Tantalum radiation shield

Methyl radicals were produced by pyrolysis of azomethane (CH3N2CH3). Azomethane was synthesized as describe earlier [18]. It was purified periodically by fteeze-pump cycles at 77 K, and the gas purity verified by RGA. The methyl radical source was similar to that developed by Stair and coworkers. [10, 11] The source was made of a quartz tube with 3 mm OD and 1 mm ID, resistive heating was supplied by means of a 0.25 mm diameter tantalum wire wrapped outside the quartz tube. The len of the heating zone was 4 cm, recessed from the end of the tube by 1 cm. An alumina tube around the outside of the heating zone served as a radiation shield. Azomethane was admitted to the hot tube at a pressure of 1x10-8 to 1x10-7 Torr via a high-vacuum precision leak valve. The pyrolysis tube was maintained at about 1200 K, adequate to decrease the major peaks in the mass sp trum of the parent azomethane at 58 and 43 amu by at least a factor of 100. 

The effective shielding of the detector system from direct and cascade radiation from the Co/Rh source is also very important. A graded shield consisting of concentric tubes of brass, tantalum, and lead was selected. The thickness and the shape of different parts of the shielding were optimized so that nearly zero direct 122 and 136 keV radiation (emitted by the Co source) was in a direct line with the detectors (see Fig. 3.16). 

Standing behind a brick wall, a technician at Brookhaven National Laboratory removes plug from lead shield containing radioactive tantalum. Man at his left holds Geiger counter to monitor radiation levels. (Credit National Archives)... 

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