4-inch diameter aluminum cylinder about three inches long. Activated charcoal is placed in the cylinder with a plug of cotton at each end. The purpose of this system is to provide routine leak-testing of a radium supply without undue exposure to the person doing the testing. A charcoal filter can easily be inserted in the vacuum line. Only a few seconds of pumping time are required to determine leakage, using an end-window G-M tube and Baird-Atomic Counter with a normal background of about 20 counts per minute. It is planned to modify the filter construction in the future for use with a well-type scintillation counter. Accumulated data which were taken with a 1.33 mg leaking radium needle, show relations between count-rate, pumping time, and leakage time. A net count rate of 22 counts per minute was obtained from a fifteen-second pumping when the needle had been in a sealed container seven days after being exhausted of its free radon. Obviously, if a leak is detected, the operator must receive some exposure in isolating the defective needle by using the old test tube (or equivalent) system.