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A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 1 9 2 0 components in electrical generators, thereby reducing the overall impact to power generation. SAFETY CONSIDERATIONS USING RADIOGRAPHY ALARA is an acronym for “As Low As Reasonably Achievable,” and de- fined in 10 C.F.R. § 20.1003, as making every reasonable effort to maintain exposures to ionizing radiation as far below the dose limits as practical, con- sistent with the purpose for which the licensed activity is undertaken, taking into account the state of technology, the economics of improvements in re- lation to state of technology, the eco- nomics of improvements in relation to benefits to the public health and safety, and other societal and socioeconomic considerations, and in relation to utili- zation of nuclear energy and licensed materials in the public interest [2, 3] . Four types of radiation are alpha, beta, neu- tron, and electromagnetic (gamma and x-ray) as shown in Fig. 4 [4] . In industri- al radiography, gamma rays and x-rays are commonly used to inspect metal components. These radiations are ion- izing and exposure to personnel must be controlled. Factors that can mini- mize radiation exposure include time, distance, and shielding. Typical termi- nology and units used for radiation in- spection and safety are given in Table 1. ELECTROMAGNETIC RADIATION (X-RAY VS GAMMA RAY) X-rays and gamma rays differ mainly in where they originate. X-rays are produced by impingement of elec- trons, created at a tungsten filament (cathode), on an alloyed target (an- ode). The electron to photons gener- ation is subsequently directed to the part being inspected by controlling a focal spot. Gamma rays are produced by radioactive decay from an excited nucleus. As their origin implies, gamma rays are consistently emitted from the source while x-rays can be controlled (turned off and on). The advantages of gamma rays include portability, lower initial investment, higher penetration, and elimination of the need of electri- cal and cooling supplies, which are re- quired in the case of the x-ray tubes. Disadvantages of gamma rays include source life/decay control of radiation generation, and logistics in handling of TABLE 1 – TERMINOLOGY AND UNITS USED IN RADIATION INSPECTION AND SAFETY Term System units CGS SI Radioactivity (strength) Curie (Ci) Becquerel (Bq) Exposure (travel through medium) Roentgen (R) Coulomb/kilogram (C/kg) Absorbed (received by individual) Radiation absorbed dose (rad) Gray (Gy) Dose (equivalent/effective) Roentgen equivalent man (rem) Sievert (Sv) TABLE 2 – MOST COMMON SOURCES OF RADIATION FOR GAMMA-RAY INSPECTION Gamma-ray source Half life Photon energy, MeV Radiation output, RHM/Ci(a) Penetrating power in steel, mm (in.) Thulium-170 128 days 0.054 and 0.084(b) 0.003 13 (0.5) Iridium-192 74 days 12 rays from 0.21-0.61 0.48 75 (3) Cesium-137 33 yr 0.66 0.32 75 (3) Cobalt-60 5.3 yr 1.17 and 1.33 1.3 230 (9) (a) Output for typical unshielded encapsulated sources; RHM/Ci, roentgens per hour at 1 m per curie. (b) Against strong background of higher MeV radiation. Source: Ref 4. Fig. 4 — Radiation types and relative levels of penetration [3] .