ASNT

(a)

(b)

(c)

(d)

Figure 30 Category II exposure device: (a) assembly; (b) steel core; (c) wire cradle and S-tube; (d) jacket color designates maximum activity.

however, a thin layer of paint is usually added to eliminate near-surface alpha and beta emissions so that shields can be safely handled. Any remaining near-surface X-ray or gamma-ray emissions are very small in comparison to the dose rate from a loaded exposure device. The category II device shown in Figure 34 has a straight-through channel and a radiation labyrinth of tungsten; there are no uranium moving parts. It is smaller and lighter than S-tube devices and meets ISO 3999:2004 standards. An easily visible green/red indicator shows whether the source is in the safe position. Some designs can be fitted with collimators to enable dual-mode operation, either as a category I or category II device. The category II device shown in Figure 35 can accept iridium 192 or selenium 75 sources and is compliant with ISO 3999:2004 .

SELENIUM 75 EQUIPMENT ISO 3999:2004 compliant selenium 75 gamma radiography exposure devices first became available in the mid-1990s. Due to lower-energy gamma-ray spectrum, selenium 75 exposure devices (Figure 36) are much smaller, lighter weight, and can be manufactured using tungsten internal gamma-ray shielding. SMALL CONTROLLED AREA RADIOGRAPHY Small controlled area radiography (SCAR) is an important technique used by gamma radiographers to predict and minimize operational safety hazards associated with taking a radiograph. It reduces the radiologically controlled safety zone around a shot and uses a category I exposure device (as a camera or lantern) to keep sources safe and secure inside the protective shielding of a device.

CHAPTER 3

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Part 3