A Method for Testing Radium Containers for Leakage Using the Well-Type Scintillation Counter

Abstract

A new method for testing for leakage of radon from clinical radium containers has been developed. Preliminary tests show the method to be reproducible and highly sensitive. It is based on trapping the radon which escapes from faulty radium containers in a mixture of water and activated charcoal and subsequently measuring the radon in a well-type scintillation counter. Method: Mix 5 ml. of tap water with 0.37 gm. (1 level 1/4 teaspoonful) of activated charcoal in a test tube suitable for well-type scintillation counter use. Place the radium container to be tested in the test tube. Stopper test tube. Leave radium in place for twenty-four hours. Remove radium. Replace stopper. Wait twenty-four hours and count in a well-type scintillation counter. Theory: If it is assumed that radon leaks from a faulty radium container at a constant rate, the rate of leakage, Rn, is related to the count rate, C, by: where K (IJ-c/count per min.) is the counter calibration factor; t is the time the radium container was left in the test tube; T is the time interval between removal of radium and counting; A is the radon decay constant; and f is the fraction of radon lost when the radium is removed from the test tube. Experimental: To establish the values of the constants involved in the equation, preliminary tests were made on 5–0.5-mg. radium cells previously known to be leaking. The half-life of the radioactivity within the test tube was observed to be 3.9 ± 0.1 days, confirming that the radioisotope was radon in equilibrium with its decay products. Leakage through the stopper was less than 1 per cent per day. Additional tests showed that f was less than 1/50. Repetitive tests on the same radium containers showed that the method was reproducible to about ± 7 per cent. The counting sensitivity for a conventional well-type scintillation counter was found to be 1.2 × 106 counts per minute per µg. by comparison with radium standards. Under the conditions given above, and for a standard error of 50 per cent of net count rate with a one-minute sample count and a ten-minute background count, the minimum detectable leak was 2.5 × 10−8 mc for the twenty-four hours of the test. This corresponds to 1.4 × 10−7 mg. of completely de-emanated radium. Conclusions: The method appears to have the following advantages: 1. With proper planning of operations, a large number of radium containers can be tested easily. 2. Handling procedures are simple, so that exposure to the operator is minimal. 3. The method is quantitative. 4. The method is quite sensitive, ample to detect significant radon leakage from clinical sources. 5. Previous methods used for testing clinical radium containers have been based on the measurement of alpha particles from Ra A-B-C (effective half-life about twenty minutes), so that counting had to be done immediately. The measurement of radon, to the contrary, allows counting to be done several days after testing. This makes it possible to use a central counting facility for radiation safety survey work, since the radium can be tested in its normal storage place and only the test tubes transported to the counting facility.