9-6Radiation Doses and HazardsChap. 96. A rule of thumb for exposure from point sources of photons in air at distancesover which exponential attenuation is negligible is as follows:-"- 6CENx= 2'rwhere C is the source strength (Ci), E is the photon energy (MeV), N is thenumber of photons per disintegration, r is the distance in feet from the source,and X is the exposure rate (R h -1).1. Reexpress this rule in units of Bq for the source strength and meters forthe distance.2. Over what ranges of energies is this rule accurate within 20%?Solution:2(a) The approximate formula can be transformed thusly:X(R/h)= 6C(Ci)E(MeV)Napprox [r(ft)]2C(Bq) ~= 6(3.7 x lOlO(BQ/Ci)) / (O:3~7"ft))-1.507 x 10-11C(Bq)E(MeV)N-(r(m)]2(b) Now calculate the exact exposure rate. The uncollided flux density of
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