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08 July 2008

Answer to Question #1317 Submitted to "Ask the Experts"

Category: Radiation Basics — Interaction Coefficients

The following question was answered by an expert in the appropriate field:

What are the attenuation factors for various thicknesses of lead for use in radiation protection when working with x rays (70-110 kV range)? Do you have a set of tables to use as reference? Also what are the compositions of various lead aprons? I understand some also use tin to attenuate, but does this mean the amount of lead used is then reduced? I was unable to bring up your Web site answers.

X-ray machines produce a continuum of photon energies resulting from deceleration and ionizating interactions within the tungsten target. As such, shielding for the equipment requires the use of empirically measured data such as those given in NBS Handbook 76 (1971) or NCRP Report 102 (1989). A significant difference in shielding effectiveness for lead can be seen even in the narrow energy region mentioned—for example, 70-110 kVp. Specifically, 1 mm of lead will attenuate the exposure rate at 70 kVp by a factor of about 5000 while the same thickness will only attenuate the exposure at 110 kVp by a factor of about 200. Reducing the thickness to 0.5 mm will result in corresponding exposure reduction by factors of 500 and 40 at 70 kVp and 110 kVp, respectively.

As for tin compared with lead as shielding material, when compared on an equivalent mass-thickness basis, tin is a slightly more effective shield for photons with energies less than about 80 keV. That is, 1 g of tin per cm² will attenuate better in this low-energy region than 1 g of lead per cm². However, lead is much more effective when identical thicknesses of material are used due to its higher density. Also, lead is considerably more effective on a mass-thickness basis above 80 keV. Aprons are typically designed for a target "lead thickness equivalence"—meaning that the apron is intended to provide the same shielding effectiveness as a pure sheet of lead at the stated thickness would provide. Regardless of the exact composition, it is important to verify that the shielding capability of the apron matches the intended application.

References

Medical X-Ray Protection up to Three Million Volts, National Committee on Radiation Protection and Measurements, 1961. Issued as: National Bureau of Standards Handbook 76, 1961.
Medical X-Ray, Electron Beam and Gamma-Ray Protection for Energies Up to 50 MeV (Equipment Design, Performance and Use, 1989, National Council on Radiation Protection (NCRP) Report Number 102 (Supersedes NCRP Report No. 33).

Robert L. Coleman Oak Ridge National Laboratory

Answer posted on 30 October 2001. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.