ASNT

(a)

(b)

(c)

Figure 29 Schematic operation of category II, S-tube exposure device: (a) source in storage position; (b) source in guide tube during crank; (c) fully cranked-out source in operating position, shown in collimator.

or selenium 75. Other models in this range have lighter-weight internal shields and different- colored jackets to designate that the maximum activity they can carry is lower. When the jacket is removed, the cylindrical stainless steel core can be used as a crawler device. The radiation shield inside is made of depleted uranium. The category II device shown in Figure 32 can carry up to 5550 GBq (150 Ci) and has a similar S-tube design and operational principles. The category II device shown in Figure 33 can carry up to 4440 GBq (120 Ci) and has a similar S-tube design and operational principles. Depleted uranium (DU) is the shielding

material of choice in iridium 192 projectors and cameras because it provides the greatest shielding power per unit mass and volume. This minimizes the size and weight of exposure devices. DU is produced from natural uranium after removing much of the fissionable uranium 235, which is separated for use as nuclear fuel. Natural uranium contains 99.2% uranium 238 and 0.7% uranium 235, whereas DU contains 0.2-0.3% uranium 235. The half-life of uranium 238 is 4.47 billion years, so it is very slightly radioac- tive. It decays with the emission of alpha particles, beta particles, and gamma rays. These are almost totally internally attenuated within the DU shield;

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Part 3