Document K64M19BYQdXL05KBrm7XLeBy6

37--Industrial Hygiene Fic 37-12 --The effect of tune on radiation expo sure is easy to understand If we are in an area when the radiation level from penetrating external radiation is 100 mR/hr, then in 1 hr we would get 100 mr (nulhrems) If we stayed 2 hr, we would get 200 mr, and so on From O S Atomic Energy Commission, "Living With Radiation--Fundamentals " will be reduced by a factor of (%)2 or % Fig 37-14 shows how the effect of distance on radiation exposure follows the inverse square law Thus, by simply putting 3 ft of space between a worker and the source, the radiation level has been reduced by a factor of almost ten Shielding is commonly used to protect agamst radiation from radioactive sources The more mass placed between a source and a person, the less radiation the person will receive If the mass is concentrated, as m lead, the barrier thickness required for the same degree of protection will be less than it would be for a less dense material such as packed earth This introduces another important term-- half value layers (HVL) Tables of HVL are given in radiation handbooks and typical val ues for two radioactive materials, cobalt-60 and cesium-137, are given in this table Lead Copper Iron Zinc Concrete Cobalt-60 0 49 in 083 087 105 26 Cesium-137 0 25 in 065 068 0 81 21 1064 Using concrete as an example, the table states that a 2 6-inch layer or thickness of concrete will reduce the gamma radiation coming from any cobalt-60 source by a factor of If the gamma emitter is cesium-137, then the half-value layer for concrete becomes 2.1 in. Why? Because the gamma radiation from cesium137 is lower in energy (not as many Mevs) than the gamma radiation from cobalt-60. A mental tool of considerable value that can be used for gamma emitters only is the formula R/hr/1 ft = 6 CE This rule of thumb states that the number of roentgen per hour measured at a distance of one foot from a source is approximately equal to six times the cune strength (C) of the source, times the energy (E) of the source It cannot be used for estimating beta radiation levels 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 are required when this formula is used Some radioactive materials, such as cobalt-60, emit more than one gamma, each with different energies The sum of the en ergy of the total emissions must be used This land of information is given m hand books Terms must be consistent If the source activity is given in milbcunes or microcunes it must be converted to cunes If milbcunes are used to work the problem, then the answer is in milliroentgens Available guides The fifth basic concept is that guides setting forth Maximum Permissible Doses (M P D ) of ionizing radiation are available It will not be possible to discuss these guidelines in de tail References that should be studied are Permissible Dose from External Sources of loruzmg Radiation (NBS Handbook 59) and Maximum Permissible Body Burdens and Max-