Document 8VR2eL0D5V71EJaO0ox88kZ8m

Ionizing Radiation (C), times the energy of the source, () If the name of the radioactive source is known and its quantity (or activity) in cunes is known, as is usually the case, the value for its energy m mev can be obtained from hand books Two words of warning (a) some radioactive materials, such as cobalt-60, emit more than one gamma, each with different energies The sum of the energy of the total emissions must be used This kind of in formation is given in handbooks (b) Terms must be consistent If the source activity is given in millieunes or microcunes it must be converted to curies Example What radiation reading would be expected at a distance of 1 foot from an un shielded 100-milhcune cobalt-60 source'* Answer C s= 100 millieunes =01 cune, E = I 1 mev +13 mev, handbook values -- Co-60 emits two gammas--one with an energy of approximately 1 1 mev, and the other with an energy of approximately 1 3 mev Hence, R/hr/1 ft = 6(0 1) (1 1 + 1 3) = 1 44 which is the same as 1,440 mR/hr For a 10-millicurie cesium-137 source? Answer R/hr/1 ft = 6(0 01) (0 66) = 0 0396 which would be the same as 39 6 mR/hr at 1 ft For a 500-microcune indium-192 source Answer R/hr/1 ft a 6(0 0005) (03 + 0 5) = 0 0024 which would be the same as 2 4 mR/ hr at 1 ft The two reasons for selecting the foregoing examples (a) to illustrate the use of the for mula in calculating dose rates, (b) to show how the application of the formula can assist m the interpretation of fractional amounts of curies The fact that a 500-microcurie source of indium-192 is going to be used in his plant may not mean much to a safety engineer He is not sure just how much 500 microcunes is, or if it ss ill be a big problem However, if the safety engineer can calculate the ex posure rate of 2 4 mR/hr and then apply the distance rule of decreasing radiation (1 divided by distance squared) he becomes aware that 2 ft away from the source the ra diation level would be 0 6 mR/hr, which is negligible The following lists a number of gamma emitters and their radiation levels measured at various distances from a one cune source The numbers were obtained by using the previous formula and then applying the in verse square law for distances of other than one foot Isotope 1 ft 2ft 4ft 8ft 10 ft Cobalt-60 14 5 36 09 023 0 145 Radium-226 90 23 06 0 14 009 Cesium-137 42 1 1 026 007 0 042 Indium-192 59 15 04 009 0059 Thulium-170 0 027 0 007 0002 00004 000027 As an example of how to use the guides in interpreting problems, consider the fol lowing recommendations given in Table 44D (page 1462) for the maximum dose of X radiation or gamma radiation permitted to the whole body, gonads, blood-forming organs, or lenses of the eve Average weekly dose--0 1 R (100 milliroentgen) 13-week dose--3 R (300mR) Total accumulated dose in roentgen is equal to 5 times (age in years, minus 18) Knowing the radiation levels that will exist at various distances from various radio active sources, one can use control proce dures to make sure that cumulative doses will not be in excess of the maximum permis sible dose (M P D ) As a final example of how M P D figures can be used, assume that a 0 5 cune cobalt60 source is involved in a fire The radiation meters have been damaged and there is rea son to believe that the source container no longer is serving as an effective shield Until someone with a good meter can make a sur vey and determine how severe the problem is, at what distance should the area be fenced off? 1 Calculate R/hr/1 ft for a 0 5 cune cobalt-60 source [R/hr /I ft = (6) (0 5) (2 4) = 7 2 or 7200 mR/hr] 2 At a M P D of 100 mR per week, for a 40-hour week, the dose rate per hour should not exceed 100/40 or 2 5 mR/hr 3 Use the following mental tool ,,. Distance ,,, (ft) = y/jm$R5/hmr/1r/hftr 1478