Theoretical calculation of the exposure rate constant for radium-226

Until recently, the photon spectrum of 226 Ra has not been known with sufficient accuracy to permit calculation of the exposure rate constant. Workers concerned with calculations involving this constant have relied upon measured values. The authors have taken compiled photon spectra authored by M. J. Martin and P. H. Blichert‐Toft in Nuclear Data Tables along with photon cross sections authored by E. Storm and H. Israel in Nuclear Data Tables and calculated theoretical values for the exposure rate constant for 226 Ra. Using the most recent 72‐line spectrum for 226 Ra and its daughter products, an exposure rate constant of 12.44 R‐cm 2 /mg‐h was obtained for unfiltered radium. A large portion of this value was contributed by some low energy x‐ray lines which would not be seen in even a very lightly filtered source. Using a 3‐mm‐o.d. sphere (90%‐Pt, 10%‐Ir, 0.5‐mm filter), a value of 8.12 R‐cm 2 /mg‐h was obtained for the filtered source. Primary, scattered, and bremsstrahlung contributions to this value were 7.610, 0.363, and 0.149, respectively. This value is within 1.6% of the presently accepted value for 226 Ra, 8.25.