Document wDY11xV620B9En1DkmYYjxNqo

IONIZING RADIATION Functions of a Radiation Safety Staff A radiation safety staff must be con cerned with establishing safe distances, de tecting and measuring radiation, making surveys, decontamination, laboratory and other special services, and record-keeping Establishing safe distance and times Work involving external sources of radi ation is often done where permanent bar riers and shielding protect personnel from harmful exposure. But it is passible that certain operations may occasion some ex posure of employees. In these cases, all unnecessary subjection to radiation should be avoided, even to the extent of barring personnel, including technicians, from ac tive areas during exposure tune, such as during use of X-ray equipment (see Fig 42-16) Maintaining a safe distance is especially critical for employees who must work near portable sources of radiation or inadequate ly shielded nonportable sources In such cases, a radiation survey should be made with an appropriate survey meter by a qualified health physicist or technician in order to provide information to establish minimnm safe distances that personnel must abide by A safe distance is die distance nonoper ating personnel must maintain from the radiation source in order to receive no more exposure than that specified in the Radiation Protection Guides even if per sonnel were to remain at that distance con tinually (see previous discussion under "Regulation and Controls"*). The distances specified for a given job may not necessarily remain constant if the radiation in use is modified Hazardous areas indicated by the protec tion survey must be barricaded or roped off to form a restricted section which must not be entered by nonoperating personnel or bystanders Large signs bearing the stand ard radiation symbol, and worded in accord ance with the Federal Register, should be posted The intensity of radiation from a point source of a gamma emitter can be calcu lated approximately from the equation" (good approximation m range 05-2 Mev [million electron-volts], fair in range 03-3 Mev) 1 = 5 ICE where I = Exposure rate in roentgens/hour at a distance of 1 ft C = Number of cunes of the source E = Total energy m gamma radia tion per disintegration, in Mev For radionuclides emitting more than one gamma, E is found by adding the energies together weighted by their relative abun dance For example, Co-60 with a 5 2 year half-life is found on p. 631 of Seaborg"s "Tables of Isotopes" to have 99+ per cent of die gammac total 2 505 Mev (11728 + 13325) Neglecting the remaining 1 per cent, the exposure rate at one foot is 128 roentgens/hour (R/hr). At 10 ft, the expo sure rate u 0.128 R/hr. The intensity is known to decrease as the square of the inverse ratio of distances where D = Distance at which the exposure rate (or intensity) is measured Subscripts refer to locations 1 and 2 I = Exposure rate (or intensity) in R/hr This is known as the "inverse square law " It is also used in the form To find the distance at which the expo sure rate is 2 nuQiroentgens/hour ( 2 mR/hr), given 200 mR/hr at 1 ft Another common practice is to reduce the exposure time and thus the exposure From the knowledge of the dose rate in a given location and the maximum dose which is acceptable for the time period under con sideration, the acceptable exposure tune can J. S. Handlosei. Heath Physics Instru mentation. New York City, Fergamoo Press, 1959 p 15 42-38