Answer to Question #3269 Submitted to "Ask the Experts"

First, it should be noted that two different types of data exist regarding this topic. Some present gamma exposure rates in air while others provide dose equivalent rates in tissue. The two will be very similar and practical implications will probably only arise when dealing with very high dose rates—but the difference is noted nonetheless.

That said, a handy reference that many health physicists employ is a 1970 edition of the Radiological Health Handbook, compiled and edited by the Bureau of Radiological Health, US Department of Health, Education and Welfare. Unfortunately, this reference is no longer in print and must be obtained from a coworker or colleague who happens to have a copy. A table is presented on page 131 of this reference that contains the gamma exposure rates in air for a variety of common radionuclides. The nice thing about this particular table is that it contains the summed contributions for the parents plus all of their daughters. This is particularly important for radionuclides such as ²²⁶Ra, ²²⁸Th, and ²²⁷Ac since the major contribution to exposure originates from their progeny. If you happen to have this reference, please note that the values in the table must be divided by a factor of ten to obtain gamma constants in units of R/h per Ci at 1 meter.

A comprehensive reference for specific gamma-ray dose constants can be found at an Oak Ridge National Laboratory Web site.* This report, generated by staff at the Laboratory, contains gamma-ray dose constants (in tissue) for approximately 500 radionuclides. Please note, however, that the report does not sum the contributions from progeny but instead lists the dose rate for each nuclide individually. One must add the individual contributions together manually to determine the gamma-ray constant for a complete series. Note also that this reference lists the gamma dose constants in units of mSv per MBq at 1 meter. As excerpted from the introduction to the report, "the data were computed on a different basis from earlier tabulations. The dose equivalent rate is given in SI units as mSv/h for a unit source of 1 MBq. To convert to the previous common normalization, one may note that 1 mCi is equal to 37 MBq and 1 Sv is equal to 100 rem. That is, to convert data in units of (mSv/h)/MBq to (mrem/h)/uCi multiply by 3.7." To convert to units of rem/h per Ci the values should therefore be multiplied by 3,700.

Robert L. Coleman, Oak Ridge National Laboratory