ASNT

Figure 28 Four-hole source changer.

rapidly transports the source-wire assembly along the guide tube into a remote shielded collimator attached to the end of the guide tube, which directs a radiation beam to the substrate being investigated while minimizing radiation in all other directions. After a shot is taken, the cranking system rapidly pulls the source-wire assembly back into the shield of the device. A category I device may contain a rotating or sliding shutter mechanism or tumbler and an internal collimator. In these devices the source never leaves the protective shielding of the device, but it may move internally within the device to direct a radiation beam to the fixture under investigation. Hybrid—or dual-mode—devices are also available, which enable exposure devices to be operated either as category I or category II devices. An exposure device can be constructed with a straight-through channel, an S-tube channel, or a helical channel. In the latter two cases, the source needs no front or back shielding in the direction of the channel, so sources can be assembled onto unshielded f lexible wires. In a straight-through channel, source assemblies must have front and/or

back shielding. This usually consists of a tungsten chain-link assembly. Devices with sources in the middle of tungsten chain links can only be used in radial (sideways) emission. In this case, the pellet diagonal is the focal dimension. S-tube devices enable both axial and radial emission to be used. Shots which are configured to use the axial emissions benefit from smaller focal spot size—such as the diameter, which is usually 1.3 × to 1.5 × smaller than the diagonal—enabling closer-proximity shots and higher dose rate at the film plane. The crank-out operation of a category II, S-tube gamma-ray projector can be represented by the schematics shown in Figure 29. IRIDIUM 192 EQUIPMENT Iridium 192 exposure devices are often approved for use with selenium 75 and ytterbium 169, in addition to iridium 192. Examples of several types of commercial exposure devices are described in this section. The category II device shown in Figures 30 and 31 has a removable protective jacket and can carry up to 5550 GBq (150 Ci) of iridium 192

CHAPTER 3

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Part 3